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MECHANICS - CASE STUDY

    Introduction


Roof Loading for Room Addition

 

You finally decided to add a room onto your house. You would like to have an exposed beam ceiling with large timbers. But after pricing 20 foot long 8x8 timbers, you realize it will be very expensive. Then you think of a great idea. Why not use four 2x6 planks to construct a simulated 8x8 timber? It will save over 70% the lumber costs.

What is known:

  • The expected roof load is 80 lb/ft2 (combined live and dead loads).
  • The beam will be constructed with standard 2x6 planks.
  • The beams will be 4 feet on center.
  • The room will be 20 feet deep
  • The wood is Douglas Fir with a failure stress of 7,500 psi.
  • 1/4" screws will be used to secure the planks together. Each screw can hold 800 lb in shear.
  • Assume the beam is simply supported at each end by the wall structure.
     
    Question

   

How far apart should the screws be spaced? Do the vertical and horizontal screws need to be spaced differently?

     
    Approach

   
  • Recall, actual lumber dimensions are not the same as the name (i.e. a 2x6 is not 2 inch by 6 inch). Use the Structural Shapes appendix to get actual size.
  • The shear stress at each joint needs to be determined using the shear stress equation,

         
  • The Q term will be different for the vertical and horizontal joints.
     
   
 
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