Seismic Drift Limits for Crane Buildings

[thaidavid40]

I have a project specifications review task for which I have a question regarding seismic drift limits for mill buildings with overhead bridge cranes. My associate insists that h/400 should be applied for the seismic drift limits, just as it would be for wind drift (measured at the crane rail elevation, per AIST TR-13). I say that an earthquake is an ultimate strength event, and this is too strict a limit. I believe that something more like h/60 is more appropriate. One isn't concerned with keeping a straight-tracking rail line during an earthquake. You only want to ensure that in addition to the building not collapsing, that the crane doesn't fall off the rails. This may take a lateral restraint or keeper to accomplish, but you still don't want to apply a limit that is too strict. What is the current thinking on this subject for mill buildings with heavy crane loads (100 tons, or more)?

 

[KootK]

I'm not an expert on these kinds of building but your logic sounds reasonable to me. It's scary to think of the P-delta effects in play when columns supporting a 100T load are shifting around to the tune of 4". Although I suppose the probability of the max carried load being in place at the same time as the design level earthquake is pretty low.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[thaidavid40]

Thanks for the concurrence. The P-delta effects for this kind of drift are considerable, but that is usually true for all seismic displacements. And as you say, the probability of a rated load during a seismic event is likely low (so low that the load is not usually considered in the seismic mass load cases). Besides, the mass acts as an un-tuned damper anyway. With separate crane columns under the runways, tied back to the building columns, and both anchored with some fixity at the base, that is about all I can do to lessen the moment demands at the column-truss connections.