special concentric braced frames

[broncosfan]

Hi,

I understand that the brace connections for an scbf need to be designed for the expected strength of the member. At the pier and foundation, the brace gusset plate is attached to the column and base plate. Does the base plate attachment to the pier and footing need to be designed to support the expected strength of the brace? The brace is currently an HSS8.625x0.322. So the expected strength is approximately 439.6kip. That's going to need some big anchor bolts and shear keys.

Also, is it correct that the foundations for an scbf do not need to be designed for the overstrength (omega) load combinations?

Finally, does the metal roof deck diaphragm and the attachment of the metal roof deck diaphragm to the collector beams need to be designed for the overstrength load combinations?

Thanks!!!

 

[Bagman2524]

I would say yes. I don't see much sense in requiring a member to support a certain load if the member's foundation is not capable of supporting the load.

 

[prsconsultant]

Below is a response I had from AISC few year ago:

Shall I use the load combinations (shown below) with amplified earthquake load (Eqs. 4-1 and 4-2 of the 1997 AISC Seismic Provisions for Structural Steel Buildings) for the design of column base plate and anchor rods?
1.2D + 0.5L + 0.2S + Omega0QE
0.9D - Omega0QE

The answer to the question depends on the expected system performance of the structure. In general the System Overstrength Factor, Omega0, is prescribed to assure sufficient design strength to allow selected members to yield. When a Building Code requires design of a connection in accordance with Special Load Combinations that include the System Overstrength Factor, the intent is to assure that the connection is strong enough and stiff enough to allow yielding of the member.
In the case of moment frames, if the building system performance intends column yielding at the base plate, the connection between the column and the base plate should be designed for the System Overstrength Factor.
In the case of braced frames, if the building system performance intends brace yielding at the base plate, the connection between the brace and the base plate should be designed for the System Overstrength Factor.
In all cases, the design of the anchor rods should consider ductile behavior, without using the System Overstrength Factor. In other words, the system performance of all frames will be enhanced by designing anchor rods to yield before they “pull out” of the concrete. Designing anchor rods to yield requires sufficient concrete embedment to preclude concrete “shear cone” failure. The top half of the anchor rod should also have a bond breaker to increase the length of rod that will strain.

 

[blake989]

in the 2005 seismic provisions section 8.5 "somewhat" addresses column base and anchor rod requirements. I have not had a chance to give a good look yet, but I did see it in there.

 

[broncosfan]

AISC seismic C8.5 states, "column bases are required to be designed for the same axial forces as those required for the members and connections framing into them. If the connections of the systems are required to be designed for the amplified seismic loads or loads based on member strengths, the connection to the column base must also be designed for those loads."

I guess this somewhat answers it. Sort of. Is "connection to the column base" referring to the gusset attachment to the base plate. Or is it referring to the anchor bolts. Is "column base" the base plate? Or is "column base" the pier and footing?

Please tell me I don't have to design my piers and footings for the expected strength of my braces That would suck.