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RE: IBC 2305.1.5

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The new shearwalls are 40 feet inside of the perimeter? Are there existing perimeter shearwalls or frames? If not, you have a problem with cantilevered diaphragms being that long. Secondly, if your walls are concrete/cmu, you will have high seismic forces and by definition your diaphragm is flexible. If you do not have a perimeter LFRS, then you need rigid diaphragm behavior which you will not get with a plywood diaphragm. Without a perimeter LFRSm your diaphragm shears could get very high at the interior walls…


What about wall anchorage and sub diaphragms?


Not sure if I’m picturing the building properly.




-----Original Message-----
From: Jared Lewis [mailto:jared(--nospam--at)]
Sent: Tuesday, December 13, 2005 3:28 PM
To: seaint(--nospam--at)
Subject: IBC 2305.1.5


I am working on an existing three story facility that will have new shear walls positioned 40’ inside the perimeter walls.  The diaphragms consist of plywood sheathing over existing straight sheathing and the existing walls are lightly reinforced concrete and urm.  The above referenced section of the IBC references torsion as an unacceptable force distribution through horizontal trusses and diaphragms.  I suspect that the IBC means flexure in lieu of torsion.  However, flexure seems an inappropriate limitation as all wood diaphragms experience flexure under lateral loading. 


In addition, a steel strap horizontal diaphragm can be designed for this application.  However, I do not feel this is necessary given that the diaphragm depth to span ratio is 2:1.  Does anyone have experience with an alternate solution to this limitation? 




jared lewis


catena consulting engineers

1111 ne flanders street

suite 206

portland, or  97232