Pre-camber concrete mid-floor query 1

[Haiden S]

Hi,

I need help with a cantilever concrete mid-floor structure please.

I have cantilever UC beams (2/250UC90's) supporting a V-Span/Unispan concrete deck. The total deflection of the cantilever is around 40mm which is too much for the glazing at the end of the cantilever, thus I will pre-camber the steel beams to allow for the weight of the concrete floor. The issue I have is that I need the full weight of the concrete to act on the cantilever beams in the temporary case, but the unispan needs to be propped in the temporary case so the full weight will not be acting on the beam. Once the props are removed the beams will continue to deflect past acceptable levels.

Has anyone had this problem before and found a good, practical solution?

I cannot increase the beam depth due to Architectural requirements.

Any help is appreciated,

Thanks

 

[rapt]

How is pre-camber going to help you? The total movement at the end is the problem, and the total movement is the same with or without pre-camber. The only difference is the starting height.

 

[Tomfh]

Pull the precambered beams down with threaded rods or chain blocks, and release them once the Concrete is in.

 

[Haiden S]

Thanks for the replies.

rapt - I'm only really concerned about the deflection "from flat" or deflection after the concrete has set, not total deflection. If I don't pre-camber the beam then the end of the cantilever beam will be too low for the joinery (which has no real tolerance..)

Tomfh - Thanks, I did consider this option - just thought the builders might not like me for it.. might be the only solution though

 

[molibden]

How do you precamber steel beams? What method do they use? Is it possible to precamber just the cantilever?

 

[r13]

Maybe I don't understand your concern, but if the beam with slab on top can be jacked up (pre-camber) and carryout strengthening/post-tensioning process, with proper consideration in designing the process, upon release the shore, the beam shall deflect to the desirable/design elevation, but not drop back to the original elevation (with excessive deflection) due to increase moment capacity at the beam support.

 

[JLNJ]

I don’t have experience with this particular floor system.

That said, my gut feel is that cambering is not an exact science and you should not rely on it to meet an architectural component’s tight tolerance. Especially with a non-uniform framing arrangement such as you have.

 

[r13]

Yes, effect of cambering is difficult to predict, but with proper design and method, it can bring the deflection within tolerable range. By the way, I think it is a science, just have too many factors involved. Remember that science never meant to be "exact", though it is the aim.

 

[structSU10]

I don't quite follow the problem. If live load deflection is causing an issue camber will not solve the problem. If live load deflection alone is acceptable, won't the window system be installed after shoring is removed / all dead load is applied and you have no issue?

 

[NathanNZ]

When you release the propping for the unispan will this not bring the precamber down to the required level?

 

[Haiden S]

Thanks again for the responses:

molibden - The local steel fabricators use cold-cambering so plastic deformation by force, and yes they can camber only the cantilever section

retired13, JLNJ & NathanNZ - I agree that when the props are released, it will undergo more deflection and will probably get to within an acceptable tolerance and so maybe I shouldn't be too concerned but for the deflection to occur, the concrete will have to crack and @NathanNZ I agree, the composite action will affect these deflections. Composite action will affect the backspan more than the cantilever as I won't get effective composite action acting where there is hogging moment.. It makes the predicted long-term deflections hard to calculate so ideally I wouldn't try predict these deflections.
The cracking may be within tolerable levels so maybe I shouldn't be too concerned but ideally I wouldn't want too much cracking within the slab (I know concrete cracks anyway but I would still like to reduce cracking where possible).

structSU10 - Live loads are not my concern as they are within acceptable limits, and yes if there was more tolerance within the joinery it wouldn't be an issue but I don't have any tolerance to work with. I would also like an even cover of concrete throughout. If I prop the beam, I will get 40mm deflection after removing the props and cracking at the point of hogging moment and if I don't prop the beam (and don't precamber), I will get more concrete at the points where deflection is greatest and still 40mm deflection.

The image below doesn't show the cantilever beam but the beam is at the same height as the circled beam, just spanning along the page.

 

[human909]

Tomfh nailed it I believe. There really isn't a whole lot of choice given your other stated constraints.... You need a load on those cambered beams while the concrete sets.

The other option might be to use additional temporary cross member members back to the UC beams to support the Unispan formwork rather than propping.

 

[Tomfh]

Yeah that could be easier. Provide additional framing to carry the deck.