×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

long concrete cantilever
4

long concrete cantilever

long concrete cantilever

(OP)
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"  

RE: long concrete cantilever

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

RE: long concrete cantilever

(OP)
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?
 

RE: long concrete cantilever

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

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

RE: long concrete cantilever

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/
 

RE: long concrete cantilever

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

RE: long concrete cantilever

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

RE: long concrete cantilever

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.

RE: long concrete cantilever

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

RE: long concrete cantilever

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

Dik

RE: long concrete cantilever

(OP)
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!

 

RE: long concrete cantilever

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

RE: long concrete cantilever

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.

RE: long concrete cantilever

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

A classic problem the world round.

RE: long concrete cantilever


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?

RE: long concrete cantilever

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.

RE: long concrete cantilever

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.

RE: long concrete cantilever

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!!!

RE: long concrete cantilever

rapt,

as always you make some very good points.

RE: long concrete cantilever

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."
 

RE: long concrete cantilever

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.

RE: long concrete cantilever

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.

RE: long concrete cantilever

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.  

RE: long concrete cantilever

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.
 

RE: long concrete cantilever

BONILL,

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

BA

RE: long concrete cantilever

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.

RE: long concrete cantilever

BAretired,

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

 

RE: long concrete cantilever

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.

RE: long concrete cantilever

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

RE: long concrete cantilever

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."
 

RE: long concrete cantilever

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

BA

RE: long concrete cantilever

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."
 

RE: long concrete cantilever

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

RE: long concrete cantilever

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."
 

RE: long concrete cantilever

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.

RE: long concrete cantilever

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."
 

RE: long concrete cantilever

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.

RE: long concrete cantilever

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?

 

RE: long concrete cantilever

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources