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Until AISC introduced the Load and Resistance Factor Design (LRFD) specification in 1986, the design of steel structures was based solely on Allowable Stress Design (ASD) methodologies. The shift to LRFD has not been readily embraced by the profession even though almost all universities shifted to teaching the LRFD specification within ten years of its introduction.

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Mass timber milestone - Civil + Structural Engineer magazine

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Mass timber milestone - Civil + Structural Engineer magazine

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Oregon rightfully earned its place as one of the greenest regions in the world years ago. A leader in architectural innovation and sustainability, Portland has the second highest number of LEED-accredited buildings in the United States and is recognized for its mix of groundbreaking and influential modern design and sustainable, eco-friendly architecture.

Perhaps most of all, Portland and its neighboring cities are known more recently for their use of mass timber as an alternative to steel and concrete. Known for its lighter environmental footprint and as an environmentally responsible building material, mass timber sequesters carbon dioxide from the atmosphere by storing carbon in the structure during the entire life of the building. It also promotes sustainable forest management practices.

The headquarters of the non-profit First Tech Federal Credit Union in Hillsboro is the latest groundbreaking project to join Oregon's architecturally renowned landscape. Weeks ago, the final cross-laminated timber (CLT) panel was placed, completing the building's structure and marking a significant milestone in mass timber architecture. At 156,000 square feet, the First Tech headquarters will be the largest CLT structure in the country, adding to a growing list of commercial buildings in Oregon to join the mass timber movement of recent years.

The decision to build such a large structure out of timber was born out of First Tech's "people first" ethic. When doors open this summer, the building will support and promote the health, happiness, and comfort of the employees located there.
The research
Swinerton Builders were brought onto the First Tech project by Portland-based architect, HACKER, who saw mass timber as a crucial part of the overall development strategy for the First Tech project. Emphasizing energy efficiency, indoor air quality, equal access to natural light, and the value of connecting people with nature, HACKER recognized the benefits of the building material from the beginning.

However, HACKER needed to optimize ways to integrate mass timber as the structural solution for this particular structure. To better understand how to leverage the value and strength of mass timber in this project, Swinerton committed to researching alternative structural scenarios that compared structural steel and nail-laminated timber (NLT) to CLT. Swinerton and HACKER were able to recommend CLT to the client for this project by unlocking several strengths of the CLT panels.

Running in both directions, Structurlam's CrossLam CLT panels simplified the edge details of the building, were incorporated as the structural diaphragm of the building for earthquake stability, and provided a lighter overall structural system, obviating the need for costly ground improvements or deep foundations. Because of the precision with which the CLT panels can be fabricated, virtual design and construction could be leveraged to produce more than 4,500 mechanical/electrical/plumbing (MEP) penetrations in the factory, reducing field time for follow-on trades.

After an in-depth research period, Swinerton's core team took a trip to Canada, where more CLT projects had been completed. Their purpose was to gain a better understanding of constructability aspects and considerations for CLT and its installation. Swinerton focused its visit to Kelowna, Penticton, and Okanagan Falls on not only Structurlam's facilities and detailing group, but also by visiting construction sites and meeting with general contractors familiar with CLT installation.
The design
Following the research period, the Swinerton team partnered with Structurlam Products to harness the benefits of a glulam and CLT structure through a holistic approach to design and construction. Structurlam provided the design of concealed glulam connections and CLT horizontal diaphragms, as well as a fully fabricated mass timber package for the First Tech project.

With the First Tech project, Swinerton Builders identified and capitalized on the ripple effects of optimizing the building's structure. Swinerton employed CrossLam CLT not only as a structural solution, but also as a path toward simplifying other building systems, such as the foundation and envelope. Through pre-planning and coordination, Structurlam implemented a shift toward a more "systems-oriented" construction process.

Swinerton managed and coordinated project partners through a fully integrated, virtual design and construction BIM model, which included the timber structure, the steel lateral systems, and the MEP services. At the Structurlam facility, glulam and CrossLam CLT were prefabricated for First Tech by CNC machines based on a fabrication-level 3D model. Like Lincoln Logs or Legos, the materials are pre-constructed to an exact fit, giving way for smarter utilization during the construction period.
The installation
Upon completion of the design, 3D modeling, and shop drawings, the first glulam columns were delivered and installed on July 27. In total, 626+ CrossLam panels and 988+ glulam columns and beams were delivered in 64 truckloads. On Nov. 16, the final CrossLam panel was installed, completing the timber and steel frame of a 156,000-square-foot structure in 15 weeks. How did we manage to construct such a large structure in just 15 weeks? Through self-performing the construction process and using optimizable materials.

CrossLam CLT is manufactured to span in two directions, with strength in both major and minor axis directions. By optimizing the frame to be as efficient and light as possible, this allows us to cut down on heavier transport and installation time and costs. Detailed preconstruction planning also helped to ensure that the installation of CrossLam CLT was easy and efficient, simplifying the foundation system and making ground improvements in the foundation unnecessary.

A milestone project, First Tech Federal Credit Union is a shining example of the possibilities of building with mass timber and the associated benefits. In a time when eco-friendly buildings are not a luxury but a necessity, mass timber structures have drawn the world's attention for their strength, light environmental footprint, and cutting-edge prefabrication technology.

Erica Spiritos, is a mass timber specialist with Structurlam( www.structurlam.com ), and William P. Silva is preconstruction manager with Swinerton ( www.swinerton.com ).

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Mass timber milestone - Civil + Structural Engineer magazine

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Oregon rightfully earned its place as one of the greenest regions in the world years ago. A leader in architectural innovation and sustainability, Portland has the second highest number of LEED-accredited buildings in the United States and is recognized for its mix of groundbreaking and influential modern design and sustainable, eco-friendly architecture.

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Tensile Structures: How Do They Work and What Are the Different Types? | ArchDaily

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Tensile Structures: How Do They Work and What Are the Different Types? | ArchDaily

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Historically inspired by some of the first man-made shelters-such as the black tents first developed using camel leather by the nomads of the Sahara Desert, Saudi Arabia, and Iran, as well as the structures used by Native American tribes-tensile structures offer a range of positive benefits compared to other structural models.

Tensile structure is the term usually used to refer to the construction of roofs using a membrane held in place on steel cables. Their main characteristics are the way in which they work under stress tensile, their ease of pre-fabrication, their ability to cover large spans, and their malleability. This structural system calls for a small amount of material thanks to the use of thin canvases, which when stretched using steel cables, create surfaces capable of overcoming the forces imposed upon them.

Predominantly used in coverings of sports centers, of arenas, and industrial and agroindustrial constructions, tensile structures are based on the old systems used during the Roman Empire. However, from the Roman period until the mid-20th century, due to the low demand, usability, and lack of manufacturers of cables, canvasses, and connections capable of resisting the forces generated, there were few technological advances. It was only after the Industrial Revolution and the triggering of the era of Fordism that new developments were able to meet the intrinsic needs of this construction system. The low cost of mass production and the demand for systems capable of adapting to the most varied terrains with large spans, such as circus tents for example, encouraged the development of the technique.

The instability caused in previous models by the application of interlaced cables and very light covers, resulting in structural deficiencies, was solved during the middle of the last century. This was done thanks to a system of steel cables and fiber membranes with a high degree of strength, together with layers of waterproof coatings, giving protection against ultraviolet rays, fungus, fire, and allowing greater or less translucency and reflectivity.

Such progress was only possible thanks to the physical-structural studies initiated by German architect and engineer Frei Otto, who since the 1950s conducted the first scientific studies and the first works of roofing using tensioned steel cables combined with membranes.

As a student, Otto visited the office of Fred Severud, where he saw the Raleigh Arena in North Carolina and was impressed by the bold aesthetics and propitious comfort of the project. Back in Germany, he began to explore small-scale physical models, empirically generating several surfaces, by means of chains, pulled cables, and elastic membranes.

Convinced by the usefulness of tensioned roofs, he developed the first large-scale project using the system, which later enabled projects including Olympic stadiums, clubs, zoo, and pavilions. In 1957 he founded the Center for the Development of Light Construction in Berlin. Seven years later, in 1964 he created the Institute of Light Structures in Berlin at the University of Stuttgart, Germany.

Author of notable projects passed by experiments and technical refinement, such as the German Pavilion for the 1967 Expo in Montreal and the Munich Olympic Stadium in 1972, the architect is renowned for his intense research work and was honored with the RIBA Royal Gold Medal in 2006 and the 2015 Pritzker Prize. Frei Otto is still responsible for the first comprehensive book on tensile structures-"Das Hangende Dach" (1958)-and intensified the idea of reinventing material rationality, prefabrication, flexibility and luminosity over internal space, and even sustainability, when the term was not yet used in architecture.

There are three different main classifications in the field of tensile construction systems: membrane tensioned structures, mesh tensioned, and pneumatic structures. The first relates to structures in which a membrane is held by cables, allowing the distribution of the tensile stresses through its own form. The second case corresponds to structures in which a mesh of cables carries the intrinsic forces, transmitting them to separate elements, for example, sheets of glass or wood. In the third case, a protective membrane is supported by means of air pressure.

Structurally, the system is formalized by combining three elements: membranes, rigid structures such as pole and masts, and cables.

The membranes of PVC-coated polyester fibers have greater ease in factory production and installation; lower cost; and medium durability-around 10 years.

PTFE-coated glass fiber membranes have superior durability-around 30 years; and greater resistance to the elements (sun, rain, and winds); however, they require skilled labor.

In this system, there are two types of support: direct and indirect. The direct supports are those in which the construction is arranged directly on the rest of the building structure, while the second case is arranged from a raised point such as a mast.

The cables, which are responsible for the distribution of the tensile stresses and the hardening of the canvasses, are classified in one of two ways according to the action which they perform: load-bearing and stabilizing. Both types of cable cross orthogonally, ensuring strength in two directions and avoiding deformations. The load-bearing cables are those that directly receive the external loads, fixed at the highest points. On the other hand, the stabilizing cables are responsible for strengthening the load-bearing cables and cross the load-bearing cables orthogonally. It is possible to avoid attaching the stabilizing cables to the ground by using a peripheral fixation cable.

Further, some nomenclatures for different cables are generated according to their position: a ridge-line cable refers to the uppermost cable; while valley cables are fixed below all other cables; radial cables are stabilizer cables in the form of a ring. Ridge-line cables support gravitational loads while valley cables support wind loads.

Here are some projects already published on Archdaily using tensile structures:

References

ARCOLINI, Tatiana; BARRADAS, Paula. Coberturas tensionadas são soluções eficientes e econômicas. Available at: <https://www.aecweb.com.br/cont/m/rev/coberturas-tensionadas-sao-solucoes-eficientes-e-economicas_7990_10_0>. Acessed on 24 Dec 2017.
Tensoestruturas: Cabos e Membranas. Available at: http://wwwo.metalica.com.br/tensoestruturas-cabos-e-membranas. Acessed on: 24 Dec 2017.
Tensoestruturas: Cobertura de Estruturas de Membrana Tensionada. Available at: <http://wwwo.metalica.com.br/tensoestrutura-cobertura-de-estruturas-de-membrana-tensionada>. Acessed on 24 Dec 2017.

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Tensile Structures: How Do They Work and What Are the Different Types?

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Historically inspired by some of the first man-made shelters-such as the black tents first developed using camel leather by the nomads of the Sahara Desert, Saudi Arabia, and Iran, as well as the structures used by Native American tribes-tensile structures offer a range of positive benefits compared to other structural models.

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The Think Wood Research Library is a central resource for research on designing and building with mass timber. The library links to research publications from around the world on mass timber building materials and technology, structural systems, performance, and market adoption. It includes research reports, books , academic journal publications and conference proceedings.

