Allow a rigid diaphragm to restrain a beam
Metal Deck and Composite Deck 2D members are often used to laterally restrain floor beams via diaphragm action. Sometimes, when setting the element behaviour to Rigid Diaphragm, beams will still deform laterally, i.e. in the plane of the 2D member. It is like the beams are not connected to the 2D member, and therefore the rigid diaphragm member is not restraining the beam.
This phenomenon often occurs because there are not enough mesh nodes along the length of the beam to connect to the 2D member. To fix this issue, make sure to set the average number of tiles of 1D element to between 4 and 10, before calculating.
After setting the appropriate 1D mesh settings, all the floor beams will behave as expected: rigidly in the plane of either the metal deck or composite deck member: