Expert: Arshad Khan - 10/2/2011

Question
QUESTION: dear sir,
hope you will be fine...good to have you back after your vacation.
actually im designing a 11 storey tower with 9mx9m span...the length of the tower is 85m and the width is 27m. zone is zero. it is a requirement to avoid expansion joint within this tower...ive analysed this buiding in ETABS but i am not sure how to consider the expansion/contraction of this tower..please note that the outer edges of the slab are not exposed to the outer side...please suggest a solution to avoid cracking of slab due to excessive length...
regards

ANSWER: Hi Kamran,

Just work out the maximum daily variation of the temperature of the slabs (worst scenario will be during construction), then apply that temperature as a load case in ETABS. The load combos should be in accordance with ACI-318, or the SBC.

What this will do is to apply horizontal displacements to the columns, working outwards from the centre of the 85m overall length- so the outer columns will have maximum lateral deflection, which will impose additional moments & shears (and possibly additional pdelta moments), so you just need to cater for these.

There will also be additional axial loads in the beams and in-plane forces in the slabs, but these are usually minimal, unless expansion/contraction is restained between RC shear walls, in which case they can be substantial.

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QUESTION: dear sir,
wat ive done is that after running an analysis in ETABS(without TEMP load), exported the model to SAFE and applied TEMP load there with a 30degree temp gradient..when i designed the slab for the ASCE Temperature comboniation, the slab FAILS in regions of SHEAR WALLS due to a higher restraint in these areas....my slab is a one way joist with 100mm deep slab....please suggest wat should i do in the shear wall areas....
Best Regards.

Answer
Hi Kamran,

That is a very high gradient- surely the daily variation will not be more than 10 to 15 degrees? In any case, what you can do is provide temporary thermal/pour strips around the shear walls (where you cast the concrete after sometime when the slab is protected from external direct thermal effects). This way you can mitigate the internal stresses due to expansion/contraction. This technique of pour strips is similar to what you have to do for any post-tensioning of slabs that are restrained by shear walls.