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TESIS ESCRITA VIGUETAS PREFABRICADAS .pdf

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(15) [ FREI OTTO : THE GERMAN PAVILION - EXPO 1967 ] - YouTube

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(15) [ FREI OTTO : THE GERMAN PAVILION - EXPO 1967 ] - YouTube

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Why Concrete Is So Much Stronger With Just a Little Bit of Rebar

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Why Concrete Is So Much Stronger With Just a Little Bit of Rebar

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It's tempting to think of "strength" as a simple variable and that various materials all fall somewhere on a spectrum of strong to weak. The reality, of course, is much more complex. Different materials are strong in specific situations, but weak in others. For a really robust material, you have to start mixing and matching. Reinforced concrete is a perfect example.

Concrete is great at putting up with stress in the form of compression. It can take a lot of squeezing abuse before it gives up. It's terrible at handling tensile stress though, and is not suited to fight against forces that would threaten to pull it apart.

This is a particular problem in construction, as Practical Engineering explains, because concrete in a building almost always experiences both kinds of stress. If you put weight on a concrete beam from above, its top will hold up heroically against the compression. Its underside, however, will experience severe tensile stress as the beam deforms. This creates a situation that engineers often refer to by the technical term "bad."

Rebar helps to alleviate this issue because, unlike concrete, rebar is pretty great taking tensile stress. As a result, rebar inside of concrete both strengthens the resulting mixture by making its strength more all-around, and also decreases the speed at which failure occurs, giving engineers crucial time to spot a disaster before it happens.

Watch Practical Engineering explain with some handy visual examples, take a moment to appreciate what's underneath you the next time you stand on a bridge:

Source:
Practical Engineering

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Why Concrete Is So Much Stronger With Just a Little Bit of Rebar

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It's tempting to think of "strength" as a simple variable and that various materials all fall somewhere on a spectrum of strong to weak. The reality, of course, is much more complex. Different materials are strong in specific situations, but weak in others. For a really robust material, you have to start mixing and matching.

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Así se cortan los troncos para obtener distintas apariencias y utilidades en la madera | Plataforma Arquitectura

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Así se cortan los troncos para obtener distintas apariencias y utilidades en la madera | Plataforma Arquitectura

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Madera Laminada Cruzada (CLT): qué es y cómo usarla | Plataforma Arquitectura

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Madera Laminada Cruzada (CLT): qué es y cómo usarla | Plataforma Arquitectura

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The Design Building brings the University of Massachusetts design programs - Architecture, Landscape Architecture and Regional Planning, and Building and Construction Technology - together in one creatively designed and sustainable building. Photo: Albert Vecerka/Esto
The UMass Design Building is the first public mass-timber building employing cross-laminated timber on the East Coast.

The University of Massachusetts (UMass) wanted to bring its design programs - Architecture, Landscape Architecture and Regional Planning, and Building and Construction Technology - together in one creatively designed building exemplifying sustainable construction practices. While a steel- or concrete-framed structure would be conventional for this building's size and use, the new John W. Olver Design Building features a timber-framed superstructure with an innovative composite floor system. The exposed wood structure emphasizes the potential of engineered wood elements while complementing and influencing the aesthetics inside and out.
Creative solution
Simpson Gumpertz & Heger Inc. (SGH) worked closely with Equilibrium Consulting, Inc. to design an 87,000-square-foot attractive, functional, and innovative structure. One challenge was designing open floor plates, originally conceived as a steel structure, with a mass-timber system. Historically, heavy timber buildings were constructed with large, closely spaced beams spanning 25 feet or less. Steel and concrete structures are commonly arranged in 30-foot bays, but easily allow engineers and architects to achieve longer spans. For the Design Building, the gravity system was changed to glue-laminated (glulam) beams and columns with an innovative approach for the floor system to construct the reimagined structure.

The solution was a composite cross-laminated timber (CLT) and concrete floor system. CLT, developed in the early 1990s, is a wood technology where sawn lumber from smaller trees are glued together in perpendicular layers to form large, mass-timber panels. Although CLT has been popular in Europe for longer than 20 years, it is a relatively new product for the United States. Taking this innovation one step further, perforated steel plate shear connectors were embedded into both the CLT panels and glulam beams and bonded with epoxy resin. A 4-inch-thick concrete slab was then placed on top of the CLT, which bonded to the steel plates and allowed the wood elements to act compositely with the concrete slab. This composite action greatly increases the strength and stiffness of the floor system, allowing the design team to achieve the required spans of up to 26 feet, typically reserved for concrete and steel construction.

The wood-concrete composite floors have excellent acoustics and dynamic damping properties. They also experience smaller deflections and limited potential for long-term creep effects as compared with a non-composite CLT floor. The steel reinforcement contained in the concrete topping helps mitigate concrete cracking and also facilitates transferring shear forces at diaphragm connections. Aesthetics were another important consideration in the development of the composite floor system. The underside of the CLT floor decks are mostly exposed, offering a warm and attractive ceiling, while the concrete slab is polished, providing a modern-looking and durable floor finish.
Dramatic design details
A defining feature of the Design Building and the showcase for its use of mass timber is the multi-story atrium. The atrium roof spans up to 56 feet to create a wide-open space below and an outdoor classroom/green roof above, complete with large pyramid skylights. A system of 3D "spider trusses" - consisting of composite concrete-glulam top chords, round glulam struts, and steel tension rods - creates a visually striking perspective from inside the space. The structural elements are left exposed as a visual demonstration of the efficient use of wood in compression and steel in tension. While one end of the spider trusses bears on typical glulam and steel columns, the other end bears on a story-deep steel truss that is visible from the atrium roof through the glass façade.

A suspended CLT stair complements the exposed structural elements in the atrium. In a hybrid use of materials, the CLT stair is suspended via steel rods from the steel story-deep truss that is in turn supported by two large glulam columns.

The structure's lateral system is composed of CLT shear walls and exposed glulam braced frames. The vertical CLT panels compose the walls of the building's stair towers, elevator core, and mechanical shafts. They presented an additional design challenge with their tall, narrow layout and relatively small gravity loads, which resulted in significant uplift forces from wind and seismic forces. Similar to the composite slab connectors, the shear wall hold downs are composed of steel plates embedded into the CLT panels, secured to the wood with epoxy, and welded to a conventional steel connection plate anchored to the concrete foundation.

The glulam braces are connected to the glulam columns by a series of embedded knife plates and tight-fit steel pins. The steel plates come together to form a true pin connection that is left exposed to provide unique aesthetic that honors the connection's structural function.
Secrets to success
As part of the design effort, SGH vetted production quality of the epoxy CLT connections by load testing one-fourth-scale mockups of the shear wall hold downs to confirm their strength and ductility. Additionally, SGH demonstrated to Massachusetts Department of Public Safety (DPS), the authority having jurisdiction, that this structural design meets the current building code's general intent. The design team developed a design basis with supporting standards, testing data, and proposed future code language for DPS's review.

Creative systems integration and willingness to try something new drove this project's success. With an exposed structure, the layout directly affects the building's aesthetics. Successful implementation required close coordination with architecture, and creative detailing of the wood connections to accentuate the visual appearance of the exposed wood framing. The design and fabrication team developed a variety of connections using embedded steel plates and dowels, tight-fit pins, screws, shear keys, and specialty cast steel clevises. These unique connections allowed the design team to cleverly hide some elements, such as beam-to-column connections, and emphasize others, such as the center hub of the spider trusses.

Engineered mass-timber structures employing CLT and glulam components allow for precision fabrication in a controlled factory environment. Computer numeric control (CNC) machining decreases wasted material volumes while increasing productivity and precision. This type of controlled fabrication ensures components fit together when delivered onsite and reduces field equipment needs and labor. Even with a moderate learning curve facing the small crew of timber erectors, construction schedules realized approximately 10,000 square feet of floor installation per week.

Several of the structural elements composing the large atrium space required additional onsite assembly and coordination. The steel truss was shipped to the site in pieces, assembled in the staging area, and lifted into place in two crane picks. The atrium roof was shored while the spider truss elements were connected to the center hub and the concrete topping was placed. Once the concrete topping achieved sufficient strength, the shoring was carefully removed. The stair was shipped to the site in large sections and erected by first suspending it from the steel rods and then field-connecting the pieces together with steel plates and epoxy in a carefully monitored procedure.

Factory fabrication also facilitated the use of the shear connectors for the composite slabs and hold downs for the shear walls, which both rely on epoxy to anchor embedded connections. The epoxy required strict conformance with environmental temperature, humidity, and volume tracking to ensure fabrication consistency and engagement within fabricated grooves - all of which were able to be carefully monitored and controlled in the factory. Most of these anchors were embedded during the shop fabrication process, with the CLT stair being the one exception.

The exposed structure required collaboration among the mechanical, electrical, plumbing, and fire protection (MEP/FP) systems. CLT shaft walls provide attractive enclosures for the MEP/FP chases, serve as bearing walls to support the floor and roof decks, and contribute to the structure's lateral force-resisting system. Openings in the CLT walls and decks were designed and detailed to allow MEP/FP systems to pass through the structure as needed. Electrical conduit buried within the composite CLT decks required additional coordination to maneuver the runs around the composite CLT-concrete shear connectors but allowed for a cleaner-looking ceiling in the end.
A striking example
As one of the larger modern mass timber buildings in the United States, and a first of its kind in Massachusetts, the Design Building uses wood products in new and creative ways to serve as a positive example for future work and provides a collaborative learning space for UMass's design programs. The Design Building offers a striking, first-of-its-kind example of many uses for mass construction methods and successfully provides an inspiring space for students, faculty, and visitors.

Watch videos about design and construction of the UMass John W. Olver Design Building at http://bct.eco.umass.edu/about-us/the-design-building-at-umass-amherst/design-building-videos.

Greggrey Cohen is senior principal, Jeffrey Langlois is senior project manager, and Nancy Varney is senior staff I with Simpson Gumpertz & Heger Inc. ().

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An education in mass timber - Civil + Structural Engineer magazine

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The Design Building brings the University of Massachusetts design programs - Architecture, Landscape Architecture and Regional Planning, and Building and Construction Technology - together in one creatively designed and sustainable building. Photo: Albert Vecerka/Esto The UMass Design Building is the first public mass-timber building employing cross-laminated timber on the East Coast.

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Brian Lemay, Research Assistant Intern, National Ready Mixed Concrete Association Credit: 1 LU/HSW Course Number: ZG082019CS Sponsored by: Course Overview:
Concrete is the material of choice for the tallest buildings in the world and infrastructure designed to last centuries. To meet demands for these cutting-edge projects, concrete must be stronger, more durable and more workable than ever before. This article explores how new products, manufacturing methods and research are developing innovative concretes to meet these new challenges. Bendable concrete, smog eating concrete and carbon capture are just a few examples of new technologies enhancing a product that is nearly 5,000 years in development.
Learning Objectives:
1. Understand new technologies used in concrete manufacturing
2. Discover how innovative concrete products can improve project performance 3. Learn how to implement the latest concrete innovations in building and infrastructure projects 4. Demonstrate the importance of incorporating new technologies to enhance resilience and sustainability in the built environment Introduction
What do the Jubilee Church and the Pantheon have in common? They are both places of worship in Rome. But besides that, they are both built with innovative concrete. The Romans mastered the use of concrete 2,000 years ago to build some of the most iconic structures ever built. Although different than today's concrete, Roman concrete used the same principals, combining aggregate with a hydraulic binder. The aggregate included pieces of rock, ceramic tile and brick rubble often recycled from demolished buildings. Volcanic ash, called pozzolana, was the favored binder where it was available. Gypsum and quicklime were used as binders also. And even 3,000 years before, the Egyptians used a form of concrete made with mud and straw to build the pyramids. Today of course, most concrete is made with portland cement, invented in 1824, and combined with high quality quarried aggregate. Most modern concrete is augmented with innovative products and additives to enhance both plastic and hardened properties.

