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RE: Deflection Limits for Studs Backing Brick Veneer

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Chapter 16 of the Florida Building Code and IBC both allow wind loads to be
taken as 0.7 times the "component and cladding" wind loads for the purposes
of determining deflection limits.  

Our firm takes the approach of meeting the L/600 deflection criteria
commonly specified but we do so by checking the deflection limit using the
components and cladding wind load reduced by 30%.  Design for moment, shear,
etc. is checked using the full wind load.

This seems to be in keeping with the idea of "service level" wind
recommended by the Brick Institute.

Sean Martin
David H. Melvin, Inc.
Tallahassee, Florida

 


-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
Sent: Wednesday, August 08, 2007 12:48 PM
To: seaint(--nospam--at)seaint.org
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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