One of the most important decisions you have to make when designing a bridge is what materials to use. Different materials have different properties, costs, and environmental footprints. You should consider the availability, recyclability, and durability of the materials, as well as their compatibility with the site conditions and the bridge function. For example, concrete is widely used for bridges because it is strong, versatile, and cheap, but it also has a high carbon footprint and can degrade over time. Steel is lighter and more resistant to corrosion, but it requires more energy and resources to produce. Wood is renewable and biodegradable, but it is vulnerable to fire and insects. You can also use composite materials that combine the advantages of different materials, such as fiber-reinforced polymers or hybrid concrete-steel systems.