Do I need any joints in a raft foundation over piles?

Not necessarily, but the raft is going to be too big to pour in one day, so you might want to consider shrinkage strips poured after the concrete on each side has set for a week or so.

If temperature is relatively constant, the only volume change will be shrinkage, so it seems like a good idea to take steps to relieve shrinkage stresses.

I think you are mixing up your terms. You say that you are on "piles".

A pile cap is a thick concrete mat that rests on piles. It is part of the foundation of a building, typically a multi-storey building, structure or heavy equipment. The pile cap distributes the load from the pillars, or piers, to the piles. A similar structure to a pile cap is a "raft", which is a foundation floor resting directly on bedrock.

http://en.wikipedia.org/wiki/Pile_cap

I don't find the term "raft" to be misleading. There are times when you might wish to design a slab supported by piles. Call it a slab, call it a raft, call it a huge pile cap...who cares? The question is: does he need joints in a 100m x 100m slab/raft/pile cap?

Usually Expansion Joints are given in super structure which exposed to weather but not the sub structure which is relatively remains at same temperature.

Construction joints will be required if concrete cannot be poured in one instance. Which meas you need many pumps and many mixer trucks to pour in one instance. So plan accordingly.

Do not pour during high temperature because of shrinkage cracking.

Hello Bakrtweet

If you are talking about an expansion joint, then the answer is NO. Dont forget that your raft foundation shall be checked for Combined bi-axial eccentricity. The CG shall not be, in any event, greater than 1/3 of the mid-span.

If you are designing pile caps then you will need joints between the pile caps and the raft. Else the raft will start acting monolithically and the caps as pin-supports, meaning it acts as slab on grade [Plate analysis is required].

Either way, if you do not have water seepage, hight water table or pore-pressures then this idea of Pile and Raft is highly expensive. You could try individual Pile&Cap foundations with strap beams instead.

Mohammed Fareeduddin

B.E; M.E Civil Strl.

Member of: ACI / AISC / TIA-EIA222 & ASCE

http://www.linkedin.com/in/mohammedfareeduddin