Distribution of horizontally applied loads through diaphragms to lateral load-resisting elements depends on relative rigidity of diaphragms and the resisting elements. In general, diaphragms are assumed to be Rigid if the distribution is directly proportional to relative stiffness of the lateral resisting members. In this case, the diaphragm is able to transfer torsional moments and diaphragm deflections are not significant compared to the deflections of the resisting members. On the other hand, the diaphragm is referred to as Flexible\None (none-rigid) if horizontally applied loads are not distributed to the resisting elements based on their relative stiffness (i.e., the distribution is independent of the relative stiffness). In this case, the diaphragm is not able to transfer any diaphragm torsional moments and diaphragm deflections are significantly larger compared to the deflection of the resisting members.

For the cases where diaphragm deflections and resisting members’ deflections are in the same order of magnitude, then such diaphragms cannot be categorized as Rigid or Flexible\None. Instead, it is referred to as a Semirigid diaphragm, which basically represents a diaphragm condition between Rigid and Flexible\None. Analysis with semirigid diaphragms considers diaphragms stiffness and thus, it reflects real diaphragm deflections and provides a more involved load distribution among the resisting members.

It should be noted that diaphragm properties (such as thickness, E, etc…) and its dimensions do not alone reflect whether diaphragm is categorized as rigid, none-rigid or semirigid. Rather, its interaction with the resisting members and relative stiffness of diaphragms as well as of the resisting members plays a major role in this decision. Many building codes provide some guidelines for determination of diaphragm types.

If a diaphragm is defined as Semirigid in RAM Frame, then the program automatically meshes the diaphragm. Most of the meshing details are taken care of by the program so that the user does not need to go over this tedious process manually. The engineer can change the diaphragm type in the Story Diaphragms dialog.

If applicable, crack factor is applied to in-plane thickness of the diaphragm. Hence, in-plane (membrane) stiffness of the semirigid diaphragm is reduced by applying crack factor.