Shrinkage & Cracking Reinforcement 1

[pappyirl]

I am investigating the possibility of post tensioning a 200 slab with quite a small footprint. The building is quite tall and a thin slab is very desirable for foundation loadings etc. The spans are roughly 10m square and each bay is continually supported at each edge i.e. the slab is essentially spanning within a box.

The obvious problem from the support arrangement is restraint. I have looked at releasing the slab from the walls but i believe it may be easier/cheaper from a builability viewpoint to provide top and bottom steel to resist shrinkage and service cracking throughout the slab. The PT providing the uplift and ULS strength.

I can work out the shrinkage strains and cracking stresses easy enough and provide sufficient steel to cater for these effects. Torsion steel at the corners will also be required. Is there anything else i need to consider that i may have missed?

I am also worried about vibration (residential) of the 200 slab over the 10m. It is quite thin, although it is supported on 4 sides and from my analysis, deflections (pinned wall connection) & ULS are adequate. The project is at quite an early stage and i don't have time to carry out a modal analysis which will probably not result in an accurate prediction regardless. Does anybody have any thoughts/experience in what i am trying to achieve?

 

[civilperson]

The thin slab is OK, but the span is excessive. The depth of the edge supports of the slab are limiting the floor to floor heights so why not divide the slab in quadrants with center line beams with depth equal to edge beams.

 

[pappyirl]

civilperson, I'm not sure how you can say the thin slab is ok but the span is excessive. The span is one variable i can't change so i assume you mean the slab is too thin.

For a post tensioned 2 way spanning flat slab spans of L/40 to L/45 for light residential loading is not uncommon. The support structure of this arrangement is what lends itself to improve on this ratio, hence the L/50 ration i am achieving.

I never mentioned edge beams, because there are none. The floor to floor height will be measured to the soffit of the slab.

 

[civilperson]

"each bay is continually supported at each edge", I mistakenly thought that meant supported with beams, i.e. spandrel type edge support. For non-human service, this may suffice. For human comfort and perception, this will feel like a trampoline.

 

[civilperson]

Floor to floor height is self defining, your definition is not used by the rest of the world.

 

[mrengineer]

If you have a restraint issue, my concern would be that the slab might not obtain the desired precompression, possibly resulting in excessive deflections.

 

[rapt]

pappyirl,

L/40 is ok for internal spans of flat slabs. For a simple slab supported on 4 sides, I would not use less than this, so 250 would be my minimum for deflections and vibrations.

Remember if you are going to take care of the restraint with reinforcing in the slab (the correct approach by the sound of the slab), the resulting stresses will crack the slab and severely affect your crack control, deflection and vibration calculations.

 

[dik]

With stiff edge members, the slab thickness should be greater than the perimeter / 200... This is for smaller spans and with regular concrete... 8" is likely OK and would have been my first guess... You have to be careful about losing prestress to the edge members and they should be designed accordingly... also have to watch out for corner levers from the two way flexure... often causes a 45 degree crack parallel to the diagonals...

Dik

 

[NewImhotep]

pappyirl

200mm can work for 10m slab,specially if you have a stiff building with a lot of walls , but analysis for sure will be required to confirm that. for my self I never give a decision like this until I do analysis.

also it is depends to the loads you have. what is building type office , residential ,parking? what is partations weight finishing type and thickness.

It is better to do analysis with adapt or ram ( usually it should not take more than 2 hours arrange a dxf file then import it from adapt and start the model and check)
If you can't do that you have to go to 250mm to avoid any thing in future.

vibration should not be an issue if your live load is small (in my experience live load >4 Kpa or deflection from live load >10mm will start to make vibration problems).
restrain is nothing to be controlled by slab thickness , usually it is controlled by the rebar , live /dead ends& pour strips.

I don't know where you are but try with concrete grade not less than 45 Mpa (cube).

Regards

Ahmed

 

[dik]

I'm not sure about 45MPa... would have thought something like 25MPa... Unless you have large amounts of reinforcing and unless for shear... concrete strength has little effect. On a 4 side stiff support, deflection is not likely going to be an issue.

Dik