If we could get inside the brain of an engineer and look at the core values driving the thought process, one of the values near the top of the list would be efficiency. Engineers are interested in efficiency in everything they do. If an engineer is building something, then that value expresses itself in the efficiency of materials. Excess materials add weight and increase the cost. A truss makes very efficient use of materials to span a gap. It is mostly air— a collection of open triangles engineered for great strength. The oldest covered bridge in the United States, and therefore the oldest bridge truss, is the Hyde Hall Covered Bridge from 1823, located in upstate New York. The bridge is 53 feet (16 meters) long and it is made using two wooden trusses, one on either side of the bridge. A roof over the top protects the wood from the elements. Trusses got their start in roof structures dating back to the Roman Empire, and it is easy to understand why. If you are trying to span the walls of a cathedral or a large room, the ceiling joists need to connect the two walls. But wooden joists, no matter how massive, start to sag under their own weight at about a 40-foot span. The solution is a kingpost truss, where a kingpost in the center of the joist ties into the peak of the rafters, supporting the joist. This is the basic idea behind any truss — using different pieces of the truss to support other members through tension or compression. It takes far less material compared to a solid beam of the same dimensions. This is why we see trusses everywhere in the modern world: in bridges like the Golden Gate Bridge, buildings like the World Trade Center, tower cranes, power line towers, etc. Engineered properly, a truss is far less expensive and lighter than a solid beam of the same size.