Chapter 31

Determining Beam Size

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

Structural Lumber• Lumber sizes

– Nominal size describes the width and depth– Net size is the final size of wood

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)

Structural Lumber (cont’d.)• Lumbar grading (cont’d.)

– Common terms• Dimensional lumber• Timbers• Post and timbers• Beams and stringers

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

Structural Lumber (cont’d.)

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

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

Methods of Beam Design (cont’d.)• Methods of beam design (cont’d.):

– Standard formulas to determine how the beam will be stressed

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

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

Sizing Laminated Beams

• Glu-lam beams are determined in the same manner as standard lumber– Different values are used

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)

Simple Beam with a Load Concentrated at the Center

Simple Beam with a Load Concentrated at Any Point