Describe the purpose of roller and rocker bearings, and what makes them vulnerable to failure.

The purpose of roller and rocker bearings is to accommodate longitudinal movement and rotation in bridge superstructures, allowing for thermal expansion and contraction while transferring vertical loads to the substructure. Roller bearings consist of cylindrical rollers that allow the superstructure to slide horizontally, while rocker bearings consist of curved surfaces that allow for rotational movement. These bearings are vulnerable to failure due to several factors, including corrosion, wear, and misalignment. Corrosion can occur if the bearings are not properly protected from moisture and de-icing salts, leading to a reduction in their load-carrying capacity. Wear can occur due to friction between the moving surfaces, especially if lubrication is inadequate. Misalignment can result from settlement of the supports or other structural movements, causing uneven load distribution and increased stress on the bearings. Regular inspection and maintenance, including lubrication and corrosion protection, are essential for preventing failures in roller and rocker bearings.