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RE: l/600 or l/720 brick veneer light gage back-up

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Ken,

You can stop looking in the IBC.

The requirement for out of plane deflection control is a serviceability issue. The building codes are focused on life safety issues. It is almost impossible to get any serviceability requirements into the building code so don't expect the building codes to ever require things like out of plane deflection limits. The only reason that there are drift requirements for seismic is that they have a bearing on structural performance. The BIA is the appropriate place to find "guidance" for brick cladding.

Some industries have a not too transparent vested interest in reducing the lateral deflection requirements. The concrete block industry would like to see the lateral deflections reduced to L/1000 to force you to back up brick with CMU. The light gauge industry likes it at L/360 to keep their products viable. The Clemson study is the study that both the L/360 proponents and the L/1000 proponents point to as "proof" of their position. The Clemson study was just a study and just reported what they observed. The Clemson study did not make recommendations.

The BIA, in an article by Brian Trimble, recommends that you use L/600, and implied that you use the full 50 year wind. In another article, "Technics Steel Stud / Brick Veneer Walls" by Trestain and Rousseay fo the Canada Mortgage and Housing Corporation, L/720 was the recommended deflection limit. There are many other recommendations regarding ties (never use corrugated ties), air space, and weeps that should be considered.

Being a serviceability issue, there is a lot of precedence to use a 10 year wind as opposed to a 50 year wind. But if you use a 50 year wind for L/600, you will be about the same with a 10 year wind for L/720.

Now the comparison has been made to residential construction. Commercial construction is different. I personally use the same performance guidance for residential as commercial. It comes down to O&M, client expectations, and what I feel comfortable with recommending. If a client wants lesser performance, it is the client's call. It is not a code requirement. I guess I just don't want to cater to clients wanting lesser performance. The residential construction in Europe is virtually identical to their commercial construction. Here in the states we have many areas where the engineer is a carpenter. So if we are to compare engineering skills with a carpenter or mason's engineering skills, it is legitimate to lower performance expectations. Then we all go to the school of "I have been doing it this way for 30 years". Granted I do very little residential, and I may starve if I would be in that market. The few residential clients I have spend money on slate roofs, and increasing the stiffness of a wall is inconsequential.

I have been told "... and the horse you rode in on." I choose not to cater to those clients.

Regards,
Harold Sprague




From: "Ken Peoples" <kspeoples(--nospam--at)lvta.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: "Seaint" <seaint(--nospam--at)seaint.org>
Subject: l/600 or l/720 brick veneer light gage back-up
Date: Tue, 7 Dec 2004 08:26:24 -0500

I know that BIA recommends l/600 to l/720 for a brick veneer back-up deflection limitation. They say "for service loads". What I am not sure about is what the design wind pressure for this condition should be. Should this just be based on the same component and cladding design load used for the strength design of the studs or can it be a reduced wind load. Table 1604.3 of IBC 2003 indicates that the deflection limit for exterior walls with brittle finishes should be l/240 and footnote f indicates that the load may be 0.7 times the design component and cladding loads. This seems to be quite different form BIA's recommendation of l/600. Perhaps a brick veneer is not considered a brittle finish?

Is the l/600 or l/720 specifically required somewhere else in the IBC?

Thanks for your help.

Best regards,


Ken

Kenneth S. Peoples, P. E.
LVTA
Lehigh Valley Technical Associates, Inc.
1584 Weaversville Road
Northampton, PA 18067

Phone: 610-262-6345
Fax:610-262-8188
Email: kpeoples(--nospam--at)lvta.net

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