Q&A: Safety Factor in Wood Construction

A.The idea that "wood structures are way overbuilt" may be the greatest myth in the wood construction field. It is possible that at one time in history wood structures were overbuilt, but it is certainly not true today. The safety factor for bending strength for visually graded dimension lumber is 1.3; by contrast, the safety factor for structural steel, which has much less variability from piece to piece, is as much as 2.

So how are safety factors applied? To arrive at the design values used in wood design, thousands of pieces of lumber of representative sizes, grades, and species have been tested. These tests are run for about ten minutes to determine the stress that will cause a piece of lumber to fail. The test data for every piece of lumber of a given grade, size, and species is recorded. In a test of bending strength, for example, the values from a batch of lumber might range from 3,000 to 15,000 psi. By convention, the value of the 5th percentile is calculated (in other words, 95% of the pieces tested fall above this number, 5% fall below). Choosing a value at the 5th percentile is a way of accounting for the wide variability in the strength of pieces of visually graded lumber (due to knots, slope of grain, etc.).

This number — let’s say it’s 4,000 psi — is then divided by 1.62 to convert it to a ten-year duration value, which is the load duration that is used in the design of wood floor systems. (Remember, the test lasts only ten minutes; lumber can resist more stress for short periods of time.) Finally, the ten-year value is divided by a safety factor of 1.3. So a 5th percentile value of 4,000 psi would become 1,899 psi. This is the number that is published in the allowable design stress tables.

It’s a grave mistake to make design decisions based on an assumption that the wood safety factor is excessive.

— F.W.