issue related to mass definition

[nsharmanishant]

Dear fmk, moderators and other members,
I am trying to analyse a simple portal frame with fixed base using ground motion data and I am exciting the structure using UniformExcitation command. i am assigning mass using the 'mass' command in the element itself as shown in the following definition.

"element nonlinearBeamColumn $eleTag $iNode $jNode $numIntgrPts $secTag $transfTag <-mass $massDens> <-iter $maxIters $tol"

However, it is seen that the structure does not displace at all and remains stationary over the entire duration of analysis.

But when the mass is assigned on the nodes (using the following command) the structure displaces and shows some results.

"mass $nodeTag $mX $mY $mZ;"

My query is that is there any problem with assigning mass in the element definition itself?
Do we have to assign the mass on the nodes itself?
Is it the same case when we use quadrilateral element for modelling soil?

I would be very grateful if you could shed some light on it.

Thank You