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SCBF Compressible Material?

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Dear Lin:
You may find material in a recent Steel TIPs report that I co-authored with Mike Cochran and Rafel Sabelli somewhat useful. The TIPs is at http://www.steeltips.org/steeltips/tip_details.php?id=96.
Another Steel TIPS that I am completing on Base Plates for CBFs, to be released in July, has very specific information on your question. A summary of that information is given below:
If the "2t" plastic hinge length of the gusset plate in SCBFs ends up being encased in the concrete of the floor slab, which is normally the case for gusset plates connected to the top flange and base plate, there needs to be a gap between the gusset plate and the concrete slab on both sides of the gusset plate to allow the gusset to bend out of plane and accommodate end rotation of out-of-plane buckling brace. The width of the gap depends on the lateral deformation of the brace which in turn depends on the axial shortening of the brace. The axial shortening of course is directly related to the story drift by geometry. Putting all of this together and going through the derivations, the equation that can be used to establish what should be the width of the gap:

g= (Teta)( Lg), where g is the width of the gap on each side, Teta is the story drift and Lg is the length of gusset embedded in the concrete measured along the axis of bracing. I have suggested a drift value of 2% for SCBF's. The 2% used here is the expected story drift for SCBFs similar to what 3% is for SMFs. Example of application: For a gusset with Lg=12", the gap will be g= (2%)(12)=2.4 inches. As for what material can be used in the gap, after calculating the needed gap, you can use any material but you have to make sure that that material deform at least an amount equal to "g" the calculated gap when subjected to compression of 500 psi. The value of 100 psi is arbitrarily selected to be low enough not to cause crushing of concrete behind the fill material. Obviously if material is soft, you would need just to add small amount to the gap size g to accommodate the thickness of the material after it has been compressed. if material is relatively stronger, if you have a feel how much it will be squeezed into, you can add that to the amount.

Please consider the above text: Copyright 2007 by Abolhassan Astaneh-Asl until I get it into the Steel TIPS in progress: " Behavior and Design of Base Plates in Concentrically Braced Frames" By A. Astaneh;Asl, July 2007.

Finally, following Albert Einstein's saying that : "everything should be made simple but not simpler", which is my motto in my professional and academic life,  I would just use 3.5-4 inches of gap on each side of the gusset plate, and fill it with  relatively soft fill material, anything with less than f'c= 200-300 psi , and forget about end rotations of half sinusoidal curves while buckling out of plane in an inclined position being a member of special concentrically braced frame with both ends partially embedded in the concrete slab and forming plastic hinges in "2t" space!  I can't spend this much time on design of a gusset. I have a building to design and finish the design yesterday!

Best wishes in your design and hope this helps.

Abolhassan Astaneh-Asl, Ph.D., P.E.
Contact info at: http://www.ce.berkeley.edu/~astaneh

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

From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
Subject: Re: SCBF Compressible Material?
To: seaint(--nospam--at)seaint.org

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J. Lin,

       I can not answer your question directly but I have seen =
styrofoam used for similar purposes where it does not work as intended.  =
I have seen it used under grade beams supported on piles to prevent =
uplift due to frost action or swelling soil where it does not perform as =
expected.  The structurally weakest (white) styrofoam type of insulation =
generally resists about 10 p.s.i (1440 p.s.f.) before significant =
yielding takes place; and stronger (structurally) grades, such as =
Styrofoam HI, can resist from four to eight times as much or more.

       These are very small stress levels compared to structural =
building materials; but if you have large enough contact (and here, I am =
not familiar with your geometry) areas the resulting forces may not be =
irrelevant.

Regards,

H. Daryl Richardson
 ----- Original Message -----=20
 From: SE3949=20
 To: seaint(--nospam--at)seaint.org=20
 Sent: Thursday, May 10, 2007 10:58 AM
 Subject: SCBF Compressible Material?


 For Steel Special Concentrically Braced Frame (SCBF), I always saw the =
Contractor using Styrofoams at both sides of the embedded gusset plate =
as the compressible material to eliminate the lateral restraint.=20

 Recently there is a project that the Contractor using the "JOINT =
FILLER" (Fibre Expansion Joint by W. R. Meadows, Inc.; similar to the =
one you saw on your concrete driveway as the concrete divider but =
harder) instead of the Styrofoam. The Contractor and the Design =
professionals claimed that this FILLER were used for MANY projects in =
the past several years in SCBF design/construction.

 Can you let me know your opinion regarding those rigid-like filler for =
SCBF?=20

 My opinion: That filler will provide excessive lateral restraint and =
will prevent the brace to buckle in pin-pin shape. If the brace did not =
buckle at the designated force level (lost the function as the FUSE =
system), then other portion of the frames (columns, beams, joints, =
connections) might fail before the actual brace buckling. The designer's =
nightmare of sudden building collapse might occur since the brace =
wouldn't buckle (dispatchs the energy and also acts as the warning sign) =
at that designated seismic force.

 J. Lin, SE
 Los Angeles




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