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RE: OMF in low seismic zones

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Thanks for the info.

I actually have the 1997 Seismic Provisions and the 1999 Supplement, but
since Michigan is basically seismically dead (typically, Av & Aa < 0.05 or
at 0.05) I just have bothered to force some time to read them...yet.  Most
of the job my company does is in Michigan, thus wind generally governs
(except for heavy concrete structures which are rare).  Also, I was just
plain lazy and new that you would have the answer! <grin>


Scott Maxwell

On Mon, 19 Jun 2000, Charlie Carter wrote:

> >[In] a seminar by AISC called "Lateral
> >Framing Systems East of the Rockies"....
> >there are sketches of connections for OMF
> >...web connection [is] a welded shear tab
> >(both to the colun and beam) w/
> >erection bolts.  This seems to imply that
> >the web connection can no longer
> >be bolted.  Is this true?
> There is  a lot of confusion as to what requirements apply where. That
> seminar is aimed at straightening out some of the confusion. Some background
> information on the philosophical differences between wind, low-seismic and
> high-seismic design can be found here:
> At the risk of being a bit too simplistic, it really all gets back to what
> value you use for the seismic response modification factor R. If the bulding
> code permits and you choose to take R as 3 or less, you can design according
> to the AISC Specification for Structural Steel Buildings. As soon as you
> take R greater than 3, though, you'll have to meet the added requirements in
> the AISC Seismic Provisions, even if wind forces control the design.
> For an ordinary moment frame or "OMF" (2 percent interstory drift
> capability, minimum), R is identified in the 1997 NEHRP Provisions (and the
> IBC 2000) as 4. So you have to use a moment connection detail that can
> achieve at least 1 percent plastic rotation (the general equivalent of 2
> percent interstory drift with the elastic portion taken as the other one
> percent). As you'll find explained in the Commentary to the AISC Seismic
> Provisions, the welded shear tab detail you describe is one of the
> connections that has been tested and shown to achieve this level of
> performance.
> >Is the only approved (pre-tested) connection
> >detail to weld the shear tab?  Or is it
> >permited to be bolted to the beam
> >web (using slip critical bolts)?
> FEMA/SAC will soon publish their guidelines documents that will give you
> other such details that can also be used, including a few, I believe, with
> bolted webs.
> Hope this helps.
> Charlie
> P.S. You can download the 1997 AISC Seismic Provisions here:
> and 1999 Seismic Provisions Supplement No. 1 here: