Shear wall failing in shear? 2

[thomaschua17]

I'm modelling an 8-Storey with Basement building, currently I have two shear walls, and both are failing in shear at the bottom from basement to 4th storey.
the base is currently at 550mm thk, and it's still failing. How can I go about this? I have attached the screenshot of my model, the top is the framing from 5th to 8th while the bottom is from basement to 4th.

Any help would be very much appreciated.

Thank you!

 

[Rabbit12]

Make it thicker. Make it longer. Add another shear wall. Check your model to make sure it's behaving correctly.

 

[jayrod12]

How many levels of basement? Are you modelling the soil pressure? Perhaps you're getting backstay effect and it is a real thing. More horizontal reinforcing might help?

 

[phamENG]

I don't do much about 5 stories, but my first instinct when looking at that is "where are the walls...oh...those little things?". You have a very small "core" that is off center from the rest of the building. Torsion is going to be a factor even in general, orthogonal load cases which only makes things worse.

I'd say find a way to increase your core size, or start introducing additional LFRS components. Is the rest of it steel? Can you introduce braced frames anywhere? Moment frames?

 

[XR250]

The shearwalls seem awfully short for a building of that size

 

[thomaschua17]

Thanks all for replying!

"Make it thicker. Make it longer. Add another shear wall. Check your model to make sure it's behaving correctly."
- all seems good. no errors or ill condition, displacements are all in place.

"How many levels of basement? Are you modelling the soil pressure? Perhaps you're getting backstay effect and it is a real thing. More horizontal reinforcing might help?"
- Just one basement. lateral soil pressure was included, but I just added it as an line load, as I don't want the retaining walls to carry some seismic forces.

"I don't do much about 5 stories, but my first instinct when looking at that is "where are the walls...oh...those little things?". You have a very small "core" that is off center from the rest of the building. Torsion is going to be a factor even in general, orthogonal load cases which only makes things worse.

I'd say find a way to increase your core size, or start introducing additional LFRS components. Is the rest of it steel? Can you introduce braced frames anywhere? Moment frames?"
- Haha. that was my thought at first too. at first it was just the small elevator core at the bottom, I just added the additional wall at the top part. I'm thinking, maybe the fact that the storey seem to split at the 5th to 8th floor level, and the only member holding them together is the small wall at the middle. All are reinforced concrete.

"The shearwalls seem awfully short for a building of that size"
- Thanks! Will try adding more.

Also for added info. It's at seismic zone 4, Na = 1.2; Nv = 1.6.

 

[Trenno]

What's the base shear? What's the fundamental period? Have you done a hand calc to verify? Have you audited the ETABS design module?

Try linking the shear walls over the lobby with coupling beams?

A 550mm thick wall seems a bit much for an 8st building.

 

[KootK]

I think that your root problem here is likely the torsion that phamENG mentioned. With just the small-ish core group resisting torsion, any load eccentricity is going to absolutely hammer your walls with a great deal of leverage. If at all possible, I'd add some additional LFRS elements as far away from the cores as you can put them to improve torsional response.

 

[MIStructE_IRE]

How is your lateral sway looking? Those shear walls don’t look long enough for 8 storeys.

Is this seismic?

 

[mrlm]

As people have already told you, your building does not appear to have a large enough core.

A very basic calc you can do to check the fundamental natural period of the structure is T = h/46 where h is the height of the building above ground or T = n/10 where n is the number of storeys. This will give you a very approximate indication of your expected fundamental natural period and how the building would respond to lateral loads. Compare this approximation to your analysis out of your design program and see where you end up. You will more than likely find it will need to be stiffened significantly.