long concrete cantilever 4

[enko]

Greetings to everyone!
I am dealing with a regular concrete 10 storey building with balconies projecting up to 3 m.
The balconies are supposed to be cantilever ribbed slab floors poured on place.
Is there any strict recommendation about the max. length of the cantilevered slab?
Nominal live load 300 kg/m2

Thanks in advance.

Ps: I know the history of the "Frank Lloyd Wright Falling Water House"

 

[hokie66]

Good article, RE. Too little tolerance, too tight design causes these problems all the time. The fixes never work well, and are always more costly than doing it right the first time.

 

[spats]

One other thought on this subject... if it would be appropriate in this situation, I would consider specifying an upward "camber" at the end of the cantilever to counter the initial dead load deflection.

 

[rapt]

Spats,

Yes,
- as long as you know what deflection you are expecting first. It is no use precambering and then have it deflect further upwards. So you still cannot use L/D ratios! You need to consider all of your pattern cases to decide if precamber is appropriate.
- precamber only assists with visual deflection. If you are worried about brittle finishes or walls, precamber does nothing to reduce incremental deflection that occurs after the props are removed.

RE
I had meant to add in comments on pattern loading but forgot as I was writing it (happens as old age sets in). Fortunately AS3600 now requires pattern loading be checked for deflections for all designs, so hopefully eventually people will start to get an understanding of the effects.

 

[BONILL]

Please see attached sketch. My back spans are in excess of 8m. and consist of a waffle slab with B = 0.16 m. (ribs) and H = 0.35m. There is no drainage issue since it is a floor slab. I am doing the calculations for strength and long term deflection. My perimeter beam is B = 0.70 m. , H = 1.40 m. so I am expecting a very large rotational stiffness at the support. I am having trouble finding information as to how to treat the vibration issues and pattern loading. Thanks all for your help.

 

[BAretired]

BONILL,

If your sketch is showing joists spanning parallel to the beam, you have no backspan.

BA

 

[csd72]

That beam would need to be designed for the torsion but then the columns need to take the bending from this.

I think you need an experienced eye to go over the whole thing before you get too far. As others have pointed out, there is only so much that can be covered in a forum.

 

[BONILL]

BAretired,

I apologize for the sketch. It should show a two way waffle slab (joist spanning both directions).

 

[hokie66]

No need to apologize. You did say it is a waffle plate. But is the cantilever also a waffle? I think it should be, just to decrease the dead load. Without doing any numbers, I think the scheme should be OK. Agree that the big beam will help, and it should be reinforced for the incidental torsion that will result.

 

[BAretired]

BONILL,

That helps, but if you can taper the exterior slab from 350 at the beam to, say 150 at the outer edge, it would reduce the negative dead load moment (see attached).

I would try to balance the interior and exterior dead load so that the deflection is minimal at the edge of the cantilever under dead load only. Live load comes and goes, but it doesn't result in permanent deflection.

BA

 

[rowingengineer]

Agree with hokie, the scheme looks possible, however I would query why there is no steps or falls for the balcony. Step and falls could reduce your effective depth such that the scheme doesn’t look so good.

In my local area (note this is Australia and in a high wind zone so doesn’t apply to your situation, recommend you consult the architect for the project) we would have a minimum 50mm step at the doors and 1:50 minimum fall after removal of formwork. Not knowing you door locations but assuming it is at the edge beam that would reduce the 350 to 240 at the edge using my numbers.

You also need to model this in your calculations, either making conservative assumptions and adjusting the dead loads as required or using a method that can adapt to the changing sections.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BAretired]

Just re-read BONILL's first post. The cantilever is ribbed, so tapering is not an option.

BA

 

[rowingengineer]

I don't see why a rib slab cannot be tapered. I understand the ribs have to be the same height is the top plate can change in thickness; I have done this in the past with success. If you did construction a flat balcony you would have to grout in the falls with tiles but this would mean a greater step at the door (if there are any).

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BAretired]

If the ribs are formed with metal pans, the pans themselves are tapered from top to bottom, making it labor intensive to form a taper longitudinally on the rib.

BA

 

[rowingengineer]

Hokie,
Is the ribbed slab the same terminology in the USA/Canada?

To me I can see no reason why the top plate of the ribs not the ribs themselves cannot be change in thickness from 100mm to 150mm to get fall as required. Sure you need to account for the extra dead weight, but you either have it in the slab or in the grout. But this is all based on Australian terminology for ribbed slabs.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[hokie66]

You could use different depth pans for the cantilever than for the backspan to achieve what you are suggesting, RE. A bit complicated, as the OP said this is a waffle pan slab, and having several depth waffle pans on site can create logistical issues. As to setdowns, I took the cantilever to be part of the internal floor.

 

[rowingengineer]

Hokie,
You could be right I just took exit way to be similar to the balcony with regards to requiring a door and step/falls.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BONILL]

The other issue I have is that the cantilever is curved in plan, so the interior ribs would not be parallel to the cantilevered ribs (if these are oriented perpendicular to the supporting beam). Please see attached sketch.

 

[csd72]

This is just a moment vector issue that can easily be calculated with trig. Having the stiff edge beam definately helps to spread the moment to the different ribs.

You may get a higher deflection because of the longer span though.

 

[BONILL]

csd72

So you would place the negative steel from the cantilever ribs perpendicular to the beam and cross them to the other side of the beam, and distribute the moment into x and y components for design of the negative moment of the interior ribs. So in a way, the interior ribs at their support would have moment flexure and torsion. Is that more or less the idea?

 

[hokie66]

I wouldn't change the direction of the top reinforcement. That just complicates placement. Extend the top rib reinforcement straight out.