Amar,

You said the report shows the value calculated according to E.4.6.1-3, which is a threshold of Ai≥0.625*Sp*I/Rwi valid for S1≥0.6

First, your S1 is less than 0.6 so you are not in that case!
Second, it seems you are right that the threshold is calculated as 0.5*S1*I/Rwi=0.5*0.375*1.25/3.5= 0.066964, even S1 is less than 0.6.

Why Tank reports such value of Ai instead one based on E.4.6.1-1? I have no idea, I cannot check your output anyway.
Here Intergraph team can help you giving an answer.


About your manually check giving value Ai=0.6*1.25/4=0.1875, I have also some remarks.

- Your SDs value cannot be SDs = 0.6, I don't know from where is coming up that value.
In fact in E.4.6.1-1, API considers SDs which is specific to ASCE 7 and failed to develop a similar formula for non-ASCE sites.
For non-ASCE sites, SDs must be substituted by Fa*Ss (the definition of SDs is given in API 650/ E.2.2 Notations as SDS=Q*Fa*Ss where Q=1 for non-ASCE sites)
I think you must consider SDs=1.2*0.75 for your site E.

-About your value of Rwi, you must see the the table Table E.4, where Rwi=3.5 for non-anchored tanks.

Best regards.