Views of Dump Truck on Bridge


A local land developer needs a single lane bridge across a small stream. He figured that it could be built with two I-beams bridging the stream and smaller C4x7.25 channel sections every few feet supporting the concrete deck. He asks you to check the channel members for both shear and bending stress. He mentions that dump trucks will have to use the bridge to bring fill dirt to the construction site.

What is known:

  • The bridge span is 20 feet long and 10 feet wide.
  • The heaviest load is a 19 ton dump truck. The front wheels will support 20% of the load and the dual back wheels 80%.
  • As a worst case, the two back wheels are assumed be a single point load and will be loaded on a single channel.
  • The wheels are 8 feet apart.
  • Assume the I-beam supports are pinned supports, 10 feet apart.
  • The maximum allowable shear stress is 18 ksi.
  • The maximum allowable bending stress is 36 ksi.
  • Assume the concrete decking carries none of the loading.


What is the maximum shear and bending stress in the channel member? Is it a safe design?


  • Construct a free body diagram to understand all loading on a typical channel.
  • Determined the location of the maximum shear load and bending moment.
  • Calculate Q for the channel. Moment of inertia for the channel is listed in the Shapes appendix.
  • Use the shear and bending stress equations to calculate the stresses.
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