Slab Top Reinforcement 4

[benjaman]

Hi,

I came across the design of a slab with edge beams which has nominal negative moment near support and the continuous positive reinforcement which is continuous through this region is theoretically adequate to cope with that moment.

My question is, in that case do we need to provide any top reinforcement in the slab at all? Can anyone quote any code reference regarding this?

Thanks.

 

[asixth]

Depending on where the job is located. Refer to Concrete Codes for this design, possibly Eurocode 2. If it is a suspended slab, they generally won't have top reinforcement midspan for slabs with regular column spacing. If your not a practising structural engineer you really should be running this design past one. Are you working in a multi-disciplinary office?

 

[benjaman]

Sorry, I guess I did not make myself clear enough.

I was talking about negative reinforcement near the supports. Theoretically no negative reinforcement is required in this particular case as the slab is very lightly loaded and the span is very small. So i was asking whether it is compulsory to provide negative (Top) reinforcement even when not required according to calculation. This design is performed in accordance with ACI. I could not find anything regarding this in ACI.

Hope I have been able to explain my problem now.

Thanks.

 

[ishvaaag]

In the past O would say a lot of structures were designed on the simplification of beams being reinforced as simply supported. It does not only works, it was even acknowledged practice at the ETSAM where a tenet was running saying that "reinforced concrete structures behave as they are reinforced". And in fact you can count as some corollary of the principle of the minimum energy of deformation that the structure as allowed by the restraints shall deform in accord to such principle in the most efficient way, reserving the maximum energy of deformation it can for additional loads. Initial restraints and the effect of local failures as the deformation advances are the milestones of meeting progressive loading of the structure.

So in a continuos beam, failure to meet the negative moment may develop a crack and put the beam in the search of if there's a positive reinforcement enough to meet the loads. If the beam is deep enough and deformation small it may become just an aesthetic problem.

Even if moment redistribution according to modern codes (and not at all modern, I have a book of the nineteen forties that demands such check) is quite limited, the practice subsists surely on its strengths of simplicity and proven usefulness. Its main weakness are of course greater cracking where moments standing from elastic analysis are not met. I would say that other than in simple spans of any kind having some restraints at ends that are disregarded the most likely element where this practice survives is in transfer beams for columns, designed for positives moment only. Also you can see it (far less frequently) in grill foundations where to avoid the analysis of the beams on the ground, only an inverted positive scheme is used, the loads then being adjudicable by hand by tributary division of those at the columns. This olden practice, that had the working rebar atop and so the farther from the soil is most rarely seen today.

 

[Lion06]

I've never done a complete concrete project, but I would imagine you would want top steel to limit crack widths over the beams. It's one thing to provide a direct and safe load path, but you also have to worry about serviceability (those pesky flexural cracks that will happen at the negative moment regions).

 

[JAE]

benjaman,

For strength, if you assume pinned ends as ishvaaag describes, then you are OK without the top bars for slabs in general. But what happens then is that the concrete will have a tendency to crack at the support ends, near the inside faces of the beams on the top surface. Placing negative moment steel there, or some nominal amount, will help control that cracking with is really a servicability issue, not a strength issue.

We've designed numerous beams like this as well, assuming pinned ends on the outer supports of continuous spanning beams. But then we came back and placed some level of top bars there, perhaps some rough approximation of what the top steel would likely have been. For continuous beams we would have assumed the pinned ends on the outer columns and then place perhaps 40% to 60% of the steel in the top that was required over the first interior support.

 

[benjaman]

Thanks Jae,

I too want to recommend the the designer to put nominal negative moment steel at support ends. It would be great if I could show him some code references regarding this. Otherwise, i have to land on series of arguments with him. But I guess there is nothing mentioned regarding this in ACI.

 

[Tomfh]

What part of ACI is it designed under?

There are a few clauses in ACI about negative reinforcement at edge beam supports.

 

[apsix]

From BS8110
Clause 3.12.10.3.2 Curtailment of bars at end support of slabs (where simple support has been assumed in assessment of moments)
Despite this assumption, negative moments may arise which could lead to cracking. To control this, an
amount of reinforcement equal to half the area of bottom steel at mid-span but not less than the minimum
given in 3.12.5.3 should be provided in the top of the slab at the support.

Strictly speaking this applies to end supports only but a similar requirement could be used for interior supports.

Personally I would put in top reinforcement as required for the negative moment, to control cracking.

 

[asixth]

If you don't put top steel over this support you will get flexural cracking at the face of the column which has already been mentioned above by SEIT and JAE, but this cracking will also lead to a major reduction in stiffness and increase the deflections midspan. The elastic deflections equation for a simply supported beam is 5*w*L^4/(384*E*I) whereas the elastic deflection equation for a beam rotationally restrained at the supports is w*L^4/(384*E*I). So you can see by allowing this cracking will dramatically increase you deflections.