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RE: Horizontal bracing seismic provisions (AISC or IBC)

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I have seen two approaches to this.  And I believe neither of them are code
supported, but the code is gray here to say the least.

First approach: design the horizontal bracing to remain elastic.  Vertical
lateral force resisting systems are generally assumed to yield before the
horizontal.  So design the horizontal members to force yielding in the
vertical system first.  This concept to me makes sense and seems to be
conservative, especially given wind will control the design of most of these
buildings.

Second approach: design the horizontal bracing the same as the vertical.  I
personally feel this is an absolute minimum.  Logically, the first approach
makes more sense to me, but I can't always force my "opinions" on others.
However, this can increase the number of potential yield points (and
therefore energy dissipation) depending on how the connections are detailed.

Lastly, no matter what direction you go, make sure they have a clear load
path into the bracing.  Out-of-plane anchorage, subdiaphragms, continuous
cross-ties all still apply.  Forces are likely to be small, but they are
still there.

HTH,
Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Rick Burch [mailto:rburch(--nospam--at)conterra.com]
Sent: Wednesday, October 22, 2003 8:31 PM
To: seaint list server
Subject: Horizontal bracing seismic provisions (AISC or IBC)


In steel buildings where there is horizontal steel bracing at the roof
level, instead of a steel deck or concrete slab diaphragm, what seismic
requirements would apply? Say this is a building in IBC Seismic Design
Category D, where the AISC Seismic provisions apply.

There are plenty of design provisions for diaphragms in the IBC, and
plenty of design provisions for the vertical bracing system (whether
bracing or moment frames) in the AISC Seismic Provisions, but I don't
see requirements for horizontal bracing.
I'm wondering about things like restrictions on tension only bracing,
kl/r limits, width/thickness limits, etc.

Any guidance would be appreciated.

Rick Burch
Columbia, SC

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