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Because of the wide range of structural possibilities, this Spotter's Guide shows only the most common fixed (non-movable) bridge types. Other types are listed in the Bridge Terminology page. The drawings are not to scale. Additional related info is found on the other Terminology pages which are linked to the left.

The four main factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types.

span (simple, continuous, cantilever),

material (stone, concrete, metal, etc.),

placement of the travel surface in relation to the structure (deck, pony, through),

form (beam, arch, truss, etc.).

The three basic types of spans are shown below. Any of these spans may be constructed using beams, girders or trusses. Arch bridges are either simple or continuous (hinged). A cantilever bridge may also include a suspended span.

Examples of the three common travel surface configurations are shown in the Truss type drawings below. In a Deck configuration, traffic travels on top of the main structure; in a Pony configuration, traffic travels between parallel superstructures which are not cross-braced at the top; in a Through configuration, traffic travels through the superstructure (usually a truss) which is cross-braced above and below the traffic.

Beam and Girder types
Simple deck beam bridges are usually metal or reinforced concrete. Other beam and girder types are constructed of metal. The end section of the two deck configuration shows the cross-bracing commonly used between beams. The pony end section shows knee braces which prevent deflection where the girders and deck meet.

One method of increasing a girder's load capacity while minimizing its web depth is to add haunches at the supported ends. Usually the center section is a standard shape with parallel flanges; curved or angled flanged ends are riveted or bolted using splice plates. Because of the restrictions incurred in transporting large beams to the construction site, shorter, more manageable lengths are often joined on-site using splice plates.

Many modern bridges use new designs developed using computer stress analysis. The rigid frame type has superstructure and substructure which are integrated. Commonly, the legs or the intersection of the leg and deck are a single piece which is riveted to other sections.

Orthotropic beams are modular shapes which resist stress in multiple directions at once. They vary in cross-section and may be open or closed shapes.

Arch types
There are several ways to classify arch bridges. The placement of the deck in relation to the superstructure provides the descriptive terms used in all bridges: deck, pony, and through.

Also the type of connections used at the supports and the midpoint of the arch may be used - - counting the number of hinges which allow the structure to respond to varying stresses and loads. A through arch is shown, but this applies to all type of arch bridges.

Another method of classification is found in the configuration of the arch. Examples of solid-ribbed, brace-ribbed (trussed arch) and spandrel-braced arches are shown. A solid-ribbed arch is commonly constructed using curved girder sections. A brace-ribbed arch has a curved through truss rising above the deck. A spandrel-braced arch or open spandrel deck arch carries the deck on top of the arch.

Some metal bridges which appear to be open spandrel deck arch are, in fact, cantilever; these rely on diagonal bracing. A true arch bridge relies on vertical members to transmit the load which is carried by the arch.

The tied arch (bowstring) type is commonly used for suspension bridges; the arch may be trussed or solid. The trusses which comprise the arch will vary in configuration, but commonly use Pratt or Warren webbing. While a typical arch bridge passes its load to bearings at its abutment; a tied arch resists spreading (drift) at its bearings by using the deck as a tie piece.

Masonry bridges, constructed in stone and concrete, may have open or closed spandrels A closed spandrel is usually filled with rubble and faced with dressed stone or concrete. Occasionally, reinforced concrete is used in building pony arch types.

Truss - simple types
A truss is a structure made of many smaller parts. Once constructed of wooden timbers, and later including iron tension members, most truss bridges are built of metal. Types of truss bridges are also identified by the terms deck, pony and through which describe the placement of the travel surface in relation to the superstructure (see drawings above). The king post truss is the simplest type; the queen post truss adds a horizontal top chord to achieve a longer span, but the center panel tends to be less rigid due to its lack of diagonal bracing.

Covered bridge types (truss)
Covered bridges are typically wooden truss structures. The enclosing roof protected the timbers from weathering and extended the life of the bridge.

One of the more common methods used for achieving longer spans was the multiple kingpost truss. A simple, wooden, kingpost truss forms the center and panels are added symmetrically. With the use of iron in bridge construction, the Howe truss - - in its simplest form - - appears to be a type of multiple kingpost truss.

Stephen H. Long (1784-1864) was one of the U.S. Army Topographical Engineers sent to explore and map the United States as it expanded westward. While working for the Baltimore and Ohio Railroad, he developed the X truss in 1830 with further improvements patented in 1835 and 1837. The wooden truss was also known as the Long truss and he is cited as the first American to use mathematical calculations in truss design.

Theodore Burr built a bridge spanning the Hudson River at Waterford, NY in 1804. By adding a arch segments to a multiple kingpost truss, the Burr arch truss was able to attain longer spans. His truss design, patented in 1817, is not a true arch as it relies on the interaction of the arch segments with the truss members to carry the load. There were many of this type in the Pittsburgh area and they continue to be one of the most common type of covered bridges. Many later covered bridge truss types used an added arch based on the success of the Burr truss.

