Q&A: Long-Span Floor Joists

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 which of the two products would have the higher strength from the samples tested. In one case, the test safety factor (SF) would be 100/55, or 1.8; in the other case, the test SF would be 2.2. Slight differences in the test SF might result from sampling error, differences in materials used to build the products, and the fact that engineering models aren’t perfect.

Strength is seldom an issue in residential floor joist products. Stiffness is more of an issue, because the lack of stiffness increases the chances for annoying vibration in the floor system (see Practical Engineering, “ Beyond Code: Preventing Floor Vibration,” 11/98).