From the stone arches of the Roman era to the inclined pylons of Calatrava, bridges have been both a technical marvel and an aesthetic delight throughout human history. Successful bridges are a balance of structural forces, rendered in materials that evince both strength and a degree of flexibility. While certain shapes, such as the arch, are naturally attuned to balancing these forces, newer bridge design displaces these forces in more creative ways, introducing elements that create truly striking designs. In order to understand how bridges work, we will first review the basic types of bridges, the forces that act upon them, and how these types and forces interact.
At their most elemental, bridges can be broken down into spans and support structures. Spans are the area that people or traffic cross to get from one side to the other. The support structures—such as towers, beams, cables and trusses—allow the bridge to carry the weight of the bridge and the weight of the traffic that crosses over it. The frequency and design of the support structures is an indicator of how the architect chooses to mitigate the structural forces. Within these basic parameters, bridges have several distinct types, including the arch, the suspension, and the beam.
There are several structural forces which impact bridge design and durability. The major forces are compression and tension. Compression, as its name suggests, concentrates the weight of the bridge into a small area, while tension spreads out this weight over a greater area. Torsion refers to a twisting or rotational force, which is due primarily to gravity. Although this effect can be minimal, over a long period of exposure to this force the span of a bridge can flip over. Resonance refers to a rhythmic wave which can disrupt the structural integrity of the span by introducing a dangerous imbalance into the dynamic between compression and tension. Lastly, weather, with its fluctuations in temperature and motion, is such an unpredictable force that bridge designers can’t fully protect against its effects, except by performing periodic maintenance.
Any bridge — whether a beam, an arch, or a suspension bridge — must grapple with all of these forces, and find a way of effectively mitigating them.
How Type and Forces Interact
The arch design is classic partially because the basic shape is such an effective distributor of compression. The semi-circular design evenly distributes the weight of the bridge into the compact form of the arch (compression), with very minimal tension distributed on the interior of the arch itself. As long as the ratio of the semi-circle to the span is about 1.57, the arch bridge may be any size, provided the materials are strong enough to support the weight of the intended traffic. This strength requirement is why most Roman arch bridges were built of stone.
Because of the thickness and strength of traditional arch bridges, resonance, weather, and torsion must be extreme before they can substantially impact an arch bridge. This is partially why these bridges have lasted for thousands of years.
Unlike the arch, a beam bridge has a more equivalent ratio between compression and tension. A traditional beam bridge is a long span resting on two beams, placed at either end of the span. Compression acts on the top portion of the span, because the span is contracting under the weight of the traffic, while the underside of the span experiences tension by expanding slightly. The forces create a structural balance by acting upon each other. Unfortunately, this balance is limited. Beam bridges can’t have very long spans, with a maximum length of about 250 feet.
Beam bridges are susceptible to torsion. To mitigate this effect, beam bridges utilize either an overhead or underside series of trusses, which can distribute tension more evenly. In a sense, trusses extend the weight and force bearing capability of the span and the beams by creating greater surface area on which the forces can act.
Beam bridges are susceptible to torsion. To mitigate this effect, beam bridges utilize either an overhead or underside series of trusses, which can distribute tension more evenly. In a sense, trusses extend the weight and force bearing capability of the span and the beams by creating greater surface area on which the forces can act.
Relatively mild resonance can destroy a suspension bridge. If an enormous marching band were to synchronize their steps exactly as they stepped across the span, the resulting wave from their rhythmic stomps would be enough to send a wave through the span, temporarily disrupting the balance between compression and tension, and causing the span to snap. Engineers discovered a way to prevent this effect by introducing ‘dampeners,’ which are irregular thick panels embedded in the span which interrupt the amplitude of the resonant wave, thereby preventing major build-up and a subsequent imbalance of the compression/tension dynamic.
However, the way suspension bridges handle tension and compression, and keep torsion from wrecking havoc on the span, requires the use of additional structural implements, including trusses, and vertical stabilizers to add strength to the cables. Both trusses and other vertical stabilizers act essentially as extensions to the towers and cables by adding additional support.
Weather — especially violent wind storms — can flip the span of a suspension bridge if it is not properly supported by trusses and reinforced cables. Provided a bridge is properly outfitted, an individual incident of weather will not destroy a suspension bridge, as long as all of the components of the bridge are checked after the storm for damage.
For structural musings on arch bridges and other structures, Karl-Eugen Kurrer’s book “The History of the Theory of Structures: from Arch Analysis to Computational Mechanics” is worth perusing, in order to glean a more precise understanding of how ratios affect structural strength. “Calatrava: The Complete Works 1979-2009” by the Taschen press features incredible permutations on contemporary bridge design, especially the architect’s Mujer Bridge, which also has a rotating section to allow the passage of boats.
“Cable Vibrations in Cable-Stayed Bridges” by Elsa de Sá Caetano will tell you perhaps more than you want to know about tension and the dangers of torsion and resonance. “Bridges: Three Thousand Years of Defying Nature” by David J. Brown will provide you with a detailed overview of historical achievements in bridge building, while “Golden Gate Bridge: History and Design of an Icon” by Donald MacDonald takes an in-depth look at one of the most famous suspension bridges in the world.
