1. Chapter 29 Determining Simple Beams

2. 2 Links for Chapter 29 Loads & Supports Properties of Lumber Beam Design Related Web Sites

3. 3 Loads and Supports A load can be uniformly distributed over the entire span of a beam or concentrated in a small area A support post from an upper floor rests on a beam of a lower floor

4. 4 Structural Lumber Nominal size describes lumber in whole inches - 6 X 10 Net size is the actual size of the lumber –A 6 X 10 is actually 5 1/2” X 9 1/2” Lumber graded as #1 is the base value for other lumber of the same species

5. 5 Lumber Grading Dimension Lumber –Ranges from 2 to 4 inches –Moisture content less than 19% Timbers –At least 5” thick –Post and timbers - –Beams and stringers

6. 6 Properties of Lumber b = breadth of rectangular beam in inches d = depth of a rectangular beam in inches A = Area of the beam, determined using bxd

7. 7 Properties of Lumber

8. 8 Extreme fiber stresses - Top of beam in compression while bottom of beam in is tension Neutral axis - Axis formed where forces of compression and tension reach equilibrium Grain - Fibers that can be visualized and are aligned in the same direction

9. 9 Properties of Lumber Forces parallel to grain is when load is applied in the same direction as the fibers Forces perpendicular to grain is when the load is applied across the fibers

10. 10 Fiber Bending Stress Measurement of bending strength Bending moment is any point on the beam and its tendency to bend at that point Design equation is S = M/F b –S - Section modulus –M - Maximum bending moment –F b - Extreme fiber bending stess

11. 11 Deflection Deals with the stiffness of the beam It is the tendency to bend due to gravity Floor beam may not deflect more than 1/360 Roof with ceiling may not deflect more than 1/240 Roof with no ceiling may not deflect more than 1/180

12. 12 Horizontal Shear The tendency for the wood fibers to slide past each other along the length of the beam The beam will fail where the compression and tension forces meet Design equation is v=(1.5)(V)/A –v - Maximum unit horizontal shear stress in psi –V - Total vertical shear in pounds –A - Area of the structural member in sq. in.

13. 13 Other Forces Vertical shear is the tendency of a beam to fail perpendicular to the fibers of the beam from two opposing forces Tension attempts to lengthen a beam Compression attempts to compress a structural member

14. 14 Methods of Beam Design Computer programs use software provided by vendors for their particular product A span computer is similar to a slide rule as specific values are aligned Books contain design information, tables, and formulas Calculations can be made using formulas and diagrams

15. 15 Sizing Wood Beams Determine the area to be supported by beam Determine the weight supported by 1 linear foot of beam Determine reactions and pier sizes Determine bending moment Determine horizontal shear Determine deflection

16. 16 Wood Adjustment Factors Repetitive use occurs when the load is spread over several members Load duration is when a load is applied for a continuous amount of time Moisture and temperature content refers to the loss of strength due to extreme moisture and temperature

17. 17 Wood Adjustment Factors Size compensates for natural properties of lumber Shear stress allows for the maximum number of splits, checks, and shakes For each of these factors, refer to specific tables

18. 18 Related Web Sites International Wood Products Association - www.inpa.org www.inpa.org Western Wood Products Association - www.wwpa.org www.wwpa.org Trus Joist MacMillan - www.tjm.comwww.tjm.com