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Re: Steel Building Bracing

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Drift is a major concern with prefabricated metal buildings.  Drift
limits must be specified by the designer.  The design of prefabricated
metal buildings is generally driven from wind load as opposed to
seismic.  Consider that the trib. dead load of a typical prefabricated
metal building is in the range of 3 to 4 psf.

The MBMA states that some owners become concerned when the lights start
swaying under relatively light winds.  I conducted an informal inquiry
recently and found some manufacturers design their buildings to deflect
h/30 in a 10 year wind (that's a 30 ft tall building deflecting 1 ft).
The increase in cost is generally small to stiffen the frames. Guidance
can be found in the AISC Design Guide #3 for wind drift.  Design Guide
#3 can also serve as the basis for probability for developing seismic
drift limits.

It is interesting to keep in mind that Butler also manufactures mill
buildings containing huge bridge cranes.  Lateral drift and deflections
are crucial to operation.  The system used within the limits of the code
is sound if the designer selects drift limits and designs substructures
to accommodate the deformations.

Harold Sprague
Black & Veatch


 ----------
From: Joe McCormick
To: seaoc(--nospam--at)seaoc.org
Subject: Re: Steel Building Bracing
Date: Thursday, October 30, 1997 1:12AM

I would be curious how well such structures do with regard to lateral
drift.  That's not to say that "Butler" type buildings are not of high
quality, but they have appropriate uses - an exception in the code
allows many
such buildings to waive drift limits if safety can be demonstrated by
calculation (94 UBC 1628.8.2).
The last metal building system I looked at had a story drift 3 times
larger
than 0.005*height which might not be very comfortable for many custom
home
owners!

Respectfully,
Joe McCormick

At 03:05 PM 10/29/97 -0800, you wrote:
>Section 2211.8.5 is limited to one and two story buildings.
>
>A while back I passed on a custom home where the architect was adamant
>about using a seismic resisting system using a "butler" type building
>company to "design"-build the frames with tension only bracing in one
>direction and bolted moment frames in the other,  based on lateral loads
>established by me.
>
>The project got built using another engineer and as expected the seismic
>system was extremely inexpensive, half the cost of ordinary frames. Now the
>architect wants to use this system on just about every job. Does anyone
>have any opinions on the use of this system for custom, high ceiling, lots
>of glass, near fault, two story residences?
>Regards,
>
>Jeff Smith. S.E.
>phone: (415) 543-8651
>fax: (415) 543-8679
>email: smthengr(--nospam--at)sirius.com
>
>Smith Engineering
>27 South Park
>San Francisco, CA 94107