Minimum unity check / minimum utilization ratio to justify the response modification coefficient R 1

[quasiblu]

Hi All,
I am reading ASCE 7-16 and AISC 341-16.
I am dealing with OCBF and OMF, so I have R=3.25-3.50, say R=3. Anyway this post could be extended to concrete SFRS as well.
In those codes there's no requirement for designing the foundations for seismic forces stronger than those that the superstructure can bring down to its supports.
Namely, if I apply R=3 to the seismic forces on the superstructure, I can go with R=3 for the foundations as well.
But if I overdesign the dissipating members of the superstructure, and I have, say, unity checks UC~0.2-0.3<1.0, I could not have enough dissipation and therefore bring down to the foundations values of the seismic loads greater than the design loads (assessed with R=3).
Note that in practice it is not always possible to calibrate the unity checks to high values for a number of reasons (constraint in the geometry, non-seismic combinations acting and maybe also governing, etc.).
Only in the ASCE 7-16 commentary I find the generic good rule of overdesigning "a bit" the foundations, as they are the least element where one wants to have issues.

Is anybody aware of a minimum unity check / utilization ratio for the dissipating members, to be achieved in a minimum number of members, in order to justify the R per ASCE 7-16 tables?

I know that the effective R could be estimated running a pushover, but I would not go there today with the discussion with my colleagues.

Thanks.

 

[r13]

IMO, you are overthinking on this matter. If you are going to purposely overdesign the supper structure, then you shall start with a lower R, so the design will be coherent.

 

[quasiblu]

Thanks for your reply, yes that may help. Unfortunately I don't decide that, can't do.

 

[Deker]

Is anybody aware of a minimum unity check / utilization ratio for the dissipating members, to be achieved in a minimum number of members, in order to justify the R per ASCE 7-16 tables?

None that I'm aware of since the ASCE 7 R factors are based mostly on judgement and historical performance. See if you can track down the SEAOC Blue Book articles titled "Development of System Factors" and "Seismic Force-Resisting Systems" for a brief overview of how the factors came to be. That said, based on your statement below, it sounds like your concern lies more specifically with the overstrength factor.

But if I overdesign the dissipating members of the superstructure, and I have, say, unity checks UC~0.2-0.3<1.0, I could not have enough dissipation and therefore bring down to the foundations values of the seismic loads greater than the design loads (assessed with R=3). Note that in practice it is not always possible to calibrate the unity checks to high values for a number of reasons (constraint in the geometry, non-seismic combinations acting and maybe also governing, etc.). Only in the ASCE 7-16 commentary I find the generic good rule of overdesigning "a bit" the foundations, as they are the least element where one wants to have issues.

I think it's great that you're thinking about this and would encourage anyone engaged in seismic design to do the same. The graph below taken from one of the Blue Book articles I referenced above illustrates this well. The ASCE 7 R factor is the product of the system ductility factor R[sub]d[/sub] and the system overstrength factor Ω[sub]0[/sub]. For simplicity, let's assume that the ductility factor is constant and depends solely on the detailing and ductility capacity of the lateral system chosen.

As you noted, if the lateral system is significantly overdesigned, any brittle elements in the load path that are designed using a prescriptive overstrength factor may fail prior to allowing the lateral system to reach full yield at a load of V[sub]y[/sub]. So if you are truly concerned about trying to keep these brittle elements from failing prematurely, I would encourage a capacity based approach which would account for the overdesign in your lateral system.

Whether foundations are one of the elements that necessitate overstrength / capacity based protection is another story. Some jurisdictions require it, most don't. If you are practicing in a jurisdiction that doesn't, at least keep an eye on the demand capacity ratios for the different limit states at the foundation and take note of which ones could be classified as ductile or brittle. Also, if you are designing systems with a low R factor of around 3, it's likely that you are not in a high seismic area, in which case this discussion is probably too academic given the low seismic risk.

I know that the effective R could be estimated running a pushover, but I would not go there today with the discussion with my colleagues.

Although you may not want to go there, some great discussion about the variables that impact the the seismic response factors can be found here: Link.

 

[quasiblu]

Thanks a lot!