Seismic Design - General Questions

[cancmm]

I don't have much background with seismic design and am attempting to begin the learning process through some reading. Will also be taking a graduate course in the fall, but am trying to get a head start. Could someone please comment/respond to my questions below?

1) I've read that buildings are designed with the strong-column - weak beam approach and bridges are the opposite. Assume this is for ease of repair? If so, are bridges more susceptible to collapse?

2) For a plastic hinge design, is it simply a detailing matter (per ACI, etc.)? Or, is an actual plastic structural analysis/design needed at that location for the member? How is the plastic capacity of a concrete section calculated?

3) What would a typical plastic hinge detail look like in concrete beam or column? More confining steel, less longitudinal?

4) Have any US codes adopted displacement-based design?

 

[JoshPlumSE]

1) I wouldn't say that SC/WB concept is for ease of repair, but rather done to ensure a greater level of frame ductility. In bridges, it is probably not desirable to have a PT box girder to hinge. Therefore, the column bents are designed to provide all the ductility.

2) Generally handled through structural detailing.

3) You'll have to go through the literature to see the various detailing requirements.

4) I imagine displacement based design is more popular with bridges as you get some issues with short / squat columns. But, I'm not really a bridge guy.

In commercial building design, I would imagine that a push over analysis would qualify as a displacement based approach. These type of analysis is usually done in retrofit situations rather than new construction.

 

[Lion06]

Josh,
It looks like the SC/WB really only applies for SMF. Is that a fair statement? That's the only section in AISC 341 where I see the requirement that the column moment capacity be greater than the beam moment capacity.

 

[frv]

The way I read ACI, I think it applies to concrete as well, although it's not called that.

 

[JoshPlumSE]

In the current versions of the steel code the SC/WB requirements seem to be limited to the SMF frames. It's possible that some of the pre-qualified connections (per AISC 358) might require it for IMF's as well. I don't think it does, but I didn't specifically check before writing this post.

Keep in mind that (for steel), this is more than just requiring that the moment capacity of the column is greater than the beam. It involves amplifying the beam moments to account for strain hardening and probable yield. Whereas the column moments don't increased by these values... in fact, they even get reduced due the to presence of axial force.

Therefore, this is actually a much more stringent requirement than it first appears. Perhaps that is why it is only required for the SMF's.

 

[cancmm]

Thanks for the responses guys.

I'm more on the bridge/bridge-related side of things so I'm not up to speed with the SMF/IMF/OMF requirements.

I have two follow-up questions:

5) How is the plastic capacity of a concrete or steel section determined? I've read that a nominal 1.25 factor is applied to the elastic capacity. Is that always the case?

6) In structural modeling (say in SAP2000 or STAAD), how is plastic displacement/deformation accounted for? The elastic modulus theory goes out the window in this range so do the programs account for this internally if a non-linear analysis is performed?

 

[sandman21]

AISC 341-05 for the code adopted equations.

FEMA 358 is what AISC 341 is based on.

FEMA 353 goes over construction.

Steel moment frames have been tested to ensure they perform the way they should. That’s why you need to test any non-prequalified connection configuration.

Those code can be found here for free

http://www.aisc.org/content.aspx?id=2886