API 650 Appendix E Seismic Design

[linus]

As per Mapped method given in API, S1 is Mapped, maximum considered earthquake, 5% damped, spectral response acceleration parameter at a period of one second, %g and Ss is Mapped, maximum considered earthquake, 5% damped, spectral response acceleration parameter at short periods (0.2 sec), %g. These are to be modified for appropriate site classes by use of Fa and Fv factors. The code does not mention this but S1 and Ss initially are to be values for site class B. I think this is covered in IBC and ASCE-07 from which Appendix E is derived. For one of our projects we have been given S1 and Ss based of Rocky Soil class A. How can this be converted to values for site B so that Appendix E calculations may be proceeded with.

Regards,

LINUS

[Arrayxxx]

i think, s1 and ss is determined by seismic map as your tank be will be build, not by class b as you said
fa and fv is depend on what class your soil is...
so, what is your problem?

[linus]

True, S1 and Ss are determined by Seismic Ground Motion and Long-Period Transition Maps. However all these maps are based on Site Class B as can be seen from maps in ASCE-7 or IBC. My query is if I have been given S1 and Ss for Site Class A for a particular location not covered by ASCE or IBC, how do I convert this to S1,Ss of Site Class B so that rest of the Appendix E calculations may be carried out.

Regards,

[Arrayxxx]

I'm sorry i'm not yet get the point
as I know, t, you can get he value of s1 and ss from asce7 map.
Then, site class is determined by site soil check

if your region not covered by asce7 map, try to find the value of sp from hazard seismic map then you can get ss = 2.5 sp and s1 = 1.25 sp

if you want to convert the value of s1 and ss for site class a to value of s1 and ss to site class b, you must calculate overturning moment with data s1,ss, and site class a
after you get the overrunning moment, then iterate with that value to find the value of s1 and ss with site class b

I think that took long time, but that's only the way you can do as I know

[linus]

Dear Arrayxxx

As stated in my earlier post, value of S1 and Ss are obtained from maps in ASCE/IBC etc. However all these maps carry a note that the values are for B site class. These values are then adapted to the required site class as per soil check by use of Fa, Fv values. My problem is that I have seismic hazard maps for a location which gives S1, Ss for site class A. How do I analyse this as per ASCE/IBC metods?

Kindly let me have details, references for conversion method explained by you.

Regards,

[marian_g]

Dear linus,

As API 650 says in E.4.4 Modifications for Site Soil Conditions, "the maximum considered earthquake spectral response accelerations for peak ground acceleration, shall be modified by the appropriate site coefficients, Fa and Fv from Table E.1 and Table E.2."
As you can see in these tables, Fa=1 and Fv=1 for Site B. That means Site B is considered as reference also for API 650.

Normally the reference being Site B you need only to apply the site coefficients specific to your site. The result will be a spectrum as suggested by Figure EC.1—Maximum Earthquake Response Spectrum.

In case your reference is Site A, that means your spectrum already included coefficients of Site A, instead to consider Fa and Fb equal with 1 (as for site B).
So your spectrum indicates values as Fa_A*Ss, FV_A*S1/T. As graph, you may see that your spectrum is "stretched" or non-uniformly scaled by Fa_A and Fv_A.
You need to calculate back Ss and S1 (which correspond to Site B), later to apply Fa and Fv for your site.

NB. It's easier to do than to explain, I'm sure you can do it quickly having Figure EC.1 in front of you...

Regards.

[linus]

Dear Marian_g

Thanks a lot. I had surmised as much but needed some peer opinion.It is a simple matter of dividing the value by Fa,Fv for site A.

Many Thanks,

Regards,