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OSHPD SCBF's using HSS Braces

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Dear Gerard: I feel that the provisions of Section 2205A.4.1.5.1 of the CBC on SCBF should be applied to the square HSS as well as rectangular sections. The reason is as Professor Goel's tests in Michigan have shown, when HSS sections are filled with concrete, their cyclic performance improves significantly, especially with regard to delaying the fracture of the locally buckled areas of the HSS at the location of the plastic hinges.
In rectangular HSS braces, usually the long side of the rectangle is vertical and since the brace buckles out -of-plane, it is this long side that is subjected to compression and local buckling. If you have a square shape, still the vertical side will be locally buckled but in square shape, the horizontal side is also as long as the vertical side and cannot provide as much restraint to the vertical side to help delay its local buckling. So, it seems to me that if we are worried about pre-mature cyclic local buckling of the rectangular HSS sections as SCBF members, we should be more concerned about square sections (given the same b/t ratio for both)

Another issue is that when we use square HSS instead of rectangular sections, for the same area, the square HSS, will have larger compressive capacity , since out-of-plane KL/r will be smaller for the square section compared to rectangular section with the same area. But, the square section will have smaller tension capacity than the rectangular section since the shear lag factor "U" for square HSS will be greater. Of course from architectural point of view, rectangular section are preferred since they have smaller width and easy to hide inside the walls.
Finally, I wanted to mention an item regarding filling the HSS braces with grout. Last year I had an e-mail from a structural engineer looking for information about performance of the concrete-filled HSS braces where the sides of the HSS section have "bulged-out" and in a way turned the 4-sided section into an 8-sided section.  Apparently this happened because of the hydro-static pressure of the liquid grout placed inside the HSS. I think the final decision in this case was to replace the deformed braces.  So, in specifying the grout fill-in, may be it is prudent to specify that the grout should be placed in sections ( may be two sections) especially if the story is high and the b/t ratio of the HSS is also relatively high. A simple hydro-static analysis can also be done to check this failure mode.

Best wishes.
"Hassan"

Abolhassan Astaneh-Asl, Ph.D., P.E.
Professor and Consultant on Structural Engineering
www.astaneh.net
www.ce.berkeley.edu/~astaneh

==============

From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)gmail.com]
Sent: Wednesday, October 15, 2008 3:26 PM
To: seaint(--nospam--at)seaint.org
Subject: OSHPD SCBF's using HSS Braces

Section 2205A.4.1.5.1 of the CBC modifies AISC 341 for SCBF's using
RECTANGULAR HSS sections. The section requires them to be filled solid with
cement grout. This is for OSHPD only projects.

Question: Do the really mean Rectangular and Square HSS sections or only
Rectangular as stated?

thanks,
-gm


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