Civil Engineering Professional Engineer Practice Exam

When calculating reactions in a truss, what direction are the reactions applied?

1. In the same direction as the load

2. Perpendicular to the load

3. In the opposite direction of the load

4. Randomly, depending on the design

The correct answer is: In the opposite direction of the load

When calculating reactions in a truss, the reactions are indeed applied in the opposite direction of the load. This is based on Newton's Third Law, which states that for every action, there is an equal and opposite reaction. When a load is applied to a truss, the structure experiences forces that exert stress on its members. The support reactions must counteract these loads to maintain equilibrium, meaning they should act in the opposite direction to balance the system. This ensures that the truss does not accelerate under the influence of the applied loads and remains stable. The correct direction of the reactions allows for an accurate analysis of the forces within the truss members, ensuring that the structure can safely support the loads it encounters in real-world applications. In contrast, considering other options is essential for understanding the concept. The idea of the reactions being applied in the same direction as the load would contradict the principles of static equilibrium. If they were perpendicular to the load, there would be ineffective counteraction of the forces, leading to unbalanced conditions. Suggesting that the reactions are applied randomly would undermine the systematic approach required in engineering analysis and design, as it is crucial to have predictable and exact methodologies for calculating forces in structures.