Reliability‐Based Design of Wood Members Subject to Ponding

Abstract

Ponding loads coupled with creep effects may contribute significantly to the nonperformance (ultimate failure) of wood roof members. Proposed load and resistance factor design (LRFD) specification provisions for the flexural design of wood beams in roof systems that may be subject to ponding loads include: (1) A check to determine if the member is subject to ponding loads as designed; and (2) a modified design equation to include a ponding moment amplification factor. These equations, however, include largely empirical treatments of creep effects using creep factors taken directly from the previous National Design Specification for Wood Construction. These creep factors were originally developed for use in deflection‐checking equations. This study investigates the effects of creep on the safety of wood roof beams subject to ponding loads. The results of the analysis are used to evaluate a creep factor for design on a more rational basis that includes time effects (i.e., the coupling of creep and moment magnification). Finally, the validity of proposed LRFD specifications is examined and suggestions are made for their modification.