(2) At support, only reactions are nonzero = to be monitored. At points loaded with forces, external forces are to be monitored.

I assume you have a supported bottom surface with many nodes. In such case, you need to monitor the reactions in all nodes and sum the values outside of ATENA (preferably using a spreadsheed) to get the total reaction. To make your life easier, you can add an elastic, relatively stiff "support plate" to the bottom. Then, you support the plate in a only a few points and monitor the reactions in 1-5 points which is much less work to sum compared to the values from all finite element nodes at the bottom of the wall.

Regards,
Dobromil

Recently I am proposing to use ATENA 3D to conduct an analysis which is similar to pushover analysis on a single shear wall, thus, please help me figure out the questions listed below, thank you!
(1). Can I apply constant vertical load on the shear wall, rather than the iterative  load case? And how?
(2). It is desirable to produce a curve expressed as base shear versus top displacement, so that the capacity of the structure can be monitored. And I know   this can be done by setting monitoring points on the specific positions on the structures. However, I was still confused how to monitor the base shear? What options should I choose, external force or reactions? And your suggested position of the monitoring point?

ATENA is very powerful in modelling concrete structures! I wish your company will continue to improve it!

Thank you very much!

Best wishes,
Jacky