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MECHANICS - CASE STUDY

    Introduction


Building Canopy with New Hoist

 

 

The manager at the loading dock thought it would be a good idea to install an electric hoist under the existing canopy. If installed on a rail system, then large loads can be easily lifted and moved to another truck.

The existing canopy is constructed from 1.5" O.D. pipes that are connected with ball joints. There is one truss for every 5 feet of canopy. The canopy roof needs to carry at least 60 lb/ft2 for possible dead and live loads (wind, weight, snow, etc.).

What is known:

  • Young modulus of members, E = 29,000 ksi.
  • Roof load (combined live and dead loads) equals 60 lb/ft2 and is distributed to truss node joints (no bending in truss members).
  • Factor of Safety of 2 is required.
  • All truss members are steel circular pipe with an outside diameter of 1.5 in and wall thickness of 1/16 in.
  • Spacing between trusses along dock is 5 ft.
  • Design for buckling (assume tension stress is below yield stress).
  • Assume failure mode is only buckling.
   
  Question

 

What is the maximum weight that the hoist can lift (ignore weight of the hoist itself) for the existing canopy with a factor of safety of 2.0?


3D Truss Example at the
Milwaukee Airport
   
  Approach

 
  • Roof load needs to be distributed to the top three node joints of the truss.
  • Check all truss members that are in compression.
  • Use Euler's Formula for simply supported (pinned-pinned) column

          
     
   
 
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