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RAM Frame Diaphragm Stiffness Clarification

Zachary Otten
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I am working on a structure that has a flooring system of 3/4" concrete board spanning between cold-form metal joists at 16" O.C. The joists span up to 20' between wide-flange moment and braced frames. In an effort to simplify the modeling for this multi-story, large structure that is changing through design, I have omitted the joists and spanned the floor in the direction of the joists. My problem is that the floor system is being analyzed as semi-rigid, and the out of plane (vertical) deflections for lateral wind and seismic loads have been very large, 25" - 30". One solution is to model the joists as lateral members, but this will be very time consuming. Is it possible to make this deflection more realistic? I looked into the "Story Diaphragm" option in RAM Frame that includes out of plan stiffness for semi-rigid diaphragms with one-way decks for lateral and gravity load cases, and while this creates the result I expected, I am not sure what the program is doing since the span is still 20' and there are no gravity members joists that I would expect to be able to improve the results to 1/2" of vertical movement. A step that I have considered is using stiffness modifiers to increase the out of plan stiffness, but I am not sure that will lead to the desired outcome and how that may affect other results.

  • 0 Offline in reply to Seth Guthrie

    That is a good suggestion. Can you clarify the difference between the include out of plane stiffness option in Story Diaphragm and increasing the out of plane stiffness modifiers through slab assign properties? Both improve the vertical deflections for the lateral cases, but I am not sure why the increasing the stiffness modifiers helps if the consider out of plane stiffness is not selected in Story Diaphragm. 

  • 0 in reply to Zachary Otten

    The option in Criteria - Diaphragm is "all or nothing", while the settings in the Modeler apply deck by deck. It's worth noting that the program attributes a very tiny amount of stiffness to the decks even when the option to consider the stiffness is turned off. This is to prevent rampant instabilities. 



  • 0 Offline in reply to Seth Guthrie

    Is the option in Criteria - Diaphragm applying a large stiffness modifier to the out of plane direction? Similar to manually applying a modifier in the Modeler Deck Menu? I was able to note the stiffness being applied even when the Criteria - Diaphragm option is off as I changed deck type from a thin to thick slab. What I am trying to get at is if the Criteria - Diaphragm option is scaling the out of plane stiffness up by some factor or if it is a different mechanism.

  • 0 Offline in reply to Seth Guthrie

    I'm sorry if I am missing the point, but I am not following what the 0.000001 is being applied to. Here's a summary of what I have taken from our discussion so far:

    1. The out of plane diaphragm deflections do not change the lateral framing results.
    2. To reduce the out of plane diaphragm deflections, I can either model the joists, apply a stiffness factor to the slab, or select the RAM Frame option in Criteria - Diaphragm to include out of plane stiffness for one way semi rigid diaphragms.
    3. The stiffness factor, applied in RAM Modeler, increases the stiffness of the material properties by a factor, but does not include gravity elements to stiffen the deck in RAM Frame.
    4. The RAM Frame Criteria - Diaphragm option allows gravity members to be considered, but also seems to increase the stiffness of the slab since I have 20' spans that perform much better with this option on.

    Here is what I am still questioning:

    1. The option in RAM Frame seems to permit the out of plane stiffness of the slab to be considered, but by using the slab modifiers and comparing different slab types with this option off, the slab stiffness seems to be part of RAM Frame's vertical deflection calculations regardless if this option is selected. I understand that gravity elements come into play when this option is checked, but the reduction in the middle of the 20' slab span does not seem to be happening only because gravity members are included. Therefore, I am wondering if this option also increases the slab stiffness?
    2. You seemed to note in your last message that by selecting the Criteria - Diaphragm out of plane stiffness option, something is scaling by 0.000001. I was expecting you to say stiffness increased by factor similar to 100x not the inverse. Can you please explain what you are refering to scaling?
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