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.
For details see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18616/ram-frame---criteria---diaphragms#two-way
I have reviewed this page, the RAM Frame Manual, and the RAM Modeler Manual. While these have provided some insight, they have not settled my questions/concerns.
If the diaphragm out-of-plane stiffness is ignored for lateral loads, and the diaphragm exhibits large out-of-plane deflections, that is of no consequence to anything else. It makes some people uncomfortable, but it does not lead to any specific problems with member forces, frame shears, etc. so long as the structure is stable and the program can solve the P-Delta and/or eigensolution.
Hi Seth, ok so even though I am seeing very large 30" vertical deflections for the wind load case in the diaphragm, the lateral load is arriving at the lateral frames? This magnitude of deflections makes it impossible to see the lateral movement of the structure, so I have been looking for a way to rationally stiffen the diaphragm in a an appropriate and realistic manner to match the increased out of plane stiffness that joists at 16" O.C. would provide. I spoke with a colleague that suggested 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 eliminates out of plane diaphragm movement. When I look at the results, I still have a little vertical deflection in the region of the building with little lateral support (we are relying on collector elements to bring the load to the wings on the structure). While this option is getting closer to what I expected in terms of vertical displacement, I am hesitating to use this option until I understand its implications. One thing that makes me unsure is that this option improves the structure where the 3/4" board is spanning 20' (for load distribution in gravity). I would expect that those areas would still be problematic even with this option since the gravity elements are far and the slab is not stiff enough for that span.
I think you can turn off the display of the deck if it's causing your problems to visualize the frame deformation.
When adding the out-of-plane stiffness of the deck, think of it like another frame beam. You already have a moment frame (I'm guessing) and now you have the additional moment stiffness of the deck. It ought to slightly reduce drift (if the total load is constant), reduce the moments in the frame beams, etc. That's why our default is to not consider that stiffness.
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.
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.
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.
I believe it is scaling it by 0.000001.