Conventional raft slab on ground with piers?

I don't know what you loads are....but are you sure you need beams? Sounds like the pier spacing will be tight so you may want to give some thought to omitting them.

By the way, what is this "slab-on-ground" for? A building? Or just wheeled traffic?

Your beam spacing will depend on how far your slab can span

Your pier spacing will depend on how far your beams can span

There is no magical formula that I know of to optimize it. You may need to run a few iterations in order to determine what arrangement offers the best economy.

Usually I work backwards, figure out my pier capacity using an optimal pier size, around here the contractors love a 16" diameter. Then determine how far I can space them apart and not exceed the capacity. Design the beams and slabs to suit these spacings.

If the unsuitable fill is really "deep" (as stated), maybe driven piling could be considered. Agree with WARose, if the (100mm) slab is thickened (a lot), perhaps a flat slab, instead of beams, would be both reasonable and less expensive.

Thanks for your replies.
It is called stiffened raft slab which is commonly used in Australia for residential projects. Normally the stiffened raft slab will simply sit on the ground unless there is deep fill then we need to have piers under. Loads are just normally 1.5kPa live load plus self weight dead loads.

I understand how to calculate the beam spacing, pier capacity etc. What I really don't understand is do we need to design it as suspended slab, which means all the loads from slab & beams will transfer to piers? I am asking because I have seen so many plans with 2 meters - 3 meters beam spacing (please refer the first pic I uploaded above. Beam sizes: 300mm x 600mm, slab thcikness: 100mm and pier sizes: 300mm or 450mm diameter). With 250kPa bearing capacity, I dont think it is gonna past if we design it as a suspended system.
Thanks

What I really don't understand is do we need to design it as suspended slab, which means all the loads from slab & beams will transfer to piers?

Click to expand...

Depends on the stiffness of the structural system, loads, the underlying soil, etc. (I.e. you need to do a analysis.) But from what you've described.....with the pier spacing being that tight.....it sounds like it would be best/conservative to just design it as only being supported by the piers. (I.e. suspended.)

With most mats on pile foundations I've done, I just do that anyway. (With all the uncertainties of settlement.)

I would be surprised if beams that size don't work at 2m spacings. Those loads are overly significant.

Zzzer,

As you are in Australia, most residential foundations are done in accordance with AS 2870 Residential Slabs and Footings Code. It is prescriptive and to an extent empirical, based on experience as much as science. I haven't done any residential in recent years, so can't advise as to the details. There is a web presentation by SRIA which comes up at the top of the Google list for AS 2870.

A caution...residential footing performance is at the top of the list for complaints about builders and their engineers.

You need to take into account the skin friction on the piles, then you calculate the depth of the pile based on what load you are chasing.

If you base the pile capacity on only base bearing you are being too conservative.

The material under the slabs/beams is uncontrolled fill, which I don't think will provide stiffness when piers are placed under beams...


Beams are fine, I am just worrying about the pier capacity.

Thanks for the information, I will have a look. Yes, for raft and waffle I will simply use what the standards say but when piers are used I think calculation is still needed. The calculations tell me piers shall be closer. And the builder complains just like what you say... So I have no idea what the assumption should be.

I didn't use the skin friction because I think it will be a bit risky, considering the skin friction and depth are uncertain (the standards says that no skin friction should be assumed to exist to a depth of 0.75Hs) and there will be negative skin friction too.