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"

 

[BAretired]

There is no recommended maximum length of a cantilevered slab in the Canadian code. You are free to extend it as far as you please but you are required to meet strength, deflection and vibration standards which I assume you have in the code you are using.

BA

 

[enko]

I have performed the checks of strength and deformation with the reducted EI and the nominal load as described by the code.
I can meet the code requirements by increasing the height of the section and by a higher concrete grade.

Beside these 2 things I mentioned, is there anything else I could do with a 3 m concrete cantilever?

 

[msquared48]

Post a sign that says "No Jumping in Unison".

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IDS]

Long tem deflections are likely to be much greater than found from a simple implementation of code provisions. I suggest checking with Eurocode 2 methods, including effects of shrinkage and creep and differential temperature. If the top of the balcony is exposed to the sun the cyclic differential temperature effects are likely to reduce tension stiffening effects to close to zero, so the section will in effect be fully cracked, and shrinkage strain will add significant curvature, even with symmetrical reinforcement.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[handex]

"Post a sign that says "No Jumping in Unison". "
Is it sad that this gave me a decent laugh?

 

[msquared48]

Hey, it's Friday. What can I say - especially after the Hyatt Regency incident many years ago.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

3.0m cantilevers are certainly not impossible and I have even seen case studies where longer cantilevers have been designed. Because it is a ribbed slab I am assuming that it is a non-prestressed slab system. I would look to comply with typical span to depth ratios (l/8) so a 375mm thick slab.

Being external, consider set-downs and falls for waterproofing and cover for durability. Ensure that you have sufficient backspan as well.

 

[dik]

IDS nailed it... you have to be concerned about long term deflections. This can be improved by using some compression steel and tapering the cantilever soffit to reduce dead load at the 'far side'. Depending on the environment, you may have to provide for corrosion resistance using increased cover, stainless or HDG bars. QC is important to see that proper cover is maintained. With parkades, I usually provide the client with a maintenance 'manual' to reduce corrosion effects.

Dik

 

[dik]

I should have included high strength 30-35 MPa concrete, low slump and W/C, good aggregate, etc. concrete.

Dik

 

[enko]

kikflip
It is non prestressed slab.Is there any link where I can find a similar case study?

Guys Thanks for the posts.

I am sceptic about posting signs because people try to dissobey the rules.
When all the ideas will be finished,I would rather carry the slab on my back!

 

[dik]

It's like a 'Wet Paint' sign, you may just encourage people to test it...

One of the more difficult things I designed was a 6 storey parkade in a seismic area with 15' cantilevers; I needed to use the parapet to stiffen the edge to force the vibration mode.

The architect did not want columns exposed around the perimeter...

Dik

 

[a2mfk]

Like dik alluded to, in the off chance your balconies are anywhere near the coast, put a lot of thought into potential corrosion. There are concrete repair contractors that make their living off condos and hotels built near the beach in Florida. Salt spray effects may be hundreds of feet or more from the actual beach.

 

[csd72]

Any the point of cracking happens to be at the worst place for salt corrosion.

A classic problem the world round.

 

[BONILL]

I am also designing a 3m. cantilevered ribbed slab floor poured in place. However, mine is an exit way, so my live load is 500 kg/m2. The dimensions of my ribs are: B = 0.16m., H = 0.35 m. and I am getting around #4 at 4" o.c for negative reinforcement. Any thoughts?

 

[csd72]

Bonill,

Sounds reasonable, small bars at close centres are better to prevent cracking than large ones at larger centres.

I think you will find that your neutral axis is quite high up due to the limited rib width. you may get a better result by modifying this.

 

[rapt]

Regarding cantilever deflections you cannot base them on an L/D
ratio.

The deflections are dependent to a large extent on the length of the back span to the cantilever length and L/D does not take this into consideration. The only way to do it properly is to do the calculations properly for long term deflection taking into account the rotations at the support from the rotation of the back span.

For a 3m cantilever with a back span of 8m, the L/D ratios would probably be fairly good. If the back span is 5m, the deflection will increase by a factor of about 3.5. If you increase the back span to 10m, the deflection of the cantilever is significantly upwards.

Any self-respecting enginerer who believes L/D ratios for cantilevers should get out of the business and probably also if he believes them at all for concrete members!!

And do not believe the scale of the benificial effects the codes say you get from compression reinforcement, unless your member is heavily reinforced and you do not want that for your cantilevers.

 

[rapt]

To continue with my rant,

People need to be careful making approving suggestions on this website without the full information and without really understanding design. You could really get someone into trouble.

As an extreme example for this case, there was once a 16 storey RC building in Asia with 5m cantilevers. Depths were determined in accordanse with the L/D ratios in BS8110. No account was made for rotation at the support.

According to normal design logic assuming no brittle walls etc, the allowable deflection would be 5000 / 125 = 40mm , total long term, so probably in the order of 10-15mm on release of the forms.

The actual deflections on stripping the forms were 160mm!!!!!!

The back span was 5m, the same length as the cantilever!

You cannot tell someone his 3000mm cantilever will work without knowing the complete span arrangement and without designing it and calculating the rotation!!!

 

[csd72]

rapt,

as always you make some very good points.

 

[rowingengineer]

These are common problems with L/d ratios for cantilever, which Rapt has discussed. If you don't complete analysis for deflection at a cantilever without taking into account the structural arrangement you can/will have large problems with serviceability.

If your back span is large as Rapt suggested the slab may tip back, this looks really bad as this means that water instead of following your assumed runoff direction, it ponds against the support.

If your back span is small the back span or not activated then the deflections looks great. But I doubt your client or his attorney will see it that way. Here not only do you have water runoff problems again, but the building will look poorly designed.

The next issue that wasn’t cover by rapt is the live load applied or not applied during calculations for deflection on a cantilever. I always check my cantilevers for pattern loading, thus that the cantilever has no live load when the back span is fully loaded and vice-versa, then I do a dead load only deflection check. I use these different deflections to asses if the drainage will work as required.

I have attached a note published by the concrete international. While this is discussing the effects of posting tensioning and problems with posttensioned slabs, this is just equally applicable to your slab, plus I really do like the pictures.

The biggest problem an engineer will face in the length of his career is not a structural failure but is a failure of the building to perform to expected levels of service. Generally you can get the structural calcs wrong to some level and your building will get through life with an issue, this is due to the law of averages and never seeing a large design load, however serviceability problems will occur from day dot.

You need to ensure you use the best design principles, and know the limitation of your quick hand calcs. You should also know the limitations of other methods. L/d ratio’s as Rapt suggested are not appropriate for cantilever slabs.


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