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RE: 97 UBC Lateral force on elements of structures

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Bill's explanation is pretty close to the intent of the authors.

Fp should be calculated at each attachment point, i.e., top and bottom of wall.
The wall should be designed for the average of the Fp at the attachment points.
This results in the same answer as Bill's explanation, unless one of the
attachment points hit the maximum (Eq 32-1) or the minimum (Eq. 32-23).

Eq. 32-1 serves as both the maximum value and a simplified formulation.

Eq. 32-2 is the "exact" equation to use if you have knowledge of where the item
is in the building.

Eq. 32-3 serves as the minimum value.

Rick Drake, SE
Fluor Daniel, Aliso Viejo


"Bill Allen" <Bill(--nospam--at)> on 10/19/99 07:17:52 AM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)
cc:    (bcc: Rick Drake/AV/FD/FluorCorp)

Subject:  RE: 97 UBC Lateral force on elements of structures

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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