Transfer Slab Modeling 1

[StayHome]

Hi guys hope you all safe during this hard time.
I have a structure that contains many levels of transfer slab and I am looking for a convenient way to build the model in FEA package, say Ram Concept.
This is what I am doing now
Analyse and design the slab based on floor by floor load run down procedure - model the top level and get the reactions and then model the lower level and input the reaction from top level as loading applied on the slab and then another lower level till the lowest floor.
Q1, Is this the correct procedure?
Q2, If the answer to Q1 is no, what is your procedure?
Q3, If the answer to Q1 is yes, what can I do to quickly input the reaction from upper floor as loads on the floor in say, Ram Concept? Is there any way to automatically get the reactions and transfer to the loads instead of manually input? I am thinking of using Ram Structural System and then export slab by slab but the problem I found is the loading from a full model is not the same as floor by floor load rundown and thus cannot be used in slab analysis and design.(Can be used for envelop purpose though). Am I wrong?

Thank you.

 

[r13]

The way you were doing is fine, but outdated by those commercially available programs with 3-D modeling capability. Maybe you shall consider updating your software.

 

[StayHome]

Thanks retired13, by outdated do you mean the floor by floor load run load method? I do use 3D FEA package but my concern is that we cannot use the loading from the full 3D model for transfer slab design because of the non-linear behaviour of concrete. What software do you use? thank you.

 

[rapt]

Stayhome,

I agree with your analysis approach. Overall 3D building analysis does not take into account construction procedure effects. The real answer will be somewhere between the 2, but load takedown will be on the more conservative side, while overall 3D model will be on the wrong side.

A better approach would be to re-educate the architect!

 

[StayHome]

Thanks for your reply Rapt. If I do load run down, is there any quick way to applied the reactions from upper floor as loads? I find it very time-consuming to manually put the loads. Not sure if there is any quick method that I don't know.

 

[r13]

Personally I'm more familiar with STAADIII, but RISA and SAP2000, all can handle FEM and 2nd order analysis (though I'm not familiar with that aspect). Maybe I didn't understand your question/concern correctly, can you elaborate more on the non-linear behavior of concrete transfer floor, and provide an example.

 

[r13]

I wonder if the linked blog provides some insight to your questions/concerns. Link

 

[rapt]

Are you sure RAM Structural System will not do a tributary area load takedown rather than full 3D analysis? Some software offer it as an option. Whether you trust them or not is another matter!

 

[StayHome]

I think the Strategy 2 in the link cannot apply to concrete structure because it doesn't take the behavior of cracked concrete into consideration unless you run non-linear analysis, and this will affect the loads applied on transfer members. Another concern is like what rapt mentioned - construction sequence.

 

[StayHome]

I am not familiar with Ram structural System to be honest. Ram concept doesn't allows me to import any loads or the like. The only way I can think of is importing Ram Structural System and that's why I mentioned it? I am not sure how can tributary area load rundown be done (floor by floor) when you have a full 3D model in FEA software? (I have been using ETABS and I don't think there is such a load rundown sorta thing unless you establish multiple floor only models).

 

[r13]

In the 3D model, you can assign member effective properties (Ie, Icr per ACI) to account for cracked behavior, and deactivate members/joints to account for construction sequencing/procedure, if I understand your concerns correctly.

 

[StayHome]

Is this the approach that you use to design transfer slabs? I did think of revising the stiffness to say 0.25 for slabs in the full model but still not confident to use the loads directly from the full 3D model. Can anyone please confirm that this method has been used in your projects without load rundown?

 

[EZBuilding]

It appears you may be using ETABS for your building modeling and Concept for your slab modeling. I had that same work flow on a similar project.

To design a transfer slab on that project, I did a "save as" of the ETABS building model and assigned pinned supports at the transferred columns. This provided a worst case scenario of the loading for these columns without the possible un-conservative effects that the 3D model can produce.

 

[r13]

Since extent of concrete crack is not predictable, if you are using reduced stiffness method, I think you need to run the model a few times with varying stiffness to capture the most critical reactions. At this time, how to select the output forces is pretty much an engineering judgement. However, once you are getting comfortable with the method, it potentially will save you a lot of problem from transferring data from one program to the other.

 

[StayHome]

It is because of crack is not predictable that I am not confident to use this 'reducing stiffness' method especially when more than one transfer slab is involved and prefer a load rundown method to 'peace my mind'. So how do you normally design the transfer slabs?

 

[StayHome]

I get your idea of assigning restraints to vertical transferred members. But what happens if you have more than one level of transfer slabs? Lets say level 2 and level 3 are both transfer slabs. To design level 3 I can assign restraints to vertical transferred members above level 3 but what do you do when it comes to level 2? Removing these restraints and assign restraints to vertical transferred members above level 2? This way of designing level 2 does not consider the situation that we consider for level 3 though (ie restraints for vertical members above level 3 not considered at the same time). Not sure if you know what I mean.

 

[EZBuilding]

StayHome, I would propose the following process.

Pinned supports above third floor. Utilize transferred column reactions as the loads for transfer elements on the third floor.

Pinned supports above second floor. Compare transferred column reactions with the slab analysis model transferred column reactions. Perform a load takedown by hand/excel. Utilize worst case reactions from three options for design of second floor transfer elements.

Would also recommend:
-Bracketing these behaviors across a few trial models which utilize different cracking factors as described by retired 13.
-Reviewing transferred elements lateral moments at attachment to transferred elements.

 

[r13]

EZ has nailed it, and paved a simpler design path. Too bad, we all wish there is a "one shot" solution, but it's not there yet !

 

[Trenno]

True column loads (transferred or not) will lie somewhere between the results from the following analysis types:

1) hand calculated tributary area with a pen, paper and scale ruler.

2) isolated floor analysis with pinned supports, which will account for the elastic distribution of the floor plate (ie 20% more to internal columns etc)

3) full construction sequenced non-linear 3D model.

There are so many other things that could affect the true column loads, so at the end of the day you just want to know you're being sufficiently conservative.

Typically speaking I'll rely heavily on method two, as it should produce conservative results for the transfer slabs.

Also you can set up auto-generated loads in RAM Concept. It's similar to building ETABS models using text files. There is a post of the Bentley wiki page about it. I've used it previously to set up 1000s of point loads and 50 odd load cases/combos.

 

[SerbGile]

How will isolated floor analysis give you 20% more to internal columns? Where does this 20% come from? My opinion is isolated floor analysis will give you more to transferred members and how much more depends on the layout(stiffness).