This question may be a little simple. But it confuses me.....

In my case,the analysis is a time-history analysis of a structure under earthquake.

First, gravity analysis, and then, dynamic analysis with constant gravity load (stress and deformation obtained by dynamic analysis will superimpose on those obtained by gravity analysis). However, if the structure is damage, the superimposition is not rational.

- the equilibrium equation is Mx''+Cx'+kdx = -ma - mg -R or Mx''+Cx'+kdx = -ma -R ??

- is [do one step dynamic analysis (t->t+dt) and do one gravity analysis at the same time, and then superimpose them] rational?
- if the 2. is not ture, How to consider gravity in a dynamic analysis?