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@hotmail.com]
Sent: Tuesday, August 07, 2007 9:40 AM
To: seaint@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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