250 mm RC Slab Spanning 9 m (One Way)

[KootK]

I have a wacky idea that needs vetting.

I've inherited a 250 mm, one way concrete slab that spans 9 m. Deflection is a problem. I'd love to thicken or post-tension it but neither of those options is possible. Instead, I'm considering packing the slab full of continuous tension and compression steel (2-15M bundled at 150 mm o/c). I intend to design the slab, for both strength and stiffness, as though it were a steel only section comprised of the tension and compression steel acting compositely (no concrete). My thinking is as follows:

1) If I use the same amount of top and bottom steel, there ought not be any curvature induced by shrinkage.
2) When creep starts to kick in, the concrete should shed it's compressive load to the compression steel.

Some concerns that I have are:

1) Usually creep is conceived of as a compressive stain. Is there a shear component to it that would invalidate my assumption of near perfect composite action between the tension and compression steel?
2) If I'm using the top bars in compression, do I need to provide ties to prevent them from buckling? That would probably be a deal breaker. If I remember correctly, there's a provision for compression members somewhere in ACI that lets you omit compression steel ties when the area of reinforcement is <1%. I'm not sure how to apply that in my case however. I've got about 1.6% reinforcing in both the top and bottom of the slab (3.2% over the whole section).

Thanks for your help.

KootK

 

[hokie66]

Sorry, KootK, but compression steel can only help so much, and it won't make your slab work if the 9 metres is a clear span. Your quasi steel beams will be only about 150 deep, so deflection will still kill the system. What are the supports for the slab? If 9000 is centre to centre of supports, can you introduce wide, shallow band beams?

As you have "inherited" the problem, can't you just give it back?

 

[IDS]

If I use the same amount of top and bottom steel, there ought not be any curvature induced by shrinkage.

Shrinkage doesn't cause curvature in a symmetrical section, but once a concrete section has cracked it isn't symmetrical any more, so shrinkage can and does cause curvature in a cracked section with equal top and bottom steel.

If the slab has not yet been designed why is post-tensioning not an option?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rowingengineer]

I like Hokies idea, maybe you could also change to bond deck or similar to increase your d. or you could use the dreaded precamber on the form work, with 60mPa concrete.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[KootK]

Yeah, I figured that I was harbouring some false hope on this one. In response to your responses:

@ Hokie: if I ignore creep & shrinkage issues, I can actually make a go of it from a deflection standpoint. 360/240 & 1.6% reinforcing top and bottom. Unfortunately, my slab supports are steel beams. My slab is pretty close to truly pinned at the ends I'm afraid. On the plus side, the slab will develop some two way action that I didn't mention. It's about 13 m in the other direction.

@IDS: great point regarding creep in cracked sections. Now that I think of it, this really isn't all that different from the situation with a regular composite beam which also undergoes creep and shrinkage deflections. Post tensioning is problematic due to a) live end access problems and b) contractor objections.

@ Rowing: It may indeed come down to that. What do yo think with regard to magnitude of camber? 0.7 x self weight?

 

[rowingengineer]

KK,
I hate camber it is so easy to over spec, I would go for 0.5 self weight excluding creep effects.

Best way to handle creep deflections is to up the concrete strength.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

Why is your depth restricted to 250? If it is because the steel is already detailed/fabricated, maybe you can just increase the depth gradually toward midspan.

As to the live ends of PT tendons, they don't have to be at the slab edge. You can stress in pockets.

 

[KootK]

The 250 limitation is a headroom issue. We're matching the floor levels of a very old existing building. That's an good point with the stressing pockets however.

 

[rapt]

Precamber is only an option if your problem is total visible deflection.

If you are worried about deflection effects on something on top of the slab such as partitions or brittle finishes, precamber has no effect as the total slab movement is still the same.

Re Shrinkage, as explained above, it is the relationship of the top and bottom reinforcemnt to the cracked centroid that is critical. For a slab, the compression face steel will not be much above the centroid so it will have very little effect.

Re Creep, the last comment above also applies, the compression face reinforceemnt will have very little effect as the compression stress at the level of the reinforcement will be very low.

Do not make the assumption that the kcs factor for long term deflection applies in thin members like this.

 

[hokie66]

Another way might be to use 250 steel beams, with concrete between. But then you probably have fire rating issues...