The Town lattice truss was patented in 1820 by Ithiel Town. The lattice is constructed of planks rather than the heavy timbers required in kingpost and queenpost designs. It was easy to construct, if tedious. Reportedly, Mr. Town licensed his design at one dollar per foot - - or two dollars per foot for those found not under license. The second Ft. Wayne railroad bridge over the Allegheny River was an unusual instance of a Town lattice constructed in iron.

Herman Haupt designed and patented his truss configuration in 1839. He was in engineering management for several railroads including the Pennsylvania Railroad (1848) and drafted as superintendent of military railroads for the Union Army during the Civil War. The Haupt truss concentrates much of its compressive forces through the end panels and onto the abutments.

Other bridge designers were busy in the Midwest. An OhioDOT web page cites examples of designs used for some covered bridges in that state. Robert W. Smith of Tipp City, OH, received patents in 1867 and 1869 for his designs. Three variations of the Smith truss are still standing in Ohio covered bridges.

Reuben L. Partridge received a patent for his truss design which is appears to be a modification of the Smith truss. Four of the five Partridge truss bridges near his home in Marysville, Union County, OH, are still in use.

Horace Childs' design of 1846 was a multiple king post with the addition of iron rods. The Childs truss was used exclusively by Ohio bridge builder Everett Sherman after 1883.

Truss - Pratt variations
The Pratt truss is a very common type, but has many variations. Originally designed by Thomas and Caleb Pratt in 1844, the Pratt truss successfully made the transition from wood designs to metal. The basic identifying features are the diagonal web members which form a V-shape. The center section commonly has crossing diagonal members. Additional counter braces may be used and can make identification more difficult, however the Pratt and its variations are the most common type of all trusses.

Charles H. Parker modified the Pratt truss to create a "camelback" truss having a top chord which does not stay parallel with the bottom chord. This creates a lighter structure without losing strength; there is less dead load at the ends and more strength concentrated in the center. It is somewhat more complicated to build since the web members vary in length from one panel to the next.

When additional smaller members are added to a Pratt truss, the various subdivided types have been given names from the railroad companies which most commonly used each type, although both were developed by engineers of the Pennsylvania Railroad in the 1870s.

The Whipple truss was developed by Squire Whipple as stronger version of the Pratt truss. Patented in 1847, it was also known as the "Double-intersection Pratt" because the diagonal tension members cross two panels, while those on the Pratt cross one. The Indiana Historical Bureau notes one bridge as being a "Triple Whipple" -- possibly the only one -- built with the thought that if two are better than one, three must be stronger yet.

The Whipple truss was most commonly used in the trapezoidal form -- straight top and bottom chords -- although bowstring Whipple trusses were also built.

The Whipple truss gained immediate popularity with the railroads as it was stronger and more rigid than the Pratt. It was less common for highway use, but a few wrought iron examples survive. They were usually built where the span required was longer than was practical with a Pratt truss.

Further developments of the subdivided variations of the Pratt, including the Pennsylvania and Baltimore trusses, led to the decline of the Whipple truss.

Truss - Warren variations
A Warren truss, patented by James Warren and Willoughby Monzoni of Great Britain in 1848, can be identified by the presence of many equilateral or isoceles triangles formed by the web members which connect the top and bottom chords. These triangles may also be further subdivided. Warren truss may also be found in covered bridge designs.

Truss - other types
The other truss types shown are less common on modern bridges.

A Howe truss at first appears similar to a Pratt truss, but the Howe diagonal web members are inclined toward the center of the span to form A-shapes. The vertical members are in tension while the diagonal members are in compression, exactly opposite the structure of a Pratt truss. Patented in 1840 by William Howe, this design was common on early railroads. The three drawings show various levels of detail. The thicker lines represent wood braces; the thinner lines are iron tension rods. The Howe truss was patented as an improvement to the Long truss which is discussed with covered bridge types.

Friedrich August von Pauli (1802-1883) published details of his truss design in 1865. Probably the most famous Pauli truss, better known as the lenticular truss -- named because of the lens shape, is Pittsburgh's Smithfield Street Bridge. Its opposing arches combine the benefits of a suspension bridge with those of an arch bridge. But like the willow tree, some of its strength is expressed in its flexibility which is often noticeable to bridge traffic.

Before the use of computers, the interaction of forces on spans which crossed multiple supports was difficult to calculate. One solution to the problem was developed by E. M. Wichert of Pittsburgh, PA, in 1930. By introducing a open, hinged quadrilateral over the intermediate piers, each span could be calculated independently. The first Wichert truss was the Homestead High Level Bridge over the Monongahela River in 1937.

The composite cast and wrought iron Bollman truss was common on the Baltimore and Ohio Railroad. Of the hundred or so following Wendell Bollman's design, the 1869 bridge at Savage, MD, is perhaps the only intact survivor. Some of the counter bracing inside the panels has been omitted from the drawing for clarity.

Also somewhat common on early railroads, particularly the B&O, was the Fink truss - - designed by Albert Fink of Germany in the 1860s.

