The downward pull is caused by which force?

• A. Gravity
• B. Load
• C. Tension
• D. Compression

Answer: (A)

The downward pull comes from gravity—the gravitational force acting on every mass. This force is what we call weight, equal to mass times gravitational acceleration. In a bridge, this gravity-driven weight shows up as dead load (the structure’s own weight) and live load (loads that act due to occupancy, vehicles, etc.), all acting downward through the structure to its supports. Internal forces inside members, such as tension and compression, arise in response to these external loads but are not the cause of the downward pull itself. So the downward direction is dictated by gravity, with the other terms describing how that weight is transmitted through the structure.