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MECHANICS - CASE STUDY

    Introduction


Observation Platform

 

In order to study the natural wildlife, a small elevated observation platform was constructed with just a single center post. There is a small hinged door on the floor to access the platform. Only one person is on the platform at one time.

What is known:

  • Maximum weight of the observer is 300 lb.
  • The center supporting post is a 6x8 wood timber (check Structure Shapes appendix for exact cross section size).
  • The platform overall dimensions are 6 feet by 7 feet.
  • The observer can move around the platform, but the furthest location from the center is 3 feet and 2.5 feet as shown in the diagram (6 inches from either railing)..
     

Top View of Platform
  Question

 

What is the maximum stress in the center wood post due to a 300 lb person standing on the platform at one of the corners?

   
  Approach

 
  • The observers weight will cause unsymmetrical bending.
  • Find the moment around both axes that define the plane of the platform.
  • Find the moment of inertia for both materials around both axes.
  • Use the unsymmetric beam bending stress equation

         
  • Check the compression stress due to the axial load of the observer on the center post.
     
   
 
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