use the equaldof for nodes in one floor

the dispalcement of nodes in the floor will be the same

This command is used to construct a multi-point constraint between nodes.

equalDOF $rNodeTag $cNodeTag $dof1 $dof2 ...

http://opensees.berkeley.edu/wiki/index.php/EqualDOF_command

If your model is 3D, You can also use from "rigidDiaphragm" command.

rigidDiaphragm $perpDirn $masterNodeTag $slaveNodeTag1 $slaveNodeTag2 ... http://opensees.berkeley.edu/wiki/index.php/RigidDiaphragm_command

Dear George Bekas

This command is used to construct a a rigid plane with the master node.

rigidDiaphragm $perpDirn $masterNodeTag $slaveNodeTag1

$slaveNodeTag2 ...

$perpDirn direction perpendicular to the rigid plane (i.e. direction 3 corresponds to the 1-2 plane)

$masterNodeTag integer tag identifying the master node

$slaveNodeTag1 $slaveNodeTag2 ... integar tags identifying the slave nodes.

And I hope this link be helpful.

https://www.researchgate.net/publication/317156066_Analysis_of_Machine_Foundation_Surrounded_by_Diaphragm_Wall_Using_OpenSees_Program

Dear all

I asked if I should define all the nodes at once or one by one. Is the latter scenario also acceptable.

Yes, that is acceptable.

Check the following reference Please.. it might's assist you in your question …

http://opensees.berkeley.edu/wiki/images/b/b1/DiaphragmsModeling.pdf