Shear load distribution between rc column and shear wall in a building

[Samuee]

Hi all,

I am modelling a 5-story rc structure with 2 lift shaft modelled as core wall at the perimeter of the building, and approximately 30 rc columns around 600mm x 1000mm deep

I carry the modelling works using ETABS, however i found that at the bottom storey, over 95% of shear load transferred to the corewall, the remaining 30 columns only accounts for 5% of the shear. I found this very weird suppose those columns also account for certain rigidity in shear deformation.

Is it due to the columns are assigned as frame member while core walls are assigned as wall member so more nodes connecting to the floor shell? I would like to seek some suggestions here, thanks

 

[phamENG]

Makes sense to me. Often times, the core is intended to be the lateral force resisting system in this kind of building.

You're dealing with rigid concrete diaphragms, so the lateral loads are going to be distributed based on the relative stiffness of the supporting elements. A 10' long wall is a whole lot stiffer than a 40" column, especially when you consider frame stiffness, cracking, etc. I imagine the effect is even more pronounced in the other direction, where you column is only 24" deep.

 

[r13]

Your core walls act as two huge columns that draw most of the lateral load. If the walls can resist the load, it is a good news for the columns, that are more likely affected by the lateral load in adverse manner.

 

[Samuee]

The huge moment generated from wind exceed my design capacity for the shear wall, I'm considering the approaches below

1) (Most conservative) As it is just a 5 storey building, beam column actions capable of resisting the wind load, i simply remove the shear wall in the model and treat it as vertical load bearing wall.


2) Adjust property modifier of shear wall to reduce its rigidity

 

[r13]

I think the second approach is more feasible (consider cracked wall?). If option one is used, it can't not be connected to the slab, otherwise, the load will get to the walls, no matter how the model looks. Also, the columns will take more axial load then it really share.

 

[phamENG]

As long as your adjustments are grounded in reality, that's fine. Don't adjust the stiffness just to get the answer you want. If you do that, but still detail it with all that rigidity, etc., your building will have some serious problems.

 

[calvinandhobbes10]

If you think your shearwall cant take the lateral moment, you should add lateral stiffness, rather than reduce it.

I suspect, however, your wall core may give you more bending strength than you think. Include axial conpression due to floor and self weight. Use the whole core box, all reinforcing. Your compressive block will most likely remain within the “flange” wall. Good luck.

 

[Enhineyero]

My 2 cents.

If I have a shear wall, I usually neglect the beam/column frame lateral stiffness and detail them as simple frames.

It is usually not worth utilising the beam/column frame to resist lateral loads mainly because it comes at a high price. Members would be significantly bigger (architect not happy), Reinforcement a lot heavier (developer/owner not happy), and detailing of reinforcement harder (builder not happy).

Personally, if shear walls are working hard I would rather add an extra 50mm/100mm of thickness to the wall rather than make the other 30 columns larger and more heavily reinforced.

 

[r13]

Enhineyero,

We think the same, however there is a code catch you should be aware - if I remember correctly, for lateral load resisting system consists of shear wall and structural framing (duel system), the structural framing must be capable of resisting 25% of the calculated base shear for that floor. Just be aware.

 

[BAretired]

Because of the way they deflect, shear walls take more load in the lower storeys, but for very high buildings, frames tend to become dominant.

BA

 

[blihpandgeorge]

i mostly agree with comments already made, i would design for the shear walls to take the lateral loads and supplement them if they dont work. This approach also greatly simplifies construction as there would not be additional reo in the columns / slabs for frame action.

A dual system of columns and shear wall may require the columns to be designed to take a minimum value depending on the standard (25% comes to mind) even if they are taking less. This may lead to having to over size the columns further.

I have recently had to design some concrete columns for frame action and found it very hard to do to address fire. This depends on what standards you are designing to. Designing for ultimate loads was ok, but designing columns for frame action was outside Australian codes, and there is only limited information in the Eurocodes.

Finally, for a 5 storey concrete building, i would have expected seismic or possibly notional horizontal loads to be larger or similar to wind? Clearly depends on where your building is located, shape and size of building and the standards you are desiging to.

 

[milkshakelake]

@Enhineyero Agreed. One additional reason I don't design "dual systems" is because of possibly increased punching shear due to lateral loads.

@OP If it's not possible to add thickness to the shear wall, it can help to increase concrete strength. A last ditch idea is to add a shear wall between lowest level to 2nd lowest level where you would put a partition wall. I'd still rather do that than increase all the slab thicknesses and column sizes.