Innovative supplementary cementitious materials (SCMs) such as fly ash, slag cement and silica fume are used to increase strength, durability and workability. Chemical admixtures affect set time, freeze thaw resistance and flowability. Tiny fibers are added to increase ductility and control cracking. Carbon dioxide is injected into concrete to improve strength and capture greenhouse gasses. Some enhancements actually scrub pollutants from the surface of concrete and from the surrounding atmosphere which is what makes the concrete on the Jubilee Church so innovative. The exterior curved surfaces are coated with titanium dioxide (TiO ₂) cement which eats smog, helping to keep the surface clean.

Concrete is the most widely used building product in the world. It's mostly made locally with local materials. It's cost effective, available everywhere, strong and durable. Although conventional concrete can tackle most jobs, it is also the material of choice for the tallest buildings in the world and infrastructure designed to last centuries. Although concrete is not always synonymous with innovation, new products and manufacturing methods are enhancing concretes performance to tackle modern challenges. This article explores some of these latest innovations.
Self-cleaning Concrete
Imagine concrete that can clean itself and even the surrounding air of harmful pollutants. That's what concrete made with titanium dioxide (TiO ₂) can do. The function of TiO ₂ cement is to break down harmful pollutants in the air via a reaction catalyzed by light, or photocatalysis due to titanium dioxide which is added to the cement during its production. This capability of TiO ₂ cements was inspired by the ability of certain microbes to break down harmful chemicals by modifying their oxidation state, also through photocatalysis. However, in photocatylitic cements the reaction is carried out by the titanium whereas microbes rely on natural enzymes. The cement breaks down organic as well as inorganic pollutants, and it is intended to be used for projects in urban centers where air pollution and poor air quality are most pronounced.

An example of how TiO ₂ cements break down pollutants can be seen in its conversion of nitrogen dioxide (NO ₂), a harmful compound mostly produced by burning fuels in cars and trucks. Nitrogen dioxide is one of the compounds responsible for acid rain, smog, respiratory problems and staining of buildings and pavements. The reaction with sunlight produces hydroxyl radicals which react with NO ₂ to produce NO ₃ which is dissolved by water after reacting with the cement surface.

Research data of TX Active®, a TiO ₂ cement marketed by Lehigh Hanson (a division of HeidelbergCement Group) in the US, indicates that "up to 50% of these atmospheric pollutants could be reduced in some cities if only 15% of the buildings and roads were resurfaced with TX Active® cement." TX Active® was first used for the curved panels on the Jubilee church (also known as Dives in Misericordia Church) in Rome, which used the photocatalytic cement panels for its stylistic shells. Since then, Italcementi (a division of HeidelbergCement Group) has dedicated decades of research to photocatalytic cement products. This cement is promising in its potential to greatly improve urban life and the environment.
Case Study: Jubilee Church, Rome, Italy
According to architects Richard Meier and partners, the Jubilee Church in Rome was "conceived as part of Pope John Paul II's millennium initiative to rejuvenate parish life within Italy." The project consists of the church itself as well as both secular housing and housing for the clergy. The church is most easily distinguished by the three large concrete shells which are meant to represent the Holy Trinity. Given the symbolic importance of the shells, their appearance is an absolute priority. Thus, due to the fact that the shells need to remain in pristine condition, it was only natural that "self-cleaning" TX Active® photocatalytic concrete was used to ensure that the shells would not accumulate stains due to smog. Completed in 2003, the photocatalytic shells have notably remained clean and white, performing constant self-maintenance.
Bendable Concrete
Bendable concrete presents an efficient alternative primarily in the construction and maintenance of infrastructure, where concrete is subject to harsh weather conditions and extreme loading. The design which gives bendable concrete, or engineered cementitious composite (ECC), its impressive ductility is based off nacre, the substance that coats the inside of abalone shells. Nacre is composed of small aragonite platelets that are held together by natural polymers, allowing it to be both hard and flexible as platelets are free to slide side to side under stress. This effect is mimicked in bendable concrete by dispersing tiny fibers throughout. Victor C. Li of the University of Michigan, where ECC was first researched and invented, states that bendable concrete "can deform up to 3 to 5 percent in tension before it fails, which gives it 300 to 500 times more tensile strain capacity than normal concrete". It is of course this incredible ability to tolerate tensile strain that makes bendable concrete unique.

This enormous increase in ductility suggests various potential applications. Firstly, in roads as well as other paved surfaces which must bear repeated loading of heavy vehicles, bendable concrete would crack less often, preventing further weathering primarily from road salts which corrodes steel reinforcement. Further, due to ECC's capacity to absorb greater quantities of energy without being damaged, it can be used to make reinforcing elements such as the dampers on the Seisho Bypass Viaduct in Japan, which is roughly 28 kilometers long. Dr. Li also states that ECC has been employed as earthquake resistance in tall buildings in Tokyo and Osaka and further suggests that ECC would be useful in underground construction as well as the construction of water infrastructure.

However, before it can be more widely commercialized for such large-scale projects, bendable concrete must become more readily available. To be economically viable, it must be supplied efficiently and not overused on projects. But, it is paramount that design professionals be made aware of the product and its potential as they might otherwise overlook a promising concrete option for structures that require the ability to deal with considerable tensile strain.

Bendable concrete also has self-healing capabilities. Because bendable concrete keeps cracks relatively small, natural reactions within the hardened concrete generate "healing" products through carbon mineralization and continuous hydration which repairs the cracks and restores the durability of the concrete. Bendable concrete is a promising technology that already has proven itself through commercialization by several companies.

In fact, fiber reinforced concrete is not new. Many companies supply fibers for use in concrete with the objective of improving strength and durability of concrete in some way. Fiber reinforced concrete accomplishes this by incorporating fibers made of steel, glass or organic polymers (plastics). Sometimes naturally occurring fibers such as sisal and jute have been used as well. These fibers are primarily used to combat plastic shrinkage and drying shrinkage which can otherwise crack and damage the concrete. This resistance to shrinkage and subsequent cracking is the key to extending the lifespan of concrete, decreasing the frequency of costly repairs. Fibers also keep existing cracks from widening and further damaging the concrete when they do appear. And more recently, steel fibers are being used in structural applications to reduce the amount of traditional steel reinforcing bars, saving time and labor.
Case Study: 42 Broad, Fleetwood, New York
42 Broad is a 16-story mixed-use development near New York City being built with Insulating Concrete Form (ICF). ICF construction is becoming more mainstream with thousands of projects built in the US, but still considered innovative by many. ICFs sandwich a reinforced concrete wall between forms made of rigid polystyrene insulation that stay in place after the concrete hardens. There are several taller ICF buildings in Canada, but at 16 stories, 42 Broad will be the tallest in the US.

The real innovation on this project is panelizing the Amvic ICF blocks and using HelixTM steel fiber reinforcement. The ICFs are assembled off-site in a nearby plant and arrive at the jobsite as custom panels up to 50 feet long which results in labor and time savings on the job site, meaning the owner can occupy the building earlier. Part of what makes this process possible is the use of steel fibers in the ready mixed concrete to replace the horizontal reinforcing steel which eliminates costly horizontal rebar slices.

LafargeHolcim is one of the first companies to commercialize bendable concrete with a product called Ductal®. Ductal® is an ultra-high-performance concrete (UHPC) that incorporates fibers into the concrete mixture in order to improve strength and ductility along with a host of other benefits. LafargeHolcim distributes the premix powder, fibers and admixtures necessary to produce Ductal® to their partners, who then mix it into concrete. LafargeHolcim states that they use "high carbon metallic fibers, stainless fibers, poly-vinyl alcohol (PVA) fibers or glass fibers" to increase the concrete's ability to withstand tensile loads and deformation.

Ductal® is also less porous than conventional concrete, making it more resistant to chlorides, acids, and sulfates. It is also generally much more impermeable to water, making it ideal for roofing as well. In addition, Ductal® has self-healing properties. This bendable concrete has been thoroughly researched and is commercialized.
Case Study: Perez Art Museum, Miami, Florida
The Perez Art Museum in downtown Miami is notable largely for its application of Ductal® ultra-high-performance concrete (UHPC). The museum houses roughly 200,000 square feet of indoor and outdoor space for the presentation of modern and contemporary art. However, the property comes with one significant challenge. The museum is built on Biscayne Bay, where it is subject to sea air and salt. Additionally, it is at risk of tropical storms and hurricanes and thus must withstand the forces associated with these extreme weather events. Ductal® was used to produce roughly 100 16-foot-long mullions to support the world's largest impact resistant window at the time of its construction in 2013. The concrete mullions were made to be thin, maximizing visibility, while also meeting the Florida building code for hurricane resistance.

Graphene concrete is concrete reinforced by graphene. Graphene is a single layer of carbon atoms, tightly bound in a hexagonal honeycomb lattice. Layers of graphene stacked on top of each other form graphite, a naturally occurring, crystalline form of carbon most commonly used in pencils and lubricants. The separate layers of graphene in graphite can be separated into sheets only one atom thick. Graphene is the thinnest compound known to man, the lightest material known and the strongest compound discovered, over 100 times stronger than steel. Graphene concrete is made by suspending flakes of graphene in water, then mixing that water with traditional concrete ingredients such as cement and aggregate.

This technology's strength largely lies with its accessibility given that it is inexpensive, and compatible with modern, large scale manufacturing requirements. According to a research paper published in the Advanced Functional Materials journal, entitled "Ultrahigh Performance Nanoengineered Graphene-Concrete Composites for Multifunctional Applications," graphene concrete impressively shows a "146-percent increase in compressive strength as compared to regular concrete, a 79.5-percent increase in flexural strength, and a decrease in water permeability of almost 400 percent." In addition to its increased strength, graphene concrete is also more environmentally friendly, since it requires less cement than is typically required to produce concrete at a specified strength. Alternatively, higher strength graphene concrete could be used to produce smaller structural elements, thus reducing the amount of material used.
Carbon Capture
Like most manmade materials, concrete is considered a carbon dioxide (CO ₂) emitter, mainly due to the cement manufacturing process. But what if you could reverse that process and capture or sequester CO ₂ in concrete through natural processes or carbon capture technologies.

Carbonation is a naturally occurring process by which carbon dioxide (CO ₂) penetrates the surface of hardened concrete and chemically reacts with cement hydration products to form carbonates. For in-service concrete, carbonation is a slow process with many dependent variables. The rate decreases over time. This is because carbonation decreases permeability and carbonation occurs from the surface inward, creating a tighter matrix at the surface making it more difficult for CO ₂ to diffuse further into the concrete. While slow, the carbonation process does result in an uptake of some of the CO ₂ emitted from cement manufacturing, a chemical process called calcination. Theoretically, given enough time and ideal conditions, all of the CO ₂ emitted from calcination could be sequestered via carbonation. However, real world conditions are usually far from ideal.

The rate of CO ₂ uptake depends on exposure to air, surface orientation, surface-to-volume ratio, binder constituents, surface treatment, porosity, strength, humidity, temperature, and ambient CO ₂ concentration. Predicting how much CO ₂ is absorbed by in situ concrete is difficult. What is known is that rates of CO ₂ uptake are greatest when the surface-to-volume ratio is high, such as when concrete has been crushed and exposed to air.

In one of the most comprehensive studies, Xi, et al, published a summary of their research in the article "Substantial Global Carbon Uptake by Cement Carbonation," in the journal Nature Geoscience, November 2016. The research quantifies the natural reversal of the calcination process-carbonation. Using analytical modeling of carbonation chemistry, they were able to estimate the regional and global CO ₂ uptake between 1930 and 2013. They estimate that the cumulative amount of CO ₂ sequestered in concrete is 4.5 Gt in that period. This offsets 43% of the CO ₂ emissions from production of cement caused by the calcination process. They conclude that carbonation of cement products represents a substantial carbon sink.

