Statics & Strength of Materials

Example 2 - Torque Review Problems

A simply supported 40 foot, 4000 lb bridge, shown above, has a 5 ton truck parked 10 feet from the left end of the bridge. We would like to determine the compressional force in each support. The weight of the bridge may be considered to act at its center.

Statics Problems: Techniques
1. Draw Free Body Diagram of entire structure, showing and labeling all external forces, including support forces and loads. Choose an appropriate coordinate system. Determine needed dimensions and angles.
2. Resolve all forces into their x and y components.
3. Apply the equilibrium conditions and solve for unknown external forces and torques as completely as possible.

1. Free body diagram is shown above.
2. All forces are in x or y direction
3. Eq. Cond:
S Fx = 0 (no external x forces acting on structure.)
S Fy = + Fa + Fb - 10,000 lb - 4,000 lb = 0
S ta = -10,000 lb x 10 ft - 4,000 lb x 20 ft + Fb x 40 ft = 0
Solving for unknowns: Fa = 9500 lb, Fb = 4500 lb

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