SCBF brace conn. design, AISC Seismic Provisions?

[MrStohler]

Ref: AISC Seismic Provisions Section 13.3.a

I understand that one option is to design the brace connection for the full tensile capacity of the brace, but the second choice of designing for "the maximum force, indicated by analysis that can be transferred to the brace by the system" is where the question arises.

We are currently designing a structure for an industrial facility that will use a mat foundation on piles (mat and piles to be designed by another firm). Seismic shear force will be transferred to the structure by shear lugs connected to the base plates and uplift will be resisted by the anchor rods.

Is it reasonable to conclude that the maximum force that can be delivered to the system is the combined shear strengths of the shear lugs (for shear) or, for tension, the lesser of the anchor pull-out strength or anchor yield strength?

It seems that were the shear lugs to fail the ability to transfer shear to the structure would be substantially reduced. This might also be the case were the anchors to yield due to overtuning.

These potential failure modes would allow us to evaluate the connection requirements for a load that might be less than the full tensile capacity of the brace and still satisfy AISC.

Thanks

 

[VoyageofDiscovery]

Hi,

Who's designing the shear lugs and anchor rods?

VOD

 

[VoyageofDiscovery]

If your lugs are finally overdesigned, then your structural loads would be greater.

Yes, I agree you would be able to use the actual load versus the capacity (compression/tension) of the member.

Regards

VOD

 

[MrStohler]

VOD: I am designing shear lugs, anchor dia. and anchor projection. Anchor embedment and foundation by others.

The following is ASIC's response to the question:


I believe the following comments from Jim Malley, Chairman of AISC Seismic Task Committee (TC9), will help answer your question (refer to item #2):

AISC TC9 recognizes that the means of determining the maximum force that can be delivered has been left unspecific. There are a number of means to determine this force, which all may be appropriate in different circumstances. They include:

1) Performing a pushover analysis and determining the load on the connections at the maximum capacity of the frame.

2) Determining how much force can be resisted before uplift of a shallow foundation (spread footing). Note that the foundation design forces are not required to resist more than the code base shear level. This is not typically applied for a deep foundation since the determination of when uplift will occur is not easy to determine with any accuracy.

3) Performing a suite of inelastic time history analyses and enveloping the connection demands.

Some would argue that applying Omega sub zero to the design forces from the code base shear is enough to satisfy this provision in all cases. This was allowed in the 1992 Seismic Provisions (and other seismic provisions), but TC9 removed it from the 1997 Provisions because of the concern that a global overstrength factor like the Omega sub zero factor would not be appropriate to use on a local critical demand like the connections in a braced frame. Individual connections may see forces much higher than this in order for the frame to achieve it's maximum overall capacity. This type of approach may be more appropriate for systems with very limited ductility expected, such as moment connections in metal buildings that will likely buckle elastically well before reaching the Mp of the members, thereby limiting the load to be delivered to the connections. This would generally not be the case for SCBF's however, since these frames are configured and designed to buckle the compression braces in the inelastic range, and yield other braces in tension.

Calculating the maximum connection force by one of the three methods noted above is not a common practice on design projects. In some cases, such an approach could result in smaller connection demands. But, from a conceptual basis, since the character of the ground motions is not known to any great extent, it is unrealistic to expect that such forces can be accurately calculated. All three approaches rely on an assumption of the distribution of forces which may not match reality (approach #3 above probably being the best estimate, but also the most calculation intensive). TC9 believes that in most cases providing the connection with a capacity large enough to yield the member is needed because of the large inelastic demands placed on a structure by a major earthquake.

Regards,

Sergio Zoruba, Ph.D.
Senior Engineer
American Institute of Steel Construction, Inc.
866.ASK.AISC

 

[VoyageofDiscovery]

So I guess you're gonna do an inelastic time history then.

Check if you can do a plastic analysis utilizing ductile frames in this situation.

Regards

VOD