Two areas of research and commercialization offer considerable enhancements to this CO ₂ uptake process. The most basic approach is enhanced carbonation at end-of-life and second-life conditions of concrete. This might not be considered innovative, since it would simply mean changing the way demolished concrete is collected and treated before re-use. If conditions are right, and particle size is small, crushed concrete can potentially absorb significant amounts of CO ₂ over a small period, such as one year or two and thus leaving crushed concrete exposed to air before re-use would be beneficial.

The other commercially viable technologies accelerate carbonation. This is accomplished either by injecting CO ₂ into concrete, curing concrete in CO ₂ or creating artificial limestone aggregates using CO ₂.

A company called CarbonCure uses CO ₂ captured from industrial emissions, which is then purified, liquefied and delivered to partner concrete plants in pressurized tanks. This is then injected into concrete while the concrete is being mixed which converts the CO ₂ into a solid-state mineral within the concrete. The minerals formed enhances compressive strength.

The process reduces CO ₂ emissions in two ways-through direct sequestration of CO ₂ injected into the concrete mixture and by reducing cement demand since CarbonCure concrete requires less cement to produce concrete at a specified strength.

The economic viability of this concrete also makes it a particularly attractive innovation. The cost of the equipment and licensing is offset by the reduction in cement. CarbonCure has installed their technology in over 100 plants across North America which have supplied over two million cubic yards of concrete. This product is sufficiently available to be used now and has already been used to great effect in numerous projects.
Case Study: 725 Ponce, Atlanta, Georgia
Completed in 2018, the office building at 725 Ponce De Leon Avenue was constructed using 48,000 cubic yards of CarbonCure concrete. Through their cooperation, structural engineer Uzun+Case and concrete supplier Thomas Concrete were able to greatly reduce the carbon footprint of this project. CarbonCure concrete sequestered 680 metric tons, or 1.5 million pounds, of CO ₂ which is roughly the amount of CO ₂ absorbed by 800 acres of US forest each year. The fact that emissions harmful to the environment could be reduced by such a significant factor on this large project, which provides 360,000 square feet of office space, is a perfect example of the viability of carbon capture and sequestration as a sustainable option for concrete construction.

Solidia Technologies offers another carbon capture technology. It combines a specially formulated cement with CO ₂ curing to produce concrete, primarily in the precast concrete products industry. Solidia cement is about the same cost as portland cement but significantly reduces CO ₂ emissions through reduced production energy. This is primarily because Solidia cement uses all of the same materials that are used to produce portland cement but in a different ratio.

Solidia cement uses less limestone than portland cement, which allows it to be fired at lower temperatures in the same rotary kilns in which ordinary portland cement is currently produced. These lower firing temperatures consume less energy and also produce 30% less greenhouse gases and other pollutants. Additionally, instead of curing in water like conventional concrete, Solidia concrete cures in contact with a CO ₂-containing atmosphere. Not only does this allow more precision during the curing process, but during curing, Solidia concrete sequesters CO ₂ equal to 5% of its weight. Between the combined factors of lower material costs, lower fuel costs, and the CO ₂ sequestered during curing, Solidia claims concrete's carbon footprint is reduced by 70%.

Solidia concrete also offers other practical benefits beyond being environmentally friendly. For example, Solidia states that their concrete experiences reduced efflorescence, meaning that salt staining will appear less severely and less frequently on the surface when it is exposed to water. Additionally, the concrete's water absorption is reduced, being less than 2%. It has a compressive strength of about 10,000 psi. It takes less pigment to color. And finally, Solidia Concrete is compatible with non-conventional aggregates and recycled glass. This allows further reduction of material cost and environmental benefits.

Another company using carbon capture technology is Blue Planet Ltd. They offer a product which "combines unpurified CO ₂ absorbed directly from power plant flue gas or other industrial CO ₂ emission sources with metal oxides to make limestone used to coat a substrate, making CO ₂-sequestered construction aggregate. The limestone coating is 44 percent by mass permanently sequestered CO ₂ waste." The substrate is usually small rock particles or even recycled concrete.

Blue Planet states that carbon-negative concrete is achievable by using their artificial limestone in concrete. They estimate that by replacing the conventional aggregate in one cubic yard of concrete, typically 3,000 pounds worth, 44% of its weight would be comprised of sequestered CO ₂, roughly 1,320 pounds. This would offset more than the amount of CO ₂ generally produced by the same amount of conventional concrete made with portland cement, which is roughly 600 pounds per cubic yard. Blue Planet's limestone-coated light-weight aggregate was specified for the Interim Boarding Area B at San Francisco International Airport in 2016. Concrete testing showed that Blue Planet's concrete met all necessary specifications.

Carbon capture and sequestration technology is a promising solution to reducing the carbon footprint of cement and concrete while improving performance. The possibility of vastly reducing CO ₂ emissions associated with the production of concrete or even going beyond by sequestering more CO ₂ than are produced during the cement manufacturing process is enticing. Many carbon capture and sequestration technologies are already commercially viable and are currently being used for construction since they can be conveniently produced by existing equipment or by retrofitting existing factories. Overall, carbon capture offers a simple but highly promising solution to reducing the environmental footprint of concrete.
Self-Consolidating Concrete
Self-consolidating concrete (SCC) is highly flowable, non-segregating concrete that can flow into place, fill formwork and encapsulate reinforcement without any mechanical vibration. SCC relies upon a combination of a high proportion of fine aggregate and admixtures called superplasticizers and viscosity-modifiers to achieve a stable and highly flowable concrete.

The increased ease of use and efficiency of SCC during construction is the basis for many of its principal benefits. First, it can be placed faster than regular concrete while requiring less finishing and no mechanical vibration. It also improves the uniformity of in-place concrete as well as the uniformity of surfaces, reducing or eliminating the need for surface work.

Additionally, using SCC allows for labor savings as well as increased jobsite safety as it does not require workers to travel the surface of slabs or the tops of walls to mechanically vibrate the concrete. SCC saves time during construction, resulting in cost savings, as well as improving the pumpability of the concrete and the turn-around times of concrete trucks.

SCC was first developed in 1986 by Prof. Okamura at Ouchi University, Japan, to address shortages in skilled labor. At first, SCC was used in highly specialized projects such as repair work or in areas difficult to reach due to its high cost of production and need for high quality control. The first high production use of SCC was in precast applications where concrete is produced and placed in controlled conditions. In ready mixed concrete applications, SCC was used primarily for heavily reinforced sections and where mechanical vibration was difficult. More recently, SCC is being used in architectural concrete since it results in a surface finish superior to that of conventional concrete. SCC still has a relatively high cost but is gaining popularity where labor is in short supply or where smooth exposed concrete is desired.

One of the highest profile uses of SCC is in high-rise building projects, proving its commercial viability and success in practical applications. Some considerations to take into account regarding this concrete stem from the fact that it is dependent upon flowability, which may be reduced by hot weather, long haul distances, or jobsite delays. Specifications required for a given job such as flowability and spread can vary, but mixtures can be tested via methods including the slump flow test to determine the extent of the concrete's plastic properties to ensure that the concrete arriving at a jobsite matches the standards specific to the project itself. SCC is fully commercialized and is used all over the world.
Case Study: 432 Park Avenue, New York
432 Park Avenue in New York City is currently the tallest residential structure in the US. It is an aesthetically simple building that features exposed white concrete columns that structurally reinforce the building in addition to providing the building its most distinctive stylistic attributes. The building is very thin for its height, having a width and length of 93.5 feet and a height of 1,396 feet. Multiple innovative structural methods were used to achieve "minimal displacement, accelerations, and vibrations to meet the most stringent standard" according to an article in STRUCTURE magazine, July 2018. These include, "five outriggers, each spanning over two stories, were devised throughout the height of the tower to serve as positive linkages between the interior core and the perimeter framing, which enhanced the overall performance of the structure".

Stiffer concrete with higher compressive strength was used on floors above the 38th to further increase resistance to movement in the upper stories. Further, all concrete cast for 432 Park Avenue was designed for enhanced durability by minimizing the ratio of water to cementitious materials to as low as 0.25 and the concrete was required to be pumpable, self-consolidating, and have a low heat of hydration to facilitate construction and appearance of the exposed structural elements.
From Waste to Worth
Supplementary Cementitious Materials (SCMs) such as fly ash, slag cement and silica fume are the keys to high performance concretes. What makes these materials so innovative is that most are derived from a waste-byproducts of a manufacturing process that would otherwise end up in landfills. But when these waste materials are combined with portland cement in concrete, they react with certain chemical compounds to produce more binder. As a result, these materials are extremely valuable as SCMs.
Case Study: Trump Tower Chicago
Chicago Trump Tower and Hotel stands at a stunning 92 stories, made entirely out of reinforced concrete. A total of 194,000 cubic yards of concrete was used on the project. Architect/engineer Skidmore, Owings & Merrill specified high-performance concrete and concrete supplier Prairie Materials designed the mixes. Columns and walls required 12,000 psi at 90 days up to level 51 with some lateral resisting elements up to 16,000 psi. SCC was specified for many of structural elements because of reinforcement congestion. To reduce heat of hydration, high volumes of SCMs were specified for the mat foundation which included a combination of slag cement, fly ash and silica fume. At time of construction, the 5,000-cubic-yard mat foundation was the largest single SCC placement in North America.

The high-performance reinforced concrete system helped minimize floor thickness creating higher ceilings. Residential floors also feature open spans up to 30 feet without requiring perimeter spandrel beams permitting panoramic vistas of Chicago and Lake Michigan. Combining several innovative concrete technologies allowed for quick, efficient construction as well as new opportunities that are not available with conventional concrete.

Silica fume is a waste byproduct of processing quartz into silicon or ferro-silicon metals in an electric arc furnace. Silica fume consists of superfine, spherical particles that when combined with cement significantly increases strength and durability of concrete. Of the three main SCMs, silica fume has the lowest supply and has the highest cost, usually at least three times that of portland cement. It's used in applications where extremely high strength is needed, such as columns in high-rise buildings or where extremely low permeability is desired for durability such as bridge decks and parking decks. It's typically combined with other SCMs to optimize performance and cost.

Blast furnace slag is the waste byproduct of iron manufacture. After quenching and grinding, the blast furnace slag takes on much higher value as an SCM for concrete. Blast furnace slag is used as a partial replacement for cement to impart added strength and durability to concrete. Some slag is used to make lightweight aggregate for concrete. About 16 million tons of slag were produced in the U.S. but less than half that was used in concrete as an SCM. Slag cement costs about the same or slightly more than cement depending on quality and location.

Coal ash is the waste byproduct of burning coal in electric power plants. Fly ash, a common SCM used in concrete, is one component of coal ash. According to the American Coal Ash Association (ACAA), in 2017, 111.4 million tons of coal ash were produced, of which 38.2 million tons was fly ash. Coal ash and fly ash have many uses, ranging from use in concrete as an SCM to synthetic gypsum for wallboard to mining applications. Of the 38.2 million tons of fly ash produced, only 14.1 million tons are used in concrete.

Fly ash is the most plentiful of all SCMs and is roughly half the cost of portland cement. However, because of increased emissions regulations on coal-fired power plants, not nearly as much high-quality fly ash is produced as in the past. In addition, with a move towards renewables and natural gas, coal-fired power plants are closing and thus many cost-effective supplies are diminishing.
Case Study: 102 Rivonia, Johannesburg, South Africa
102 Rivonia Road consists of two main buildings with connected walkways in-between to create a sense of connectedness and encourage collaboration between different areas of the office. It was designed with sustainability in mind, being 50% more sustainable than the average office building with a 4-star Green Star SA (South Africa) rating. Air cooled chillers and fire system that recycles used water also contributed to the project's energy efficiency. Notably, the use of fly ash in the concrete reduced the overall material use of the project by 30%, which also heavily contributed to the project having a lower carbon footprint.

