6.1 Introduction

The design of rectangular sawn wood beams and straight or slightly curved rectangular glulam beams is covered in this chapter. Glulam members may be somewhat more complicated than sawn lumber beams, and the special design procedures that apply only to glulam design are noted. Where no distinction is made, it may be assumed that essentially the same procedures apply to both sawn lumber and glulam design.

Glulam beams are sometimes tapered and/or curved for architectural considerations, to improve roof drainage, or to lower wall heights. The design of these types of members requires additional considerations beyond the information presented in this book. For the additional design considerations for these advanced subjects, see the Timber Construction Manual (TCM) (Ref. 6.5).

The design of wood beams follows the same basic overall procedure used in the design of beams of other structural materials. The factors that need to be considered are

1. Bending (including lateral stability)

2. Shear

3. Deflection

4. Bearing

The first three items can govern the size of a wood member. The fourth item must be considered in the design of the supports. In many beams the bending stress is the critical design item. For this reason, a trial size is often obtained from bending stress calculations. The remaining items are then simply checked using the trial size. If the trial size proves inadequate in any of the checks, the design is revised.

Computer solutions to these problems can greatly speed up the design process, and with...