Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition

Chapter 3 — Wood: Design approaches

The simplest way to model loads and their supporting elements is to simply add the loads together and make sure that the stresses in the elements that support them are less than the stresses that would cause the elements to fail. These stresses that are allowed to be present within a given structural element — allowable stresses — are simply the stresses that would cause failure multiplied by a factor of safety that is less than 1 (or divided by a factor of safety that is greater than 1). More sophisticated design methods have been developed (see Chapter 2), but there are still two reasons for continuing to use allowable stress design (ASD) for wood structures.

First, much of the engineering profession in the U.S. still uses this method, in spite of the fact that both ASD and LRFD (Load and Resistance Factor Design was described in Chapter 2) are available and sanctioned by industry groups such as the American Wood Council (AWC) and the American Forest & Paper Association (AFPA). The second reason has more relevance to an academic text than to practical applications in the field: even though ASD does not provide the same nuanced approach to risk as does LRFD, it has the great advantage of utilizing section properties that are part of the canonical repertoire derived within strength of materials texts. In particular, ASD uses the moment of inertia or section modulus (see Chapter 1) in the computation of bending stresses, since all "working" stresses within the wood cross section are assumed to be linear.

Even though the section modulus is retained within LRFD for wood beams (unlike LRFD in steel or strength design in reinforced concrete, where nonlinear stress-strain relationships at the limit state are made explicit and therefore preclude the use of the section modulus), much of the naive elegance of the ASD method is lost, especially since allowable stress design values, already tabulated and widely distributed, are retained within the wood LRFD procedures by multiplying them by a "format conversion factor" rather than explicitly tabulating limit state stresses as is done in steel and reinforced concrete.