RAM Frame Diaphragm Stiffness Clarification

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 22:04:40 GMT

Let me illustrate with an example.

Double cantilever structure with loads applied along the edge. Slab is 2-way but behavior is one way due to wall supports.

1. Using full stiffness for the slab and knife supports for the walls, delta Z at end of cantilever = 0.615"

2. Using a modified deck with 0.25 out of plane factor:

delta z = 2.42" (4 times as much, as expected).

I could have used 0.25 factors in the M Stiffness Modifier section anch achieved the same thing.

3. using the original deck, but with the out-of-plane stiffness ignored in the Ram Frame criteria:

delta z = 300,307", ~500,000 times as much. At this point the deflections are so large that some of the assumptions for finite element analysis are not entirely valid, but it makes no significant difference to the the wall forces, reactions, etc. at least for a flat diaphragm.

Here's my file if you want to try other tests.

communities.bentley.com/.../Diaphragm_5F00_deflection_5F00_check_5F00_v1703.zip

RE: RAM Frame Diaphragm Stiffness Clarification

Zachary Otten — Mon, 28 Feb 2022 19:52:01 GMT

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:

The out of plane diaphragm deflections do not change the lateral framing results.
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.
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.
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:

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?
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?

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 19:30:04 GMT

I believe it is scaling it by 0.000001.

RE: RAM Frame Diaphragm Stiffness Clarification

Zachary Otten — Mon, 28 Feb 2022 19:07:07 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 18:02:57 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Zachary Otten — Mon, 28 Feb 2022 17:51:00 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 17:22:40 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Zachary Otten — Mon, 28 Feb 2022 17:18:08 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 16:32:33 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Zachary Otten — Mon, 28 Feb 2022 16:27:39 GMT

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.

RE: RAM Frame Diaphragm Stiffness Clarification

Seth Guthrie — Mon, 28 Feb 2022 16:21:33 GMT

For details see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18616/ram-frame---criteria---diaphragms#two-way