A diaphragm constraint creates links between joints located within a plane such that they move together as a planar diaphragm, rigid against membrane (in-plane) deformation, but susceptible to plate (out-of-plane) deformation and associated effects. Diaphragm constraints relieve numerical accuracy problems which result when floor diaphragms are modeled with very high in-plane stiffness. They also enhance the computational efficiency of dynamic lateral analysis by reducing the size of the eigenvalue formulation.

...

Reported numerical problems for diaphragm constraints

CSI Software creates a joint at the center-of-mass of a diaphragm constraint to enable lateral-load application. Since this joint, denoted by a ~ prefix, is constrained against in-plane deformation, the out-of-plane stiffness components are set to zero. Given a diaphragm constraint along the global Z-axis, these components correlate with the UZ, RX, and RY DOF.

...