Etabs Shear Wall Design 1

[TFP_Byrd]

Hi,

I've been having some interesting discussions with my colleagues about the design of shear walls for a 6-storey, earthquake governed, precast concrete structure. With regards to the image I've attached, the PT slabs (that are designed as one-way) are looking to be creating bending moment peaks in the shear walls. This makes sense, Etabs doesn't know that the direction that the slabs are spanning and is treating the junctions between the walls and the slabs as fixed. So, any bending moments in the slabs, near the junctions with the walls, end up being transferred into the walls.

One guy in the office is saying that we should model in columns, completely released so that they don't take any lateral loads, so that they can prop up the slabs, preventing them from deflecting and transferring moment into the nearby walls. This makes sense to me as we are only using Etabs for the shear wall design.

Someone else is recommending finding a way to "release" the connection between the walls and slabs, but I can't think of a way to do this without affecting the amount of lateral resistance the walls apply.

A third person has recommended increasing the sectional stiffness of the slabs to something like 10 (we typically crack the slabs to about 0.4). This way, the slabs are very stiff and don't deflect, and hence don't put any additional moments into the walls.

Can you please let us know your thoughts on this? The one thing we (think) we do know for sure is that the moments going into the walls from the slabs aren't actually "real" because the slabs are designed as supported by pins and are running the other way.

 

[TFP_Byrd]

On a related note, from my perspective, whether the walls are precast or in-situ is irrelevant - either way, the slab-to-wall connection is more like a pin support than a fixed support due to the nature of the typical slab-to-wall connections (i.e. even with in-situ walls, the core is often jumped and the slabs are then dowelled into the walls).

 

[Celt83]

I'd suggest releasing moment at the walls or modify the weak axis stiffness of the walls to a small value. In addition recommend modelling all of the gravity columns and beams, without the vertical load resisting elements in place to support the slab you are going to get secondary load effects due to the exaggerated deflection from the slab not having any support other than the walls.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[TFP_Byrd]

Thank you!

"I'd suggest releasing moment at the walls" - do you know how to do this in Etabs? There are many different options when moment releasing the wall and I've never used this before:

"In addition recommend modelling all of the gravity columns and beams" - do you think that modelling in columns that aren't actually there will distort the results? I'm thinking that they won't - in my mind, the column locations in Etabs are irrelevant because we release all the columns anyway so that all the lateral resistance is being provided by the slab.

 

[Celt83]

I'm not well versed in Etabs, try their manual or reaching out to their support staff.

do you think that modelling in columns that aren't actually there will distort the results?

absolutely it will distort the results, why would you model anything other than the actual support conditions?

the column locations in Etabs are irrelevant because we release all the columns anyway so that all the lateral resistance is being provided by the slab.

This statement is not accurate, especially so if a 2nd order elastic analysis is being performed, the displacement of the slab in all directions will impact the secondary forces.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Trenno]

I'd agree with Celt83 but be careful of the fundamental period. It may now be an out-of-plane wall bending mode which will lower your seismic forces if using auto-generated ETABS seismic loads. I think you can override what period ETABS considers in the equivalent static analysis.

In the wall section properties, reduce the M11, M22 and M12 stiffness down to say 0.25. Test a few different values to check the moment that is developing between the slab and walls.

In saying that, depending on slab and wall thicknesses, you can still develop a fair moment connection with embedded dowels top/btm into the slab. Example

 

[Retrograde]

I think you need to release the bending moment and twisting moment at the slab/wall connection.

Have you modeled the slabs as rigid diaphragms?

 

[TFT_01]

Retrograde, do you have a suggested method of releasing the moment at the slab/wall connection? Do you suggest using links or edge releases?

 

[Retrograde]

I think you just need to tick those two check boxes in the image you posted earlier.

 

[TFT_01]

Seems to have done the trick! I assume this method is only valid if we design our slabs to be pinned instead of fixed to the modelled walls. However in reality I understand that some moment will still transfer into the shear walls, would this method then be unconservative?

Would you also suggest releasing the edges for all shear walls as we have designed the slab to be pinned to them?

 

[May10May]

I think you can simply revise the out-of-plane stiffness by changing the stiffness of m11,m22 and m12 to 0.1 when defining the wall sections. There is a stiffness modifiers option.