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Re: uplift straps/ home builders

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Gary,

Thanks for chiming in.  My concern is that the NDS lists 21 pounds (per inch) as the withdrawal value for a gun framing nail into SPF lumber.  A perfectly driven toenail only results in 2" of fastener penetration, including the tip length. I haven't seen the change in writing, but the first normal truss on a typical roof  (~5 in 12) that's only 20' long in a 90mph wind zone should have about 400 lbs of uplift on each end. That would be at least 10 perfectly driven, L/3 at 30degree toenails with a .0131x3.25" gun nail. More if you're using SPF(S).  I don't feel that's reasonable to expect in the field.

It sounds more like a GC coordination problem, not getting the truss manufacturer to properly design the trusses.  Around me, it's like pulling teeth to get truss mfrs to account for end conditions, wind exposure C (which we have a lot of), or snow drifting. I've yet to see a truss package where a manufacturer has overdesigned a whole job "because it was easier". I suppose it's possible where there are several wind contours and the GC->Truss Engineer path is so convoluted or disjointed that the manufacturer has gotten no data on the actual location. Again, that sounds like lazy GCs or sales staff, not a code problem. Would you buy a framing gun that needed a 140psi source, or allow the lumber yard to order (and charge you for) S. Pine Select Structural for wall studs? Of course not, you'd check that before you bought it - why not do the same with trusses?

If I sound intolerant, it's because I am.  There are a lot of new builders in the industry thanks to the hot market conditions we've seen. It has lasted long enough that people with little to no experience in building have been enticed by the quick riches and have taken a written test and started building spec houses. Even small fix-it guys have stepped up into the game. Many of these folks don't have a clue about what makes a house structurally sound, and some that do have expanded to the point that their on site supers don't.  When plumbing or electrical or HVAC gets put in wrong, the results are usually pretty easily found out in the year warranty period required in my state, but structural systems only see design loads 2% of the time (even less for seismic). Even for liability, there's only a 1 in 10 chance of an event occurring to test the building's structure in a 5 year statute of limitations. Most structural failures will occur after the builder is long gone, which is why it needs to be done right the first time.

Katrina was just the most recent demonstration of how wind can cause failures, and the common areas where "traditional" framing techniques fail. Toenailed roof connections rate right up there with unreinforced CMU basement walls in the "bad ideas" list.

Jordan


Ehrlich, Gary wrote:

Having both been involved in the hearings and having contributed to the article, I guess I should de-lurk and respond to the comments here.

 

First, I should note that we're not opposed to code provisions for hurricane clips and straps where there is a demonstrated need; hurricane-prone areas or longer-span roof trusses, for example.  And certainly any engineer or builder who wants to provide ties & straps even in a low-hazard region is welcome to (and we have members who do, just like we have members who fully sheath in OSB even when they could easily use the other braced wall methods).

 

Our concerns with the proposals were that they appeared to lower the current triggers in the code for strapping versus toe-nailing and did not appear to incorporate reasonable assumptions for dead loads resisting uplift.  The result was that in 90mph and less areas it looked like you weren't going to be able to have much more than a 20 foot span before you were going to have to start strapping.  We were working at the hearings with a bunch of folks on a modified proposal that would have increased the nailing requirements in lower-hazard areas but still would have kept what we felt was an appropriate trigger for requiring straps. Unfortunately, the moderator for the IRC committee was taking a hard line on floor modifications, so we'll have to come back with a public comment on that issue.

 

The roof truss proposal we were concerned about mandated the use of the roof uplift values from the truss design drawings over the values obtained from the current Table 802.11 in the IRC.  Feedback from our members suggested there are truss designers in states with multiple wind zones who either for simplicity or by local code are designing all trusses to the maximum wind speeds in the state.  Thus, under the proposal a builder could have been required to provide uplift connections based on (for example) a 120mph wind on a dwelling in a 90mph zone just because the truss engineer opted (or was required) to use the higher loads.

 

As for Katrina, it should be noted that much of the affected area had no statewide building code in force at the time.  And even with a building code adopted, in a hurricane-prone area you'd be outside the prescriptive limits of the IRC anyway and be designing per the Wood Frame Construction Manual or SSTD-10 and certainly would (and rightfully) be strapping everything.  So the effects of the 2004 & 2005 hurricane seasons shouldn't be used to justify upping the connection requirements in areas of the country where even a 90mph wind gust is extremely rare, and where existing toe-nailed rafter and truss connections have performed well.

 

Gary

 

Gary J. Ehrlich, PE

Program Manager, Structural Codes & Standards

National Association of Home Builders (NAHB)

1201 15th Street, NW, Washington, DC 20005

ph: 202-266-8545  or 800-368-5242 x8545

fax: 202-266-8369

gehrlich(--nospam--at)nahb.com

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