ETABS release slab (pin) at core walls for a tower

[philski888]

Hello all,

I am new to ETABS and I am trying to setup a lateral stability model for a tower. I have modelled the building, with the slabs as shell elements. I would like to release the slabs, eg pin them, to the core walls as the slabs will be connected via a corbel. There will be no moment transfer and I want to model it as such. Is this possible in ETABS and if so how do I do it? I see you can assign frame releases but I am not sure how this works. Essentially I need to release the slab for rotation at the core wall.

Any advice would be greatly appreciated!

thank you

Phil

 

[li0ngalahad]

Have a look here

http://www.eng-tips.com/viewthread.cfm?qid=374257

 

[philski888]

I understand the above method of changing the slabs to "membrane" instead of "shell", however I still want to maintain the stiffness of the slab, as the slab is physically there. Therefore I would prefer to keep the slab as a shell, but "release" it at the walls. is this at all possible?

 

[li0ngalahad]

Not sure if that is possible (I'm a beginner as well), I would change the slab property to membrane on ETABS to have a good global lateral analysis of the structure (slabs shouldn't contribute to it at all unless you are designing the slab to take part of the core moment), then export and design the slab separately on SAFE.

 

[philski888]

no worries, thanks I will give that a try!

If anyone does know of a 'release' type joint for shell plates let me know.

cheers

 

[MrHershey]

If you want to check overall stability this way, I'd apply stiffness modifiers to the slab to artificially lower its stiffness to 1% or lower. You'll still be a shell so all your loads will distribute correctly, but you'll just be a very flexible shell. Once stability is confirmed, you can switch modifiers back to normal.

One thing to keep in mind is that while it's okay to check as pinned to be conservative and make sure you're not getting helped by the slab in seismic events, are you really pinned for non-ultimate events? If the connection really is going to attract moment and you don't want it to fail under service conditions, you would need to account for that in your modeling.

Especially at top of wall conditions where you have uneven spans (or slab on one side), you're going to attract moment at the joint that would then transmit into your walls unless you do some special detailing to specifically release it. Whether you like it or not. The built structure doesn't really care how it was modeled. For thicker walls with two layers of reinforcing it likely wouldn't make a difference. But for thin walls that are lightly loaded and one layer of reinforcement, it absolutely could.

 

[nathan7]

Hi all

We are going through the same problem on a model that we are doing. We don't want the slab to take unrealistic moments during lateral movement but it is also not realistic to assume that the slab is fully pinned. We have used modification factors on the moment capacity of the walls and we set them to 0.1. However the slab still take moment because it is fixed in two directions and especially in the corners you can stop it to be stiff and attract moment. It is good to put some top and bottom steel on the corners to avoid crackings during lateral movement.

You should also apply modification factors to the slab stiffness in order to account that the slab is cracked or partially cracked.

 

[AqibUsmanMoosa]

i think we may use the link element.
i have attached, may it help.