A. Frank Woeste, a professor of wood construction and engineering at Virginia Tech in Blacksburg, responds: There’s really no difference between the two products, assuming both are designed and manufactured properly. The design system used by the entire lumber industry is based on rules that are believed to produce the same safety factor for the same application, regardless of the product selected. However, for a given condition, you would still expect slight differences in the actual safety factors of the two products.
For example, assume that floor trusses and I-joists were designed for a certain span and spacing and the total design load was 55 psf. If you took a sample of five floor trusses and five I-joists from production and tested them to failure, the average failure load might be 100 psf for one product and 120 psf for the other. No one could predict...
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