Because coal power generation started in the early 1900s in the US, but the use of fly ash in concrete was only started to any significant volume in the late 1,900s, it is estimated that about 1.5 billion tons of coal ash has been placed in landfills, of which some is fly ash. And that's where the innovation comes in. Several companies, understanding the demand for fly ash in concrete is likely to increase have begun to recover fly ash from landfills and treat it using a process called beneficiation.

Beneficiation simply means taking coal ash from landfills and processing it so it meets the necessary standards for beneficial use. For fly ash, that typically means reducing the amount of unburned carbon in the ash. Carbon tends to have an absorptive quality which inhibits air entraining and water reducing admixtures. There are also other chemicals such as ammonia in some coal ash deposits which must be reduced before use in concrete.

Several companies have developed processes for harvesting ash from landfills and reducing the unburned carbon and ammonia, calcium, sulfur and other impurities. The simplest process is to burn off the excess carbon. Still other methods use chemical treatment to mitigate the effects of carbon and ammonia and one company uses low-frequency sound to reduce particle size to make them more uniform, a desired characteristic of fly ash. Companies include Boral Resources, Waste Management/Fly Ash Direct, SEFA Group, SonoAsh LLC, and Charah Solutions according an article "Digging Through the Past: Harvesting Legacy Ash Deposits to Meet Future Demand," authored by Rafik Minkara and published in Issue 1 2019 of Ash at Work magazine.

Minkara concludes "While the variety of technologies now exist to beneficiate land-filled and ponded ash, the cost and complexity of doing so can be challenging." He goes on to say "Beneficiation processes can be as simple as using off-the-shelf equipment or as involved as developing customized solutions with high capex requirements." In the end, it will depend on demand for fly ash. As low-cost supplies diminish over time, the demand is likely to be filled by harvesting and beneficiating the vast supply of coal ash in landfills.
Cementless Concrete
Although we are likely years away from widespread commercialization, one of the more interesting areas of research and development is on geopolymer concrete which uses fly ash and/or slag and chemical activators as the binder in place of portland cement. Geopolymer concrete is made by using a source of silicon and aluminum, usually fly ash or slag, and combining it with an alkaline activating solution which polymerizes these materials into molecular chains to create a hardened binder. The more common activating solutions include sodium hydroxide or potassium hydroxide which liberates the silicon and aluminum.

Compressive strength of geopolymer concrete is comparable to portland cement concrete or higher and strength gain is generally faster with strengths of 3,500 psi or higher at 24 hours. Compressive strengths at 28 days have shown to be 8,000 to 10,000 psi. Research shows that geopolymer concrete has lower drying shrinkage, lower heat of hydration, improved chloride permeability, and is more resistant to acids. And its fire resistance is considerably better than portland cement concrete, which is already highly fire resistant, making geopolymer concretes ideal for special high temperature applications.

To date, most of these products have not developed beyond the research and development stage. One company, Ceratech launched geopolymer concrete in in 2002 but later closed. A product called Pyrament® was launched in the 1980s but was not successfully commercialized. Some of the drawbacks include the high cost and energy to produce the chemical activator, the difficulty and safety concerns in handling a highly alkaline solution and the need to control temperature during the curing process. In addition, building code approvals are always a hurdle. Currently the most promising applications are in severe environment applications such as precast concrete bridges or other specialty applications such as high acid or high temperature environments or for rapid repair.
Case Study: Global Change Institute, Brisbane, Australia
As Australia's first carbon neutral building, the Global Change Institute at the University of Queensland was designed to meet the highest level of sustainability. It is one of the first buildings to be registered for The Living Building Challenge. Some of the green building features include operable sun-shading, bio-retention basin, onsite greywater system, solar energy and thermal chimney. And it is the first building to include structural geopolymer precast concrete, significantly reducing the carbon footprint of construction materials.

The key to geopolymer concrete commercialization will be to develop low cost, easy to use activators. One promising development is at Rice University where engineers have developed a geopolymer concrete that requires only a small fraction of the sodium-based activation chemicals used in other geopolymer concretes. According to the researchers, they used sophisticated statistical methods to optimize the mixing strategies for ingredients. This resulted in an optimal balance of calcium-rich fly ash, nanosilica and calcium oxide with less than 5% of the traditional sodium-based activator.
Conclusion
More than 20 billion tons of concrete are produced around the world each year. As a result, concrete construction contributes about 5% of global CO ₂ emissions primarily due to the cement manufacturing process. The demand for concrete will likely continue to grow as population grows. In addition, the demands on strength, durability and workability will continue to increase. A combination of traditional and advanced technologies will help meet these new demands. Technologies such as TiO ₂ cements, SCC, SCMs and fibers are being used now to varying degrees with outstanding results. Carbon capture and sequestration are in their infancy but show great promise. Fly ash beneficiation will help meet the demand for affordable, high performance concretes and geopolymer concretes may one day help make concrete carbon neutral without sacrificing performance.
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Concrete Innovations | Civil + Structural Engineer magazine

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By Lionel Lemay, PE, SE, LEED AP. Executive Vice President, Structures and Sustainability, National Ready Mixed Concrete Association Brian Lemay, Research Assistant Intern, National Ready Mixed Concrete Association Credit: 1 LU/HSW Course Number: ZG082019CS Sponsored by: Build with Strength, a c ...

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En caso de incendio, proteger las vidas de las personas es lo más importante. Todos los ocupantes del edificio deben tener la oportunidad de evacuar a tiempo, y el tiempo disponible depende en gran parte de los materiales escogidos y su comportamiento frente al fuego.

Con el fin de facilitar y optimizar este proceso, la Unión Europea ha adoptado la Norma EN 13501, introducida en la década del 2000, donde se especifican una serie de 'clases' que determinarán las propiedades anti-incendio de los distintos materiales de una solución arquitectónica. Sus clasificaciones se unifican y comparan con base en los mismos métodos de prueba, y actualmente se toman como referencia en muchos países del mundo.

Hemos recopilado sus nomenclaturas más importantes, para comprender de mejor manera el nivel de 'seguridad' de nuestro entorno construido, y la protección que entregamos como arquitectos al elegir los materiales de nuestros proyectos.

Reacción al Fuego

Los materiales y productos se pueden clasificar en 7 'Euroclases' distintas, según la manera en que reaccionan al fuego. Para entender esta clasificación, es importante considerar la combustión súbita generalizada o Flashover, que es el momento en que los materiales combustibles -no involucrados en el incendio original- comienzan a arder, acrecentando la temperatura en la habitación y aumentando su velocidad de propagación. [1]

A1

No combustible y no contribuye al fuego. Entre ellos podemos encontrar materiales y/o productos de concreto, vidrio, acero, piedra natural, ladrillos, y cerámicos.

A2

Poco combustible y muy baja contribución al fuego, sin causar Flashover. Entre ellos podemos encontrar materiales y/o productos similares a los Euroclase A1, pero con un pequeño porcentaje de componentes orgánicos.

B

Poco combustible y muy baja contribución al fuego, pero si causan Flashover. Entre ellos podemos encontrar materiales y/o productos como las placas de yeso y algunas maderas con protección al fuego.

C

Combustible, causa Flashover a los 10 minutos. Entre ellos podemos encontrar materiales y/o productos como la espuma fenólica, o placas de yeso con revestimientos superficiales más gruesos.

D

Combustible, causa Flashover antes de 10 minutos. Entre ellos podemos encontrar materiales y/o productos de madera sin protección, variando su reacción según su espesor y su densidad.

E

Combustible, causa Flashover antes de 2 minutos. Entre ellos podemos encontrar materiales y/o productos como tableros de fibra de baja densidad, o sistemas de aislamiento compuestos de plástico.

F

Comportamiento indeterminado. Materiales y/o productos no testeados.

S

Opacidad del Humo. Capacidad del elemento de no producir humos. Se divide en:

S1. Baja opacidad y producción de humos.

S2. Mediana opacidad y producción de humos.

S3. Alta opacidad y producción de humos.

D

Caída de Gotas. Capacidad del elemento de no producir ni gotas ni partículas inflamadas. Se divide en:

D0. No produce gotas ni partículas.

D1.Produce gotas y/o partículas no inflamadas.

D2. Produce gotas y/o partículas inflamadas.

Resistencia al Fuego

Cada elemento constructivo se comporta de manera distinta al entrar en contacto con el fuego, y su resistencia se mide en base a los siguientes parámetros (o sus combinaciones). Esta clasificación es acompañada habitualmente por un número (en minutos: 15, 30, 45, 60, 90, 120, 180, 240 o 360) que indica el tiempo en que estos parámetros se cumplen. [2] A modo de ejemplo, un REI 90 indica que un elemento constructivo mantiene su estabilidad (R), integridad (E) y su aislamiento térmico (I) durante 90 minutos.

R

Capacidad Portante. Capacidad del elemento constructivo de resistir mecánicamente, sin perder sus propiedades estructurales. Aunque los criterios de rendimiento varían según la solución constructiva, y las cargas que reciben -axiales (como muros o pilares) o en flexión (como pisos o vigas)-, en los dos casos se mide su tasa de deformación y su deformación máxima.

E

Integridad. Capacidad del elemento constructivo de impedir el paso de fuego y gases calientes hacia un recinto no afectado por el incendio.

I

Aislamiento. Capacidad del elemento constructivo de impedir el aumento de temperatura en la cara no expuesta directamente al fuego.

W - M - C - K

Otros parámetros importantes a considerar:

W (Radiación).Capacidad del elemento de evitar la transmisión del fuego hacia un recinto no afectado, a causa del exceso de calor irradiado a través del elemento.

M (Acción mecánica). Capacidad del elemento de resistir el impacto por la falla estructural de otro elemento cercano.

C (Cierre automático).Capacidad de puertas y ventanas de cerrarse completamente sin intervención humana.

K (Protección contra incendios de revestimientos). Capacidad de los revestimientos de muros y techos de entregar protección a los componentes que se encuentran detrás de ellos.

La intención de este artículo es acercarnos de manera sencilla a la protección anti incendios, a través de una de las normas más referidas al rededor del mundo. Es importante aclarar que este es un tema serio, que debe ser siempre evaluado por expertos, considerando las normativas locales y las especifícaciones técnicas de cada material y producto utilizado.

[1] Referencia ejemplos: InnoFireWood. (2019). EUROCLASS System. April, 2019, by Virtual.vtt.fi. Website: http://virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/euroclass/euroclass.html
[2] Referencia: Fire resistance classes. April, 2019, by RISE Research Institutes of Sweden. Website: https://www.sp.se/en/index/services/fire_classes/sidor/default.aspx

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Cómo se clasifican los materiales ante un incendio: reacción y resistencia al fuego

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En caso de incendio, proteger las vidas de las personas es lo más importante. Todos los ocupantes del edificio deben tener la oportunidad de evacuar a tiempo, y el tiempo disponible depende en gran parte de los materiales escogidos y su comportamiento frente al fuego.

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Something Old and Something New

Mass timber is not a new idea, just a new name. Many people have heard the term "mass timber" batted around in the last few years, but not everyone has a clear understanding of what it means. Mass timber used to be referred to as "heavy timber" and the International Building Code (IBC) classifies it as Type IV construction.

Mass timber is a type of structure made up of big, fat pieces of wood that, unlike light wood frame construction, burn slowly. Because mass timber structures maintain their integrity during a fire without the need for layers of fire-resistant materials, they are suitable for larger buildings and the IBC recognizes that.

Timber buildings have been built for over 4,000 years - since bronze-age man developed the technology to forge sharp tools that could hew trees into square timbers and fashion mortise and tenon joints. Timber structures were the dominant construction type in Europe, Asia, and North America until 1850 when balloon-frame wood structures began to displace timber construction. By the turn of the 20 th century, timber construction was nearly extinct and structural iron construction was becoming commonplace for larger structures.

