Seismic weight

[phuduhudu]

I have an industrial structure with a couple of levels of braced steel frame on a concrete moment frame on a big pilecap on piles. The pilecap is also the base slab. There is a big vessel supported on the top level which is about 30% of the total weight when full. Does this and the mixed framing system preclude me from using the ELF method and require a 3D modal analysis? It is only in a seismic design category B location. The initial base shear and ELFs I am getting are higher than cyclonic wind forces which surprised me a bit for the relatively low ground accelerations. Could I be doing something wrong or is the large vessel the thing that is giving me high ELFs?

 

[phuduhudu]

OK. I kind of worked out answers to my own first questions but I have another. When using ELFs you are allowed to reduce the moments for the base at the soil structure interface to 0.75 times the base moment. What if you have piles? Does that mean you keep the base shear as it is but reduce the push pull in the piles by a factor of 0.75? Or does this not apply to piles?