The question comes up as to what is the Av and TL values in our ITSdesign program? These values are listed under the seismic input section on the design page. Av is the vertical acceleration value and is entered as a value of g. This input is different than the Ss and S1 values which are entered as a % of g. In API-650 it is up to the client to specify if a vertical acceleration is to be included in the seismic calculations. If it is, they are to provide this value. In AWWA D100 there is a minimum value that is to be used based on the other seismic values. A client can specify a higher value though.
The second value, TL, is the Region-Dependent Transition Period for Longer-Period Ground Motion. This value depends upon the location of the tank and can be 4, 6, 8, 12, or 16. Areas from Eastern Texas to Eastern Kentucky and from Central Minnesota to the Gulf Coast are listed as a 12. The Northeast states are a 6 while the Southeast states down to Florida are an 8. The Southwest is a 6 and the upper Midwest is a 4. West of the Rocky Mountains it is a 6, 8, 12, or 16 depending upon your location. AWWA D100 printed the map of these values in the seismic section, Section 13 Figure 19. Unfortunately, API 650 did not include this map in their standard. If you do not have the AWWA D100 standard, you would need to look in ASCE7 for this map.