Cantilever types - truss
A cantilever is a structural member which projects beyond its support and is supported at only one end. Cantilever bridges are constructed using trusses, beams, or girders. Employing the cantilever principles allows structures to achieve spans longer than simple spans of the same superstructure type. They may also include a suspended span which hangs between the ends of opposing cantilever arms.

Some bridges which appear to be arch type are, in fact, cantilever truss. These may be identified by the diagonal braces which are used in the open spandrel. A true arch bridge relies on vertical members to transfer the load to the arch. Pratt and Warren bracing are among the most commonly used truss types.

The classic cantilever design is the through truss which extends above the deck. Some have trusses which extend both above and below the deck. The truss configuration will vary.

Suspension types
The longest bridges in the world are suspension bridges or their cousins, the cable-stayed bridge. The deck is hung from suspenders of wire rope, eyebars or other materials. Materials for the other parts also vary: piers may be steel or masonry; the deck may be made of girders or trussed. A tied arch resists spreading (drift) at its bearings by using the deck as a tie piece.

Though Pittsburgh has been a pioneer in bridge design and fabrication, it has had few suspension bridges. The Pennsylvania Mainline Canal entered the city on John Roebling's first wire-rope suspension bridge in 1845 (replacing a failing 1829 wooden structure). A similar structure still stands at Minnisink Ford, NY, crossing the Delaware River. Roebling and his son Washington Roebling, later famous in building the Brooklyn Bridge, began their work in Saxonburg, PA, north of Pittsburgh.

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Bridge Basics - A Spotter's Guide to Bridge Design

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Because of the wide range of structural possibilities, this Spotter's Guide shows only the most common fixed (non-movable) bridge types. Other types are listed in the Bridge Terminology page. The drawings are not to scale. Additional related info is found on the other Terminology pages which are linked to the left.

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by Chris Woodford. Last updated: October 16, 2020.

Over, under, or straight through the middle? It's a simple-sounding question, but it's challenged every great engineer since ancient times. We like highways and railroads to be straight and level, but Earth's bumps and wiggles make that kind of construction an amazing challenge. How do you take a highway through a valley or make a railroad cross a creek? The simplest answer is to use a bridge. Sounds easy, perhaps, but which type of bridge do you use? Why are there so many different types and how do they all work? Let's take a closer look and find out more!

Photo: One of the world's greatest bridges. Over 150 years after it was completed in 1859, Isambard Kingdom Brunel's amazing Royal Albert Bridge still carries railroad trains 30m (100ft) over the River Tamar, separating Cornwall and Devon in England. But is it a suspension bridge, or is it a truss bridge? Well, it's certainly a truss bridge (notice the thick, tubular, "lenticular" trusses at the top). The vertical ties running from the top curve ("chord") of the trusses, through the bottom curve, down to the deck mean that the bridge does not push outward on its supporting towers, though it does push its loads down onto them. But there are elements of other bridges in here too-bits of suspension bridge, bits of bowstring (tied-arch)-and I think it's a good example of how some bridges are actually hybrids incorporating several different types of bridge in one structure. A modern suspension bridge was built alongside in the 1960s to ferry cars across too (see the photos below).

The wonder of bridges

Photo: Bridges do more than simply bear loads: with soaring towers and graceful spans, their inspiring designs are a triumph of architecture as well as engineering. This is the Palladian Bridge at Prior Park, Bath, England, built in 1755, and reputedly one of only four such bridges in the world. You can see that the lower part of the bridge-essentially its deck-rests on five separate stone arches.

In the endless war of people versus nature, there will only ever be one winner-but humans can still console themselves with occasional victories, which is what the world's greatest bridges represent. Whether we need to cross rivers or valleys, connect islands to the mainland, carry cars, people, or manmade waterways, bridges are a brilliant solution whenever nature gets in our way. Historians suppose people invented bridges when they saw how fallen trees could help them cross shallow rivers. Since then, bridges have grown longer, technically more sophisticated, and much more awe-inspiring, slowly evolving from simple stone arches to gracefully swooping suspension bridges several miles long. Buffeted by winds from above, scoured by rivers from below, pounded by traffic all day long, it's a miracle that bridges stay upright as long as they do.

How bridges balance forces

Forces make things move, but they also hold them still. It's far from obvious, but when something like a skyscraper looms high above us or a bridge stretches out beneath our feet, hidden forces are hard at work: a bridge goes nowhere because all the forces acting on it are perfectly in balance. Bridge designers, in short, are force balancers.

The biggest and most pervasive force in the universe, gravity, is constantly tugging things down, which isn't such a problem for a skyscraper, because the ground underneath pushes straight back up again. But a bridge spanning a river, valley, sea, or road is quite different: the huge deck (the main horizontal platform of a bridge) has no support directly beneath it. The longer the bridge, the more it weighs, the more it carries, and the bigger the risk it'll collapse. Bridges certainly do fall down from time to time, and quite spectacularly, but most stand happily still for years, decades, or even centuries. They do it by carefully balancing two main kinds of forces called compression (a pushing or squeezing force, acting inward) and tension (a pulling or stretching force, acting outward), channeling the load (the total weight of the bridge and the things it carries) onto abutments (the supports at either side) and piers (one or more supports in the middle). Although there are many kinds of bridges, virtually all of them work by balancing compressive forces in some places with tensile forces elsewhere, so there's no overall force to cause motion and do damage.

