Bridges can expand or contract due to several factors, such as traffic loading, materials used in construction, and temperature. For example, when the temperature is hot, a bridge expands in length, and when the weather gets colder, it contracts. An expansion roller lets a small rigidly-constructed bridge expand and contract in length safely.
An expansion roller is a mechanism built into the support system of a bridge’s truss. The truss is the triangular structure of the bridge, with all parts connected together by intersections called nodes. The expansion roller consists of joints constructed with bars that move when the bridge expands. Expansion rollers are included in bridges of 80 feet (24.4 meters) and longer to ensure the safety of the bridge at all times. While the expansion and contraction of each section of the bridge may be very small, as bridges get longer, this can become quite a large problem.
When temperatures increase, materials used to build the bridge can expand, or increase in length. The more the temperature rises, the more the materials will increase. The expansion rollers allow the sections to move so that the extra length does not cause the bridge to buckle and even break. As temperatures get colder, the bridge building materials will contract or shrink in size. In this case, expansion rollers allow the bridge parts to move so they mesh together and gaps do not occur in the bridge.
Other factors besides weather can also cause the expansion and contraction of a bridge. Expansion and contraction may be caused by the curing process of concrete, the earth moving, and heavy loads crossing the bridge over a long period of time. An expansion roller is a kind of bearing that renders these otherwise damaging factors that are not harmful to the bridge.
Bridges have to be inspected and checked regularly, like the rest of transportation and highway infrastructure. An expansion roller may become corroded and ineffective due to road salt, dust, and aging. Corrosion is the most common damage that affects the durability and life of a bridge. For example, in 2007, the Interstate 35 West Bridge, in Minneapolis, Minnesota, collapsed due to frozen, corroded, 40-year-old expansion rollers that were not functioning properly. The lack of ability to expand was because of a damaged expansion roller, and led to pent-up stresses in the bridge and the buckling and collapse of an integral horizontal beam.