A Vote for Double Top Plates
I loved David Joyce’s article “Building a High-Performance Shell” (5/10). I am a big fan of advanced framing techniques and learned a thing or two from his article. But when it comes to using single top plates, it seems that the extra studs needed to carry small point loads from upstairs would cause you to wind up with as much lumber in the wall structure as if you had just doubled the top plate. The doubled plate also provides significant strength to the wall, while the extra studs only increase thermal bridging. And using a double top plate saves the labor of site-cutting all those studs.
A Better Builder
Flush Framing With Steel
I have flush-framed with steel many times, so the article “Replacing a Wood Beam With Steel” (On the Job, 4/10) held particular interest for me. On a job we just completed, we had a similar situation — 2x8 floor joists, but the ceiling was continuous through the entire first floor. We used two pieces of MC7x19.1 C-channel drilled with matching holes, packed with 2x8s, and bolted back to back. This assembly is more costly but allowed us to forgo packing down the ceiling.
Careful With Long Beams
After reading “Replacing a Wood Beam with Steel” (On the Job, 4/10), I wish to offer some cautions.
First, long beams can have problems with sagging. Beyond a certain length the designer must give a beam’s stiffness more consideration than its strength. In general, beams with a length of more than about 20 times their depth begin to sag or bounce more than most people prefer. The beam described in the article has a length of about 32 times its depth.
Second, running members continuously over center supports reduces deflection for a given span. For instance, assuming the same materials and loading, 20-foot floor joists running over a dropped beam at center-span will sag only about 40 percent as much as 10-foot joists framing into a flush beam.
Finally, be careful when using any “prescriptive” design table: It limits you to the precise conditions the table was developed for. Often, these design conditions do not match the reality in the field.
According to my calculations, the steel beam used in “Replacing a Wood Beam With Steel” (On the Job, 4/10) is inadequate. For a 22-foot span supporting a live load of 40 pounds per square foot with an allowable deflection of L/360, the tributary width could be only 7.6 feet. The photographs show a wider load area. Also, even if the tributary width were only 7.6 feet, the moment for a total load of 50 pounds per square foot would be 23 kips, which exceeds the allowable moment for the W8x21. The fact that two LVLs were added to the steel doesn’t help — loadings for 7 1„4-inch LVLs are not even listed in the manufacturer’s tables for 22-foot lengths.