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RE: 97 UBC Lateral force on elements of structures

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Hi, Sam. Must be new to the list. Welcome.

This particular section of the code is soooo confusing and unclear, I sure
wish there would be a 1997 UBC FAQ on the 'Net somewhere. Oh well.

Anway, based on the Seismic Design Manual, it is acceptable to design the
wall section based on an average height, halfway between bottom and top, so
the answer would be (2), hx=0.5 * the wall height. Of course, prior to the
publication of the design manual, there have been many, many interpretations
but it is my understanding that the above best reflects the intent of the
author(s) whom I believe are Bob Bachman and Rick Drake.

If I am incorrect in anything I have stated above, please feel free to
correct me.


Bill Allen, S.E.
Laguna Niguel, CA

||-----Original Message-----
||From: Sam young [mailto:ag6w10(--nospam--at)]
||Sent: Monday, October 18, 1999 11:55 PM
||To: seaint(--nospam--at)
||Subject: 97 UBC Lateral force on elements of structures
||Section 1632.2 of 97 UBC gives two formulas for calculating the total
||lateral force --- (32-1)  and alternatively, (32-2). Using the second
||formula we need to determine hx, which by definition is the element or
||component attachment elevation w/ respect to grade (hx>=0). A recent
||discussion in our office yielded three different values for hx when
||designing an exterior masonry wall:
||1. hx = 0
||2. hx = 0.5 * wall height
||3. hx = wall height.
||notice that ap/Rp per table 16-o is 1/3 in this case, differences betw
||32-1 and 32-2 are:
||1. 4.0 vs 1/3
||2. 4.0 vs 5/6
||3. 4.0 vs 4/3
||since code also states Fp>= 0.7 Ca Ip Wp, it seems that
||choice 1 is out,
||and #3 is the most conservative.
||Any discussions is highly appreciated.
||Sam Young
||CBH Consulting Engineers
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