Ch 4. Beam Stresses Multimedia Engineering Mechanics Bending Strainand Stress BeamDesign Shear Stress Built-upBeams
 Chapter 1. Stress/Strain 2. Torsion 3. Beam Shr/Moment 4. Beam Stresses 5. Beam Deflections 6. Beam-Advanced 7. Stress Analysis 8. Strain Analysis 9. Columns Appendix Basic Math Units Basic Equations Sections Material Properties Structural Shapes Beam Equations Search eBooks Dynamics Fluids Math Mechanics Statics Thermodynamics Author(s): Kurt Gramoll ©Kurt Gramoll

 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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