Structural reliability of light-frame wood systems with composite action and load sharing

Graduation date: 1993In a majority of light-frame wood buildings, studs\ud and joists are the basic structural components in walls\ud and floors. The walls act as bending and compressive\ud panels and transmit lateral wind and gravity loads to the\ud foundation. Joists are used in floor systems, and\ud together with the sheathing member act as an orthotropic\ud plate in supporting live and dead loads. Mechanical\ud fasteners form most of the joints and provide semi-rigid\ud connection between the framing members and the sheathing.\ud Current design methods do not incorporate the system\ud behavior within the wall or floor, that is, both the\ud composite action of the framing with the sheathing and\ud load sharing between the framing members. Yet, system\ud strength and stiffness rely on structural interaction.\ud This study of light-frame wall and floor systems\ud introduces a probability-based evaluation, including the\ud interaction of components as well as the nonlinearities of materials and connectors. Load sharing in the wall\ud and floor systems was modeled by a series of elastic\ud springs with the sheathing as a distributor beam.\ud Stochastic distributions were used to represent certain\ud properties and loadings.\ud Reliability analyses of wall systems under bending\ud and compressive loads were conducted. It was found that\ud composite action and load sharing contribute to a\ud reduction in failure probability. Reliability studies\ud verified the hypothesis that wall systems are highly\ud reliable and can sustain loads exceeding those expected\ud under 50-year and 100-year wind loads.\ud Reliability levels were examined for floor systems\ud in a bending limit state with 16-in. and 24-in, joist\ud spacing, under a Type I extreme value distribution for\ud live load. Floor system capacity was sensitive to the\ud variability of the lumber modulus of rupture. The degree\ud of load sharing was grade dependent; No.1 joists\ud exhibited higher load sharing than No.2 joists as the\ud coefficient of variation in strength of No.2 joists was\ud much higher than No.1 joists. Based on these results,\ud the 15% increase in the allowable bending stress for\ud repetitive light-frame members as specified by the\ud National Design Specification for Wood Construction\ud appears to be conservative