RELIABILITY-BASED DESIGN OF WOOD MEMBERS SUBJECT TO PONDING

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