Following World War II, glulam timber construction became popular for long-span and architecturally exposed applications such as church roofs. At the time, if you wanted a structure that looked like timber, glulam construction was your only choice. That is no longer true.

In the 1970s, traditional timber frame construction experienced a revival. A small, core group of timber craftsmen, mostly located in northern New England, rediscovered and mastered the lost art of crafting timber structures with intricate joints. By the 1990s, timber frame construction had re-entered mainstream construction and was displacing glulam construction for architecturally exposed structures.

So, What's New?

Timber panels are new - Cross-Laminated Timber (CLT) and Dowel Laminated Timber (DLT). CLT has been used in Europe since the 1990s but has only been available in North America for a little more than a decade. Today, there are five major manufacturers of CLT in North America and one producer of DLT. The availability of timber panels has stimulated interest and excitement in the architectural community for building with mass timber.

Modern mass timber structures typically consist of a glulam timber frame supporting CLT or DLT floor and roof panels, often with CLT shear walls. This type of construction is extremely versatile and is suitable for both large and small structures - anywhere that an architecturally exposed structure is desired. It does not make much sense to build with timber and then cover it up with sheetrock and hung ceilings. By the way, industry experts continue to express that mass timber construction is also very sustainable, sequesters carbon, and is cost-effective.

CLT, NLT, and DLT

Cross-Laminated Timber has often been described as plywood on steroids. It is made up of alternating plies of dimension lumber that has been planed to approximately 1 3⁄8-inch thickness. Like plywood, each ply is oriented perpendicular to its adjacent plies. Common CLT layups are 3-ply (41⁄8-inch), 5-ply (6 7⁄8-inch), and 7-ply (9 5⁄8-inch). CLT panels are typically 8 feet or 10 feet wide and up to 60 feet long.

The structural design of CLT panels is covered in Chapter 10 of the National Design Specification® (NDS®) for Wood Construction. Effective section properties and reference design values can be found in ANSI/APA PRG 320-2018, Standard for Performance-Rated Cross-Laminated Timber.

Nail-Laminated Timber (NLT) panels are made up of dimension lumber (typically 2x6s) laid side-by-side and spiked together with nails. NLT is not a new thing and has been used infrequently for over a century. The advantage of NLT construction is that it does not require specialized fabricating equipment to manufacture, and it can even be built on-site. The disadvantage is that NLT panels require a significant number of nails that are labor intensive to install and make it impossible to cut the panels with Computer Numerical Control (CNC) equipment without the embedded nails destroying the cutter-head.

Dowel-Laminated Timber panels are the newest alternative. DLT is similar to NLT, except that it contains no nails. Transverse hardwood dowels are used instead of nails to bond the panels. Unlike NLT, DLT panels lend themselves to CNC fabrication.

Reaching for the Sky

High-rise construction has long been the exclusive domain of structural steel and reinforced concrete, but that is starting to change. Mass timber is now a player in the high-rise market. Brock Commons in Vancouver is currently the tallest mass timber building in North America, topping out at 18 stories. In the world of tall buildings, 18 stories may not sound all that impressive, but, for wood construction, it is a big deal.

It is unlikely that mass timber is going to displace structural steel and reinforced concrete for high-rise construction completely, but we will see more tall, mass timber projects and will probably be seeing a lot more mass timber and structural steel hybrids, especially with anticipated changes coming in the 2021 IBC. The 2021 IBC will permit buildings up to 18 stories to be constructed of mass timber.

Getting Connected

Timber engineering is all about the connections. Sizing the timbers and panels is the easy part. Designing timber connections is the challenging part. It has been said that a structure is essentially an assembly of connections that happen to be separated by beams and columns, and that is especially true of timber structures.

Many engineers that are inexperienced with timber engineering will attempt to connect timbers in a fashion similar to structural steel construction, with bulky side plates and lots of bolts. While this approach sometimes works, it is seldom the most practical or efficient way to make a timber connection, and it is rarely the most aesthetically pleasing solution for an exposed structure.

It is smarter to configure connections and connection hardware so that structural loads are transferred primarily in bearing and bolts are only relied on to resist incidental loads.

Timber is an organic material that shrinks and swells seasonally with changes in humidity. Failure to consider timber dimension changes associated with moisture content when designing connections can lead to disappointing (or dangerous) results. Steel gusset plates can restrain dimension change movements resulting in the splitting of the timbers (for more on this, see the February 2004 issue of STRUCTURE).

Fired Up

It is a common misconception that because wood is combustible, wood buildings perform poorly in a fire. While that may be true of light wood frame construction, it is not at all true of mass timber.

Timbers will develop a char layer on the surface when exposed to a flame. The char layer progresses slowly and insulates the wood beneath it from the heat of the fire, permitting the timbers to continue to carry load. When timber structures do eventually fail during a fire, they do not fail suddenly. They typically give firefighters ample warning prior to a collapse by making loud cracking and hissing noises. The exception is, when steel connection hardware is exposed to the fire, the connections will fail suddenly. It is important to protect steel connection hardware either with an intumescent coating or preferably by having all steel hardware embedded inside the timbers where the wood can protect the steel from the fire.

Unlike most other structural systems, the fire-resistance rating of mass timber assemblies or elements is based on a structural analysis rather than on the listed results of an ASTM E119 fire test. Chapter 16 of the NDS has a procedure for calculating the fire-resistance rating. The thickness of the char layer on the timber is stipulated for different time intervals. For instance, if a 1-hour fire rating is required, the NDS stipulates that the char layer on a timber is 1.8-inch-thick after 1 hour of fire exposure. It is then a simple matter of calculating the remaining section properties of a timber with 1.8 inches of wood removed from the exposed perimeter and determining if the reduced section is capable of supporting the applied dead loads with a stress increase to convert allowable stress to ultimate stress. The fire-resistance calculation for a CLT panel is similar, except the stipulated char thicknesses are a little different. A 5-ply CLT is needed if a fire rating is required since there is not much left of a 3-ply CLT if the bottom ply burns away.

Serviceability Considerations

The design of mass timber floor systems is typically not controlled by strength, but by serviceability - principally sound transmission and vibration.

Bare timber floors will readily transmit sound, particularly impact sound associated with footfalls. It is common to install an acoustic mat over the timber floor with a non-composite concrete topping slab or gypcrete topping to address sound transmission.

Floor vibration associated with foot traffic must be evaluated. Designing to an arbitrary static deflection limit such as L/360 or L/480 will not ensure that a floor structure does not feel bouncy, particularly if the floor structure has a period of vibration less than 9.0 Hertz.

Hybridization

Mass timber plays well with other structural materials. Often the right structural solution for a project is not a pure mass timber structure, but a hybrid solution. CLT floor and roof panels with glulam joists, structural steel girders and columns, and a concrete topping slab often make for a very efficient structure. The structural steel elements often require an intumescent coating to achieve a fire resistance comparable to the timber elements.

For the 18-story Brock Commons project, reinforced concrete shear walls resist wind and seismic loads. Mass timber shear walls could have done the job, but for a variety of reasons, it just made more sense to use concrete. There is no shame in not being a purist.

Design or Delegate

It is not uncommon for a Structural Engineer of Record to delegate design responsibility to a specialty timber engineer engaged by the contractor. Sometimes, responsibility for engineering just the timber connections is delegated, but occasionally an engineer will delegate responsibility for engineering the entire timber structure. In such cases, it is best to engage the specialty timber engineer during the design phase of the project. Otherwise, the bidding process can turn into a circus.

So how much of the timber engineering should be performed by the Engineer of Record and how much should be delegated? The answer depends on how much prior experience the engineer has with timber structures. The best advice is for the Engineer of Record to do as much as he is competent to do and delegate only what he is inexperienced at.

Resources

So, where can an engineer turn to get more information on mass timber engineering? The Timber Frame Engineering Council (TFEC) is an organization of structural engineers who specialize in timber. The TFEC has produced a library of documents that offer design guidance, including a Standard for Design of Timber Frame Structures, Code of Standard Practice, master specifications, and technical bulletins, all of which are available free of charge at https://bit.ly/2S7H7fp.

The CLT Handbook produced by FPInnovations offers a wealth of practical information.

The WoodWorks team is also always willing to offer design assistance and to point engineers and architects in the right direction.■

Mass Timber Floor Vibration

By Lucas Epp, P.Eng.

The maximum span of many floor systems is often controlled by limiting occupant perception of vibrations rather than strength or deflection. This is the case for long span concrete and steel floor framing, high-performance, light frame floors, and mass timber floors. The unsettling performance of a bouncy floor is typically caused by resonance between human walking frequencies (1.6-2.0 Hz) and the floor's natural frequency. If the walking pace is a multiple of floor frequency (e.g., walking at 2.0 Hz causing a floor at 6.0 Hz to vibrate), resonance can occur and the resulting accelerations can be disconcerting, particularly in longer-span structures with low damping.

For mass timber floors, rule-of-thumb guidance such as limiting the floor's fundamental frequency to above 8.0 Hz, or limiting the deflection under a unit point load, has been used in an attempt to avoid floors which perform poorly. However, simply limiting the natural frequency of the floor does not guarantee good performance. A better estimate of floor performance is the accelerations, which result from walking activities. These accelerations depend not only on the natural frequency, but also the amount of floor mass being mobilized, the damping in the system, the length of the walking path, and the frequency of occurrence. Human sensitivity to accelerations is a grey area and recommended limits on permissible accelerations for various use cases vary significantly between different guidelines. ( See Figure)

Guides such as AISC Design Guide 11 ( Vibrations of Steel-Framed Structural Systems Due to Human Activity, 2016) provide detailed approaches to calculate accelerations on floor plates. There are also several European guides which provide similar guidelines with different calculation approaches.

Calculating accelerations on a given floor plate can be done using finite element software, but this is an involved analysis and simpler guidance is needed to assist in early stage design and scheming.

Current Design Guidance

For mass timber floors, there are various guidelines in North America and Europe which have been developed to determine allowable vibration-controlled spans.

The CLT Handbook (FPInnovations, 2013) presents a formula which limits the span based on a combination of deflection under a unit point load and natural frequency. This criterion, however, ignores the contribution of different damping levels, the weight of any superimposed mass (such as concrete topping), and any added flexibility of supporting structure, all significant factors in vibration performance. The CLT Handbook formula results in the following maximum spans for typical CLT layups (ANSI/APA PRG 320, 2018):

4 1⁄8-inch-thick, 3-ply CLT - 11 feet to 12 feet

6 7⁄8-inch-thick, 5-ply CLT - 16 feet to 17 feet

9 5⁄8-inch-thick, 7-ply CLT - 20 feet to 21 feet

Concrete topping is often required on mass timber floors to generate sufficient acoustical separation between floors. This topping increases the modal mass (i.e., participating mass that needs to be excited by footfall), directly affecting the accelerations which would be felt on the floor plate.

In Europe, the guidance for mass timber floors in Eurocode 5 (EN 1995-1-1) states that, for floors with a natural frequency (f) > 8.0 Hz, the deflection under a 225-pound point load should be limited to less than 1⁄16 to 1⁄32 of an inch. For floors with f < 8.0 Hz, the Eurocode recommends calculating accelerations and limiting these to 0.5 to 1.5% gravity.

For early-stage design with panels supported on bearing walls, the CLT Handbook formula can be used only as an initial estimate of the maximum vibration-controlled span. For panels spanning onto beams, this formula is unconservative and more detailed acceleration analysis should be undertaken, as the beams contribute significantly to the flexibility of the system.

Currently, there is ongoing research to develop more accurate simplified criteria, as well as a robust guidance document for vibration design of mass timber floors. The US Mass Timber Floor Vibration Design Guide will be published by WoodWorks in 2020 and will contain more details on both analysis methods and acceptance criteria.

For further information and design guidance, refer to TFEC Bulletin 2019-14, Vibration Design of Mass Timber Floor Systems.

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Mass timber is not a new idea, just a new name. Many people have heard the term "mass timber" batted around in the last few years, but not everyone has a clear understanding of what it means. Mass timber used to be referred to as "heavy timber" and the International Building Code (IBC) classifies it as Type IV construction.

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El hormigón, material de construcción por excelencia, nos ha ofrecido por décadas la posibilidad de dar forma a nuestras ciudades de forma rápida y efectiva, extendiéndose rápidamente hacia las periferias urbanas o alcanzando alturas antes impensadas por la humanidad. En la actualidad, las nuevas tecnologías en madera están comenzando a entregar oportunidades similares -e incluso superiores- a las proporcionadas por el concreto, entre ellas, la madera laminada cruzada (también llamada madera contralaminada o CLT).

Con el objetivo de profundizar en sus propiedades y beneficios, conversamos con Jorge Calderón, Diseñador Industrial de la Pontificia Universidad Católica de Valparaíso y Gerente de CRULAMM, quién nos revela algunas de las prometedoras oportunidades que podría entregar el CLT a la arquitectura en el futuro.

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¿Cuál es la diferencia entre la madera laminada y el CLT?

La madera laminada es el resultado de la unión de tablas o láminas para conformar una sola unidad estructural, generando elementos lineales, curvos o rectos, pero siempre lineales. Con el CLT, en cambio, la unión de tablas en capas perpendiculares permite la fabricación de placas, planchas, o superficies, es decir, muros. Es un contrachapado hecho de tablas que permite alcanzar enormes dimensiones; de entre 2.40 m y 4.00 m de altura, y de hasta 12.00 metros de largo, o incluso más si se requiere.

Debido a la orientación en cruz de cada una de sus capas longitudinales y transversales, se reducen a un mínimo irrelevante los grados de contracción y dilatación de la madera a nivel de los tableros, mientras que la carga estática y estabilidad de la forma mejoran considerablemente. [1]

Para su traslado, las planchas son cortadas en piezas que permitan su desplazamiento por carretera, en contenedores o camiones de plataforma baja.

¿Cuál es el impacto ambiental del CLT?

El CLT comenzó a fabricarse en Austria, con el objetivo de reutilizar la madera de menor valor. En la actualidad, sin embargo, el uso de la madera, en general, está volviendo a ser un factor relevante dentro de la industria de la construcción por su impacto ambiental.

Generalmente diseñamos y construimos con hormigón, y la huella del hormigón es enorme en comparación con la madera. Por cada m3 de hormigón se emite a la atmósfera una tonelada de CO2. Cuando se construye con CLT, se utiliza "carbón secuestrado", ya que el árbol mientras crece, se alimenta de carbono. Por lo tanto, a pesar de toda la energía utilizada en el proceso de extracción y fabricación, la madera nunca llegará a igualar la cantidad de carbono que mantiene "secuestrado".

Comparación de los diversos soluciones constructivas en términos de consumo energético (GJ/m2) para su elaboración y producción [Holzbau, Rubner. Universidad de Trento, Italia]:

¿Cómo se comporta estructuralmente el CLT, en comparación con otros materiales?

Al CLT se le ha llamado "el hormigón del futuro" y en cierto sentido es cierto, ya que entrega -al menos- la misma resistencia estructural que el hormigón armado. Es un material con un alto grado de flexibilidad y que tiene que sufrir grandes deformaciones para romperse y colapsar, al contrario del hormigón. Por otro lado, 1 m3 de hormigón pesa aproximadamente 2.7 toneladas, mientras que 1 m3 de CLT pesa 400 kg, entregando la misma resistencia. Lo mismo ocurre con el acero.

En cuanto a sus propiedades físicas, para lograr el mismo grado de aislamiento que tiene un muro de CLT de 100 mm de espesor, por ejemplo, necesitaríamos construir un muro de hormigón de 1.80 m de espesor (relación de 1/18).

¿Cómo se comporta el CLT ante el fuego?

El fuego en la madera avanza a una razón de 0.7 a 0.8 milímetros por minuto. Si un muro de CLT tiene 100 mm, terminaría de consumirse luego de más de 2 horas, incluso si hablamos de madera no tratada. Este proceso de carbonización es la reacción natural del árbol para protegerse a sí mismo.

En relación al humo, principal causante de la muerte de personas en un incendio, este se mueve de una habitación a otras por todas las rendijas o espacios libres que resulten del encuentro de los distintos materiales. Construido correctamente, el CLT puede llegar a ser completamente hermético, por lo que es muy relevante la selección y manejo de todos aquellos elementos que conformen la estructura final, como herrajes, sellos, uniones, y otros. En relación a la resistencia de una construcción con CLT, podríamos atribuir un 90% de la responsabilidad a los herrajes y uniones, y un 10% a la madera misma.

¿Cómo proteger el CLT frente a las condiciones ambientales?

La humedad y la intemperie son algunos de los enemigos más importantes de la madera. La madera al exterior sufre, y como el CLT es un componente estructural, debemos protegerla para evitar su desgaste, corrosión, y colapso. Por un lado, podemos agregar nuevas capas de revestimiento sobre la madera, como fibrocemento, ladrillo, piedra, u otros, pero si se quiere dejar el CLT expuesto, existen una serie de protecciones y acabados que la protegen efectivamente.

Aceites vegetales y pinturas minerales pueden cumplir estos objetivos, aplicándose en una única pasada cada 5 años, garantizando incluso 25 años de protección sin desprendimiento ni decoloración.

Los aceites vegetales son recomendados para su uso en interiores, mientras que las pinturas minerales funcionan de mejor manera en exteriores, principalmente sobre muros. Estos productos, inoloros y de alto rendimiento, pueden ser aplicados por cualquier persona, siguiendo instrucciones básicas y tomando los cuidados necesarios.

Los paneles de CLT funcionan como piezas perfectas, con un alto nivel de precisión

Cuando comienza a fabricarse una obra en CLT, en la fábrica todo está completamente decidido y predeterminado, y no es posible realizar ajustes en obra. Entonces, más que constructores, las personas que trabajan en una obra en CLT son montajistas, que deben articular piezas virtualmente perfectas. El CLT se comporta con la precisión de un mueble, trabajando con márgenes de error de 2 milímetros.

Mientras la etapa de proyecto puede demorar un poco más, el montaje es de una velocidad sorprendente: en el caso de una casa de 200 m2, el montaje puede demorar 5 días y ocupar una mano de obra mínima (al rededor de 4 personas instruidas).

En relación a la normativa, existen en el mundo regulaciones que guían el trabajo de diseño y construcción con CLT, pero son la suma de diferentes aspectos presentes en las normas utilizadas habitualmente en obras de hormigón y madera laminada. En 2017, se publicó la norma actualmente utilizada en los Estados Unidos, que es simplemente un resumen de las normas europeas.

Recomendaciones de diseño y construcción

Como hemos dicho, es fundamental entender que todo el proceso previo a la construcción con CLT debe ser cuidadosamente desarrollado. El diseño, la planificación, y la colaboración permanente entre los distintos actores es fundamental, ya que la construcción misma será llevada a cabo tal como fue definida en las etapas anteriores.

Durante su fabricación, el CLT debe elaborarse con madera estructural -conociendo el grado estructural de cada tabla-, ya que la calidad del panel será el resultado de la calidad de las maderas utilizadas. Además, es necesario considerar que la alta precisión del CLT debe poder conjugarse con los cimientos que lo recibirán, evitando, por ejemplo, losas de hormigón que presenten imperfecciones. Incluso variaciones milimétricas podrían generar enormes dolores de cabeza durante su instalación.

Los paneles de CLT están permitiendo actualmente la construcción de edificios de hasta 30 pisos, en Canadá, y de hasta 40 pisos en Finlandia. El futuro es prometedor y seguiremos atentos a sus avances. Quizás en algunos años nuestras ciudades se vean inundadas por la calidez y la textura de la madera, cambiando además la manera en que se concibe el diseño y la construcción de nuestras obras.

[1] Exposición de Jorge Calderón dictada en el seminario inaugural del programa de difusión tecnológica "Proyectar, manufacturar y construir de forma sostenible con sistema de madera contralaminada (CLT)" | 07.11.2017. Disponible en Youtube.

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¿Es la madera laminada cruzada (CLT) el hormigón del futuro?

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El hormigón, material de construcción por excelencia, nos ha ofrecido por décadas la posibilidad de dar forma a nuestras ciudades de forma rápida y efectiva, extendiéndose rápidamente hacia las periferias urbanas o alcanzando alturas antes impensadas por la humanidad.

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WoodWorks has developed a series of checklists to assistin the design and cost optimization of mass timber projects. The design optimization checklists are intended for building designers (architects and engineers), but many of the topics should also be discussed with the fabricators and builders. The cost optimization checklists will help guide coordination between designers and builders (general contractors, construction managers, estimators, fabricators, installers, etc.) as they are estimating and making cost-related decisions on a mass timber project.

The paper also includes links to a variety of mass timber design resources, most of which can be found on the WoodWorks website ( www.woodworks.org).

The checklists cover:

* Pre-design - including design and builder team coordination, cost estimating considerations, contractual considerations, and design goals

* Schematic design (SD)- including material optimization/panel sizes/grids, system coordination, structural systems, fire resistance, acoustics, finish quality, schedule savings, and cost optimization

* Design development (DD) - including additional considerations on material optimization/panel sizes/grids, hybrid considerations, system coordination, fire resistance, MEP systems, finish quality, specifications, and cost optimization

During pre-design, for example, the paper includes:
Design Goals
* Establish design goals through discussions among the owner, designer and builder, and use them as a guide in decision making throughout design and cost analysis. Consider the relative value of the following benefits based on the project's objectives and constraints. Decide if these benefits are project goals or value-adds.

Following is an excerpt from schematic design:
SD Design Optimization Checklist Material Optimization/Panel Sizes/Grids
* Identify options for mass timber fabricators and suppliers. o Maximize panel sizes to minimize machine cutting, material waste and costs. Unique geometries and varying slab edge conditions can increase costs; balance design with budget.

* Determine the thickness and grades of roof/floor panels based on preliminary loading. Minimize the thickness of floor/roof panels to control costs. Vibration often controls floor panel thickness rather than structural span capability.

* Approach mass timber design as a modular system. Start to think of your design on a module of 8 or 10 feet. For comparing material systems, the 8-foot module also works well for steel and concrete. The module can be refined early in the design development phase. For projects that include structured parking, consider modules that work well with the parking layoutto minimize structural transfers.

* For panels sourced from overseas, shipping container size limits will govern maximum panel sizes. Actual panel size will be limited to a little less than8 feet by 40 feet.
Resources:
Ask an Expert: How can I create an efficient structural grid for a mass timber building ( www.woodworks.org/experttip/2019-efficient-structural-grid/)
System Coordination
* Optimize structure, vibration, fire resistance and acoustics while minimizing the thicknessof the floor/roof panels. These areas usually have the biggest influence on cost. o Coordinate the structure, vibration, fire resistance and acoustic systems. Meet with the project's consultants/experts to discuss system options and how each impacts the others. o Correlate acoustic and fire-resistance solutions. (They are interdependent.)
Structural Systems
* Determine where to use mass timber systems on the project. Consider: floor and roof systems, load-bearing walls, lateral force-resisting walls.

* Select the lateral system and consider how quickly it will be erected. Is the lateral system compatible with the fast speed of mass timber erection? Construction schedule savings can be significant with mass timber (and are a main driver for its use) and the choice of lateral system can greatly impact the construction schedule.

