Bridges are very complex structures that must take into account many stressors throughout their service lives. From varying load sizes to weather conditions such as floods and earthquakes, bridges must be engineered to withstand many loads and forces, structural limitations, and environmental challenges, over a longer lifespan than most structures–no one wants to build a bridge twice in the same decade! . 

What Are the Stressors Bridges Need to Endure?

Every bridge needs to endure two main stressors: gravity on the structure itself (called “dead load”) and varying and changing “live” loads, such as heavy moving trucks, traffic jams or parades with thousands  of people, winds, currents, waves, floods, etc. Bridges counteract loads and stresses by means of bearing internal tension, compression, and bending. Compression has the effect of compressing a member, like squeezing a foam earplug between your fingers. Tension has the effect of stretching, like stretching out a rubber band. Bending is like what you do when you draw a bow back, and the bow bends. All of these stressors demand the materials (like steel, wood, concrete, and fiberglass) to act in a resilient and elastic way–meaning that they can absorb the stress repeatedly without internal changes in the material.. Almost all bridges utilize at least compression, as do all bridge abutments, some act primarily in tension, like a suspension bridge, some primarily in bending, like a beam bridge. Most larger bridges utilize all three in different types of resilience..  

6 Types of Bridges 

There are numerous types of bridges that are all engineered in slightly different ways, applying compression and tension in various areas on the bridge.

  • Beam Bridge — The most straightforward bridge, you would be making a beam bridge if you put a loading pallet over a tiny creek to walk over it. The beam bridge suspends the deck over two abutments (side pillars). The deck and beams are in bending on top of two abutments that are compressed.. 
  • Arch Bridge — This bridge uses primarily compression in the main arch, but often has other forces in the internal trusswork and the deck to support loads. 
  • Suspension Bridge — The suspension bridge’s deck and beams are in bending, while its two towers are primarily compressed by the main cables which hold the suspended cables in tension.
  • Cable-Stayed Bridge — This bridge is similar to the suspension bridge, but the cables hang from the towers. 
  • Truss Bridge — This bridge has both vertical and diagonal trusses connecting two beams. Ideally, the  trusses are all in pure compression or tension. The top beam is in compression, while the bottom bean is in tension. 
  • Cantilever Bridge — This bridge has tension forces on top beam of the bridge with compression forces on the bottom beam. It is often made with internal trusswork like a truss bridge. This is not very commonly seen, but some older drawbridges worked this way. . 

These six types of bridges all use varying forms of tension, compression, and bending, creating forces that counteract the stresses placed on the bridge. Many bridges combine all three forms of load-bearing. 

How Is the Bridge Style Chosen?

Choosing the style for your bridge is dependent on many factors, including how long the bridge needs to be, what height it must be, the space needed between the piers, if there are piers at all, the dead and live loads applied, the capacity of the soils and rocks at the abutments, etc. A shorter span, such as a small road over a creek or gully, would warrant a beam or truss bridge, whereas a freeway bridge crossing a lake or major river may warrant a cable-stayed or suspension bridge. 

Looking For Expert Bridge Engineers?

Areté Engineers are experts in bridge engineering. We inspect and make recommendations to state, local, and federal entities about bridges of every size and type. Also, Areté Structures division designs and builds fiberglass bridges (generally of the truss type). If you’re interested in learning more about Areté Engineers and what they do, you can explore more here or reach out here