Lag Screws in a 2 Hour Wall

[Engrman]

The local bldg dept inspector told the contractor for one of my condo projects that the pre-engineered wood roof trusses couldn't penetrate the sheetrock and bear on a two hour wood stud fire wall even though plan checker approved the architect's detail of it. The contractor decided, unbeknownst to me and with the help of the truss supplier, to support the trusses (1530# total end reaction, trusses spaced at 24" o.c.) on a 2x8 ledger lag screwed thru the sheetrock into the wall studs (two 1/2" lags ea. stud, studs spaced at 16'' o.c.). Now the inspector of course says he needs a stamped letter approving the change. Several things about this that're bugging me:

1. Since they didn't bother to include me in on this I figure the truss supplier's engineer should provide the stamp and accept the liability. They say it's my problem. The inspector, the @#$*&%$, agrees and wants a letter from me. Any suggestions?

2. I have no way of verifying the quality of the lag screw installation. The NDS says minimum edge distance on lags, load parallel to grain, is 1.5D, which means they have to be dead bang on each one in the studs. Also, I think the NDS requirements were based on lags penetrating the wide face of each element, not the narrow. Opinions?

3. My opinion is that the 2 hour wall's 2 layers of 5/8" rock doesn't provide any support to the lags and that places them in bending, moment arm of 1-1/4" + the ledger thickness. Yes, no? Right, wrong?

4. The NDS says lag screws shall conform to ANSI/ASME B18.2.1-1981. ASME's web site says that standard is for large rivets. What are the properties of the steel lag screws are made of? The lag screws you get at Home Depot are pretty brittle. I've torqued the heads off of smaller diameter ones with just a small rachet. I don't like the possibility of brittle failure here. Am I being to old womanish?

5. The NDS lag screw values are based on shear thru the shank not the threads. If the assumption that the sheetrock provides no support is correct, then the shear is now thru the threads. Do I just reduce the NDS table values by the ratio of the root diameter to the shank's?

 

[FalsePrecision]

Legal nonsense from Code officials and inspectors is very common. It is something that one learns to deal with. Otherwise you get unproductive.
If your lag screws at Home Depot are made in China, that explains why they are brittle. I would never buy any fastener made in China (specifically mainland, not Taiwan).
In fact, for other reasons, I never set foot in Home Depot. I always got to the small independent hardware stores.

 

[SperlingPE]

If you are the EOR, the letter comes from you.
If you are the EOR, you need to talk the matter over with the owner. This is kind of a sticky situation. You have approved plans, a field inspector who has red tagged some framing, and a contractor who has taken it upon himself to provide the fix. Get all involved parties together and see if there is some way to resolve all of this. If the contractor's fix is accepted, you are going to have to sign off on it. The contractor will need to provide you with all information to evaluate his fix. Goo dluck

 

[LPPE]

I agree with you on Items 2 and 3. And if you can't penetrate the 2 hour wall with the roof trusses, why is it acceptable to penetrate it with lags? Just food for thought....

 

[DaveAtkins]

My responses:
1. You are the EOR, so you have to engineer the fix.
2. You can install 1/2" lag screws into the narrow face of a stud. Force in the ledger board will probably govern, since it is perpendicular to grain.
3. You must check the lag screw for bending.
4. I wouldn't worry about brittle failure. Once you check for bending in the lag screws, their capacity will go way down anyway. You may need to add ANOTHER ledger board undrneath the ledger board that is already there, in order to get more lag screws into each stud.
5. Are you sure the shear is through the threads? The unthreaded portion of a lag screw is pretty long.

DaveAtkins

 

[FalsePrecision]

I disagree with the concept of merely checking bending in a lag screw, regarding the fact it goes thru 2 layers of gypboard. The mechanics of failure if far more involved than that, and you will be geeting an unconservative answer. I would never allow that situation in a detail. IF you could get a fire-preservative treated 3x ledger directly lagged to each stud, that would be better. Unfortunately, that interrupts the gypboard. So, I always get into a discussion with the Inspector on this issue. Any other ideas?

 

[Engrman]

DaveAtkins: I agree I should have been the one to engineer the fix, but that didn't happen. I'm being asked to stamp somebody else's fix that I am uncomfortable with. The only solution I've come up with so far is to replace the lag screws with thru bolts. At least that way I have more confidence with the bolt material. Typical shank length on a 1/2"x5" lag screw is 2". Bolt head is 2-3/4" from face of stud.

False precision: I agree with you. I don't think this can be handled with a simple unity equation and I don't know of any research on the subject. I asked the contractor to have his supplier provide the req'd info on the lags. I'm guessing that since I haven't received it yet all he got was "huh?"