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MECHANICS - CASE STUDY

    Introduction


Truck Bed Failure due to Loading
(© 2003, Kurt Gramoll)

 

"Well, this is a new one for me" said the highway patrolman when he was called to investigate the accident. "I have never seen a semi snap in two due to the load. I wonder why it failed near the front and not in the middle?" His partner added, "I am sure the investigation team will let us know. I wonder if the truck was overloaded?"

What is known:

  • The trailer is 53 feet long (does not include the tractor).
  • The trailer gross weight (total including both load and trailer) is 79,500 lb.
  • Assume weight is evenly distributed.
  • The truck bed is designed to withstand a bending moment of 200 ft-kip (= 200,000 ft-lb).
  • Assume the truck bed can be modeled as a simple supported beam with supports at the left edge and at the center of the back wheels.
     
    Question


Assumed Location of Trailer Supports
 

Does the failure location agree with a basic beam analysis? What is the value of the maximum bending moment and does it exceed the design moment of 200 kip-ft?

   
  Approach

   
 
  • Construct a free body diagram.
  • Using static equilibrium equations, determine the assumed support reactions.
  • Determine how many sections are needed and make a cut in each section.
  • Sum forces and moments at each cut in terms of position.
  • Plot the resulting moment and shear equations.
   
 
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