Cantilever Concrete Slab 2

Hi
I have a 5ft span cantilever concrete slab. How far I should extend the reinforcement past the point of maximum moment? Should that be only the development length? In order the clarify, I'm asking about the negative reinforcement that goes to the next span.
thank you very much
Nathan

 

[austim]

You need to calculate the bending moment distribution over the full length of your slab (allowing for all possible variations of live loading), and ensure that at EVERY position there is sufficient properly anchored top and bottom reinforcement to resist the maximum/minimum moments.

 

[trilinga]

Generally the negative reinforcement shall be extended into the adjacent span for a length of 25% of the adjacent span or the development length of the reinforcement rod whichever is greater. If the adjacent span happens to be a long span of an one way panel (requiring only secondary steel) it is enough that the rods extend for a distance equal to the development length.

 

[DaveAtkins]

ACI 318-95 Sections 12.10 and 12.12 have explicit instructions for this situation, so that all live load patterns are accounted for.

 

[JAE]

trilinga - I hate to take exception directly to anyone, but simply taking the top steel from the cantilever into the adjacent span the development length, or even 25% of the span length, is not a correct application.

A cantilever may create negative moments into the adjacent span that extend some distance from the support, sometimes even all the way across the span where you never develop any positive moment in the entire length at all.

Thus, what austim stated (and what DaveAtkins referred to in the ACI code) is the proper course here. The top bars must extend a distance beyond the point where they are no longer required. This distance in the ACI code is the greater of:

depth of member/slab
12 times the bar diameter
1/16th the span

 

[trilinga]

JAE

I sincerely apologise for my half-baked reply. I wrote the post when my body temperature was a few degrees more than 98,4 Deg. F, but that is definitely not the excuse. You are right about short spans adjacent to the cantilevers subject to full negative moment. For an adjacent span of comparable dimensions my suggestion may hold good. The distance has to be calculated based on the moment distribution and the location of point of contraflexure.

Thanks.

 

[JAE]

trilinga - an apology isn't necessary - Your reply is probably appropriate for 90% of cantilever situations and we basically follow what you suggested for the most part. I was just a little nervous that NathanEIT "might" have a situation where the negative moment went further - and the simple development of 25%L wouldn't cut it.