RAM: Rigid Diaphragm vs. Pseudo-Flexible Diaphragm and Their Effect on Period

I have a model of a wood framed building with masonry walls bracing the building.  I have run the RAM Frame analysis with rigid diaphragms and with pseudo-flexible diaphragms, inputting the effective % of story force to each frame.  I noticed that the base shear for the pseudo-flexible model was much less than the base shear for the rigid diaphragm model, and this was due to the building's period.  Why does the building's period change depending on the diaphragm being rigid or pseudo-flexible?  I have the same masonry shear walls bracing the building.  Intuitively, I would think that a building's vertical resisting elements, masonry shear walls in this case, influence the building's period much more than the diaphragms. 

Thank you.  Jeff

RAM v14.06.02

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  • Hi Karl,
     
    I changed the percentages in the dynamic tab so that no out of plane walls see any mass.  I did not change the other tabs, though.  This did not seem to have an effect on the periods; I'm still seeing much larger periods than when I had rigid diaphragms.  I created a service request and attached my model.  The service request number is 7000285885.  Thank you.
     
    Jeff
     
    Jeffrey C. Chung, P.E.
    Senior Associate
     
    Goldstein-Milano LLC
    Structural Engineers
    125 Main Street
    Reading, MA  01867
    T:  (781) 670-9990
    F:  (781) 670-9939
    C:  (781) 883-4144
    www.gm-se.com
     
     
     
  • Jeff,

    I reviewed the pseudo-flexible diaphragm percentages and did not find a percentage that would apply mass out-of-plane of the walls. If you create a dynamic load case for the Eigen Solution, you will be able to display the mode shapes. When this is done, you will see that some out-of-plane wall behavior is occurring. See below for the mode shape for the fundamental mode in the x-direction:

    The transfer walls and the fact that the diaphragm is not included in the analytical model when pseudo-flexible diaphragms are used is causing this behavior. The transfer condition for the walls along Grid C are especially problematic. These walls transfer onto a steel beam on Level 1 as shown below:

    Elevation View

    Plan View at Level 1

    The steel framing provides little lateral stiffness since the beams are pinned. Load applied to the walls above the transfer level engage the perpendicular wall at the left end of the model. The fact that out-of-plane wall stiffness is ignored (RAM Frame - Criteria - General) compounds the problem. The issue does not occur when rigid diaphragm are used because the diaphragm engages other x-direction walls that are connected to the diaphragm. Those walls are not engaged when pseudo-flexible diaphragms are used because the diaphragm is not included and the walls are not interconnected with other frame members.

    To resolve the problem, I recommend checking the box for out-of-plane wall stiffness in RAM Frame - Criteria - General and releasing the rotational wall fixity at the foundation nodes. This will reduce the fundamental period in each direction to around 0.27 s.



    Answer Verified By: Jeff_C 

  • If you are specifying that all of the diaphragms at all of the levels are Pseudo-flexible, I would also suggest that you consider running the analysis with the diaphragms as Rigid to get the building periods and then rerunning the analysis with the Pseudo-flexible diaphragms but with the lateral load generators specified to Use those building periods previously calculated rather than using building periods based on the Pseudo-flexible diaphragm model. The story forces calculated using the Rigid diaphragm model periods are likely to be more correct than those calculated using the Pseudo-flexible diaphragm model periods.

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  • If you are specifying that all of the diaphragms at all of the levels are Pseudo-flexible, I would also suggest that you consider running the analysis with the diaphragms as Rigid to get the building periods and then rerunning the analysis with the Pseudo-flexible diaphragms but with the lateral load generators specified to Use those building periods previously calculated rather than using building periods based on the Pseudo-flexible diaphragm model. The story forces calculated using the Rigid diaphragm model periods are likely to be more correct than those calculated using the Pseudo-flexible diaphragm model periods.

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