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/ft^{2} (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.
