queries on modelling of table top compressor structure

huangyhg

queries on modelling of table top compressor structure
iam doing a study for feasibility of using a existing table top compressor structure, while the motor, gear box and compressor will be upgraded to a higher rating.
i would like to seek the opinion of the forum members on two queries namely:-
1) which is the best way to model the top slab(8.5m long x 3.6m wide x 1.2m thk) in staad pro.if i were to model it as a plate element, the thickness to span ratio of the plate will not satisfy the definition of plate (span to thickness ratio of > 20) .
one alternative option is using a grillage of beams ,1.2 m thick and varying widths.
another option is to use plate elements with reduced thickness, but by increasing the young's modulus of elasticity e, we can make up for reduction in depth.
using three dimensional solid elements is not viable, because it seems that  staad results for solid elements are not reliable.
2)is it necessary to model the forces on gear box? there will be two kinds of forces.first one due to out of center eccentricity of gear rotors. the second due to the gear teeth transferring force from one gear wheel to other. is it necessary to model this force as pulse loads, or these forces will cancel each other out?
regards
shinde