... larger area. The size of the area that the force is spread over is called stress. The Beam Bridge This is the simplest form of bridge; it is a horizontal structure that is resting on a pier at each end. When the bridge is under load, the shape of the beam changes as the beam curves in the middle. The upper part of the bridge shortens as the lower portion lengthens: The top part of the Beam Bridge needs to be strong in resisting compression forces while the bottom has to be strong in resisting tension forces. The bridge needs to be stiff and has to resist twisting and bending. Concrete and steel are the most popular materials for bridge building. Concrete is a resistor for compression but it is poor at resisting tension. Therefore concrete is used to build the top of the bridge, while it is reinforced with steel rods ...

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