In the context of sloped framing with rigid diaphragms, it is important to note that the diaphragm is always assumed to be rigid on a horizontal plane, even though the members in the floor may be sloped.

The following figure illustrates the effects of this.

In addition, any story mass associated with the diaphragm is assumed to occur on the horizontal plane only. Therefore, both user-specified and generated story lateral forces are applied to the horizontal plane of the sloped diaphragm, at the height of the story.

Any sloped beam attached to the diaphragm will be constrained in the horizontal plane. However, unlike the case of a horizontal beam, the sloped beam will experience varying axial force along its length due to the component of gravity load acting along the longitudinal axis of the member. In addition, under lateral loads the sloped beam member may have the same lateral displacements with the diaphragm, but the relative vertical displacements of the beam-ends could again result in axial forces in the beam.

If the effects of the rigid diaphragm assumption are too severe, the user should specify No Diaphragm (when selecting the Criteria > Diaphragm command in RAM Frame) for this level, and apply any story forces as Nodal Loads (Nodal Loads are defined in Elevation View in the Modeler).