Comprehensive Load Distribution Model For Wood Truss Roof Assemblies
Keywords:Trusses, load sharing, roofs, composite action, connections
This study developed and verified a comprehensive structural analysis model to predict the distribution of loads in wood truss roof systems. Experimental testing was performed to provide for model verification and included full-scale tests of individual trusses and a complete roof system.
Nine trusses were individually tested to their design load, then placed in a typically constructed and sheathed assembly for roof system testing. The roof assembly tests consisted of loading individual trusses within the roof using various combinations of concentrated vertical loads. Truss reactions, truss loads, and interior bottom chord deflections were electronically measured. The roof assembly was also tested with two types of gable end truss supports.
The load distributions within the roof assembly were found to remain constant up to and beyond twice the roof's design load. The load distributed through load sharing was found to vary from 60% of the applied load in stiffer trusses to 80% of the applied load in relatively limber trusses. Gable end truss support significantly influenced the load carried by the trusses near the ends of the roof.
By considering the truss members as three-dimensional frame elements with semi-rigid end connections, and by rationally assigning rigidity factors to the connected joints, a model was developed in this study that can accurately predict individual truss stiffness and roof assembly load distributions. Distributions predicted by the model compared closely to those obtained experimentally for both individual truss loadings and for superimposed truss loadings. The model developed does not require experimental connector plate parameters and once fully verified, may prove useful in roof system design procedures.
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