"The fundamental building materials for bridges are high-string steel, carbon steel, and concrete," Whittaker explained. "However, there are various modification devices that may be employed to lessen the impacts of excessive loadings." Backup systems, according to Chajes, are an important component in bridge design. They provide a safety net in case of emergency situations or damage to primary components. Backup power can come from a variety of sources, such as generators, batteries, and fuel cells. Generators produce electricity automatically if the main source fails. Batteries store electrical energy for use at a later time. Fuel cells combine oxygen and hydrogen into water and electricity simultaneously without any pollution. The best part is that they have no limit on how much power they can generate so they are very efficient. Bridges must be designed to handle extreme conditions. Strong winds can cause severe damage to bridges. To prevent this from happening, engineers reinforce bridges with tie rods and cables. These components are attached to major support points like piers or arches and serve to hold the bridge together in case of an emergency. They also help to distribute loading evenly across a large surface area which reduces the risk of structural failure. Tie rods are connected to each other with cable ties. There are two types of tie rods: straight and curved. Straight tie rods connect two parallel structure members together rigidly but not directly overhead.