The Design Tandem is assumed to be uniformly distributed over a width of how many feet?

• A. 10 ft
• B. 12 ft
• C. 6 ft
• D. 8 ft

Answer: (A)

In bridge design, a heavy vehicle’s tandem axle group isn’t treated as a single point load. Instead, the Design Tandem is modeled as a load that is spread out over a portion of the deck width to reflect how the tires contact the deck. The standard assumption is that this tandem load is uniformly distributed over a width of 10 feet. This width provides a practical representation of the effective footprint of a tandem axle group within a lane, balancing accuracy with simplicity in calculations. If the width were assumed smaller, the resulting stresses could be overly conservative locally; if it were much larger, the distribution would smear the load too much and misrepresent the actual effect.