Safe Shutdown Earthquake

[rgerk]

I'm working on a prefabricated room for a nuclear facility in a relatively low seismic area in the lower 48. At least per ASCE 7-05. The specs indicated using a Safe Shutdown Earthquake loads with spectral accelerations as follows:
Horizontal = 3.5154g, Vertical = 1.179g.

This is where my question comes in. Normally, seismic forces would be determined using Ss, S1, site class, etc to come up with a base shear. (V=cs * W). How do the spectral accelerations given in the spec play into deriving the seismic forces. Do I use the Horizontal spectral acceleration that was given as my Ss or S1 value?

Any thoughts are appreciated.

 

[vaquers]

Dear rgerk,
I think that they are asking you to design your structure for a 3.5 g's horizontal aceleration regardless of the structure period (and 1.2 g's vertical). The horizontal force (Fi) you should apply for each floor diaphragm is: Fi = acel x tributary weight.

In a way it's a kind of Cs factor. If I'm not wrong is like having a constant response spectrum.

Nevertheless it looks like a very conservative design, meaby this is because it's a nucler facility. Regards,

Sebastian

 

[Qshake]

Agree with vaquers, the design requirements are actually giving you the ordinate of the response spectrum they wish for you to use.

However, rather than thinking of this as a constant response spectrum, think of it as the upper limit for the portion of the spectra that is for low periods or very stiff structures.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[JedClampett]

When I worked in the nuclear world, we used to assume the peak spectra values most of the time. There were just too many assumptions that affect the natural frequency to refine the numbers very much. And then we would multiply the peak value by 1.5 to account for other modes. Plus the business is naturally conservative.
Another factor to consider is that not all SSE are even seismic events. In BWR's there were all kind of pipe break and SRV events that caused building accelerations. There's a lot of energy being tossed around in a nuclear power plant that can shake and bake the whole plant.
I'd jsut the values they give you and not try to analyze to much into it.

 

[paddingtongreen]

I echo Jed. On the nukes, we did a lot of up front analysis to provide the designers with conservative working numbers. The only time a structure got the full individual treatment was if we were trying for a higher qualification.

I designed the pipe break anchors for a BWR and I was amazed at what it took to hold them, huge even by power plant standards.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msucog]

3.5154g is low seismic? i'm assuming the structure is not safety related too?

they usually reference PGA and not Ss or S1 since Ss and S1 were developed to correspond with a 2 and 10 story structure. most nuclear structures are "short" and rigid...therefore PGA at/near zerio period is more appropriate. that statement is neither always true nor completely accurate but it is what it is for now.

see asce4 or asce 43 or some of the nuregs or nrc design manuals. maybe

http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6926/cr6926.pdf

will help too.

 

[rgerk]

Thanks everyone. You all confirmed what I was already thinking. I was anticipating just using the acceleration times the weight of the structure. Just hard to justify the use of such high accelerations for a 10'x14'x9'tall pre-fabricated metal panel office.

As for low seismic area. Yes, per ASCE7-05, the Ss and S1 are very low. The accelerations I was given in the spec are very big and significant. That's why I was having such a hard time justifying the use of the accelerations.

Thanks for all the responses.

Ryan

 

[bones206]

If the accelerations were intended to be applied as equivalent static loads, it's possible they already bumped them up by the 1.5 factor that Jed mentioned.

 

[rgerk]

I spoke with the Engineering firm that specified the accelerations yesterday. They agreed that the SSE values they gave would most likely not be used on the pre-fabricated office. They gave me a design spectrum to use and told me to use the frequency of the structure and then use the spectrum to locate the corresponding acceleration. I am required to take the acceleration and multiply it by 2.0 in order to get back to the SSE value. I'm guessing I'll be about 20% of the original specified SSE.

Jed is right. TVA requires that peak accelerations be multiplied by 1.5.

Thanks for everyone's input.

 

[Bagman2524]

Nuclear Plant are designed to NRC design criteria not IBC. 10CFR50 Appendix A is the main governing design requirement document. There is a Safe Shutdown Earthquake (SSE)and Operational Basis Earthquake (OBE) that need to be designed for. The SSE is the maxium earthquake expected, which that plant needs to be designed for to be able to safely shut down. The OBE is the maximum earthquake expected for which the plant needs to be designed for such that it can continue to operate. Typically a Required Response Spectra (RRS) are developed prior to construction which plot accerlation levels vs. frequency (or period) of site structures. As previously, mentioned taking the peak acceleration and multiplying by 1.5 is an accepted conservative approach. All of this however, shouldn't apply to an office building, unless its' directly tied to plant structures or components. An office building should be able to be designed per standard commercial requirements such as IBC.