Combining two or more models in ETABS
1) Open two ETABS sessions. For this example, let's say Model A and Model B.
Model A
Model B
2) If required add number of stories needed in Model B, using Edit>Edit Stories>Modify>Insert stories, Select the entire Model A by using the Edit > Copy Option command or Ctrl-C.
3) Now go to Model B and the use Edit > Paste Option or Ctrl-V to merge Model B into Model A.
4) Next, change the coordinates and rotation for the pasted model in the Paste Coordinates form. You can see a preview of the model if you place the cursor over OK.
5) Click OK and the two models will be merged.
6) Additional models can be added by repeating this process.
Note: The procedure above assumes that section properties used in Model B are also present in Model A. All frames that have same sections defined in both models will be pasted correctly, but slabs, walls and load assignments should be reassigned.