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RE: Cantilever Wall Forces-ASCE 7-05, Ch 15

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Steve,
 
There was discussion in the code development process whether a cantilever wall such as you describe was a non-building structure or a non-structural component.  Regardless, Table 15.4-1 is titled "Seismic Coefficients for Nonbuilding Structures Similar to Buildings".  A cantilever wall is not what I would categorize as similar to buildings.  So the bottom line is that Table 15.4-1 does not apply.  One could argue that you could use Table 15.4-2 "Seismic Coefficients for Nonbuilding Structures Not Similar to Buildings".  But that is not what was intended for a cantilever wall. 
 
We tried to avoid duplication in the ASCE 7.  It was decided that a cantilever wall should be listed as a nonstructural component.  As such there is an Rp value and an ap value contained in Section 13.5.  You should be referencing Table 13.5-1 and you should be calculating the Fp value to design the wall.  I would suggest an Rp of 2.5 and an ap of 1.0 using ASCE 7, Section 13.3.1 to calculate the seismic design force. 
 
Further discussion is contained in the NEHRP, Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450), Part 2: Commentary, Section 6.3. 

Regards, Harold Sprague
 

Subject: Cantilever Wall Forces-ASCE 7-05, Ch 15
Date: Wed, 13 Jul 2011 09:47:20 -0700
From: swidmayer(--nospam--at)willdan.com
To: seaint(--nospam--at)seaint.org

I’m trying to clarify the base shear requirement for a cantilever wall (concrete or masonry) using the “Nonbuilding Structures” section of ASCE 07.  Table 15.4-1 does not describe a cantilever wall as a Structure Type.  I assume that I would use this table to get the R value and that I wouldn’t use Sec. 12.11 (Structural Walls and their Anchorage), Ch. 13 (Nonstructural Components/Architectural Components).  If I’m going to use Table 15.4-1, what is the correct R & Omega values for a cantilever wall?  Thanks.

 

Steve Widmayer, PE