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RE: Joist Girder Deflection at CMU wall[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Joist Girder Deflection at CMU wall
- From: <William.Sherman(--nospam--at)CH2M.com>
- Date: Tue, 4 Jul 2006 08:26:01 -0600
I too have the same concerns with joist girders so close to a wall. I try to keep the girders at least a couple of feet from the wall to allow for some rotation and deflection at the joist girder.
I can't argue that what you describe is not often done nor that there are known failures. But it doesn't make sense to me to build something that can't structurally behave as designed. The wall connection will carry load as the joist deflects and something will have to "give" for the system to work.
From: Greg Mosier [mailto:gmosier(--nospam--at)bcraengineering.com]
Sent: Mon 7/3/2006 11:25 AM
Subject: Joist Girder Deflection at CMU wall
Fellow list server subscribers,
I am currently working with a large retail client who has a set of prototypical drawings that I am supposed to adapt to the specifics of each project. The general building system is a light metal deck and bar joist/joist girder roof with an approximate 50 foot by 50 foot bay. The lateral system for the building is CMU shearwalls and the building is in seismic design category D per the IBC 2003/ASCE 7. I have an issue with one of the details included in the set and was hoping to get additional thoughts on it.
The issue I am having is that there are 50 foot long steel joist girders that are approximately 8” from a CMU wall and steel bar joists bear on the joist girders. There are embedded plates in the CMU wall which the heel and toe of an angle gets welded to. The other toe of the angle gets welded to the top of each bar joist. The roof deck gets welded to the top of the angle and the angle serves to anchor the walls out of plane to the joists and to transfer the building lateral loads to the CMU wall. The problem I am having is justifying what happens when the joist girder deflects. The joist girder is supposed to be designed to an L/360 live load criteria which would result in a 1.67” deflection at midspan. When the joist girder starts to deflect, it will begin to pull down on the toe of the angle that is welded to the tops of the joists. In the design load condition, I calculate loads that will either cause failure of the weld to the top of the joists (or failure of the angle) or an overload condition in the CMU walls themselves due to the moment caused by the eccentricity of the load. If this connection fails, there is potential loss of out of plane wall anchorage and loss of capacity to transfer lateral loads to the CMU walls.
I would think that some sort of vertical slip connection would be needed to allow the joist girder to deflect, but still allow out of plane wall loads and in plane shear loads to be transferred. I have spoken with the joist manufacturer that typically supplies the joists and joist girders and they have led me to believe that this is a very standard method of accomplishing the connection for this condition. They added that there is no history of failures that would indicate a problem. I would prefer to avoid anything that is a potential failure issue. Has anyone seen this before and been able to justify that what I am talking about is not a problem? Are there any studies or testing to show that this is ok? Any thoughts would be appreciated.
Greg M. PE, SE
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2106 PACIFIC AVENUE, SUITE 300
TACOMA, WASHINGTON 98402
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Civil Engineering • Structural Engineering
- Joist Girder Deflection at CMU wall
- From: Greg Mosier
- Joist Girder Deflection at CMU wall
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