Bridges

Bridges exist for one main purpose: to connect the gap between two different spaces separated by an obstacle of lower elevation than the surrounding terrain – whether a body of water such as a river or valley. As they are sometimes essential for connecting two places, bridges have been around for a long time, the oldest one found being from 4000 years ago (The British Museum). 

As humanity progressed, however, there came a need for bridges that could cover wider distances and hold more weight. This is when bridges began to diversify into different types – the six main types being beam bridges, arch bridges, truss bridges, suspension bridges, cantilever bridges, and cable-stayed bridges (Britannica). 

The beam bridge is the most common bridge form and is relatively simple. A beam on top of the bridge carries vertical loads by bending. As it bends, it has horizontal compression on the top. Simultaneously, the bottom of the beam has horizontal tension. To prevent the bridge from collapsing due to the weight of what is on it, supports carry the loads from the beam by compression vertically to the foundation of the bridge (Britannica). 

The truss bridge is somewhat similar to the beam bridge in that it also operates by bending in a certain manner to carry loads. Truss bridges have a beam on the top and a beam on the bottom, with vertical and diagonal support beams between them. When the truss bridge bends from a load, it leads to compression at the top and tension at the bottom. Depending on the configuration of the support beams, they can either experience compression or tension (Britannica). 

The arch bridge is unique among the bridge types in that it mainly uses compression to carry loads. The foundation, in the shape of an arch, works to prevent both vertical and horizontal sliding since due to the arch shape, the load from the bridge usually exerts both vertical and horizontal force. Despite the more complex design, arch bridges usually require less material than beam bridges of the same span (Britannica). 

The suspension bridge carries loads by using curved cables that experience tension. Those cables are attached to towers that support the bridge – those towers undergo vertical compression to the ground. The vertical compression leads to the anchorages at the bottom, which resist the inward and vertical pull of the cables. With a suspension bridge, the deck is hung in the air, so ideally, it should be heavy and stiff, so it does not move excessively when carrying a load (Britannica). 

The cantilever bridge is usually made with three spans, of which the two outer ones are anchored at regular ground and extend out over the area to be crossed. The central span rests on the extensions from the outer spans. Just like supported beams, it carries vertical loads with compression at the top and tension at the bottom due to bending. The two outer spans carry their loads by tension in the upper chords of the spans and compression in the lower ones. Towers inside both spans carry those forces to the foundation by using compression and exterior towers carry the forces by tension to the far foundations (Britannica). 

The cable-stayed bridge uses nearly straight diagonal cables from the horizontal deck to the vertical towers that carry loads by using tension. The vertical towers transfer the forces from the cables to the foundations through vertical compression. The forces in the cables also work to put the deck into horizontal compression (Britannica). 

Most modern bridges today are made of steel and concrete (Britannica). However, sometimes this is not enough to avoid collapse. Bridges can collapse for a few reasons. There may be a structural defect or design flaw, as with the Minneapolis bridge collapse in 2007 due to a weak connection point. An unusually heavy load may have also contributed to the collapse, another potential factor in bridge failures (National Transportation Safety Board). Finally, the bridge may be hit with a force that is too great to handle, such as with the Baltimore bridge collapse recently when a cargo ship hit one of its support beams (CNN). 

Sources:

The British Museum, https://www.britishmuseum.org/blog/worlds-oldest-bridge-being-preserved-iraq 

Britannica, https://www.britannica.com/technology/bridge-engineering 

National Transportation Safety Board, https://www.dot.state.mn.us/i35wbridge/pdf/ntsb-report.pdf 

CNN, https://www.cnn.com/us/live-news/baltimore-bridge-collapse-03-26-24-intl-hnk/