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RE: ASCE 7-05 rho

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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