Artwork: Balancing forces in a bridge: Different types of bridges carry loads through the forces of compression ("squeezing"-shown here by red lines) and tension ("stretching"-shown by blue lines): 1) A beam bridge has its beam partly in tension and partly in compression, with the abutments (side pillars) in compression; 2) An arch bridge supports loads through compression; 3) A suspension bridge has its piers (towers) in compression and the deck hangs from thick suspension cables by thinner cables, all of which are in tension. 4) A cable-stayed bridge is similar but the deck hangs directly from the piers from cables. The piers are in compression and the cables are in tension. 5) A truss bridge is a kind of reinforced beam bridge. Like a beam bridge, the top is in compression and the bottom in tension. The diagonal trusses are in tension and the vertical ones are in compression. 6) A cantilever bridge balances tension forces above the bridge deck with compression forces below.

Carrying loads

If a bridge is unloaded, all it really has to do is support its own weight (the dead load), so the tension and compression in its structure are essentially static forces (ones that don't cause movement), changing little from hour to hour or day to day. However, by definition bridges have to carry changing amounts of weight (the live load) from things like railroad trains, cars, or people, which can increase the ordinary tensile or compressive forces quite dramatically. Rail bridges, for example, bend and flex every time a heavy train crosses over them and then "relax" again as soon as the load has passed by.

Environmental forces

Bridges also have to bear ever-changing environmental forces. Arch bridges over rivers, for example, have to cope with water backing up behind them (their abutments often have strategically placed openings to let high flood water drain through). Suspension bridges that carry cars tend to bear the same loads all day long, though, often sited in windy estuaries, they also have to endure squalling gusts of wind, which can set up a twisting force, called torsion, in the bridge deck. (Modern suspension bridges tackle this problem by having decks with aerodynamically designed cross sections, tested in wind tunnels, and may be reinforced with trusses underneath.) Loads that cause a bridge to move back and forth can be particularly dangerous if they make it vibrate wildly at its so-called natural or resonant frequency. Resonance, as this is known, is what makes wine glasses shatter when opera singers get a bit too close; the "singing" of the wind can have equally catastrophic effects on a bridge.

Bridges through history

People find bridges bewitching and bewildering at the same time. Why are there so many different types? How do engineers choose one kind over another? Why have people tended to build different kinds of bridges in different periods of history? There's are easy answers to all these questions-and complex ones too.

Photo: Bridges are a product of their place and time: place, because they have to be tailored to specific local conditions; and time, because they have always drawn on the most advanced materials and engineering technology available. This is a view of the revolutionary Brooklyn Bridge in New York City, taken from the Brooklyn side. Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

One simple answer is that, over thousands of years of human civilization, engineers have gradually developed more sophisticated bridge designs that can span ever greater distances. The oldest bridge types, beams and arches, can only stretch so far before they collapse under their own weight; more sophisticated versions of these designs (truss, box girder, and cantilever bridges) can reach further; and suspension and cable-stayed bridges can go further still. This gradual evolution-and extension-of bridges has been made possible partly by a deeper understanding of engineering, but also by the development of far stronger materials. Arch bridges were popular in the Middle Ages, for example, because they were quick and easy to build from locally sourced materials and lasted a long time with little or no maintenance. When Ironbridge, the world's first cast iron (arch) bridge, was built at Coalbrookdale in Shropshire, England, in 1779, it revolutionized bridge construction; during the 19th century, hundreds of other bridges were built from iron and later steel, including New York City's famous 1883 Brooklyn Bridge, with a span of 486m (1595ft). Suspension and cable-stayed bridges rely on those most dependable of modern materials, reinforced concrete and steel. Some of the newest bridges naturally use the very latest composite materials.

Types of bridges

While it's easy to discuss bridges in this fairly abstract and theoretical way, it's much more interesting to look at some specifics by examining each major type of bridge in turn.

Beam

Photo: A beam bridge carrying a railway line over a road in Dorset, England. Note the abutment on the right-hand side that stops the bridge from collapsing down the hill toward us.

A beam is the simplest (and often cheapest) kind of bridge: a deck, spanning a relatively short distance, held up by a pair of abutments (the vertical supports at either end). Stand on a plank (the deck) stretched between a couple of chairs (the abutments) and you'll make it flex downward in the middle, so it's slightly longer underneath and slightly shorter on top. That tells us that the bottom of a beam is in tension (pulled longer than it would ordinarily be), while the top is in compression (squashed shorter). The load on a bridge like this is transmitted through the beam to the abutments at either end, which are also compressed (squashed downward). The longer the beam, the more likely it is to sag in the middle, which is why basic beam bridges are usually quite short. Modern beam bridges can be much longer, if they're built with box girders (huge hollow boxes made from repeating sections of steel girders and/or reinforced concrete) or braced with trusses (diagonal reinforcements) either on the side or underneath. Beams are explained further in our article on how buildings work.

