Expert: Arshad Khan - 12/28/2009

Question
QUESTION: DEAR MR. ARSHAD
WE HAVE 26 CM POST TENSION SLAB , BUT AFTER 3MONTHS SINCE WE HAVE CASTED THIS SLAB WE NOTICE IN ONE OF SPANS (9.7M LENGTH)THAT THE DEFLECTION IS ABOUT 5CM (FROM SELFWIEGHT OF SLAB).
-WHAT.S THE ACCEPTABLE VALUE FOR DEFLECTION IN THIS CASE?
-IS THE THICKNESS OF THE SLAB ACCEPTABLE
-WHATS THE SOLUTION

ANSWER: Hi Maher,

You have not indicated whether the span in question is on-way or two-way spanning, continuous internal span or edge span, what the effective depth is, and whether this is a flat slab or one with downstand beams all around. Even with the best scenario, your slab is on the limit for span/depth ratio. I suspect that creep effects and loss of tension have exacerbated the situation.

You have also not stated the function of the slab and the design super-dead and live loading. All this determines the acceptable deflection. If the slab is a roof, then you may get away with just filling the deflection with lightweight concrete. If the tendons are not bonded (grouted), you you may be able to go back and re-tension them. Otherwise the only other option (apart from demolish/re-build) is to apply external reinforcement such as carbon fibre panels, as was done for the lower floor slabs in the Burj Dubai tower.

---------- FOLLOW-UP ----------

QUESTION: MR ARSHAD
KINDLY CHECK THE DRAWING OF THIS TYPICAL SLAB ,AND HELP ME WITH ACCURATE ANSWERS FOR THIS CASE ,
WHAT.S THE ACCEPTABLE VALUE FOR DEFLECTION IN THIS CASE?
-IS THE THICKNESS OF THE SLAB ACCEPTABLE
-WHATS THE SOLUTION
-COULD I ADD BEAMS TO SUPPORT
THANKING YOU
BEST REGARDS

Answer
Hi Maher,

From what I can make out, this is a flat slab and the span in question is an end-span. In this case the thickness is insufficient, and the long term deflection (including live loading) should be restricted to about 40mm. Incorporating beams may help, if you can somehow post-tension them, or you can incorporate I-beams, with either pre-cambers or inserts at mid-spans between beam and slab soffit to create some sort of load sharing even before live load is applied.