Arch and truss bridges are used for crossing medium-sized spans. Both are designed for humans, motor vehicles and locomotives to cross them. Steel arch and steel truss bridges can span up to 550 m. The longest concrete arch bridge is 425 m. Neither type of bridge can achieve massive spans like those of suspension and cable-stayed bridges. Truss bridges tend to run straight across an expanse, but arch bridges may curve in different directions.
Arch bridges were originally made of individual stones, which were force fitted together. When the Romans mastered concrete, it was used to reinforce and join the stones together. Truss bridges, which were created primarily as an alternative to expensive stone, were initially made of wood. Iron was used for only a short time in construction, as it was found unsuitable. Iron was replaced with steel, which is still used for both arch and truss bridge construction.
The construction of an arch bridge relies on the arch shape to support the bridge and load. The pressure from the load is diffused from the center, or keystone, of the bridge, down the arch to the abutments at the end of the bridge. A truss bridge uses diagonals, horizontals and verticals to manage both compression and tension. The load causes compression, which creates tension. Both bridges transfer the weight to the ground.
Arch and truss bridges deal with torsion, or twisting, relatively well. Resonance, or vibration, could be problematic for both types of bridges and could cause collapse. Weather-related problems are the most common cause of bridge failure, and both arch and truss bridges can be susceptible to extreme heat and cold, which cause bridges to expand or contract. Natural disasters such as earthquakes and tornadoes could certainly destroy either type of bridge.
