Permanent bracing design for MPC wood roof truss webs and chords

摘要

Permanent bracing of metal-plate-connected trusses is required to stabilize specific members of each truss throughout the life of the roof structure. The objectives of this research were to: 1) determine the required net lateral restraining force to brace multiple webs or chords in a row braced by one or more continuous lateral braces (CLBs); and 2) develop a methodology for permanent bracing design using a combination of lateral and diagonal braces. Three system analogs used to model multiple truss chords braced by n-CLBs and one or two diagonal braces were analyzed with a finite element analysis program. Single member analogs were analyzed that represented web members braced by one and two CLBs and chord members braced by n-CLBs. For analysis and design purposes, a ratio R was defined as the net lateral restraining force per web or chord divided by the axial compressive load in the web or chord. For both 2 by 4 and 2 by 6 webs braced with one CLB, the R-value was 2.3 percent for all web lengths studied, and for webs braced with two CLBs, the R-value was 2.8 percent for all web lengths studied. Calculated R-values for truss chords braced by n-CLBs, assumed to be spaced 2 feet on center, ranged from 2.3 to 3.1 percent. A design procedure was offered for determining the net lateral restraining force required for bracing multiple webs or chords in a row based on the results of the single member analogs studied.