Arch

Photo: The Pulteney Bridge in Bath, England is made up of three stone arches. Completed in 1773, it's modeled on the Ponte Vecchio in Florence, Italy.

Arches are the only kinds of bridges supported entirely by forces of compression. There is some tension underneath an arch, but it's usually negligible unless the arch is large and shallow. That makes sense, if you think about it, because an infinitely wide arch would just be a horizontal beam, with its lower side in tension. A bridge deck resting on an arch pushes down on the curve of stones (or metal components) underneath it, squashing them tightly together and effectively making them stronger. The load on a stone arch bridge is transmitted through the central stone (called the keystone), around the curve of other stones, and into the abutments, where the solid ground on either side pushes back upward and inward. Like beam bridges, arches are relatively simple and cheap to construct, and don't need to block a road or river with central piers. They can easily exceed the span of a basic beam, though their big drawback is that they need large abutments, so they're not always an efficient way of bridging something like a highway if a lot of clearance is needed underneath. Viaducts and aqueducts (water-carrying bridges) are often variations on arch bridges, with multiple arches side by side.

Examples of arch bridges include the Mostar Bridge in Bosnia Herzegovina and the Charles Bridge in Prague.

Truss

Photo: A truss bridge carrying a pedestrian walkway over a railroad line in Dorset, England.

One way to extend the reach of a basic beam bridge is to reinforce it-and engineers have found the best way to do that is with a system of diagonal, triangular bars on the sides, which are called trusses. There are many ways of arranging trusses to support a bridge, giving a variety of intricate and often attractive lattice patterns; lenticular (curved) trusses, used in the Royal Albert Bridge in the top photo, are one example. A typical truss bridge looks like a hollow box with open or closed vertical sides and roof, the sides reinforced with diagonal trusses, and the base resting on girders.

Cantilever

Photo: The Huey P. Long cantilevered bridge on the Mississippi River near New Orleans, under construction in the early 1930s. Note how the apparently unsupported spans at either end are extending outwards into thin air from the piers-the cantilever principle at work. Picture restored from an original by Marcus Lamkin courtesy of US Library of Congress.

Two back-to-back beams extending outward from a pier can balance one another-just as a tightrope walker can balance by holding both arms straight out from her body. That's the basic idea behind the cantilever bridge. Normally, when we talk about a cantilever, we mean a beam supported at only one end, like a diving board or see-saw only much more rigid. In a cantilever bridge, there's usually a pair of cantilevers extending from each pier, with a short beam bridge in between, linking them together; alternatively, some have a cantilever extending out from each abutment toward the middle, with a beam bridging them. Cantilever bridges are sometimes hard to recognize because they're typically reinforced with girders and trusses, but easier to spot if you remember that they have multiple sections and often have at least one pier in the middle.

The world's most famous cantilever bridge, the Forth Bridge in Scotland, has three cantilevers (reinforced with a lattice of trusses) with two shorter beam bridges in between them. The world's longest cantilever is the very similar Quebec Bridge, at just under 1km long (987m or 3239ft to be exact). Other examples of cantilever bridges include the Queensboro Bridge in New York City and the Crescent City Connection in New Orleans.

Suspension

Photo: The Tamar Bridge, completed in 1961, spans the River Tamar, the boundary between Cornwall and Devon, England, alongside Brunel's 1859 rail bridge (which you can see in the background). Notice the truss and girder reinforcements under the deck.

If you need a bridge that spans even further, a suspension bridge of some kind is really your only option. The genius of a suspension bridge lies in using very tall piers with huge, curving main cables strung between them. Dozens of thinner vertical suspension cables of varying length hang down from the main cables and support the immense weight of the deck and the loads it carries. (And although people always notice the cables in a suspension bridge, they often fail to spot the girders and trusses reinforcing the deck underneath. This is a subtle and quite important point: most bridges are actually composites of two or more of the basic bridge types.) The biggest bridges all use the suspension approach; the world's longest, the Akashi Kaikyō in Japan, is 3.9km (2.4 miles) long.

Famous suspension bridges include the Humber Bridge and the Clifton Suspension Bridge in England, the Golden Gate Bridge in California, and the Brooklyn Bridge in Manhattan, New York City.

Cable-stayed

Photo: The Arthur Ravenel, Jr. cable-stayed bridge in Charleston S.C. Picture courtesy of Carol M. Highsmith's America Project in the Carol M. Highsmith Archive, US Library of Congress.

A big drawback of suspension bridges is that they need to be anchored to the ground on either side. That's not always possible if there isn't room for the cables or appropriate bedrock to anchor them into. A different kind of suspension bridge, known as a cable-stayed bridge, does away with this by balancing two sets of suspension cables either side of each pier, which supports the load. In a "normal" suspension bridge, the deck hangs from cables of varying length that are themselves supported by the immensely strong main suspension cables. In a cable-stayed bridge, there's only one set of cables that fan out, diagonally, from each pier to the bridge deck, which tends to be stronger and bulkier than in a suspension bridge.

