Wood Diaphragm Deflections. II: Implementing a Unified Approach for Current CLT and WSP Practice

Abstract

Horizontal wood diaphragm systems, whether decked with conventional or mass timber panels, transfer wind and seismic loads to vertical elements of the lateral force-resisting system, in flexible, rigid, or semirigid ways. Characterizing and calculating the resulting diaphragm deflections will help determine the distribution of forces to critically loaded components and a significant portion of lateral building translations and rotations. Deflection equations for sheathed wood structural panel (WSP) diaphragms are well established in US design standards in a four-term expression that models flexural, shear, and fastener-slip deformations, but similar equations for cross-laminated timber (CLT) diaphragms have yet to unfold, despite growing industry consensus that CLT panels make efficient slabs and decks. Building code standards require CLT diaphragm deflections be computed using the principles of engineering mechanics. The current three-term and four-term deflection equations for WSP diaphragms are based on various assumptions that are often outpaced by current design practices. This is the second of two companion papers, in which the first paper provides the full generalized derivation of the current four-term WSP diaphragm deflection expression with a mechanics-based expansion to unify both potential WSP and CLT applications. This second paper builds on the first paper by expanding the generalized equation with implementation insights unique to WSP and CLT diaphragms. The various challenges of calculating diaphragm deflections associated with the current design practices are discussed with suggestions to assist in implementation. In general, the computation of diaphragm deflections has become increasingly important ever since equations were first developed for WSP systems around 70 years ago. Deflections are routinely computed to evaluate building separations, property line setbacks, P-delta instability, and evaluation of nonstructural damage from interstory drift. Additionally, appropriate engineering modeling requires accurate diaphragm stiffness values to characterize diaphragms as rigid, semirigid, or flexible. The mechanics-based equations in this paper are presented in a form that will be useful to today’s practitioners to produce more rational building designs.