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Re: FW: CMU Site Wall using 2001CBC

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I will offer another opinion:
 
The purpose of the term in (  ) is to address in-structure amplification of the input ("base") motion.  The in-structure amplification is the amplification of the input motion by or within another structure, which serves as the "base" or "brace" of the item in design.
 
If the item in design is supported at the base only, and is not associated with the amplified response of any other structure that supports or braces it, then hx=0 is appropriate, and all of the seismic force amplification is intended to be accounted for in the a-sub-p term.
 
But, if your wall is independent and base supported only, it may be better considered as a structure itself, the non-building variety, under 1634.5 (1997 UBC), which is where I would go.
 
Mark D. Anderson  PE
Anchorage, AK
----- Original Message -----
Sent: Wednesday, August 04, 2004 10:02 AM
Subject: Re: FW: CMU Site Wall using 2001CBC


Molly,

As your wall gets taller and taller it is subject to more and more seismic force amplification therefore using hx = 0 would not be appropriate.  There are two ways to calculate this:

The more technical way would be to calculate Fp at the top where hx = hr then at the bottom where hx =0 (note that Fp minimum will probably govern) then apply as a trapezoidal loading which could be resolved into rectangular and triangular sections.

The simpler way which is generally acceptable would be to calculate Fp at the top and bottom, add them together, divide by two, and apply at  mid-height.

Thomas Hunt, S.E.
ABS Consulting




"Molly Skinner" <MSkinner(--nospam--at)kpff-irvine.com>

08/04/2004 10:44 AM

Please respond to
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FW: CMU Site Wall using 2001CBC






I am designing a free-standing site wall using the (UBC/CBC) Fp equation
of section 1632.2.  I know the value of ap=1.0 and Cp=3.0 per Table
16-O, but what value of hx should be used?  The code states that hx is
the element of component attachment elevation with respect to grade, so
to me, that means that hx should be 0, and as a result the Fp,min
equation will govern.  I've read the blue book, plus looked at many
books, but this situation never sees to be addressed for free-standing
(non-building walls).  Is using hx=0 appropriate?  I had a plan checker
tell me that it isn't and should be using hx = hr.  Just curious what
others are doing.

Molly Skinner, P.E.
Irvine, CA  



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