Cable-stayed bridges are significantly shorter than conventional suspension bridges and generally don't span distances much greater than 1km; the world's longest is currently the Russky Bridge in Vladivostok, Russia, at 1.1km (3622ft). Other examples include the Vasco da Gama Bridge in Portugal, the Millau Viaduct in France, the Hangzou Bridge in China, and the Chords Bridge in Jerusalem.

Pontoon

Photo: A pontoon bridge laid across the Euphrates River in Iraq. Photo by Kevin C. Quihuis, Jr. courtesy of US Marine Corps.

Boats obviously float on water, so if you need to build a temporary bridge in a hurry, floating a deck on a series of boats is one possible solution. A floating bridge like this is called a pontoon-and it's widely used by the military for improvised river crossings (such as when existing bridges have been blown up for strategic reasons). Well-organized armies have prebuilt sections of pontoon bridges that they can float into place and bolt together, wherever and whenever they need to. The main problems with pontoon bridges are basic instability and the relatively light loads they can carry. Since the deck floats very close to the waterline, a pontoon bridge automatically blocks boats from using a river, though it's usually possible to detach a section or two from the middle and float it aside to let river traffic navigate through.

Through arch and tied-arch (bowstring)

Photo: Hell Gate Bridge, a through-arch railroad bridge in New York City, pictured around 1915-1920. Note how the bridge deck cuts horizontally through the arch (so some of the arch is above the deck and some below) and the big abutments at either end holding the arch in place. Photo by Detroit Publishing Co. courtesy of US Library of Congress.

Hang a beam bridge from an overhead arch and what you get is called a through-arch bridge (if the deck cuts through the arch) or a tied-arch bridge (if the deck ties the arch in place at its base). Both kinds are a bit like suspension bridges, because the deck and its load hang from the arch. However, although they look very similar, they balance forces in different ways.

In a through-arch bridge, just like with a conventional stone or brick arch, the ends of the arches push ("thrust") outward and need pushing back by the abutments. Bridges like this are sometimes called thrust arches for this reason.

Examples of through-arch bridges include the Sydney Harbor Bridge in Australia, the Hell Gate Bridge in New York City (shown here), and the Tyne Road Bridge in Newcastle, England.

Photo: Daniel Carter Beard Bridge, a tied-arch bridge over the Ohio River. Note how the deck sits at the base of the arch (in the other words, the whole arch is above the deck) and ties it in place; for that reason, no big abutments are needed to hold the arch in place. Picture courtesy of the Carol M. Highsmith Archive, US Library of Congress, Prints and Photographs Division.

In a tied-arch bridge, while the arch supports the deck, the deck also stops the arch from pushing outward, holding it in place, so the arch and deck balance one another. Just as a cable-stayed bridge is more self-supporting than a suspension bridge, because it does away with the anchoring cables, so tied-arch bridges are more self-supporting than a conventional arch, because they have less need for sturdy abutments. Tied-arch bridges are sometimes called bowstrings because they resemble the arch of a bow pulled out ready to fire an arrow, and because the crossbar ties the bow together in a similar way.

Examples of tied-arch bridges include the Puente de la Barqueta in Spain and the St Georges Bridge in Delaware, USA. The Chaotianmen Bridge in China is both a through-arch and a tied-arch bridge.

Lifting and swinging bridges

Photo: The El Ferdan Swing Bridge carries a railroad line over the Suez Canal in Egypt. Spanning 340m (1100 ft), it's the longest swing bridge in the world. Photo by Daniel Meshel courtesy of US Navy.

Conventional bridges are impractical if something like a low road has to cross a river or canal through which tall boats need to pass. In that case, we need a mechanical bridge with a deck that can lift up or swing aside whenever necessary. Tower Bridge in London, England is a kind of double drawbridge: it has a split deck that lifts up in the center. There are many examples of swing bridges all over the world. Some have two moving parts that swing to the sides, leaving a central channel clear; others swivel on a central support to open up one or two clean channels of a waterway either side.

How do you design a bridge?

Just as bridges balance competing forces from different directions, so engineers have to balance all kinds of considerations when they plan a new bridge.

Type

How far does the bridge need to stretch? That will usually determine the type of bridge that's needed. A very short span (over a small river, road, or rail track) could merit just a low-cost beam or truss; suspension and cable-stayed bridges will generally be unnecessarily complex and expensive; and arched bridges are built much less often than they were in the Middle Ages, partly because other types of bridges use the available space more efficiently. As we've already seen, the type of bridge determines the materials used, to a very large extent. Even so, there may be scope for using local materials so a bridge blends into its environment. That was certainly a feature of traditional arched bridges, often built from local rock or stone. Modern bridges are usually built from steel and concrete and have to rely on design to integrate themselves into their surroundings instead. Box-girder bridges are often manufactured in sections, off-site, which means they can be very rapidly erected. Unfortunately, it also tends to mean that they look very similar and generic.

