Influence of Changes in Allowable Stresses on Wood Truss Design

Sensitivity studies were conducted on wood truss design to determine the influence of changes in allowable properties on truss performance. The load duration factor was used as an example of a property modifier of contemporary interest. The sensitivity of metal plate wood truss design to changes in the factor was affected by the type and loading of the truss. In the “common” light-frame house truss, the influence of bending predominated over compression and tension, in that order. For trusses more lightly loaded on the bottom chord, tension was the most critical. Parallel chord trusses provided varying response; the 4 by 2 flat truss response paralleled that of the lightly loaded pitched truss, while the parallel chord roof truss exhibited a mixed response. Definition of significant amount of change in end-use performance, such as 3 inches of span or 5 pounds of load, is critical to designing experiments on allowable stresses or their modifying factors. Preliminary experiments and theoretical analysis are needed to provide estimates of variability; levels of type I and II errors must be selected. If high degrees of sensitivity to change or low type I or II errors are desired, experimental sample sizes may be large and costs high.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member