A diaphragm constraint creates links between joints located within a plane such that they move together as a planar diaphragm which is rigid against membrane (in-plane) deformation, but still susceptible to plate (out-of-plane) deformation and the 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.

A plate constraint is different from a diaphragm constraint, and has the reverse effect. Plate constraints are rigid against out-of-plane bending while in-plane deformation remains unaffected. These constraints are useful for connecting frames and shells to solid elements, and may be implemented in detailed beam models to ensure that plane sections remain plane.

Reported numerical problems for diaphragm constraints

Software creates a joint at the center-of-mass of diaphragm constraints 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.

An error message may occur which informs users of this zero stiffness condition. This message may be ignored in that analysis proceeds accordingly, and results are not affected in any way. Analysis may be run using the Advanced Solver to avoid the reporting of this message.