A bridge is an argument settled between force and material. Every design must carry its own weight and the loads that cross it, then pass those forces safely down to the ground. Beam bridges rest on supports and work well over short spans. Arch bridges push their load outward into the abutments, which is why Roman examples still stand after two thousand years. Suspension bridges hang the deck from cables draped over tall towers, allowing them to cross wide straits and deep gorges.
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