Structural Design Methodology for Mass Timber

This chapter introduces the fundamental design code in the United States for all wood and wood products, including mass timber – the National Design Specification (NDS) for Wood Construction. It describes how designers make structural decisions using one of two possible design methodologies: Allowable stress design (ASD) or Load and Resistance Factor Design (LRFD). The NDS has a companion document called the NDS Supplement or simply the NDS-S. LRFD replaced ASD for steel and concrete because it offers a more rational approach to safety and reliability. The only way to truly know how strong something is, is to break it. As such, reference design values for all mass timber products are derived from experimental testing. Due to wood's many unique characteristics, there are a large number of adjustment factors for wood: some are applicable to certain products or applications while others are more universally applicable.