I was pleased to see Mike Guertin’s “Better Deck Piers” article in the Feb/Mar 2015 issue raise awareness of footings. Sizing footings based on tributary area has been a topic of interest for many involved with the development of DCA6; in fact, an Appendix and Commentary to DCA6 is being developed in that regard.
As far as the DCA6 prescriptive approach, we believe, based on the example presented in Figure 4 of the article, that a joist span of 14 feet rather than 18 feet should have been used. Just as the cantilever is not added to the joist span for the beam-sizing table, so too it should not be added to the joist span in the footing-sizing table. This correction means that in DCA6, the prescriptive middle footings should be 18 inches square or 21 inches in diameter. A footnote to the footing-size table in DCA6 allows for end-footing sizes to be multiplied by 0.9, resulting in 16 1/4 inches square or 19 inches in diameter. A difference still exists but not as significant as presented in the article. The difference is primarily because the DCA6 footing-size table assumes the maximum joist cantilever is present, which adds to the tributary area. In other words, the smaller the actual cantilever, the larger the difference will be between the calculation method and the prescriptive method.
Also, Mr. Guertin commented that the new beam spans offered in the 2015 IRC are “a bit more generous.” The difference between the IRC and the DCA6 table is 3 inches because span is defined differently. The IRC defines the span as being from center-of-post to center-of-post; whereas DCA6 defines it as clear span (from inside face of post to inside face of post.) With that in mind, the difference between the IRC and DCA6-09 isn’t significant.
Hopefully this letter adds some clarity to DCA6 and the tools available to deck builders. The tributary-area method is extremely useful and will always be more precise than a prescriptive method, and new DCA6 Commentary provisions will aid in this approach as well.
Manager of Engineering Research
American Wood Council