Shear wall length? 1

[FreshMan2020]

I know that the column-flat slab is not considered a lateral system normally but wall-flat slab can. So I am wondering how long the wall need to be so 'wall'-flat slab can be considered a reasonable lateral resisting system generally speaking? Below is the structure I am working on. On the ground floor it is a retail so the architect doesn't want walls, which is fair. But as the nearest shear wall is around 14 meters away I am not comfortable with that. The architect allows me to put some columns in with 800mm max depth. I am not sure with this depth whether it can be considered lateral resisting system? I understand that if I do it in FEA, as long as I put a frame with rigid connection it will have lateral stiffness that attracts lateral loads and thus will be lateral resisting. But I also understand that in real world, column-flat slab connection is not reliable for lateral resisting. So normally how long a 'wall' should be to be considered lateral resisting system? Another thing worth noting is the first floor will transfer slab, with shear walls above these forest columns. What do you think about this structure? Do I have to put shear walls in retail area within 14 meters on both ends ? It is a 7-storey building.

 

[FreshMan2020]

Sorry forgot to upload pics.

 

[KootK]

So I am wondering how long the wall need to be so 'wall'-flat slab can be considered a reasonable lateral resisting system generally speaking?

1) Some codes will define walls and slabs based on their aspect ratio in plan. A vertical element that is 4X as long as it is wide might be designated a wall for example. That said, my understanding is that these provisions are about vertical load design not lateral load design. I know of know formal definition of when a shear wall is long enough to be considered a shear wall.

2) Functionally, I believe that whether or not a shear wall is appropriately long to avoid the lateral load punching shear issue that you mention is about the kind of deflected shape you see in your shear wall under load. See the sketch below. If the deflected shape of your shear wall looks a lot like that of a moment frame column, then you may get slab punching shear issues and your shear wall is probably too short. If the deflected shape of your shear wall looks like that of a cantilever, then you're probably good to go.

But as the nearest shear wall is around 14 meters away I am not comfortable with that.

3) This doesn't look so bad to me. The spread between the two long walls will make for fairly efficient torsional resistance. If there's a problem, I suspect that it's with the limited amount of wall length in the E-W direction. There's no reason not to just let your numbers be your guide on that though.

I am not sure with this depth whether it can be considered lateral resisting system?

4) Probably not since the lateral response can be expected to be dominated by the much longer, and much stiffer, shear walls near by.

Another thing worth noting is the first floor will transfer slab, with shear walls above these forest columns.

5) This is usually fine provided that:

a) The shear in the discontinued wall is transferred through the floor diaphragm over to the walls at the level below.

b) The axial loads in the discontinued shear walls are transferred into the columns at the level below, including any force amplification required for seismic effects.

 

[r13]

You need columns, if more wall is not desirable. Use drop panel to increase shear capacity of the columns, and design as RC moment frame. If you work with an architect, usually you won't have much voice in determine the wall length, which is the main concern in space utilization. But, in general, the shear wall length is the length required to generate adequate lateral load resistance, I don't think there is a rule to it, but maybe check code definition of walls may help.

 

[FreshMan2020]

Thanks Kootk.
Torsional effect is exactly the concern I have here. I feel more comfortable with shear walls near ends when a structure is this wide. I have some shear walls in E-W direction that I didn't show for clarity. It is not just the ground floor only actually, I also have this torsional concern for the structure above, which is typical apartments layout with corridor in the middle and apartments on both sides as shown below.
The lines in green are party walls that I can make them RC shear walls. So I try to make them all shear walls, and then adjust a bit to make the rigid center and mass center to be very close. However after I run analysis the first mode is still in torsion and the direction factor is over 0.99! Why is that and what will you suggest to avoid this?

 

[FreshMan2020]

Thanks retired13. I am actually not worry about the shear capacity of the columns but more of lateral resisting system. The code has definition of walls & column but it is more for fire etc. Not really what I was looking for.

 

[KootK]

However after I run analysis the first mode is still in torsion and the direction factor is over 0.99! Why is that and what will you suggest to avoid this?

I don't think that this is a legitimate problem in this instance. Torsion isn't your first mode because your building is torsionally flexible but, rather, because it is insanely stiff translationally. Having torsion as a first mode isn't a problem if it's at a frequency that only dogs can hear.

 

[r13]

Here is a reference of limits of aspect ratio for shear walls. Link

 

[FreshMan2020]

Isn’t it true that if the mass center is close to rigidity center the torsion wont be in the first mode? I can change some of these walls to be light weight instead of shear walls or make them shorter as long as I can avoid torsion in the first mode. What will you suggest if so?
Can you please elaborate on this? I was told that torsion should be avoid in first mode especially when it is high rise. Or is there any books that cover this 'torsion under earthquake' particularly like why and when torsion should be avoid that you would recommend, better with some examples?

 

[FreshMan2020]

Thanks retired13. The link doesn't work though.

 

[JLNJ]

Where is your building located? What code are you bound to? What is its seismic design category?

 

[KootK]

Isn’t it true that if the mass center is close to rigidity center the torsion wont be in the first mode?

It'll certainly help but it no guarantee. Your torsional and translational modes all have their frequencies and it will always be possible to make the translational modes stiff enough to make the torsional mode become dominant. That's just not a common scenario for most building layouts that typically have much less shear wall that you are contemplating.

What will you suggest if so?

I suggest that you do nothing as that is the correct approach when you don't actually have a problem.

Can you please elaborate on this? I was told that torsion should be avoid in first mode especially when it is high rise.

That is generally sound advice and will be applicable to most buildings. But you can't lose sight of what the actual goal of that recommendation is: to avoid torsionally flexible buildings that rely on relatively unpredictable and unreliable torsional modes of deformation for energy dissipation.

The building that you're proposing is not torsionally flexible even if torsion is the dominant mode of vibration. And any sensible engineer is going to design this building so that 100% of the direct shear seismic energy can be dissipated through the transnational modes alone.

Or is there any books that cover this 'torsion under earthquake' particularly like why and when torsion should be avoid that you would recommend, better with some examples?

Yeah, you wish. This is what all the young ETAB'ers want these days: a clean, published reference to point to for definitive advice. Sometimes that exists but often it does not. Much comes down to the application of engineering judgment, either your own or that of your mentors.