Crossing

The place where a bridge is being built is also a critically important factor. Is the ground firm enough to take large abutments for an arch? Is there solid bedrock into which suspension cables can be anchored (and, if not, would a cable-stayed bridge be better)? If the bridge has to cross a river, how can piers and towers be safely sunk into its bed so they're not scoured away by the rushing water. The exact location of a bridge is carefully chosen to simplify construction, reduce cost, and ensure the bridge is strong and durable. It's not always possible for bridges to cross in a perfectly straight line, however; bridges sometimes have to cross at an angle (which gives what's known as a skew bridge), curve, or change direction from one section to another. Modern box girder bridges, built from modular deck sections, are easy to curve through even quite dramatic angles.

Photo: The Calstock Viaduct has carried railroad trains high over the River Tamar at Calstock, Cornwall in England since its completion in 1908. Imagine the engineering challenges here. You have to carry very heavy loads (trains) at a high level over quite a wide river valley while allowing enough clearance for sailing boats to pass beneath. A single arch so high and wide would have been impossible to build, so the designer opted for a 12-arch viaduct instead. An ugly bridge could easily have spoiled the beautiful landscape here, but this elegant structure serves as a perfect focal point. Although it looks like stone, the material is actually precast concrete.

Load

Apart from the dead and live load, what kinds of occasional, transient forces might the bridge need to withstand? Are there earthquakes or hurricanes and, if so, how can the bridge be designed to survive them? Will a river bridge be able to cope with floods? And what about the loads it will carry? If it's a road bridge, how much is traffic likely to increase over the coming years and decades, and will the bridge always be strong enough to handle them? What if several of these transient forces occur at the same time? For example, suppose a bridge simultaneously has to handle high winds, immense pressure from rising water levels, and heavy traffic?

Other factors

Engineers have to consider all kinds of other factors beside the basic type, location, and strength of a bridge. For example, does a bridge have to carry different types of traffic (a railroad, cars, and pedestrians) and how will it separate them? What about safety considerations (stopping speeding cars from plunging over the edge), and issues like minimizing the risk of suicides (a particular problem for some of the world's tallest bridges)? What kind of maintenance will the bridge need, from regular concrete inspections to systematic painting to protect against corrosion?

Bridges in harmony

Science, technology, and engineering give us confidence we can build stone, iron, steel, or concrete bridges that will survive for many decades. But there's much more to a bridge than merely staying upright as humdrum loads shuttle over it. Think of some of the world's greatest bridges-the Stari Most arch at Mostar, the Brooklyn suspension bridge in Manhattan, the Forth Railway cantilever bridge, or the very recent, cable-stayed Millau Viaduct in France, for example-and you'll quickly realize that great bridges are as breathtakingly memorable as great buildings. Sitting in a river or straddling a valley, you could argue that a bridge disrupts the balance of nature. But bridges connect people and communities together, and many would contest that great bridges are true world wonders that enhance their environment. Who, for example, can imagine San Francisco Bay without the Golden Gate Bridge? So it's maybe just as true to argue that the genius of a great bridge lies in forging a partnership between people and place so that engineering and nature sit happily, side by side.

Why do bridges collapse?

Bridges don't fail very often, but when they do, the results are spectacular and unforgettable. Once you've seen the footage of the Tacoma Narrows bridge resonating in a gale, bucking back and forth before the deck breaks up and crashes to the river below, you'll never forget it. Imagine how terrifying it would have been if you'd been on the bridge at the time!

Bridges always collapse for exactly the same reason: something happens that makes them unable to balance the forces acting on them. A force becomes too great for one of the components in the bridge (maybe something as simple as a single rivet or tie-bar), which immediately fails. That means the load on the bridge suddenly has to be shared by fewer components, so any one of them might also be pushed beyond its limit. Sooner or later, another component fails, then another-and so the bridge collapses in a kind of domino effect of failing materials.

Photo: This is the remains of the I-35W Mississippi River bridge, a steel-trussed arch bridge that used to carry a very busy highway over the river. It collapsed unexpectedly in 2007, killing 13 people and injuring 145 more. A report into the disaster found that a metal plate had ripped along a line of rivets, causing a catastrophic failure. Ironically, the bridge was carrying a massive extra load of construction equipment for repairs and reinforcement at the time. Riddled with fatigue cracks and corrosion, it had been deemed "structurally deficient" as far back as 1990. Photo by Joshua Adam courtesy of US Navy.

There are two different ways in which a bridge component can fail catastrophically: weakness and fatigue. First, and simplest, it might be too weak to cope with a sudden transient load. If a bridge is designed to carry no more than 100 cars, but 200 heavy trucks drive onto it instead, that creates a dangerous, transient load. Or if hurricane-force winds buffet the bridge, twisting the deck much more than it's designed to cope with, that can be catastrophic too. So a bridge can fail through weakness because a force exceeds what's called the ultimate tensile strength (the most you can pull) or compressive strength (the most you can push) of the materials from which it's constructed.

