camber in hollow core slabs 2

[Lutfi]

I have a building where we are using hollow core labs. We are requiring 2 inch topping and counting on the topping to carry the horizontal shear from wind.

1. How does the camber affect the topping? Is it normal practice to place te inches ay the highest point and add more topping to achieve level away from plank center line?

2. Is it more popular and cheaper to rely on the plans carrying the shear in lieu of the topping by adding welding the plans along their joints using embed plates?

Please advise.

 

[MotorCity]

Lutfi,

I definitely would not rely on a 2" thick topping to transfer or carry any loads. Generally, toppings are not structural elements, especially when they are only 2" thick. When calculating the camber, you should include the deflection due to the weight of the topping. The desired minimum topping thickness should occur at the peak of the camber and increase in thickness toward the edges. Your statement 1. above is typical construction practice to achieve a level floor.

However, a word of caution......be careful if the finished flooring surface is tiled. The joints between the hollow core planks and the topping should be carefully detailed to prevent damage to a tiled floor.

 

[haynewp]

You need 2 1/2" over precast to be used as a seismic diaphragm if it is not acting composite with the precast.
ACI 02 21.9.4

 

[jike]

I would recommend that you check with your local precaster as to what he anticipates the maximum camber to be.

If you have any significant camber, you may have to use 3" topping.

The topping is usually the specified thickness at the plank ends and so would be less in the center (topping minus camber). The Architect may not like to see the floor crowned. You don't want thicker topping at the ends since it may affect the doors or alignment with adjacent floors.

 

[JedClampett]

For camber, ideally the camber should be negated by the topping load and dead load of the slab. In place the slabs sit totally level. In practice, the camber is driven by the number of strands, the precast engineer needs to carry the moment. They should roughly coincide.
As far as the shear transfer, I've used the grout key that ties and seals the gaps between the hollowcore slabs. See the report below:

http://www.icc-es.org/reports/pdf_files/ICBO-ES/PFC2755.pdf

 

[KarlT]

Some things to keep in mind.....

1. While cambers can be calculated they are only estimates at best. I have seen groups of identical slabs (in terms of length and strands) in the precast yard with varying cambers resulting from things such as which one was at the top of the pile & in the sun, winter ot summer conditions during storage, a slightly different mix, the way in which the slabs are blocked (blocking a couple feet from each end can affect the camber calcs significantly).

2. The precasters will sometimes rely on a nominal 2" composite topping to increase the slab spans. The topping would need to be fully bonded to the slabs below in order to count on it for composite action either for gravity loads or lateral stability loads.

3. When we specify a 2" topping that implies a minimum 2" at the mid span of the slab, and it would then increase to more than 2" at each end. Most clients want a level floor, so we don't ususally see someone trying to provide a 2" uniform thickness over a cambered slab.

 

[Lutfi]

The project is in Florida. We are using 2 inch topping minmim with WWF.

I apprecaite the input from everyone.

Great forum.

 

[mrengineer]

My standard practice with planks is to use a nominal 4" topping, with a note on the drawings that this is the thickness at ends of planks and will be less at mid-span depending on amount of camber. This may seem like a lot, but I have yet to have anyone question it, and it solves a lot of problems with differential camber, topping too thin to place w.w.f., adequate room for conduits etc. On a recent project I tried to be a good guy and specify 3" at roof condition and 4" at floors. I got a call from the contractor asking if it was o.k. to use 4" at roof due to conduits and excessive camber. (I asked the design team at start of project if there were conduits and was told "probably not"). Another thing that has happened to me more than once is that the contracor decides on his own to place control joints in the topping. Also, 4" topping in combination with 8" or 12" planks details out nicely in multi-story masonry buildings. On double tee projects with long spans you also want to be very generous with topping thickness. Some of the differential cambers can be downright jaw dropping, especially where short tees are next to long tees.