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Back Anchorage for Cantilever Slab

 
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George.Philip
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PostPosted: Sun Feb 11, 2018 5:42 am    Post subject: Back Anchorage for Cantilever Slab Reply with quote

Dear All,

I have a condition at the site where I have to remove the formwork support provided for the portion of slabs left as cantilever due to the construction joint. It will take 3-4 months to cast the remaining slabs after the construction joint. The length extending from the column line is 2.70m. The slab has enough top reinforcement to take the moment from the probable dead and live loads. But the top reinforcement has a back anchorage to the interior slab is more than 100xdia of the bar and but less than the length of the cantilever. But usually, the back anchorage provided is 1.5x cantilever length.
Since the slab will be temporary cantilever, is it mandatory that the back anchorage should be 1.5x cantilever length or Can I consider the slab with the capacity to act as a cantilever?

Thanks in advance.



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es_jayakumar
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PostPosted: Sun Feb 11, 2018 1:57 pm    Post subject: Reply with quote

Dear Er.George,
Theoretically speaking, you have to extend the top bars to a distance of 12# beyond the point of contraflexure in the adjoining slab, even though the thumb rule is to provide the extension equal to the cantilever length. In your case, the extension already provided has to be checked for adequacy. In case the cantilever slab is provided in between cantilever beams (that means, supported on 3 sides), the point of contraflexure may not extend long into the adjoining slab. You need to locate the point of contraflexure and see if the criteria is satisfied. You may please post a detailed dimensioned plan of the structure showing slabs and beams - present as well as future.

Regards,
E S Jayakumar
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spsvasan
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PostPosted: Mon Feb 12, 2018 2:59 am    Post subject: Re: Back Anchorage for Cantilever Slab Reply with quote

George.Philip wrote:
Dear All,

I have a condition at the site where I have to remove the formwork support provided for the portion of slabs left as cantilever due to the construction joint. It will take 3-4 months to cast the remaining slabs after the construction joint. The length extending from the column line is 2.70m. The slab has enough top reinforcement to take the moment from the probable dead and live loads. But the top reinforcement has a back anchorage to the interior slab is more than 100xdia of the bar and but less than the length of the cantilever. But usually, the back anchorage provided is 1.5x cantilever length.
Since the slab will be temporary cantilever, is it mandatory that the back anchorage should be 1.5x cantilever length or Can I consider the slab with the capacity to act as a cantilever?

Thanks in advance.



Dear Er.George

This is to supplement Er.Jayakumar's comments.

Is this an actual problem? The reinforcement detailing shown in your sketch seems to indicate that this is a hypothetical problem. There is no bottom reinforcement for the proposed span and the top bar curtailment is not consistent with the span arrangements. However, assuming that you are facing a real problem, I give the following comments.

Since the temporary cantilever seems to be pretty large, it is advisable to carry out an analysis of the existing slab with the temporary cantilever, work out the bending moments at the point of curtailment to check whether there will be a problem.

My initial reaction is that curtailment at 100 times bar diameter will be grossly inadequate and there might be cracks in the top of the slab at the curtailment point if the props are removed.

If the slab is already cast, it is better to have it propped till the balance is concreted. The props can be suitably arranged to minimise inconvenience to the users if any.

Another option is to add a temporary heavy load on the adjacent span so that positive moments will develop quickly. Here again we have to check the safety of the various sections and reinforcement with calculations

Calculations are essential to arrive at a safe solution

S.P.Srinivasan
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George.Philip
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PostPosted: Mon Feb 12, 2018 5:01 am    Post subject: Re: Back Anchorage for Cantilever Slab Reply with quote

Dear Jaykumar Sir and Srinivasan Sir,

Thank you for replying.

The slab arrangement and details are provided in attached images. I forgot to mention that the top bars are spliced at 100xdia distance from the column face and the bars are not curtailed. The bottom reinforcement was also not shown in the previous post.
However, I understand that from your reply that I need to check the moment and find the point of contra-flexure without assuming anything.

Thank You.



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es_jayakumar
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PostPosted: Mon Feb 12, 2018 12:20 pm    Post subject: Reply with quote

What I understand is that you have provided top and bottom bars throughout in the slab. Then, you need not have to do the exercises such as locating the point of contraflexure etc. However, you may check the adequacy of the top bars provided in the 2.70m long cantilever slab, to take the hogging moment.

E S Jayakumar
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George.Philip
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PostPosted: Tue Feb 13, 2018 5:44 am    Post subject: Reply with quote

Dear Sir,

The steel reinforcement is continuous throughout the slab(already casted). I have checked the capacity of the slab itself for taking the hogging moment. But the concern is only that location of the splicing is occurring at 100xdia length.

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vijaya_vittal
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PostPosted: Thu Feb 15, 2018 6:41 am    Post subject: Back Anchorage for Cantilever Slab Reply with quote

SO FAR THE BALANCING MOMENT IS MORE THAN THE DESTABILIZING MOMENT AND THE THE REBAR HAS SUFFICIENT ANCHORAGE IN COMPRESSION ZONE OF THE CONCRETE U CAN REMOVE THE SHUTTERING TO THE CANTILEVER SLAB

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