Select Criteria > Diaphragm to control the diaphragm in-plane and out-of-plane (bending) behavior. Regarding in-plane actions, a diaphragm can be designated as a Rigid, Semirigid, Pseudo-Flexible, or Flexible\None. For out-of-plane behavior, it is dominated by slab\deck type in the program. If a two-way slab\deck exists inside a diaphragm, the out-of-plane stiffness of the diaphragm is always included. For one-way slab\decks, it is optional (i.e., you either include or ignore out-of-plane stiffness for one-way decks\slabs). Further information about diaphragm out-of-plane options can be found in Diaphragm Out-of-Plane Stiffness .

The Rigid option provides a means of specifying that the diaphragm is sufficiently stiff in its in-plane to act as a unit; the lateral members at each level are rigidly "linked" together in the plane of the diaphragm. It also provides the mechanism for distributing lateral story forces to the various frame members based on their relative stiffnesses.

A Pseudo-Flexible diaphragm is not able to transfer story torsional moments to frame members as the rigid diaphragm does. Also, story shears are not distributed to the frames members based on members’ relative stiffness, but rather based on tributary areas of frames. A more detail discussion is provided at Section 6.12 in Technical Notes.

A Semirigid diaphragm is able to transfer loads to frame members through its in-plane stiffness. Therefore, a Semirigid diaphragm is not assumed to be infinitely rigid, but it has finite stiffness, so that the diaphragm deformation is considered while transferring applied loads. Further information is provided at Section 6.12.3.

The Flexible/None option provides a means of specifying that there is no diaphragm or that the diaphragm has insufficient stiffness to act structurally as a unit.

The diaphragm in-plane stiffness selection has a significant impact on the way RAM Frame functions. If a diaphragm is defined as Rigid, Pseudo-Flexible, or Semirigid, then the analysis can be based both on the diaphragm forces specified for that diaphragm by the load cases created in Loads - Load Cases in RAM Frame and Nodal loads specified by the load cases created in PropTable - Nodal Load Cases in RAM Modeler. If Flexible\None is selected for a diaphragm, the analysis can only be based on the nodal loads specified for that diaphragm by the load cases created in PropTable - Nodal Load Cases in RAM Modeler. If there is no diaphragm at a level, then the story forces specified by the load cases created in Loads - Load Cases in RAM Frame are not applied to this level. Regardless of the Diaphragm setting, the Dead, Live, and Roof load cases are generated based on the gravity loads specified in the RAM Modeler, and are available for analysis.

The program allows some levels to be declared to have a rigid or pseudo-flexible or Semirigid diaphragm and others to have no diaphragm. Levels that have no diaphragms can be loaded by nodal and gravity loads only. Dynamic analyses can be performed only if some or all levels in the building have a rigid, semirigid or pseudo-flexible diaphragm associated with some mass.

Refer to Analysis of Models with Diaphragms , for more information regarding analysis with rigid and semirigid diaphragms.

It should be also noticed that semirigid diaphragms are always meshed for analysis. In addition, if there exists any two-way slab\deck inside a diaphragm (whether it is rigid or semirigid), then it is also meshed. This is necessary to carry out a two-way load distribution for loads on two-way slab\decks.

For a one-way semirigid diaphragm, the program inserts vertical springs for gravity column and wall under the one-way deck when it runs an Eigenvalue analysis. These springs are removed for all other load cases (e.g., dead, live, seismic, wind, and center of rigidity calculations). This is only intended to provide stability in the out-of-plane direction for one-way deck.

In addition, a special section is added to the dialog for two-way decks: it is possible to consider gravity columns and walls to be included in the analysis as springs for those columns and walls under a two-way deck. It should be noted that these springs are included in analysis for only gravity load cases, and they are excluded for lateral load cases (wind and seismic load cases) expect that they are used as well for an Eigenvalue analysis (i.e., dynamic load case). Also provided is the ability of including gravity members (columns and walls) directly in the analysis. This is only applicable for those gravity members under a two-way deck. Again, these members are included for gravity load cases and they are excluded for lateral load cases except Eigenvalue analysis (i.e., dynamic load case). Refer to Diaphragm Out-of-Plane Stiffness for more information.

Making changes to the Diaphragm criteria after a model has been analyzed necessitates a re-analysis of the structure.