RAM Frame - Pseudo Flexible Diaphragms [FAQ]

What are pseudo-flexible diaphragms?

Pseudo-flexible diaphragms were added to simplify the process of modeling flexible diaphragms, which require manually calculating lateral loads and placing them as nodal loads in Elevation mode of RAM Modeler. They are referred to as “pseudo-flexible” since the behavior of the flexible diaphragm is not directly captured by considering the finite stiffness of the diaphragm, but rather by considering frame portioning of the diaphragm (i.e., by use of independently acting frames with distributed story shears).

What are the steps for modeling a pseudo-flexible diaphragm?

1. Click on RAM Frame – Criteria – Diaphragm and change the diaphragm type to “Pseudo-Flexible."
2. Assign a separate frame number to each braced frame, moment frame, or shear wall. In most cases, the same frame number will be used for the members of a given brace at all levels.
3. Click on RAM Frame – Loads – Pseudo-Flexible Diaphragm Properties and assign a percentage of the story force that will be directed into each frame (listed by frame number) that is connected to the diaphragm at each pseudo-flexible diaphragm level. This should be done for all load cases, including the dynamic load case, which is used to calculate modal periods and frequencies used by program generated seismic and wind load cases. Typically, the percentages are defined according to the tributary area of each frame.

Why are only certain frame numbers listed in the pseudo-flexible diaphragm properties dialog at each level?

In order for the pseudo-flexible diaphragm properties to recognize the frames within the diaphragm boundary, frame numbers must be assigned to a vertical frame member (wall, column, or brace). If all vertical frame members assigned to a given frame number are outside the diaphragm boundary, then that frame number will not appear in the dialog. If only beams are assigned to a given frame number, then that frame number will not appear in the dialog. If no frame numbers have been assigned, then only Frame 0 will appear in the dialog.

How do I assign frame number to a mutual column that is part of two different frames?

In many instances, a single column is part of two different frames. One example is a corner column that is part of a both a x-direction frame and a y-direction frame. RAM Frame permits only a single frame number assignment to each member. When a frame is part of two different frames, consider one of the following approaches:

1. Assign the mutual column the same frame number as one of frames. The total force in the frames will be correct and the overall behavior will be accurate. However, the axial force in the beam will be impacted and might not be correct. This may create an unconservative design and may require you to design the beam by hand for the proper axial force.
2. Assign a unique frame number to mutual column and assign it percentages in both directions. For example, imagine frame 1 is parallel to the X axis and frame 2 is parallel to the Y axis. Each frame takes 50% of the story shear in its respective direction. If they share a mutual column, it could be assigned to frame 100. Then, the percentages in the X direction would be entered as frame 1 and frame 100 each with 25% in the X direction and frame 2 and frame 100 each with 25% in the Y direction.  

Can I use pseudo-flexible diaphragms for frames that are skewed in the XY plane?

Nodal loads that are generated for pseudo-flexible diaphragms are applied in the global x or global y direction only. If a frame is skewed in the XY plane, there will always be a force or mass component that acts in the out-of-plane direction. This out-of-plane component will result in large deflections or instability errors. The following are better options for skewed frames:

1. Use a semirigid diaphragm instead of a pseudo-flexible diaphragm (see RAM Frame - Criteria - Diaphragms and RAM Frame - Semirigid Diaphragms)
2. Model nodal lateral loads in RAM Modeler - View - Elevation View making sure the resultant force is exactly in the plane of the 2D frame. Then, define the diaphragm as Flexible/None in RAM Frame.
3. Assign 0% to the skewed frame and design it in another program. Note that pseudo-flexible diaphragm percentages do not need to add up to 100%.

Why is there an instability error, Eigenvalue error, or large displacements when I run the analysis with pseudo-flexible diaphragms?

The important thing to remember about the pseudo flexible diaphragm in RAM Frame is that it is no diaphragm at all. The behavior is exactly the same as if you set the diaphragm to "Flexible/None." Unless the frames are interconnected with frame members or horizontal braces, they will displace independently.

The following are common issues that lead to errors that prevent the RAM Frame Analysis from running to completion:

1. A frame with a pinned column base is attached to pseudo-flexible diaphragms only and not connected to other frames with frame beams or horizontal braces. Since there is nothing to stabilize the frame in the out-of-plane direction, the frame is free to rotate about the frame axis. To resolve the problem, fix the out-of-plane moment at the base of the column. If the pseudo-flexible diaphragm properties are defined so that the frame is loaded in-plane only, this will not result in a moment reaction at the column base. The base of columns that are part of frames in two directions can remain pinned.

2. A frame wall is attached to pseudo-flexible diaphragms only and is not connected to other frames with frame beams or horizontal braces. Also, out-of-plane wall stiffness but rotational fixity at the base of the wall has been released. This generates an instability error similar to issue 1 above. To resolve the problem, click on RAM Frame – Criteria – General and uncheck the box for releasing rotational fixity at the foundation nodes.

3. Percentages in the Dynamic tab are divided equally among all frames or load one frame in the out-of-plane direction. This will either result in a very long fundamental period or an Eigenvalue error (see RAMSS Eigenvalue Error). To resolve the problem, review the percentages for the Dynamic tab and verify that mass is not attributed to the frame in the out-of-plane direction.

See Also

RAM Frame - Criteria - Diaphragms

RAM Frame - Semirigid Diaphragms

RAMSS Eigenvalue Error