OCBF Brace Connection - AISC 97 Seismic Provisions?

[MrStohler]

Thanks in advance, I would appreciate some feedback about interpreting the 1997 AISC Seismic Provisions.

The project in question is covered by IBC 2000 and is in Seismic Design Catagory D. As a result the 1997 AISC Seismic Provisions also apply. The structure is to be designed as a ordinary concentrically braced frame (this question would also aplly to a SCBF). Most of the vertical bracing will be wide flange sections turned so the flange surfaces are vertical, which are connected to a gusset with 4 "claw" angles at thw WF's web (long spans make double angle bracing less feasible and welded HSS bracing is not an option).

AISC-97 SP Section 14.3c indicates that the connection needs to provide flexural strength of at least 1.1*Ry*Mp in the "direction" of the brace buckling. I'm OK with this concept, but it creates alot of work (our firm designs all connections, this is not left to a detailer). It could lead to a very complicated analysis if the buckling is out of plane wrt the gusset, but the design configuration could address this.

There is an exception to meeting the Flexural Strength requirements:
"Exception: Bracing connections that meet the requirements in Section 14.3b., that can accommodate the inelastic rotations associated with brace post-buckling deformations, and that have a design strength that is at least equal to the nominal compressive strength Fcr*Ag of the brace are permitted."

Finally, the question: The first and last items of the exception can be addressed easliy but what would be an appropriate method to evaluate whether the connection can accommodate the inelastic rotations associated with brace post-buckling deformations?

Thanks again.

 

[Taro]

See the commentary on page 84 for a discussion of gusset plate ductility detailing. Also see the applicable Supplement(s). The Astaneh Steel Tips article is a good reference (

http://www.steeltips.org).

 

[MrStohler]

Thanks for your response Taro, but I have a follow up question.

The AISC details for an SCBF show that a distance of 2t is required between the end of the brace and a line perpendicular to the braceat the corner of the gusset(this is to develop a "hinge" in the gusset)

Question: If angles or plates are used to connect the brace to the gusset (the AISC detail shows a pipe or tube directly connected to the gusset) do these connecting elements also need to clear the "line of bending"?

Is anyone aware of prototypical details for different types of braces (the AISC detail is good for tube welded to gusset or it can be adapted for doubl angles bolted to a gusset, but that leaves alot of unanswered questions).

 

[Taro]

The angles or plates must also clear the yield line by at least 2t so that the rotation associated with brace buckling does not adversely affect the gusset connection. However, be aware that this may cause the gusset plate dimensions to get very large which can be a problem if aesthetics are an issue. Sometimes you can end up with very large gussets at each end of a short brace and it looks rather silly.