Camber of precast slab 1

[JStructsteel]

I have 3 precast slabs for a project. They vary in length from 28'-7" down to 21'-4". Obviously the 28' span will deflect the most. I calculate the DL only deflection, and its not horrible, only about 1/2" due to dl. When I add the LL to it, obviously it changes my Icr, and the deflection zooms up. I am trying to have them cast with a camber, but the range of spans will make that hard. I was thinking just camber the first slab, and then the other 2 smaller spans let them be. With a 1/2" camber in the first, I would get the first and second to match up, but the 3rd will be the problem, it will still be less deflection. Plus, I doubt these will ever see the full LL, thus that deflection not ever be seen.

Thoughts on how to handle this?

 

[r13]

Don't understand what you meant "match up" the beams. If the service deflection criteria is met, I wouldn't worry too much on live load effect - a transient phenomenon.

 

[JAE]

All three will have some level of natural, inherent camber.

Because of the different spans, none will ever match up at the peak, or along the length of span.
I assume you are using a topping slab?

 

[JStructsteel]

Retired13, these are 6'-6" precast planks, with a varying length. Sort of trapazoidal. Delfection is met, but the longer one will deflect more, and thus a lip between the pieces.

JAE, no topping, the Architect didnt want to use one.

 

[Brad805]

Is this conventionally reinforced, or stressed? If conventional, call the precaster. Precast decks are perfectly flat, and most will not add to their decks to precamber the slabs. Secondly, your calculations are unlikely to closely predict the actual deflections immediate or long term.

 

[r13]

As mentioned, if the live load deflection is within code specified allowable (that not likely to affect structure above), I wouldn't worry about it, as if to chamber without strength concern, it only enhances aesthetic impressions, an un-necessary but good practice in most cases. However, the longer and the shorter panels will deflect a different amount, thus I suggest to camber the longer panel to the absolute deflection between the two produced by dead weight; or to chamber all 3 panels, with the shorter panel chambered to the deflection by its own weight, and the longer chambered the sum of this amount plus the absolute difference mentioned above. After set in place, the instantaneous deflection of the longer panel will then take care of the matter to flatten the finished surface.

 

[BridgeSmith]

The simple solution is to add an overlay to level it out after placing the panels and grouting the shear keys. If that's not an option, then either temporarily shoring the longer span or preloading the shorter span(s) could work to get the panels aligned for grouting.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[SlideRuleEra]

I have 3 precast slabs for a project. They vary in length from 28'-7" down to 21'-4".
...these are 6'-6" precast planks, with a varying length. Sort of trapezoidal.

Is this the plan view, with supports at the ends of each slab?

Differential deflection between slabs may not be a big a concern as assumed. Note that where the slabs adjoin (shown in red and blue) the span on each slab is exactly the same. Look into this.

http://www.SlideRuleEra.net

 

[r13]

Yeah, missed that part - zero differential at joints for rigid bodies.

 

[JStructsteel]

Yes, that makes sense. For the first piece,(long side)28'-7" I am at L/187 on the long side, and L/252 at the short side (first joint). I think using a L/220 total deflection number is safe, like retired13 said its unlikely that will ever happen. Im using live load 100psf, its a outdoor walkway.

Thanks

 

[r13]

I am surprised not to use topping for this situation. Is this a roof top application?