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Re: FW: CMU Site Wall using 2001CBC

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Although the structure and the design of property walls are quite basic, the damage to those walls is the most visible and the most common after each earthquake.  Although not exactly an inverted pendulum, these walls are quite sensitive to lateral loads.  As documented in the 1998 NEHRP Conference proceedings, in SF Valley they even evaluated the actual seismic forces based upon the mode of damage to the walls. 
And how about that:
1) The weird UBC Formula 30-8 for natural period of 6.87'-tall CMU structure gives T=0.08 sec (amazingly close to T=0.087 by Timoshenko's formula). 
2) The UBC Formula 30-5 gives seismic force of 0.379W (0.379/1.4=0.271 for ASD).  The use of Section 1632 results in the seismic force of 0.308W (hx=0), somewhat close, but not conservative.  In any case, even from its name, this UBC section is definitely inapplicable to the property walls.
3) The Exposure C, 70 MPH wind load produces pressure of 18.6 PSF at 6-ft height; the seismic load on the same wall (normal-weight, 45 PSF) is 12.2 PSF<<18.6 PSF.   At the same time, I don't think we ever had a 70 MPH wind in Southern California; Exposure C in 90% of the cases exists only in the jurisdiction (e.g., City of Yorba Linda) requirements.
Steve Gordin, SE
Irvine CA 
----- Original Message -----
Sent: Wednesday, August 04, 2004 7:36 PM
Subject: Re: FW: CMU Site Wall using 2001CBC


I would not sign off on that LA city wall.  I have a problem out here in La Quinta where they built the city spec wall and they want to change height up to 8' . I told them the city spec wall was underdisiged and they had two choices for a fix.
1. Remove and rebuilt 8" cmu wall
2. Install a new 6" wall directly behind the existing wall attached with hairpins ties to verts of each wall and enlarged footing doweled together.  I probably wont here back from the client, but I can sleep soundly in the evenings.

Joe Venuti
Johnson & Nielsen Associates
Palm Springs,  CA