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