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Earthquake design for Importance Level 4 building

Earthquake design for Importance Level 4 building

Earthquake design for Importance Level 4 building

Sorry for the long post, but here goes:

I am looking for some comments on how to address a concrete shear wall building with importance level 4 under seismic, and specifically the AS1170.4 and AS3600 requirement to remain serviceable for immediate use following an importance level 2 event.

I am thinking this would require 2 models / designs and designing for ULS members and drifts for the worse outcome of either (with model 2 expected to govern):

1) Model 1 - IL 4 loads (1500yr return) at the adopted mu/sp = 0.38 (ie a limited ductile shear wall) or possibly even 0.22 if it is worthwhile doing as a ductile shear wall noting the additional detailing requirements.

2) Model 2 - IL2 loads (500yr return) and a mu/sp of TBC?, including review of drifts etc.

I expect Mu=1, but not sure if Sp = 1 as well to be fully elastic, or Sp = 0.77 to align with that used for Model 1? (I note a response to another post in the last few days that suggests Sp/Mu = 1.0 being a IL 4 design Link)

I found a paper on the new Adelaide Hospital (attached) that outlined how it was one of the first buildings to have to address this and it used Mu = 1 and Sp =0.77 but notes that It is envisaged that reinforced concrete elements could develop some minor cracking during the serviceability earthquake, but without significant yielding of reinforcement or crushing of concrete. I’m not immediately sure if this is consistetn with immediate use following an IL=2 event, however it does go to length to say that drifts / displacements and detailing were rigorously checked. It is also a 1.3bn project that would have been reviewed forever so it is hard to ignore as good standard.

Further to above, where I get tripped up in my mind, is it correct to be using ULS limit state design with model 2? Ie the limit state design assumes things are at yield and there is some cracking from resdistribution, but how well does this doesn’t align with an elastic response?

RE: Earthquake design for Importance Level 4 building

Does the AS1170.4 standard not have the SLS2 limit state like NZS1170.5? Maybe go and look at NZS1170.5, it's possibly a little clearer in there what you need to be designing for or at least the philosophy.

Generally in NZ the approach I've taken in the past is make the base shear coeeficient for the ULS and SLS2 the same, this accepts some level of ductility at SLS2. However elements that are required for the ongoing operation of the building, facades, stairs, etc should have sufficient clearances for the drifts associated with these limit states so they are not damaged (staiurs won't be a problem as they will need sufficient clearances for the ULS (and beyond to MCE in NZ at least).

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