Permanent Bracing Design for MPC Wood Roof Truss Webs and Chords
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SAP2000 (CSI, 1995), a finite element analysis program, was used to analyze structural analogs for three sets of truss chords braced by n-CLB's and one or two diagonals, one web braced by one and two CLB's, and j-truss chords braced by n-CLB's.
System analogs used to model five eight-foot truss chords braced by three CLB's and one diagonal, six twenty-foot truss chords braced by nine CLB's and two diagonals, and eleven twenty-foot truss chords braced by nine CLB's and two diagonals were analyzed. For each of the three cases analyzed, the chord lumber was assumed to be 2x4 No. 2 Southern Pine (S. Pine) braced by 2x4 STUD Spruce-Pine-Fir (SPF). Chord load levels of 10% to 50% of the allowable compression load parallel-to-grain assuming le/d of 16 were studied. All wood-to-wood brace connections were assumed to be made with 2-16d Common nails. A nonlinear load-displacement function was used to model the behavior of the nail connections.
Single member analogs were analyzed that represented web members varying in length from four-feet to twelve-feet braced by one and two CLB's. The web and CLB's were assumed to be 2x4 STUD SPF. The web members were also analyzed assuming 2x6 STUD SPF.
Single member analogs were analyzed that represented chord members varying in length from four-feet to forty-feet braced by n-CLB's spaced twenty-four inches on-center. The truss chord was assumed to be No. 2 Southern Pine and the CLB's were assumed to be STUD SPF. The chord size was varied from 2x4 to 2x12 and connections were assumed to consist of 2-16d Common nails. The system analog analysis results were compared to the single member chord analysis results based on the number of truss chords and the diagonal brace configuration.
For the three cases studied involving multiple 2x4 chords braced as a unit (and believed to be representative of typical truss construction), the bracing force from the single member analog analysis was a conservative estimate for bracing design purposes. It was concluded that the single member analysis analog yields approximate bracing forces for chords larger than 2x4 and for typical constructions beyond the three cases studied in this research.
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 2x4 and 2x6 webs braced with one CLB, the R-value was 2.3% for all web lengths studied. For both 2x4 and 2x6 webs braced with two CLB's, the R-value was 2.8% for all web lengths studied. The web and CLB lumber species did not affect the R-values for the braced webs.
Calculated R-values for truss chords, 2x4 up to 2x12, braced by n-CLB's assumed to be spaced two feet on-center for chords four to twelve feet in length ranged from 2.2% to 3.0%, respectively. For chords from sixteen to forty feet in length, R ranged from 3.1% to 2.6%, respectively. The lumber species and grade assumed for the chord and CLB did not affect the R-values for the truss chords.
A step-by-step design procedure was developed for determining the net lateral restraining force required for bracing j-chords based on the results of the single member analogs studied. The required total lateral restraining force for j-compression members in a row can be calculated based on the R-value for or the number of CLB's installed at 2 feet on-center, the design axial compression load in the chord, and number of trusses to be braced.
- Masters Theses