API 650 10th Ed, Addendum 4 Seismic Loads

[SteveGregory]

I have calculated the seismic loads using Appendix E (Addendum 4) for a series of tall small diameter tanks. The overturning moment Ms is supposed to be used for foundation design. This number is based on allowable stress design (ASD). How do use this number with the load combinations?

This addendum is compatible with ASCE 7-05. Should I multiply Ms by 0.7 for the working stress concrete and foundation design or use the straight value for Ms?

 

[SteveGregory]

Another thought on Load Combinations, Appendix R (Add 4) seems to say use Ms without a seismic reduction factor.

 

[PEStructural]

API 650, 10th Edition, Addendum 4, Section E.5.1.1 defines the response modification factors to be used with the standard. If you compare these values for the impulsive component (3.5 for self anchored and 4 for mechanically anchored) to ASCE 7-05, Table 15.4-2 for flat-bottom ground-supported steel tanks (2.5 for self anchored and 3 for mechanically anchored), I believe there is a 0.7 (or divided by 1.4) factor built in to the API 650 Rw values. This means that the seismic values (overturning, shear, etc.) are in working stress levels and does not need to be multiplied by the 0.7 for the basic combinations in ASCE 7-05, Section 2.4.1. If you choose to use the factors from Section 2.3.2, you would need to divide the E values (seismic) by 0.7 (multiply by 1.43).

That's my $0.02. Hope it helps.