• To: <seaint(--nospam--at)seaint.org>
• From: "Mike Valley" <mtv(--nospam--at)skilling.com>
• Date: Tue, 19 Jun 2001 14:29:14 -0700
```It seems to me that calculation of the seismic base shear is the least
step further into the process.

The steps for a simple seismic design are:
1. Determine the seismic hazard at the site (by zone, map, etc.).
2. Calculate the building weight.
3. Calculate the seismic base shear.
4. Distribute the seismic base shear (as seismic forces).
...

The steps for a simple wind design are:
1. Determine the wind design req'ts (wind speed, exposure, etc.).
2. Determine the building characteristics (height, area, etc.).
3. Calculate the wind base shear.
4. Distribute the wind loads to the main wind force resisting system.
...

At step 3 of each process a comparison may be made.  If the wind base
shear is noticably larger, wind will control the design forces, but
seismic detailing commensurate with the assumed R value (and
associated prescriptive rules) is still required.  If the base shears
are comparable, seismic loads may still control the upper stories.  If
the seismic base shear is larger, seismic design will control the main
wind force resisting system, but more severe wind loads on smaller
pieces of the structure and cladding will still need to be considered.

The calculations must proceed to at least step 3 in order to make a
meaningful comparison (unless the comparison can be made "by
inspection" as in a tall Hong Kong building [all wind] or a short,
heavy L.A. building [all seismic]).

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Tuesday, June 19, 2001 12:20 PM
To: SeaInt Listserver (E-mail)

Somewhat related to the discussion of submittals of calculations: IBC
2000,
Section 1603 lists a number of design loads and related items which
must be
shown on the construction documents. For earthquake design data, the
"design
base shear" is required to be shown "regardless of whether seismic
govern the lateral design of the building".

Isn't this a bit of overkill for low seismic regions? In some areas,
it can
be shown that wind load will govern over seismic loads with a brief
comparison of loads but without a detailed analysis of seismic base
shear.
This requirement forces the engineer to perform more detailed seismic
design
loads even when they do not govern design. Is this really a practical
requirement?

William C. Sherman, PE
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

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