Formwork Camber for Flat Slabs

[ajk1]

Does anyone have a typical detail that lists the required formwork camber for flat slabs with drop panels (and perhaps also for flat plates)?
I am reviewing a typical detail sheet that we have for formwork camber for flat slabs with drops, giving cambers at critical locations such as mid-panel and mid-span of interior and exterior spans of flat slabs with drops, for 3 sets of spans, viz. 5.8 m to 7.6 m, >7.6 m to 9.0 m, and >9.0 m to 10.7 m. However I do not know how this was developed. Some of the cambers seem suspect because where I would expect a lesser deflection than another location, the specified camber is greater.
I would expect that the camber should equal to the immediate dead load deflection...but I am not sure. Perhaps the dead load deflection should be determined by running SAFE for a number of geometries/loadings.
Or is it the practice to to take a ratio (span/360±?) of the orthogonal spans and diagonal span to determine the cambers, and if so, what would that ratio be?
So to repeat my question, does anyone have a typical detail that lists the required formwork camber for flat slabs with drop panels (and perhaps also for flat plates)? And is there a span ratio should the camber be made equal to?

 

[JAE]

Camber in flat slabs/plates is purely based, or should be, on engineering judgement and calculations. Calculations will only get you so close to the actual deflections that a slab will incur.

Here's a link to a story about a project where rather intense camber calculations were developed:

https://www.structuremag.org/?p=11615

Here's some other links on the subject:
thread507-108861

https://www.forconstructionpros.com...roach-form-camber-specifications-with-caution

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[mijowe]

If I was putting together a typical detail for camber, any camber would reference the floor plan where spot cambers would be shown. As noted above, the camber may be betied to the span, but so many other variables of the design would go into it that tieing it just to the span would be an oversimplification.

I would also be surprised to see camber at all for the lower end spans that you list, deflection with a reasonable slab thickness should not be a problem.

 

[JAE]

If you read through the Structure Magazine link, you get the distinct impression that the engineer on that project developed a camber plan for the whole floor vs. pinning a camber amount to a span distance.
(i.e. end spans are different than internal spans).

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[ajk1]

Thank you all and thanks particularly to JAE for the attachments, one of which has specific suggestions for amount of camber.
In the old days (20th century)we never cambered but we did not span so far or reduce thickness less than Code empirical equations indicated. We had no deflection problems then. But thise days are gone forever.

 

[WARose]

Here is a pretty good article that discusses tolerances (including floor slabs) for forms:

https://www.awci.org/cd/pdfs/8003_e.pdf

 

[DTGT2002]

We should be careful in how we approach our wording on all of this - I may camber my formwork to counter anticipated deflection in my formwork and call it formwork camber, but when we want the structure to have a certain initial camber, it must be clear that this is the intent. On a recent revision of ACI 301, there was quite a bit of discussion as to the context and use of the word camber in the document.

When the specifier requires a camber, that calls out an indicated profile of the final work, so the formwork should be made (and priced) to provide that profile. But calling to formwork camber could seem to some that the actual camber of individual members of the formwork or falsework would be per the direction of the speficier.

Yes, the initial camber is created by the proper setting of forms, no I wouldn't call it formwork camber.

 

[hokie66]

Much too complex. I did a lot of flat slabs, never cambered one. But that was in the "old days", for which I yearn.

 

[ajk1]

Thank you all. I will be more careful. I should not have said "formwork camber".

 

[hokie66]

There can be unintended consequences in cambering concrete. What happens if it doesn't deflect as expected? I remember cambering a cantilevered balcony once, it never deflected as predicted, so the water ran back toward the interior. Just saying, deflections in concrete are not as predictable as in steel, no matter how sophisticated you get.

 

[ajk1]

Well said. I realize this.

 

[ajk1]

Has anyone established what the approximate percentage cost extra is to camber the forms vs. not cambering them for a flat slab with drop panels, say for a 9 m span? Any formwork contractors on this forum? Anyone who perhaps tendered a project both ways, or has obtained these costs for a project from a formwork contractor? If not I suppose I can ask a local formwork contractor.

<5% ?
5% ?
10% ?
>10%?

 

[DTGT2002]

I'll see what we would rough out from our estimating department.

There are times when the geometry of the structure happily aligns with logical breaks in equipment dimensions and the costs would be negligible. And there are times when the whole world would need to be tweaked to accommodate a camber so small it could get lost in construction tolerances. So any feedback will come with quite a few grains of salt.

 

[ajk1]

to DTGT2002 (Structural) - that would be very helpful of you if they have such cost data. If the calculated camber is small, I don't think it should be specified. Cambers < 18 mm probably should not be required, in my opinion. What do you think?

 

[DTGT2002]

I would agree that a target camber that small would be difficult to appreciate (and frankly measure!). But I have seen 1/2" listed in specifications a time or two.

On one hand, it can help someone think about a potential issue and trend towards the best result. Or it can confuse them.

 

[KootK]

@DTGT2002: are you actively involved in formwork design? I'm in the process of getting into that space myself. I would be good to know if we've got an expert on staff.

 

[BAretired]

With fly forms, two way camber might be difficult to achieve. One way camber may be more practical.

BA

 

[ajk1]

to BAretired (Structural)- an interesting point you raise, about flying forms.

On another point: Regarding a flat slab without edge spandrel beams, should a camber be specified for the edge column strip or should it be specified as zero camber? Is there a concern that the camber may not come out when the slab deflects, making problems for connecting the cladding? On the other hand, if the camber is specified as zero, and then the slab deflects, doesn't that also cause a problem for the cladding connections?