AISC-05 Seismic Provisions 1

[Scullysguy]

In designing SCBF and OCBF, both sections talk about designing the brace connections for the lesser of (2) loads. (1) the expected yield strength of the brace "Tu" and (2) the maximum load effect "Pu" that can be delivered to the brace by the system" (see sections 13.3a and 14.4). What is meant by "maximum load effect"?
In the "seismic design manual" example 3.4 (OCBF) on page 3-13 they calc. "Tu" and then calc. the "max. load effect, Pu" based on the, I assume, static lateral analysis of the building, and then say to use tth "Pu" to design connections. But then in example 3.6 (SCBF) on page 3-53 they calc. "Tu" and then just use that. Why not the "Pu"?
Under what conditions can you design for the "max. load effect"?

 

[FranzR]

Scully,

For the SCBF case, the max load that can be delivered to the system is either the shear capacity of the diaphragm, or a load that creates overturning of the footing. Remember that this "out" only applies to the tension force in the brace and not compression forces. In SCBF, the connections will always have to be designed for the expected axial compressive strength of the braces. As far as OCBF's are concerned, the connections are only required to be designed for omega level seismic forces. These forces are generally considerably less than forces described in the SCBF case.

 

[sandman21]

"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"

 

[Scullysguy]

The second part of this is related to the 2002 Seismic Provisions. If I am reading ASCI 341-02 correctly, for an OCBF system, the brace connections (brace to gusset, gusset to beam, gusset to column) must be designed for the "expected tensile capacity of the brace member"....and with the 2002 provisions there are no "outs". Is this a correct interpretation?