I think Mike is right you’ll have to do some testing if you really want to know a value. I assume the 15# or 30# felt, or layers thereof, were put there to absorb some irregularities in the bearing/mating surfaces btwn. the pier cradle and the tank. Temperature and bearing pressure will cause the felts to bond to each other and to the steel tank and the concrete saddle, and the failure mode will be a shearing failure or flow in the felt thickness. I’ve seen and done things like this in the past and just assumed there could/would be some movement or flow in the bearing material layers without over stressing the steel tank or conc. saddle, but never really had numbers to put on it.
This tank will be outside and under the effects of mother nature year around. So I will treat the pier analysis as solid contact with no slippage eventho it is not correct and the results will be conservative. By the way, the asphalt felt is 1/2" thick.
