Q. To differentiate my company from our competitors, I like to explain to our customers that we go "beyond code" when we build decks. One example that I use is the fact that my company typically includes diagonal bracing at each post, which I feel results in a much stiffer deck. But recently I noticed a note to Figure 10 in DCA 6 that prohibits diagonal bracing on center posts. Why is this a bad idea? On a wide deck with multiple intermediate posts supporting the beam, would it still be true that only the corner posts should be braced?
A. Andrew Wormer, PDB editor, responds: To get an answer to your question, I reached out to the American Wood Council, the organization that publishes DCA 6 (the Prescriptive Residential Wood Deck Construction Guide). According to Loren Ross, who is manager of engineering research with the AWC, the prohibition in DCA 6 against diagonal bracing on center posts is to address concerns of potential overstress from lateral loads on the center posts, which receive more vertical load than the posts at the corners.
I was a little skeptical, so in order to get an independent third-party perspective on this question, I also contacted Paul Bennett, who is principal engineer office director for Exponent, an engineering and scientific consulting firm in Denver, Colorado. When I asked Bennett if he thought bracing the center posts to make the deck feel stiffer would put those posts at risk of buckling, his answer was a little surprising to me. “We ran some sample scenarios, and concluded there is concern with bracing – which supports the AWC comment to not brace it.”
According to Bennett, the issue is that although a braced column’s buckling capacity is increased (because the bracing effectively makes the column shorter), it also is susceptible to increased lateral loading due to wind loads. So while a braced post results in a stiffer deck, the post will be more susceptible to buckling under wind loads, which are very specific to each deck’s geometry and location.
Are wind loads really a concern on a deck? Research by Don Bender at Washington State University suggests that even storm-force winds don’t create particularly high loads on decks (see "Wind Loads on Decks", PDB Jan/2014). But as Bennett points out, there could be many different loading scenarios, with no one-size-fits-all solution, which is why AWC makes this recommendation against center post bracing. “The bottom line is that every situation and deck is unique, which is why a deck builder should really consult with an engineer when in doubt (I know that's not an answer anyone wants to hear). In this case, I think AWC is erring on the side of caution (as would I), since a deck failure could potentially result in the loss of life.
When installing diagonal (or "knee") bracing on those corner posts, remember to follow DCA6 guidelines, where the brace is fastened to the post and the beam with 1/2-inch-diameter lag screws (or the equivalent), with the fasteners placed at least 2 feet away from the post-to-beam intersection, as shown in Figure 10 in DCA 6 (drawing above).
