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PT Slab on Ground on piles - Detailing Query / General Input

PT Slab on Ground on piles - Detailing Query / General Input

PT Slab on Ground on piles - Detailing Query / General Input

Hi guys,

Currently working on a portal frame factory founding in very poor soil conditions, supporting heavy stacking loads. The site requires approximately 2.3m of fill over soft compressible clays, and the soils report states that due to long term settlements in the clay layers the floor slab must be designed as suspended on piles.

The client wants to investigate using a PT suspended slab supported on screw piles or driven timber piles. Our initial concept was to use a one way slab and band beam arrangement, with the beams being at equal centres to the portal frames (7500mm). We have run some numbers and come up with a preliminary design, but my main concern is detailing the slab to limit soil restraint from the grade beams as much as possible and give the slab a chance to elastically shorten when stressed.

Does anybody have any rules of thumb or references for the maximum gradient between underside of band beams (or edge thickenings) and the slab to prevent restraint?

Alternatively, all I can think of is using some sort of compressible material around the band beam edges. Haven't given it too much consideration yet, but if anyone has done this or heard of something similar any input would be much appreciated.

We will also be providing a flat slab arrangement with a tighter pile grid for a comparison with the client, which obviously wont have the same soil restraint issues.

Thanks in advance

RE: PT Slab on Ground on piles - Detailing Query / General Input

All the industrial PT slab on ground examples I have seen are flat slab construction with no beams.

One big issue is all the detailed excavation required to form the beams.

EDIT: Refer PTIA seminar notes:


RE: PT Slab on Ground on piles - Detailing Query / General Input

Some people have suggested about 1 in 6 but I do not agree. Any slope is going to cause problems. You are also going to get significant restraint from the piles and the subgrade on the flat soffits.

So cracking is inevitable. .6% reinforcement + PT is about the only solution to control the crack width.

RE: PT Slab on Ground on piles - Detailing Query / General Input

For calculating restraint from the subgrade on a flat soffit you can use a friction coefficient of 0.5, which assumes a layer of polythene over a sand blinding layer.

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