2. For the small time-step, dt of earthquake has been used for the last transient analysis. However, very large time steps have been used for gravity analyses which are conducted as transient. Should I change the type of gravity analysis from transient to static?
You can turn them to static. But sometimes you can't, for example when you use U-P mixed elements for soils. In case you use transient analysis for quasi-static problems, you can use very large time steps so that inertia effects are negligible.
3. Mumps system and Krylov-Newton algorithm have been used for all analyses, is that right? (openseesmp solver will be used).
Krylov-Newton is just more stable than standard Newton. Mumps is much faster than other solvers, but it is available only in OpenSeesMP/SP, that's why I used it.
4. EqualDOF or rigidLink constraint should be used to connect the top and bottom faces of mat foundation? The lateral faces of mat are connected using EqualDOFs, is that right?
No.
You have to use beamToSolidCoupling to connect the columns to the foundation.
You don't need EquaDOF at the link foundation-soil, otherwise you are simulating a perfect adherence. I noticed that you are using both contact and equalDOF at the interaction between foundation-sides and soil. Doing so your foundation will be attached to the soil, and the contact will be useless. Using only contact is more realistic.
However, you should also do the same at the interaction between the bottom-face of the foundation, otherwise, your structure will penetrate the soil.
5. What is the correct order of Analysis steps for using InitialStateAnalysis and updateMaterialStage together?
There is not a unique way.. It really depends on what you want to simulate. Typically you do:
1) initial state on
2) gravity elastic
3) turn to plastic
3) initia state off
4) dynamic analysis
So that you will see displacements relative to the gravitational ones.
6. Should any masses be assigned to the soil and structure? Loads are assigned to frames by eleLoad_beamUniform, and by body force b3 in the SSPbrick elements definition for soil and foundation in addition to mass density for material definitions.
Yes, you need to assign mass-per-unit-length to the beams, either in the beam element property or using the condition.EdgeMass
7. For the force time history applied to the base of the soil, is it correct to use a unit area of the soil column to calculate damping coefficient regardless of mesh size? (biased mesh is used).
Yes, if you use a FaceForce, it is supposed to be a force-per-unit-area (Pressure), and STKO will take care of converting it into equivalent nodal loads.
8. Is rayleigh damping necessary in my case? If it is necessary, is rayleigh damping of soil enough (using frequencies 0.2 and 20 Hz)? Or should I enter it separately for both the building and the soil using region command?
It depends... since you never model all the physical nonlinearities, you may need artificial damping. And you should do it with regions if the damping coefficients are different for soil and structure.
here is an ERROR: the analysis did not converge.
Try to do the changes I suggested for the connection between foundation and soil, and let's see what happens