API-650 Standard Seismic Parameters Discrepancy

[iceberk]

Hello everyone,

I have an argument with our civil department about the seismic parameters Ss and S1 which should be used in API-650 tank design according to ASCE 7. In Clause E.4.1 it says:
"For sites located in the USA, or where the ASCE 7 method is the regulatory requirement, the risk-adjusted maximum considered earthquake ground motion shall be defined as the motion due to an event that is expected to achieve a 1 % probability of collapse within a 50-year period. The following definitions apply."

But in clause EC.4 "Historically, this Annex (and the U.S. standards) was based on ground motion associated with an event having a 10 % probability of exceedance in 50 years. This is an event that has a recurrence interval of 475 years. In seismically active areas where earthquakes are more frequent, such as the west coast of the US, this was a reasonable approach. In regions where earthquakes are less frequent, engineers and seismologists concluded that the hazard was under-predicted by the 475 year event. Thus, the maximum considered ground motion definition was revised to a
2 % probability of exceedance in 50 years, or a recurrence interval of about 2500 years. The economic consequences of designing to this more severe ground motion was impractical so a scaling factor was introduced based on over-strength inherently present in structures built to today’s standards. See the NEHRP Provisions for a more extensive discussion of this rationale." is written. According to that sentence, actually 2% should be used.

In addition they used 2% and 10% values in the examples.

I am designing Tanks located in Turkiye but the parameters are calculated according to ASCE 7. The problem is that, there is only 2% probability criteria not 1%. Can I use 2% probability of exceedance in 50 years with Q=2/3?

Thanks in advance.

 

[HTURKAK]

. The problem is that, there is only 2% probability criteria not 1%. Can I use 2% probability of exceedance in 50 years with Q=2/3?

- The MCE ( Maximum Considered Earthquake ) is defined as 2 % probability of exceedance in 50 years . That is , 2500 years return period.

In addition they used 2% and 10% values in the examples.

- I think you are mentioning EC.9.1 Example Problem #1. The worked example compares two resources . One is http://eqhazmaps.usgs.gov/ and the other one is IBC 2000 CD-ROM. Eventually uses the first reference ( more update and gives the relevant info. for a specific location) and chooses MCE use Ss = 103 % g, S1 = 42 % g and S0 = 38 % g. The multiply with 2/3 to get 500 year EQ. As we are structural engineers we like to multiply with 1.5 then somewhere we multiply with 2/3 and get the same.
In this case , the MCE is reduced with a factor 2/3 as wishful thinking .

In your case ,
- Choose 10 % probability of exceedance in 50 years. ( event that has a recurrence interval of 475 years. ),
- Calculate Ai and Ac . Dont forget the damping ratio for sloshing would be 0.5 % ( for Ac calculation).