1. Chapter 31
Determining Beam Size

2. Introduction
Need to determine structural member size occurs frequently in residential design
Skills to determine structural member size
Ability to distinguish loading patterns on beams
Recognize standard engineering symbols used in beam formulas
Recognize common causes of beam failure
Understand how to select beams to resist these tendencies

3. Structural Lumber Lumber sizes
Nominal size describes the width and depth
Net size is the final size of wood

4. Structural Lumber (cont’d.)
Lumbar grading
Most structural lumber is visually graded when it is sawn at a mill
Some structural lumber is tested nondestructively by machine and graded as mechanically evaluated lumber (MEL)

5. Structural Lumber (cont’d.)
Lumbar grading (cont’d.)
Common terms
Dimensional lumber
Timbers
Post and timbers
Beams and stringers

6. Structural Lumber (cont’d.)
Properties of lumber
Breadth (b)
Depth (d) and area (A)
Extreme fiber stresses (Fb)
Neutral axis
Moment of inertia (I)
Section modulus (S)
Grain

7. Structural Lumber (cont’d.)

8. Loading Reactions of Wood Members
For every action there is an equal and opposite reaction
Fiber bending stress
Bending strength
Deflection
Modulus of elasticity
Horizontal and vertical shear
Compression

9. Methods of Beam Design Methods of beam design: Computer program
Wood design books
Pencil, paper, and a few formulas
Wood association books
Wood Structural Design Data
Western Woods Use Book

11. Methods of Beam Design (cont’d.)
Standard formulas to determine how the beam will be stressed

12. Sizing Wood Beams Using Standard Formulas
Determine:
Area to be supported by the beam
Weight supported by one linear foot of beam
Reactions
Pier sizes
Horizontal shear
Bending moment
Deflection

14. Sizing Wood Beams Using Standard Formulas (cont’d.)
Refer to the text for:
Complete instructions on determining wood beams
Wood adjustment factors
Review and practice questions
Sizing beams using span tables

15. Sizing Laminated Beams
Glu-lam beams are determined in the same manner as standard lumber
Different values are used

16. Working with Engineered Beams
Most manufacturers of engineered joists and rafters supply beams made of:
LVL (laminated veneer lumber)
PSL (parallel strand lumber)
Refer to Figure 31-10, a table for sizing beams made of PSL
LSL (laminated strand lumber)

17. Simple Beam with a Load Concentrated at the Center

19. Simple Beam with a Load Concentrated at Any Point