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[SEAOC] Re: [SEAOC] Rw factors[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)power.net
- Subject: [SEAOC] Re: [SEAOC] Rw factors
- From: BCASE1356(--nospam--at)aol.com
- Date: Fri, 8 Nov 1996 21:21:43 -0500
We typically use Rw = 6.0 for plywood shear wall systems when combining with steel frames and masonry shear walls. Just because the loading is distributed by tributary area does not mean you could argue using a different Rw value for that lateral resisting element. Depending on the aspect ratio of the plywood diaphragm, the actual loading to the lateral resisting elements may not be distribute by tributary area, especially if you consider deflection compatability of the lateral resisting elements. I observed damage in two 5 story office buildings after the Northridge Earthquake, which had flexible diaphragms and truss moment frames on the front of the building and masonry pier/spandrels on the back side of the building. Since the deflection of the two lateral resisting elements was significantly different (ex: the masonry pier/spandrel system was more rigid than the truss moment frames) when the truss frames deflected, the floor diaphragm acted as a drag transfering all load to the masonry pier/spandrel beams at the back of the building overstressing them to failure. In this case, the distribution of lateral load was more 90% to the masonry system and 10% to the moment frame system, instead of the 50 - 50 distribution of load normally assumed by tributary area for the two lateral resisting systems as allowed by the code. There was no damage to the truss moment frames or the glass curtain wall which it supported. Based upon these observations, I would still be hesitant of using stictly tributary area without considering deflection compatibilty, especially in building more than one story. Obviously this is more work, and hopefully you can get a fee to cover this. My two cents worth. Michael Cochran Brian L. Cochran Associates Los Angeles, CA ------------------------------------- separator -------------------------------------- In a message dated 96-11-08 13:50:09 EST, you write: << Subj: [SEAOC] Rw factors Date: 96-11-08 13:50:09 EST From: MGFran(--nospam--at)aol.com Reply-to: seaoc(--nospam--at)power.net To: seaoc(--nospam--at)power.net I would like to know what other engineers have been using for Rw with the following conditions: Flexible diaphragms using plywood shear walls mixed with steel frames on both residences and commercial structures. We have conservatively used the lowest value for Rw for all lateral force resisting elements in the structure in the past, but it seems that one could justify using an 8 for the plywood shear walls and a 6 for "Ordinary" steel moment frames, for example, since the diaphragms are flexible and the shears are distributed based upon tributary area. ...
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