Tall wall unbraced for half its length

[Lion06]

I have a condition where a 6 story tall concrete wall that is about 18' long. The problem is that only half of it is within the footprint of the slab and the other half is out in space unbraced laterally. The wall that extends beyond the slab is not supporting anything except for its own weight and a brick veneer. The portion under the slab is supporting a nominal floor load (less than 100 square feet of trib).

I'm not worried about out of plane wind, I have the horizontal bars designed to allow it to cantilever horizontally from where it is braced by the slab. My concern is general lateral stability under self-weight and the brick veneer.

At the start of the project it had a 2 ft extension, which I didn't blink an eye at. It's slowly crept longer and longer and hasn't been on my radar while detailing the rest of the building.

What are your thoughts as-is? I have my own, which I'll keep to myself for now, but am hoping someone may see something I don't.

 

[Ron]

Ideally this wing wall elevation would be narrow at the top and wide at the bottom....obviously not what the architect wants.

I would consider "framing" the unrestrained perimeter (top and outside edge) as a beam/column bent. Bottom fixity for the "column" is not an issue; however, getting sufficient fixity of the "beam" at the top slab might be an issue. Might require "flaring" the beam width at the top where it meets the slab.

 

[Lion06]

Thanks for the feedback, Ron. I was considering a return of the wall at the unframed end to provide some lateral stability there, I was just worried that for a 60' height the return would need to be larger than I can realistically get.

The compression stress in the wall is less than 150 psi.

Are you thinking a beam at each floor line or just at the roof level?

I did consider a couple things. I thought about bringing a thin piece of slab out to a return wall to maintain the floor to floor bracing height. I thought about having a WT laid sideways on the outside doubling as a brace spanning from floor line to return wall and the brick relief. I also thought about detailing the stair (which is why the slab is cut away from the wall here) framing to be rigidly enough attached to the floor lines to brace the wall. I liked this idea a lot, as I thought it was the most benign, but I quickly realized that because the stair stringers are sloping that it will impart additional vertical forces on the floors they are attaching, too.

 

[Lion06]

Here is where I'm at with this wall. I would appreciate any thoughts.

I built a full height finite element model with the portion outstanding unbraced. It's an 18 foot long wall with 9 ft engaged with the floor slabs and 9 ft oustanding. The wall is 12 inches thick and supporting very little floor load and even less roof load in addition to the brick veneer. The self-weight of the wall is much greater than any other load on the wall. I have all of the loading that will be applied to the in the model in a manner to be the most de-stabilizing to the tip of the wall. What I'm finding is that the movement of the wall is very, very low and it doesn't appear to be at all unstable. I have the loads in fictitiously high (gravity and lateral) and the wall in with only 3/4 of the thickness. I was a little surprised at how little it is moving, but the load is so small. I have the lateral loading applied to initiate out of plumbness to allow instability to be observed through excessive displacements.

Is this type of second order analysis enough or is having that much outstanding wall still just a bad idea regardless of what the analysis shows?

 

[hokie66]

I wouldn't worry about it any more. As long as the wing wall is secured to the building and well reinforced horizontally, it is fine. There could be some issues if in a highly seismic area, but for gravity and wind loading, no problem.

 

[Ron]

Agree with hokie66. You've checked it adequately. Seems substantial

 

[BAretired]

I would consider a 12" thick concrete wall with a 9'-0" cantilever to be laterally braced by the floor slabs as it easily meets the span to depth ratio for a solid concrete slab. In the CSA code, l_n/10 would be the minimum thickness required where l_n is the cantilever length.


BA

 

[Lion06]

Thanks everyone.

 

[a2mfk]

12" thick concrete and 9ft cantilever would work even if it were a floor with a big live load, no?

Sounds like you checked your deflection pretty carefully. Just a thought since you are supporting brick.

Plus, check out this concrete cantilever in a Calatrava building!