Background information on truss bridges building
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. 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 , 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 Brooklyn Bridge , with a span of m ft. Suspension and cable-stayed bridges rely on those most dependable of modern materials, reinforced concrete and steel.
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Some of the newest bridges naturally use the very latest composite materials. 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. 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. Beams are explained further in our article on how buildings work. Completed in , 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. Photo: A truss bridge carrying a pedestrian walkway over a railroad line in Dorset, England.
Startling Information About Warren Truss Bridges
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.
Photo: The Huey P. Long cantilevered bridge on the Mississippi River near New Orleans, under construction in the early s. Note how the apparently unsupported spans at either end are extending outwards into thin air from the piers—the cantilever principle at work. 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 m or ft to be exact. Photo: The Tamar Bridge , completed in , spans the River Tamar, the boundary between Cornwall and Devon, England, alongside Brunel's 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.
Photo: The Arthur Ravenel, Jr. Picture courtesy of Carol M.
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Highsmith's America Project in the Carol M. 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. Photo: A pontoon bridge laid across the Euphrates River in Iraq. Photo by Kevin C. Quihuis, Jr. 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.
Basics of Truss Bridges
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. 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. 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. 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. The Chaotianmen Bridge in China is both a through-arch and a tied-arch bridge. Trusses are also suitable for use in several other structures like roof supports and space stations.
Truss Bridge Construction
Amongst the modern bridges, truss bridges are considered to be included in the older kinds. The famous truss bridges are relatively inexpensive due to effective utilization of the bridge materials. The truss bridge designs are an important factor in architecture. Truss bridge construction is initiated with a detailed soil analysis to determine suitability for the bridge and traffic loads.
Truss Types & Truss Bridge: Experiments, Labs, Studies and Background Information
The truss bridge is designed with either the road being at the truss bottom, or alternatively with the road being at the truss top. The former design has the bridge elements under compression, while the elements of the later design are mostly under tension. The bridge materials are selected and either welded or bolted, according to the requirement.
The concrete is poured in the abutments, with the anchor bolts firmly inserted. Finally, the trusses are built, and the bridge is ready for use.