Cantilever slab - how far back into the anchor span to extend rebar 3

[ajk1]

Given: Cantilever reinforced concrete balcony slab of typical condo apartment. Say balcony cantilever spans 1.5 m north-south and there is no parallel supporting wall at the east and west ends of the balcony slab. The anchor slab spans 6 m east-west and is supported by reinforced concrete shear walls. Assume there is a reinforced concrete beam at the supported end of the cantilever. Assume live load of cantilever is 4.8 kPa and on condo floor is 2.4 kPa.

Question: Is there any rule of thumb or general practice used by those designing condo/apartment buildings, as to how far the top rebar in the cantilever should be extended into the anchor span? Should it be 1.5 times the cantilever span, or 2 times the cantilever span? I suppose a finite element analysis might give and answer, but without doing that, is there an answer? This question pertains to a typical detail I am reviewing.

 

[MotorCity]

The top bars should extend at least the development length of the bar beyond the support.

 

[MIStructE_IRE]

Our old BS Code used to say 2 x cantilever length. Eurocode i think says 1.5 times... I stick with 2!

 

[JAE]

Technically the cantilever top bars should extend back into the back-span a distanced beyond the point where the top negative moment ends (i.e. the inflection point) plus an extended distance beyond that - we use a full development length beyond the inflection point.



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[ajk1]

I would not think that a development length (300 mm for a 10M bar) was anywhere near sufficient, because it will result in significant tensile stress/cracking in the top fibre of the anchor span, at the point of termination of the top bars of the cantilever. Also if a significant extent of the anchor span slab is not mobilized to balance the cantilever moment, I would expect significant rotation at the cantilever support location resulting in excessive deflection of he cantilever.
In the balcony slabs that you design on your condo projects, do you extend the top bars only a development length? If so, how have the slabs performed with respect to deflection of the cantilever and cracking in the anchor span?

 

[ajk1]

Yes I would tend to go with the 2 times, so thanks for your comment. I wonder if anyone has done any analysis of this.

 

[JAE]

2 times the development length past the support may not be adequate depending on your cantilever length.

You must go back to the inflection point of the negative moment and then some.

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[JAE]

Since your "back span" is spanning orthogonal to the cantilever span, you still need to pay attention to how the negative moment in the cantilever is dispersed back into the interior and extend your bars far enough to account for the neg. moment. You could idealize your back span to be 2 times the cantilever span.

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[ajk1]

I will go with 2 times the cantilever span.

 

[CANPRO]

I will go with 2 times the cantilever span.

Why take a guess at it with some multiple of the cantilever? Maybe you want to use 2x as a minimum, but JAE gave you the answer you need to do this proper...

Technically the cantilever top bars should extend back into the back-span a distanced beyond the point where the top negative moment ends (i.e. the inflection point) plus an extended distance beyond that - we use a full development length beyond the inflection point.

 

[slickdeals]

Agreed with JAE. If you have a short cantilever with a long back span, 2x cantilever length will not cut it.

 

[BridgeSmith]

I concur with CANPRO and Slickdeals, JAE hit the nail on the head - "...use a full development length beyond the inflection point." IOW, design the RC slab. We would just consider a strip of some convenient width (say 1m) by the 7.5m length, and design that as a beam.

 

[atrizzy]

My 'rule of thumb' is the 2x distance of the cantilever, OR, a full development length beyond the point of inflection, whichever is greater.

With balconies, don't forget that pattern loading can shift your point of inflection dramatically.

 

[JAE]

atrizzy is correct - looking at a case where the balcony is loaded but the backspan is not can create a very long negative moment zone in the backspan.

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[BridgeSmith]

Don't guess or assume - apply the worst case loading and design it. With a load on the cantilever and none on the interior span, you may have tension in the top of the slab through the entire width, so you may not be able to discontinue the reinforcement, at least not all of it, before you reach the end of the back span.

 

[EDub24]

I agree with JAE. You need to determine the inflection point with the worst cast loading and develop the bars beyond that. Sometimes what I have done is to extend the top bars from the back span into the cantilever slab using a 2-part dowel system.

 

[YuleMsee]

B.S code allows you to curtail 50% of the required steel at 0.75 times the cantilevered length and 100% at 1.5 times the length. I only do this on projects where clients are very cost conscious, other times I take all the steel to 1.5 times the length.
But make sure the Bending moment envelope shows you no longer need top steel at this point as has been said above

 

[ajk1]

Thanks everyone for their response. Much appreciated.

 

[ajk1]

Also special thanks to YuleMsee for providing specific info on the British Code.Very much appreciated.

By the way, I think some of the responses were more appropriate for the case where there is a backspan that spans in the same direction as the cantilever In the case I described in the original post, I noted that the backspan spanned at 90 degrees. Is not so easy to locate the inflection point in this case.

 

[ajk1]

...I see that JAE gave a way to model the backspan to find the inflection point. So thanks for that too.