But a bridge can also fail even if the forces on it are relatively modest and well within these limits. Everyday materials usually have to undergo repeated stresses and strains-for example, a bridge deck is loaded (when a truck drives across) and then unloaded again immediately afterward, and that can happen hundreds or thousands of times a day, hundreds of days a year. Just as a paperclip snaps when you repeatedly bend it back and forth, the endless cycles of stress and strain, flexing and relaxing, can cause materials to weaken over time through a process known as fatigue. Eventually, something like a metal cable or tie in a bridge will snap even though it's not experiencing a particularly high stress at that moment. Fatigue is often compounded by gradual corrosion (rusting) of metal components or what's informally known as concrete cancer (such as when reinforced concrete cracks after the metal reinforcing bars inside it start to rust).

Engineers try to protect against bridge failures in two main ways. If we learn to see bridges as "living structures," constantly aging and being degraded by weather and the environment. it's easy to understand that they need regular maintenance, just like our homes and bodies. Periodic inspections and preventative maintenance helps us spot problems and correct them before it's too late. Engineers can also protect against bridge failure by building in a factor of safety -designing them so they can cope with forces several times larger than they're ever likely to encounter. That might include extra "redundant" components or reinforcements so that even if one part of the structure fails, others can safely share the load until the bridge can be reinforced or repaired.

Further reading

Books

Why did the bridge collapse? by Elizabeth Stawicki. MPR News, Aug 2, 2007. A very interesting article exploring the reasons for the I-35W Mississippi River bridge collapse.

Why Buildings Fall Down by Matthys Levy and Mario Salvadori. Norton, 2002. Chapter 7 "Galloping Gertie" explores the Tacoma Narrows bridge collapse and Chapter 9 "Thruways to Eternity" looks into the Mianus River bridge fall in 1983.

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by Chris Woodford. Last updated: October 16, 2020. Over, under, or straight through the middle? It's a simple-sounding question, but it's challenged every great engineer since ancient times. We like highways and railroads to be straight and level, but Earth's bumps and wiggles make that kind of construction an amazing challenge.

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A bridge is a structure that is used to get from one side of an obstacle (such as a river, lake, or road) to the other side. There are six basic types of bridges shown below. The word Beam comes from the old English word for tree (the first bridges were probably fallen trees or logs).

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The bridge is a structure that provides passage over obstacles such as valleys, rough terrain, or bodies of water by spanning those obstacles with natural or human-made materials. They began being constructed in ancient times when modern civilizations started rising in Mesopotamia. From then on, knowledge, engineering, and manufacturing of new bridge-building materials spread beyond their borders, enabling bridges' slow but steady adoption worldwide.

In the beginning, bridges were simple structures built from easily accessible natural resources- wooden logs, stone, and dirt. Because of that, they could only span very close distances, and their structural integrity was not high because the mortar was not yet invented, and rain slowly but constantly dissolved the dirt fillings of the bridge. A revolution in bridge construction came in Ancient Rome, whose engineers found that grinding out volcanic rocks could be an excellent material for making mortar. This invention enabled them to build much sturdier, more powerful, and larger structures than any civilization. Seeing the power of roads and connections to distant lands, Roman architects soon spread across Europe, Africa, and Asia, building bridges and roads of very high quality.

One of the defining successes of Roman bridge architecture was the discovery of arches. Using this type of building, load forces of the bridge were conveyed to move along the curve of the arch, meeting with the ground where supports on the end of the arch canceled them. Because of that, Romans were able to create bridges that were much lighter than before and held loads twice as heavy as the bridge itself. In constructing their numerous aqueducts, Roman architects even managed to create water-carrying bridges with multiple arched tiers that reached incredible heights!

Using this new building technique, Romans could quickly produce cheap, light, and very powerful bridges from materials they found near the project. The only material that had to be imported from Italy was mortar dust, which was combined with water and inserted into the bridge structure.

After the fall of the roman empire, bridge-building techniques in Europe and Asia stagnated until the 18th century (if we ignore the introduction of Rope suspension bridges that were brought back to Europe from Central and South America) when the new age of science and engineering swept across the world. Architects of that time started using new construction material - cast iron! Iron enabled the creation of new bridge designs, such as truss systems. Sadly, wrought iron did not have the tensile strength to support heavy structures, which was fixed with the advent of steel and the ideas of famous French architect and engineer Gustave Eiffel.

Modern bridges are usually made with concrete, irons, and cables. They can be built from very small sizes to incredible lengths that span entire mountains, rough landscapes, lakes, and seas.

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The bridge is a structure that provides passage over obstacles such as valleys, rough terrain, or bodies of water by spanning those obstacles with natural or human-made materials. They began being constructed in ancient times when modern civilizations started rising in Mesopotamia.

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What Makes Bridges So Strong? | Engineering for Kids | STEAM | SciShow Kids

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People may cross bridges everyday to get to work, some boring and some simply amazing, here are 12 of the most amazing bridges ever built! Subscribe for weekly wacky videos and learn interesting facts about the world with awesome top 10 lists and other amazing videos.

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There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the dist…

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