RC slab deflection check EC2

[peppast]

hello everyone,

I have calculated the required reinforcement at the mid-span of the slab and trying to do the deflection check. At EC2-1 @ 7.4.2, i need to find ? which is the required tension reinforcement ratio at mid-span to resist the moment due to the design loads. Is it ?= As/(b*d) ?

 

[apsix]

You've already found it, now you just need to use it.

Yes, ? = As/(b*d), where As is the tension reinf't area

 

[peppast]

Thank you very much for the reply. Now the problem is that when I am calculating the maximum allowable length/depth according to the chosen reinforcement area, is extremely high(40m length) and it is increasing as I am reducing the reinforcement area which is absurd.

 

[peppast]

As required= 263

As provided= 523

length of the slab =5200mm

depth = 175mm

effective depth= 130mm

 

[IDS]

There is a design chart for deflections here:

http://www.concretecentre.com/PDF/5 Slabs.pdf

that might be useful.

I presume that the span/depth ratio goes up for reducing steel on the basis that less steel indicates less bending moment, but that doesn't make much sense to me either, since they seem to be working on the basis of a reference stress, and similar sections with different moments and steel percentages, but equal steel stress will have similar curvatures.

What makes even less sense is that it seems to allow an infinite span/depth ratio if p = p'.

It might be worth contacting the Concrete Centre for their comment, but I'd recommend doing a proper deflection calculation, taking account of creep and shrinkage, anyway.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

I attended a presentation on this by the person you wrote the CC documents on EC2 a few years ago. His example was completely wrong at that stage. The basic result of the way he approached the calculations was that long term deflection is directly related to the steel percentage. This is absolute rubbish. Doubling the area of tension reinforcement does not halve the deflection as it did in his example.

I do not know if they have improved this paper since then.

Doug, I agree. Do it properly. Simplified L/D methods are a good way to end up in court!