Truss Bridges
Introduction/Motivation
After the Industrial Revolution, bridges became more and more sophisticated as iron and steel became more commonly available. By using iron and steel, engineers could design bridges capable of supporting larger loads and spanning greater distances, making it possible to link cities and communities through shorter, more direct routes and crossing obstacles such as waterways or other natural features that had previously blocked passage. Sometimes we take it for granted that bridges provide important links between places. They enable us to get to resources, conduct commerce, travel and visit other people. The design of bridges is important to the transportation networks we depend upon.
Beam bridges are the most common type of bridges, and include truss bridges. Truss bridges distribute forces differently than other beam bridges and are often used for heavy car and railroad traffic. In a truss bridge, the beams are substituted by simple trusses, or triangular units, that use fewer materials and are simple to build.
Truss bridge construction rapidly developed during the Industrial Revolution; they were first made of wood, then of iron and finally of steel. During this time, different truss patterns also made great advances. Many truss systems originated in the mid-1800s are still in use today. The Howe Truss, one of the more popular designs, was patented by William Howe in 1840. His innovation was his use of vertical supports in addition to diagonal supports (see Figure 1). The combination of diagonal and vertical members created impressive strength over long spans; this made the truss design ideal for railroad bridges. Howe's truss was similar to the existing Kingpost truss pattern. However, he used iron for the vertical supports and wood for the diagonal supports. Although iron and wood are not used as much today in modern bridges, the Howe Truss pattern is still widely used. See Figures 2-4 for other truss patterns.
Today, we are going to act as teams of engineers making bridge models. We have been hired by a city to create a bridge to cross one of the local rivers. However, the city does not want the bridge to affect the fish population in the river below it. Engineers always consider the design objective when creating models.
Our design objective is to make a bridge that spans the river (scaled down to a distance of 10 inches [25 cm], supports the most weight for the cars that will pass over it, and does not disturb the river's fish. To simulate the load of the cars, our bridge must have a place to securely hold a small cup in the center of the span. To demonstrate environmental limitations on the design, no part of the bridge may touch the "water" (or bottom of the wooden support structure) and the bridge cannot be taped to the wooden support structure. Engineers often have many design constraints or limitations that are part of their job assignments. Today, our design constraints not only include the environmental and weight constraints, but also limited budget and materials using straws and tape as our construction materials.
Types of Truss Bridges
