It is assumed that flexible diaphragms are not able to transfer torsional moments and that story shears are transferred to vertical members based on tributary areas of the frames. Under these observations, it is fair to conclude that each frame acts independently from the others. Therefore, behavior of a flexible diaphragm can be represented by the help of independently acting frames in the diaphragm followed by resolving story shears for each frame.

Based on this observation, a feature is provided in RAM Frame to capture the behavior of flexible diaphragms. It is referred to as pseudo-flexible diaphragm since the behavior of the flexible diaphragm is not directly captured by considering finite stiffness of the diaphragm, but rather by considering frame partitioning of the diaphragm (i.e., by use of independently acting frames with distributed story shears).

To this end, frame effective participation ratios are required for each frame, which explicitly define what percentage of the story forces are resisted by each frame at the diaphragm. Note that the effective ratios can be approximated due to tributary area of each frame.

It is essential and expected that the user first assigns frame numbers to the members. It should be noted that this information directly affects analysis results. The effective participation ratios are then entered in the Pseudo-Flexible Diaphragm Properties dialog.

After building base shear for a wind or seismic load case is calculated, the program distributes the base shear over the height of the building according to the chosen design code. Once diaphragm load (which is a fraction of the base shear) is calculated for each diaphragm, it is further distributed among the participating frames in the diaphragm if the diaphragm is flexible (otherwise, for rigid diaphragms, the calculated diaphragm load is applied at the diaphragm’s mass center considering any eccentricities). The effective participation ratios are used to determine how much of the diaphragm load goes to which frame. Thus, it is important to recognize that the participation ratios assigned for each frame directly affect the validity of analysis results.

Once the load for each frame is resolved, it is further distributed among members in the frames, based on either members' relative rigidities or on equal distribution among members (the user can specify which method is to be used in the dialog). Note that calculated story masses are always distributed among members equally. If distribution based on relative rigidities of members is chosen, the following equations are used to evaluate member rigidities:

• For columns and braces:
  $K_{maj}=\frac{3E{I}_{maj}}{L^3}$
  $K_{\min }=\frac{3E{I}_{\min }}{L^3}$
• For walls:
  $K_{maj}=\frac{1}{\frac{L^3}{3E{I}_{maj}}+\frac{6L}{5GA}}$
  $K_{\min }=\frac{3E{I}_{\min }}{L^3}$

Finally, frame joint forces are calculated and they are applied as global nodal loads at member joints. This completes the analysis with flexible diaphragms procedure implemented in the program.

It is always advised to check the Loads and Applied Forces report after analysis completed to review nodal forces applied at frame joints for pseudo-flexible diaphragms.

In computation of interstory drifts and drift\height ratios for flexible diaphragms, the diaphragm member displacements are averaged and these averaged values are used as the flexible diaphragm displacements. Thus, all drift related quantities are based on these averaged values.