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RE: OMF in low seismic zones

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Title: RE: OMF in low seismic zones

>[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:

    http://www.engr.psu.edu/ae/steelstuff/seismic.htm

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:

    http://www.aisc.org/documents.asp?mode=docdetail&doc=188

and 1999 Seismic Provisions Supplement No. 1 here:

    http://www.aisc.org/documents.asp?mode=docdetail&doc=153