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Re: Deflection Limits for Studs Backing Brick Veneer

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This is consistant with the Canadian research. The size of the cracking due to the stud deflection made little difference in the quantity of the moisture that penetrated past the masonry. The important part for moisture ingress of the wall as a system is the moisture barrier on the side opposite the masonry in the air gap. Again, the L/720, L/1200, etc. made no significant difference to the moisture penetration of the system.

Regards,
Harold Sprague





From: "Dave Handy" <dhandy(--nospam--at)trg.ca>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Re: Deflection Limits for Studs Backing Brick Veneer
Date: Tue, 14 Aug 2007 16:53:40 -0400

It is interesting..possibly..to note that the latest Canadian code for masonry has reduced the deflection limit for flexible structural backing systems to L/360 providing the veneer is not used as part of the moisture management system. L/720 + tie deflection was used in the earlier code based upon the veneer being used to limit water penetration. We always have an air barrier membrane of some sort which would deal with any moisture that makes its way through the veneer.

David Handy, P.Eng.

----- Original Message ----- From: "Harold Sprague" <spraguehope(--nospam--at)hotmail.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, August 08, 2007 12:48 PM
Subject: RE: Deflection Limits for Studs Backing Brick Veneer


Bill,
I agree that L/600 is too stringent for out-of-plane bending for a serviceability issue. The Canadadian research "Technics Steel Stud / Brick Veneer Walls", by Trestain and Rousseau is one of the best studies and drew from the McMaster University studies. The McMaster studies actually constructed veneer stud walls and tested with wind pressure and simulated rain.

The result was that there was no increased system vulnerability due to excessive leakage from the flexural cracking. The L/720, 600, 360 or whatever does not elmininate flexural cracking. The deflection limit is intended to reduce the flexural cracking size. But as the McMaster study indicated, the size of the flexural cracking did not increase the system vulnerability.

What did have a more significant effect on the system were the elements to control and manage the moisture that enters through the brick from rain and dew point and provide corrosion resistance. The Technics article did recommend L/720 for the full wind load, but (as stated earlier) actually provided evidence that the crack width was not an issue for system performance.

A case can be made to use L/400 for the 50 year design wind (inferring the L/600 for a 10 year service). I also suggest a look over the architect's shoulder to see if the system is properly accounting for water management and corrosion resistance.

Regards,
Harold Sprague





From: <William.Sherman(--nospam--at)CH2M.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Deflection Limits for Studs Backing Brick Veneer
Date: Wed, 8 Aug 2007 07:12:18 -0600

I feel that a reference to "service level wind loads" without a
qualifier means code based wind loads without load factors applied.
Thus, it would mean a 50-year wind load as written.

But I do agree that the issue of "serviceability" is much more
subjective.  I think that a deflection limit of L/720 makes more sense
for vertical deflection of lintels than for out-of-place deflection of
masonry walls, due to greater wall flexibility in the out-of-plane
direction.  I would prefer to see the deflection limit defined for full
code level, "service level wind loads", than define it for a lesser wind
frequency, even if the lesser wind frequency is part of the basis for
the defined limit.  This just keeps requirements more "user friendly".

Ultimately, I tend to feel that L/600 is too stringent a limitation for
out-of-plane deflection.


Bill Sherman
CH2M HILL / DEN
720-286-2792

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Tuesday, August 07, 2007 9:40 AM
To: seaint(--nospam--at)seaint.org
Subject: Deflection Limits for Studs Backing Brick Veneer

There has been some good discussion on the maximum deflections of studs
that back up brick veneer.  There have been many good papers on the
topic.
Promulgated deflection limits include L/360 (steel stud mfgrs.), L/600
(BIA), and L/720 (Canadian Research).

Interestingly, the BIA guidance (TEK Note 28 B) limits the lateral
deflection of the stud to L/600 for "service" wind loads.  Per BIA 28B,
"Therefore, to obtain sufficient backing stiffness, the allowable
out-of-plane deflection of the studs due to service level loads should
be restricted to L/600."  But BIA does not define "service level loads".

For wind the IBC and ASCE 7 have us calculate the variable "p" that is
defined as the "design" wind pressure and is the 50 year Mean Recurrence
Interval (MRI).  Serviceability is discussed in the ASCE 7 Section
C6.5.5 and in the AISC Design Guide 3.  The general consensus of the
AISC is that service level winds are 10 year MRI winds and are about 75%
of the pressure calculated from "design" 50 year MRI winds.

If the above logic is considered valid, the L/600 BIA limit at a
"service"
10 year MRI wind would be about the same as a L/400 at a 50 year MRI
"design" wind load.

I know it is conservative to use the 50 year MRI for the L/600, but it
also increases the cost.  I would welcome discussion and any performance
studies on systems constructed.

Building codes focus on life safety.  This is a serviceability issue.

Regards,
Harold Sprague

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