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Re: OCBF & SCBF warnings..

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Howdy Ben.

Rafael Sabelli at Dasse is the Braced Frame guy and I think he did some Steel Tips that came out this year with your old Professor Astaneh. I only skimmed it, but I believe it focused on the 2T issue on the gusset plates, but there may be some more info for ya there.

I've seen the buckling restrained braced frame on a few buildings going up around here in a few places and in the south bay (on hospitals) but still predominately EBF and CBF's.

Looks like core tube shearwall buidlings are in the crystal ball...

Anyways, hope all is well.

Your former drafter from "Industrial Wall & Roof"

On 3/29/07, Benjamin Maxwell < enginerd666(--nospam--at)> wrote:
Has anyone on the List seen or discussed the Steel Concentrically Braced Frames Preliminary Recommendations article published in the March 2007 SEAONC News by the Seismology and Structural Standards Committee? 
Big stuff...I think the results have been in for a while, but the committee has only recently decided to move forward with their early recommendations (perhaps someone with a deeper inside knowledge would comment).
"Recent test results on braces used in steel concentrically braced frames (CBF) indicate that many commonly used sections and brace configurations do not meet seismic performance expectations.  Specific parameters that [sic] were shown to affect the ductility of braces, notably the net-section, section type, width-thickness ratio of the cross section, and member slenderness".
"...recent experimental test results indicate that bracing members that comply with current width-thickness limits fractured at drifts of 2% to 3% at design level forces."
Square and rectangular HSS shapes exhibited low cycle, fatigue-induced fracture. Oops. Not good.  It looks like the committee is going to study possible modifications of b/t ratios and slenderness requirements, but also recommends using shapes less susceptible to local buckling such as round HSS and wide flange shapes.  Apparently grout-filled tubes performed better as well.
Their recommendations:
- Reinforce the net section zone at the ends of the brace (another fracture-susceptible area)
- Use braces with low width-thickness ratios and moderate slenderness (too stubby and too slender can lead to poor behavior in their own ways)
- Use more stable cross sections (round HSS and wide flange, or grout-filled HSS round, square, or rectangular)
The following link is cited in the article, but I could not get it to work.
Maybe it's time to develop some wood moment connections?

Benjamin H. Maxwell, S.E.

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