ASCE 7 SDC Classification

[Althalus]

In seismic design, we use Tables 11.6-1 and 11.6-2 to categorize SDC. But I just noticed a quirk I hadn't noticed before. And it generated a question.

Sds = 0.32 (SDC B)
Sd1 = 0.20 (SDC C)

Normally, I'd just pick the higher classification and go with that. But I just noticed that when Calculating Cs, the equation that controls is 12.8-2 which is determined by Sds, not Sd1 (which determines Cs in eq 12.8-2&3).

If the controlling load is based on Sds, it would seem reasonable to consider this to still be in SDC B rather than C.
We still have the statement:

Each building and structure shall be assigned to the more severe (SDC) in accorance with Table 11.6-1 or 11.6-2

But the remainder of the sentence is interesting.

"Irrespective of the fundamental period of vibration of the structure, T."

When the Sds is low enough that T doesn't even enter into the consideration, then it is already irrespective of T. It is just flat out in SDC B. Note that Eq 12.8-2 does not include T.


What say you?

 

[phamENG]

I welcome a correction from those more experienced in seismic design than I am, but here's my thought:

No, you need to pick the worst case design category. The reason being is that the design category and the seismic load are not directly related. The seismic design category is used to define minimum detailing requirements and prescriptive requirements. It also limits the permissible LFRS choices you have. The short spectrum response is more representative of the peak inertial loading the structure will experience (which is why the loads are based on that), but the 1 second response includes more of the response spectrum and can define more stringent ductility requirements.

 

[MotorCity]

There are a number of criteria to determine the SDC. Some of the criteria involve T, some involve Sds, some involve Sd1.
ASCE 7 is saying that any of the various parameters may control, depending on the particular site.
It is up to you to determine which of the parameters governs when determining the SDC. Your first quote is pretty clear that you should select the most severe SDC.

 

[Althalus]

but the 1 second response includes more of the response spectrum and can define more stringent ductility requirements.

That is interesting. I was not aware of that.

Well, that kinda kills it.

Thanks, I appreciate the enlightenment.

 

[HTURKAK]

If you check the formulas (11.4-1) and (11.4-2), SDS and SD1 are related to the site class. Your case , implies site class D and risk category is I,II or III..


It is true that , the calculation of Cs is based on Sds , But, the upper limit of Cs (computed in accordance with Eq. (12.8-2) ) is calculated with (12.8-3) and (12.8-4) which are based on Sd1.