"Strong, Safe Decks" by JLC Staff with Michael Chotiner (Sep/13)
I appreciate the Best Practices Decks guide in the September issue. [But] one detail needs some qualifications—and a slap on the wrist for the cover photo.
The cover photo drives me crazy! I've spent the past eight years doing presentations at the JLC Live and Deck Expos circling back to the groundbreaking article in JLC by Virginia Tech professors on guardrail post connections. One of the many points made in the article is "don't notch 4x4 guardrail posts." Another is that screw attachments won't secure posts adequately to meet codes. What's on the cover? Someone preparing to mount a notched guardrail post with screws. Perhaps art trumps best practices for cover shots. It is a good image aside from the poor practices illustrated.
The illustration on page 39 showing a post bearing on the foundation footing may not be best practice. I used to use this detail until I got spanked by an engineer. Unless the footing is designed for the deck posts, there may be issues with concentrated loads and off-center bearing on the edge of the footing.
—Mike Guertin, East Greenwich, R.I.
JLC Editors respond: Art does not trump best practice at JLC, but sometimes we miss what is right in front of us. You are right (as are others who commented online) about the less-than-best practice depicted in that image. We missed it and we apologize for not getting it right. Fortunately, as you also point out, the railing details in the article itself are correct.
Regarding the illustration on page 39, while it is best to excavate and reinforce the existing footing under the deck posts, in most cases it would not be required unless the potential for increased future loads is high (adding a hot tub, for example). If the through bolts remain tight (they can loosen as the posts dry out), the wall picks up some of the load; and assuming the wall is reinforced, the small lateral load is negligible. Unless the load on the deck is very high, the capacity of the footing and soil should be adequate to resist shear and carry the point load (see "Footing Fundamentals," Oct/00).
"Q&A" Simple vs. Continuous Joists, (Oct/13)
Congratulations on an excellent magazine. I have read JLC for many years (back to newspaper format) and enjoy the sharing of experience and expertise.
Mr. Mark McKenzie's response to a recent question regarding simple vs. continuous span joists provided plenty of good information. However, it should be noted that the bending forces are not reduced. It is fairly common to confuse strength with stiffness. Although using continuous joists does reduce the deflection and will make the floor feel more stiff, the maximum bending force is the same. For a simple span, this force—which wants to break the board—occurs at the middle, whereas for a continuous span, it occurs over the center steel beam—but in both cases it is the same (M = WL2 ÷ 8).
Interestingly, it changes from a "positive to negative moment," but that is a different conversation. We could also discuss wood grading rules, whereby the board can vary in the middle vs. the ends, but "I-joists" are not affected by this. —Ralph Schmidt, SE, PE, Wauconda, Ill.
Mark McKenzie responds: Mr. Schmidt is correct. If the two spans are exactly equal on either side of the support (say 14 feet), the maximum moments are the same. However, if the spans are unequal (say 12 feet and 14 feet), then the maximum moment in the continuous span is reduced compared with the maximum in the two simple spans.JLC Online comment:
Some of the most popular lick-'em/stick-'em stone designs ("ledge stone," for instance) promote capture and hold of water. They have all kinds of protruding horizontal surfaces that won't drain or dry, and they're not grouted in any way. So water (or snow) sits on them and eventually soaks in (or freezes). Around here [Pennsylvania], I've seen problems with freeze-thaw spalling the surface off of the "rocks," which exposes an even more porous surface, etc., more porous surface, etc. Early failure guaranteed. —jstoddard
Online response to comment: Joe, you are correct. The dry stack, albeit cool looking, will allow more moisture to collect in the assembly and enter the scratch coat. This will in turn allow additional exposure and collection. In an environment that does not experience freeze-thaw it may not be such an issue, but in our area [Iowa] it can make a world of difference. —Mark Parlee