Civil Engineering Professional Engineer Practice Exam

What key structural principle do truss bridges rely on?

1. Load is applied only at supports

2. Small parts arranged in triangles

3. Curved members for load support

4. Compression in all structural members

The correct answer is: Small parts arranged in triangles

Truss bridges are fundamentally based on the principle of utilizing triangular shapes to efficiently distribute loads. When small parts, such as beams and members, are arranged in triangles, they form a truss structure that can effectively transfer loads from the bridge deck to the supports. This is due to the inherent stability of triangles; they maintain their shape and do not deform under load. Each triangle within the truss acts to spread and balance the forces throughout the structure. When a load is applied, the forces in the members can be calculated with great accuracy, allowing for optimized material usage and minimized weight without sacrificing performance or safety. This design approach lends itself to both tension and compression within the members depending on their position in the truss, thereby increasing the bridge's overall strength and stability. Other options mentioned do not represent the primary structural principle underlying truss bridges. Load being applied only at supports pertains to reactions in structures generally, while curved members are typical in arch bridges rather than trusses. Although compression occurs in some members of a truss, it is not universal across all structural members, as many act in tension too. Thus, the unique configuration of triangles is what specifically characterizes the structural integrity of truss bridges.