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RE: ASCE 7-05 rho

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I don't work with the ASCE 7-05 or IBC 2006, but my interpretation is that if you have a somehow slender shear wall (floor height / wall length >1) that is providing you with more than 33% of story strength (one of only 2 shear walls, stiffer one of 3 or more walls, etc.) then you need more redundancy or you have to design you system for more lateral load to be on the safe side. The same thing if this somehow slender wall is crucial for the regularity of your system.

Reza Dashti P.Eng

From: dtopete(--nospam--at)
To: seaint(--nospam--at)
Subject: RE: ASCE 7-05 rho
Date: Tue, 20 Mar 2007 09:08:37 -0700


My guess, since I don’t have a copy of ASCE 7-05 or 2006 IBC yet, is that if you remove one of 4 walls, it would result in a 33% reduction in strength.  Less redundant system, hence higher value of rho = 1.3.

WRT story height, wouldn’t the height h be the story height, or the height at which the wall is braced laterally and/or the height at which there would be lateral load being transferred to the wall?

Congrats on the “SE.”  Haven’t seen that until now.


David A. Topete, SE


From: Jake Watson [mailto:jake.watson1(--nospam--at)]
Sent: Tuesday, March 20, 2007 7:30 AM
Subject: ASCE 7-05 rho


Utah has moved to the 2006 IBC and ASCE 7-05 for its building code.  ASCE section & table 12.3-3 contain new provisions for rho.  The application regarding shear walls has me confused.  Basically there are two conditions now, you either have a rho of 1.0 or 1.3.  The shear wall item reads:

"Removal of a shear wall or wall pier with a height-to-length ratio greater than 1.0 within any story, or collector connections thereto, would not result in more than a 33% reduction in story strength, nor does the resulting system have an extreme torsional irregularity... "

If you fail that test, you have a rho of 1.3.

I have two questions about this statement: 1 - does the h/l ratio apply to walls or just piers?  2 - if the h/l does not apply to walls, do you only "remove" one bay of a wall as defined in paragraph b above the table (length of one bay = story height)?  Also, I read the "h/l" value the same as defined by ACI for concrete.  In other words, h is the total height of the wall, not the story height.  Although, I'm not sure if that is the intent when we get to designing homes & the bay definition is clearly story height, not total height.  So who knows?

Thanks for any help,
Jake Watson, S.E.
Salt Lake City, UT

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