api storage tank roof loading

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api storage tank roof loading
is there a way to determine the load imposed on a flat tank roof (type 316l material) by a 300 lb. person? i need to verify that current design will allow for such loading. a customer is insisting on a 250 lb. loading over a 4" x 4" square area, but i feel that is excessive.
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i'm confused.  the load of a 300 lb person on a roof is 300 lb.  there is some impact from him/her stomping around, but traditionally that's ignored.  as far as the 250 lb.(actually also 300 lb.)over a 4 in. by 4 in. area, that's pretty standard for stairs and exits if you look at table 1607.1 in the 2006 ibc.
if your question is how to determine whether a roof is adequate for that load, you need to know how thick the roof is, how many feet it spans between purlins and estimate how much area is effective in carrying the load.  you probably need to get a structural engineer to do this.
generally, you'd assume that the roof can deflect into a catenary shape to carry the load.  i think roark's formulas for stress and strain has some equations applicable in that case.