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RE: truss uplift straps 2

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And never forget, these connections are subject to the zipper effect - one goes, they all go.

 

Bob Garner, S.E.

 


From: Andrew Kester, PE [mailto:akester(--nospam--at)cfl.rr.com]
Sent: Tuesday, October 17, 2006 8:06 AM
To: seaint(--nospam--at)seaint.org
Subject: re: truss uplift straps 2

 

Gary,

With all due respect to you as a PE, and I am sure your position puts you in some tough spots between industry demands and public demands, and there are a lot of politics involved...

 

And sorry this got a little long.

 

Jordan and I have the same opinion on this issue I believe, as I think we have both seen a lot of poorly constructed wood frame buildings (or minimally constructed). We also are familiar with the difference between what our calcs may prove will work, and how things really get built. I only use toe nails to transfer small shears in blocking, never in a gravity or uplift connection. I have done enough carpentry to know how easy it is to split wood when toe nailing and how hard it is to correctly install a toe nail. Also in my framing experience, I don't see the big deal with installing uplift straps. I don't swing a hammer every day but I can put in a pretty big strap in 2-3 minutes tops. I would think a decently fast framer could do a normal house in a day or less. With straps running a $1-$2 each, and pay for framers not what it once was, I just don't see the big economic advantage of not installing straps? $500 on a house? People will pay that to upgrade the bathroom counter to granite.... Also insurance companies are now offering deductions for those kind of connections (in FL they are at least).

 

Lets just say for arguments say, calcs show that toe nailing the trusses will actually work with some reasonable number of nails, maybe 2 per side. Which per Jordan's calcs would be about 4x21lb/inx2"= 160lb. Not very much. Is there an engineer on this list would would put that detail on their dwgs and sign and seal it? I assume few if any, and I wonder why.... Even in non hurricane areas I see California gets Santa Anna winds, in the mountains they can get 90 mph gales, and tornadoes in the corn belt and mid west. Now of course a direct hit by a tornado, nothing you can do, but what if one passes by and throws a few seconds of 110mph winds at the 90mph toe nailed trusses? A few extra bucks on straps may save roofs, which will save walls and structural collapses, which could save lives.

 

It has been my forensic experience that wood framed roofs are the achile's heel of the structure. I have seen pieces of plywood ripped off with the shingles still attached, some with plywood still attached to the framing, uplift straps snapped or pulled away with the nails still left behind, sometimes stripping a house from the foundation only leaving the base plate and a few straps if anything... Extreme examples but it shows what is predicating failures. Once you have a hole in the roof envelope, and trusses or roof framing start becoming dettached, your walls are in serious trouble..

 

As far as wind speeds and uplift pressure being OVERDESIGNED by truss manufacturers, this seems a little ridiculous. My first job out of college was for a truss mfr, albeit for about 2 months. They all use software and just plug in the local building code mandated speed into their program. It is their first step and they do not just blindly use a higher wind speed because they did that last project. They are trying to use as little wood and as few plates as possible, this is how they make the most money. I have never got the impression from the dozens of truss calc packages I have reviewed as EOR that they were OVERdesigning, usually the opposite. For a typical wood truss house with wood sheathing on top and gyp ceiling on bottom chord, most people I know and truss mfrs use about 7psf of DL to reduce the uplift. In my opinion and experience this seems pretty reasonable, if not a little high. Ceilings will often collapse in a wind event so they can be of little use. I don't think this is underestimated.

 

Furthermore, it gets back to what seems to be a constant theme of residential builders to get away with the bare minimum. What other industry strives to be so mediocre? Thank god our car manufacturers don't look to constantly reduce government safety minimums, or drug manufacturers, etc. (Well, as a cynic, I am sure some of them do too.) But you would not expect the automanufacturers to announce "victory" by getting the collision safety requirements reduced. It would be refreshing to see more builders advertise and flaunt that their homes were designed by a licensed structural engineer to exceed all building codes, instead of just talking up their open floor plans, vaulted ceilings, and upgraded countertops. Or how they saved the owner $400 by lobbying to get reduced strapping requirements.

 

 

Andrew Kester, PE
ADK Structural Engineering, PLLC
Lake Mary, FL

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