Minimum Seismic Design Category

[CURVEB]

I'm in a jurisdiction that uses IBC 2015 and requires a minimum seismic design category of B. I'm curious how other engineers are implementing minimum requirements.

I'm currently working on a project that would be SDC A based on the soil site class and acceleration parameters. Do you increase the Ss and S1 factors so that you have the minimum Sds/Sd1 to correspond with Design Category B? Or do you keep Ss and S1 and increase Fa/Fv? Either of these options would increase your Cs factor, but I'm not sure which ones are appropriate to change. S1 is used to calculate the lower-bound limit on Cs in some cases, so it seems like that variable would need to be adjusted as well.

The other option would be to keep all of the parameters the same, use Cs for the SDC A calculation, then simply make sure you are meeting all of the detailing requirements for SDC B.

 

[TehMightyEngineer]

The other option would be to keep all of the parameters the same, use Cs for the SDC A calculation, then simply make sure you are meeting all of the detailing requirements for SDC B.

I would do the above unless the wording of the rules of your jurisdiction require specifically you also increase the associated factors (which it sounds like it does not).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

Not sure how much the IBC differs from AASHTO, but in the AASHTO Bridge design and seismic guide specs, the site class is based on an average of the material over a 100ft depth. Are you sure you have better information about the proper site classification than the local jurisdiction?

Even if you are confident in your site classification, in order to avoid the potential for compliance issues, the prudent thing would be to adjust your site classification to match the assumptions used by the jurisdiction to arrive at Cat. B and proceed with the corresponding values and requirements.

 

[CURVEB]

@HotRod10 The Site Classification is coming from the geotechnical engineer on this project and is in compliance with the jurisdiction's requirements for minimum site classification (less than 15' of overburden on top of bedrock).

 

[BridgeSmith]

If the jurisdiction has criteria for others to set a site classification, I'm somewhat confused by their dictating an SDC. Are they using a higher acceleration coefficient? Is the Cat. B perhaps just the default if a subsurface investigation isn't performed?

 

[MotorCity]

I would press your jurisdiction for more information. In the jurisdictions I have worked in where minimum seismic parameters were mandated, they provided ALL of the parameters, not just the seismic design category. Seems like you are only getting part of the picture.

 

[Shu Jiang PEng SE]

Seismic design category is determined by seismic and soil date of the site.

seismic design category affects: reponse modification coefficient R, overstrength factor and deflection amplification factor. it also may affect structure maximum height. Refer Table 12.2-1 of ASCE 7 for more details.

 

[RobertHale]

My gut tells me, they are just trying to make sure you perform an ELFA per chapter 12 of ASCE 7 on the structure instead of only the stability design requirements of Section 1.4. I would use the spectra accelerations and the site coefficients that lead you to an SDC A but use Chapter 12 to get your seismic loads.

Robert Hale, PE

 

[CURVEB]

Thanks All. I will check with the jurisdiction, assuming I can get in touch with someone who understands the issue.

 

[CURVEB]

So the response from the AHJ is that we should increase the site-specific parameters (either SS & S1, or Fv & Fa) to the point where the minimum forces required by SDC B are used in the design. They noted that changing the site class from C to D could actually increase your forces BEYOND the minimum required for SDC B, which is an interesting point.

Any thoughts as to which set of site-specific parameters should be modified?

Mathematically, I believe that either modifies Sds and Sd1 which ultimately affect your Cs factor, however, S1 does come into play in one of the chapter 12 equations (that I'm aware of). Although I think that equation would only apply in higher seismic areas, for which a minimum requirement of SDC B would not be relevant (see eqn. 12.8-6 of ASCE7-05).

 

[Shu Jiang PEng SE]

Changing seismic design category form "A" to "B" may cause many affect many things. For example, lateral force calculation method, seismic force resisting system selection, response modification factor, and most importantly, applicable code regarding detailing requirements

 

[azcats]

Is this requirement from the AHJ going to cause seismic loads to control over wind in any case? I'm not a fan of arbitrary minimums in general. But I really get annoyed when they start to control designs and add significant cost to structure.

 

[CURVEB]

@Shu Jiang: Absolutely, but my question is specific to the magnitude of forces which must be increased, and how one should go about doing that.

@azcats: it might, and I agree - it seems arbitrary. I do not, however, think it will be a significant change in force in this case. My calculations have a precast parking garage going from Cs = 0.05 to 0.056.

 

[Shu Jiang PEng SE]

Equivalent lateral force method is applicable. Fa would be jump from 1.2 for soil class “c” to 1.6 for soil class “d”. That means the lateral force could be (1.6-1.2)/1.2=33% increase. It is hard to say exactly without whole picture of the structure in mind.

 

[bones206]

I would just scale up the 2/3 factor that converts MCE to design level earthquake until S_DS or S_D1 is reaches SDC B. In that way you are just simply increasing the overall factor of safety.

 

[EDub24]

I would just increase S1 to whatever it takes to get to the minimum Sd1 which will give you a SDC B and keep the Sds the same. Then I would proceed as normal and design per Ch. 12. There are 2 tables to determine SDC and you have to take the worst one so going that route still meets their 'minimum' requirements. Typically Sds controls the seismic load anyways unless you have a long-period structure where the code allows you to cap the Cs variable based on Sd1.

 

[JLNJ]

If I had not read this thread, I would not think to increase the parameters regarding the seismicity of the site.

I would simply use the requirements in the various codes for SDC "B" rather than "A". Mostly this would have to do with detailing, minimum connection forces, lateral force continuity, etc.

If, for example, SDC "A" permitted unreinforced masonry but SDC "B" required some minimal reinforcement, I'd forgo the use of unreinforced masonry and use the minimum reinforcement provisions for SDC "B".

 

[bones206]

I guess it’s up to the local AHJ to clarify if the requirement is intended to be a minimum design force or minimum detailing requirement or both.

 

[CURVEB]

Yes - in this case the AHJ's intention was to increase the forces, not just the detailing requirements. I guess it goes to show you that a call or e-mail to the building department is usually a good idea to get clarification on these kinds of things.