Modelling Flat plate ETABS as lateral load resisting system with Columns

[B_B012]

Hi There,

I intend to make use of a flat plate slab (approx 300mm thk) on ETABS so that I can use this as portal frames with my columns to resist lateral loads. (Note not the conventional beam and column system unfortunately).

If I were to model this on ETABS what would be the best way to do so?

I'm thinking of either creating frame elements with an effective width of a column strip with slab modifiers set to 0.4?

Or keep slab modifiers to ~0.4 and let ETABS distribute the load out?

What are your thoughts / experiences?

Thanks

 

[slickdeals]

If you're using ETABS Ultimate, it will let you draw design strips and you can get the forces from the design strip to design your reinforcement. You should follow the ACI recommended stiffness modifiers.

 

[milkshakelake]

Column moment modifier: 0.7. Slab moment modifier: 0.25. This is per ACI suggestions; your code might be different. Using stiffness modifiers of 1.0 will lead to an unconservative design in terms of story drift and columns. 1.0 stiffness modifier might be conservative for the slab, but I don't think you need an enveloped approach in this situation. Using 0.4 for both modifiers is similar to using 1.0 in terms of load distribution, which is unconservative for strength but too conservative for drift; it's just not entirely correct, though it's still better than using 1.0.

Strip width is for design. It does not effect the analysis of the moment frame of column + slab. But I'd just keep the normal column/middle strip widths. No need to do anything outside the ordinary.

 

[bookowski]

There are a few papers, by moehle and others, about modeling slab frames and acceptable effective widths and stiffnesses. If you modify the shell elements in etabs you will get much higher contribution than recommended by the papers. Even if you put your slabs at 0.15 you'll get much more. Typical accepted practice in my area is to use your slab for load distribution and diaphragm by modifying the out of plane to something very soft, say 0.01, and then overlay frame elements of your chose effective width and stiffness - say 48" wide x 8" deep at 0.35 etc. Slab frames are usually a last resort in my area as it's typically uneconomical, especially if it creates the need for studrails (usually columns are sized to be 99% D/C for gravity so making them be part of the lfrs tips them over).

 

[milkshakelake]

@bookowski Why wouldn't you consider the whole slab?

 

[BulbTheBuilder]

The ETABS tutorial manual has an example of a concrete frame with a flat slab.

1. Model your shell Element.
2. Draw your strips
3. Assign your diaphragm

Regarding the stiffness modification factors, I'd reference to the code you use. Doesn't sound like you use ACI

 

[BulbTheBuilder]

@milkshakelake (Structural) I am guessing it's because of the effective stiffness of the shell contributing to overall stiffness leading to lesser reinforcement or higher moment resistance by the shell.

But I am also curious to know. Can you share the article @bookowski (Structural)?

 

[bookowski]

I didn't say you can't, I said that it's not typical practice in my area. Not sure I should be posting any papers on here but if you google slab frames and moehle, rossenwasser, youssef you'll find papers. Some of those papers talk about using fe vs the effective width. Any of it is probably fine, there's a lot of bs baked into any of them. I'd just be cautious about the modifier if using the whole slab, it will give you a lot of stiffness and also make sure that all the forces are accounted for, you're going to get a lot of contribution from twisting. and like I said, slab frames are usually a last resort here so maybe others doing them often would have some good info on modeling the entire slab as effective.

 

[milkshakelake]

Thanks, will look it up if I do it one day. And I agree, I'm in the same area as you and we'd never dream of using a slab frame. Shear walls as LFRS make more sense from every point of view, at least with the way buildings are laid out here.

Edit: A bit off topic, but there's an older high rise (designed by SOM) that uses slab moment frame as outriggers; went to a presentation where they mentioned it in passing. I forgot the name of the building. I always thought it was a bit weird of an idea, but I'm sure it made sense for that particular case.

 

[Trenno]

MSL - perhaps they were referring to virtual outrigger systems?