* Consider a mass timber core as an alternative to systems that have long cure times. o Review the sequencing of erection. Do the lateral systems need to be installed priorto the mass timber?
Resources:
An Approach to CLT Diaphragm Modeling for Seismic Design with Application to aU.S. High-Rise Project ( www.woodworks.org/wp-content/uploads/Approach-to-CLT-Diaphragm-Modeling-forSeismic-WoodWorks-Jan-2017.pdf )
Fire Resistance
* Optimize the construction type to minimize fire rating impacts.

* Consider fire protection approaches and what should be discussed with the Authority Having Jurisdiction (AHJ) during design. AHJs may be unfamiliar with mass timber systems. Earlypre-application meetings can help to align expectations and mitigate concerns far in advanceof permit application.
Resources:
Fire Design of Mass Timber Members - Code Applications, Construction Types and Fire Ratings ( www.woodworks.org/wp-content/uploads/Wood_Solution_Paper-Fire-Design-of-MassTimber-Members-WoodWorks-Apr-2019.pdf)

Ask an Expert: When designing a mass timber building, what are the key design considerations related to fire ratings, panel thickness/member size and occupancy? ( www.woodworks.org/experttip/ designing-mass-timber-building-key-design-considerations-related-fire-ratings-panel-thickness-membersize-occupancy )
Acoustics

Consider floor topping options and finishes.

Determine how the wet vs. dry weight of toppings impacts schedule and mass of structure.

Decide whether the mass timber floor/roof panels will be exposed on the ceiling side or concealed.

Consider the minimum level of acoustical performance required by code and expected by the owner/occupant.

Resources:
Acoustics and Mass Timber: Room-to-Room Noise Control5 } Inventory of Acoustically-Tested Mass Timber Assemblies ( www.woodworks.org/wpcontent/uploads/Acoustically-Tested-Mass-Timber-Assemblies-WoodWorks.pdf )
Finish Quality

Start to consider the appearance of exposed mass timber elements and the speciesavailable from different fabricators. Selecting species provided by local fabricators willtypically yield shipping/transportation cost savings.

Specify desired finish grades (architectural, industrial, etc.) for budgeting purposes. o Work with potential manufacturers to determine their appearance grade options andassociated cost premiums in order to arrive at a solution that meets both aesthetic goalsand budgetary constraints.

Resources:
Ask an Expert: How should desired appearance grades and characteristics be specifiedfor CLT, NLT and glulam? ( www.woodworks.org/experttip/desired-appearance-grades-characteristicsspecified-clt-nlt-glulam )
SD Cost Optimization Checklist
Mass timber buildings are relatively new to the U.S. and, as such, not enough data is available to predict costs with the unit cost method. Cost estimating mass timber systems requires a holistic approach to compare costs and yield savings. This approach takes into account the considerations/ questions in these checklists, such as the financial benefits of a shorter construction schedule.A direct comparison of the price of a mass timber structural frame to the price of a steel/concrete frame will not provide an accurate overall project cost comparison.
Schedule Savings = Cost Savings

Mass timber buildings are erected more quickly than buildings made from other materials. Consider the financial benefits of shorter construction schedule and earlier lease/occupancy.

No cure times means follow-up trades can start working as soon as floor/roof panels are placed. According to Lendlease, Candlewood Suites at Redstone Arsenal was built 37% faster overall and with 40% fewer construction workers than a similar cold-formed steel military hotel. DLR Group projects that the Seattle Mass Timber Tower wouldbe constructed 25% (5 months) faster than thebaseline posttensioned concrete building in its case study.

Resources:
Candlewood Suites Hotel at Redstone Arsenal ( www.woodworks.org/wp-content/uploads/4-Story-CLT-HotelWoodWorks-Case-Study-Redstone-Arsenal-01-05-16.pdf)

Optimized Design, Efficient Construction: How One Company Successfully Constructed a Massive Timber Project ( www.woodworks.org/wp-content/uploads/webinar_slides-EVANS-SILVA-OptimizedDesign-Efficient-Construction-Webinar-180613.pdf)

Tall with Timber: A Seattle Mass Timber Tower Case Study ( www.fastepp.com/wp-content/uploads/181109-Seattle-Mass-Timber-Tower-Book.pdf)
Aesthetic Value

The unique aesthetic of an exposed wood structure can offer market distinction with potential profits from leasing velocity, less tenant turnover and/or higher leasing rates. 4,12,13
Less Weight = Cost Savings

Mass timber buildings weigh less than concrete buildings and some steel buildings. The lighter mass requires smaller foundations and can result in lower seismic forces.

For sites with problematic soils, significant savings are possible because of the lighter foundations, which can result in less soil remediation costs, and an earlier start to the above-grade structure.

Fabrication

For a better estimate of panel fabrication costs:

o Include allowance for CNC fabrication of panels.
Shipping/Trucking
o Include allowance for finish of mass timber, if known. If unknown, either provide a range orinclude the assumed appearance grade that is factored in the quote (architectural, industrial, etc.).

The cost of shipping can vary widely. Will material be shipped on standard trucks or will panels require oversized trucks? Can oversized trailers access the site?

For overseas manufacturers, consider the cost of:

o Shipping containers and rail transit in addition to the typical manufacturing and trucking costs
Installation & Labor
o Storage in a warehouse or the extra room and logistics to store on site (climate-controlled storage may be needed depending on project location)

Include allowance for installation/labor. Mass timber frames are usually installed with one team lead and 6 to 10 team members.

o Labor shortages are common and predicted to increase. Relatively small crews are needed to install prefabricatedmass timber systems.

♣ Lendlease trained unemployed veterans for the Redstone Arsenal project.

♣ Swinerton was able to use entry-level electricians for the First Tech Federal Credit Union.

Start thinking about who will install the mass timber, GC or subcontractor.

Consider whether on-site training of crew by the manufacturer or an install specialist will be necessary.

Capture savings from pre-fabrication. Installation costs of MEP elements can often be reduced.

Contact WoodWorks for Free Project Support
To download the checklists in their entirety, visit the WoodWorks website at https://www.woodworks.org/publications-media/solution-papers/ .

WoodWorks - Wood Products Council provides education and free technical support related to the design, engineering and construction of commercial and multi-family wood buildings in the U.S. A non-profit organization staffed with architects, structural engineers and construction experts, WoodWorks has the expertise to assist with all aspects of wood building design, including (but not limited to): allowable heights and areas/construction types, structural detailing of wood and hybrid systems, fire resistance and acoustical-rated assemblies, efficient and code-compliant lateral system design, alternate means of code compliance, and energy-efficient detailing. For assistance with a project, visit www.woodworks.org/project-assistance or email help@woodworks.org . Visit our website at www.woodworks.org .

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Mass Timber Cost and Design Optimization Checklists | Civil + Structural Engineer magazine

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WoodWorks has developed a series of checklists to assist in the design and cost optimization of mass timber projects. The design optimization checklists are intended for building designers (architects and engineers), but many of the topics should also be discussed with the fabricators and build ...

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Hemos notado una mayor atención al uso de la madera en la construcción desde hace algunos años. Con todas las preocupaciones relacionadas con la sostenibilidad y la huella de carbono en los edificios, se han desarrollado nuevos métodos constructivos y nuevas posibilidades innovadoras en el uso de este material. De hecho, la madera puede considerarse un material renovable, siempre y cuando la tala sea sostenible y se tenga especial cuidado en su manejo, permitiendo que los bosques se regeneren naturalmente. Pero es la versatilidad de la madera lo que la hace tan utilizada en todo tipo de edificios. Desde tableros, vigas, pisos, hasta azulejos y aislantes térmicos y acústicos, la madera puede estar presente en varias etapas del proyecto y con diferentes grados de procesamiento y acabado.

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En el caso de la fabricación de muebles, revestimientos de paredes, techos e incluso pisos, el uso de tableros es una forma económica y funcional de incorporar madera a los edificios. Hay varias opciones en el mercado y cada uno utiliza un proceso de fabricación distinto, ya sea a través de fibras, partículas, fragmentos u hojas, resultando así en usos específicos. A continuación, seleccionamos los tableros de madera más utilizados, sus características y usos principales:

MDF (Medium Density Fiberboard)

Las láminas de MDF están hechas de fibras de madera unidas por resina sintética y comprimidas por presión y calor, dando como resultado tableros sólidos y muy uniformes, con buen acabado y durabilidad. Debido a que las fibras están orientadas aleatoriamente, esto permite que la máquina las pueda cortar en cualquier dirección, entregando una superficie lisa al tacto. Naturalmente no resiste bien el agua, pero existen opciones en el mercado que son más resistentes a la humedad e incluso al fuego, por ejemplo.

Actualmente es el material más utilizado para la carpintería, ya que permite la aplicación de distintos acabados, como pintura simple y lacada, pegado de láminas naturales o melaminas, o incluso la impresión de patrones. Pero también se puede utilizar en revestimientos de paredes y puertas, por ejemplo.

Aunque es muy similar al MDF, su diferencia está en el proceso de fabricación. Al recibir una presión más alta, estos tableros son más resistentes, soportan más peso y pueden cubrir luces más grandes.

Aglomerado y MDP (Medium Density Particleboard)

El aglomerado es el resultado de prensar los desechos de la madera, como el aserrín y el polvo, con resina y pegamento. Con el tiempo, este material ha dado paso a otras soluciones como el MDF o su reemplazo más cercano, el MDP. Puede recibir pinturas y barnices, pero no se pega, ya que la superficie no es tan lisa y uniforme. La principal ventaja del aglomerado es su bajo costo. Sus usos son similares al MDF, pero se deben utilizar herrajes y conexiones específicas para obtener un resultado satisfactorio.

Al igual que el aglomerado, los tableros MDP también están hechos de partículas de madera comprimidas con resina sintética y prensadas con calor, con la diferencia de que las partículas finas se depositan en las caras y las más gruesas en el núcleo. Esto permite un mejor acabado, un mejor ajuste del panel y más posibilidades de aplicación para revestimientos. No es tan resistente a la humedad y puede tener algunas imperfecciones.

Los paneles resultantes son ampliamente utilizados en la producción de muebles simples.

Contrachapados

Los tableros de madera contrachapada están hechos de láminas de madera superpuestas, pegadas perpendiculares y prensadas con calor. Utilizando el mismo razonamiento que la madera laminada cruzada (CLT), las fibras en direcciones cruzadas permiten que la placa resista mayores tensiones.

Su uso es bastante variado. Se pueden utilizar para muebles, pisos, techos, puertas, encimeras, entre otros. Pueden recibir pinturas y pegamentos de láminas naturales o de melamina.

OSB (Oriented Strand Board)

Los tableros de OSB tienen una estética muy característica que se ha incorporado cada vez más en la arquitectura. Estas son astillas de madera prensadas en capas perpendiculares y unidas con resina aplicada a alta presión y temperatura. Tiene buena resistencia mecánica y rigidez. Además de su buen aislamiento acústico, no presentan espacios vacíos en su interior, ni nudos ni grietas, por lo que son muy uniformes. También son ecológicos y duraderos frente a exteriores o la acción de la lluvia (humedad), el viento y el calor. Además, los tableros son totalmente reciclables. Sin embargo, como tienen una superficie rugosa, es imposible aplicar productos laminados sobre ellos.

Las principales aplicaciones del OSB se encuentran en paredes y techos, bases de pisos para aplicación de alfombras, pisos de madera, ladrillos, cobertizos de revestimiento y construcción, embalaje, estructura de muebles, entre otros.

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Tableros de madera: diferencias entre MDF, MDP, Contrachapado y OSB

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Hemos notado una mayor atención al uso de la madera en la construcción desde hace algunos años. Con todas las preocupaciones relacionadas con la sostenibilidad y la huella de carbono en los edificios, se han desarrollado nuevos métodos constructivos y nuevas posibilidades innovadoras en el uso de este material.