Insulated raft foundation

[Cain]

Greetings 

 

I am in the early stages of building a single storey oak framed outbuilding around 90m2.  I intend to do everything myself to keep the cost down and as I have always wanted to build a cruck frame, indulge and try and enjoy the process.

 

The ground has some clay content and is close to trees so I was thinking about going down the route of an insulated slab which doubles up as the envelope to form the raft.  You may be familiar with passive house foundation stuff.

 

The problem that I am finding is that despite being ‘just a large shed’, building control will need to be satisfied that structural calculations are done.  To this end I have had some some quotes done and most are eye watering, or reasonable with the proviso I then spend a small fortune on a bespoke polystyrene system.

 

Is anyone aware of any generic calcs for rafts (after all there are tables for timber beams!). 

 

 

Any help of any kind would be very welcome

 

Cain

 

 

 

assembly.pdfFetching info...

[Triassic]

Talk to Hilliard Tanner, he’s the go to guy for insulated foundations, he did mine and was reasonably priced.

 

Once I had the design I bought all the insulation, fixed the steel reinforcing and poured the concrete myself. All fairly straightforward.

 

http://www.tsd.ie/

[Cain]

Fantastic, will do, thanks

[Conor]

Speak to Kore. Quote came out competative and quote for engineering calcs was £600 for 115m² slab.

 

[Jeremy Harris]

Hilliard certified our Kore passive slab, too.  This is a section through it:

 

Structure - Detail for BC.pdfFetching info...

 

and this is the report Hilliard wrote on the Kore passive slab system:

 

Kore_Insulated_Foundations_Report.pdfFetching info...

 

[Russell griffiths]

The only problem I can see is your cruck frame, will it be transferring point loads down to the slab, as I believe a lot of these systems like a nice evenly distributed edge load, you may need to calculate the weight of your frame with associated roofing and other stuff so the engineer can see where the load bears on to the slab. 

[Jeremy Harris]

Could always transfer the bearing load from the frame into padstones on the slab to spread the load.

[scottishjohn]

think it will have to more like founds for a portal frame building ,

deep post holes then slab poured after.

this is what your S/E will work out 

[Jeremy Harris]

  On 20/07/2019 at 12:05, scottishjohn said:

think it will have to more like founds for a portal frame building ,

deep post holes then slab poured after.

this is what your S/E will work out 

Expand  

 

The snag with that would be the combination of a nasty thermal bridge and cold ends to the timbers, which may well result in interstitial condensation.  Better to ensure that the ends of the timbers are always kept warm, and hence dry.

[scottishjohn]

  On 20/07/2019 at 12:09, JSHarris said:

 

The snag with that would be the combination of a nasty thermal bridge and cold ends to the timbers, which may well result in interstitial condensation.  Better to ensure that the ends of the timbers are always kept warm, and hence dry.

Expand  

not suggesting wood is buried in ground ,but just a pad stone  on top of simple slab won,t do -it will need a big lump of concrete under it to spread the load and slab will not really be load bearing for the main house structure .

that my guess anyway 

[Jeremy Harris]

  On 20/07/2019 at 12:26, scottishjohn said:

not suggesting wood is buried in ground ,but just a pad stone  on top of simple slab won,t do -it will need a big lump of concrete under it to spread the load and slab will not really be load bearing for the main house structure .

that my guess anyway 

Expand  

 

 

It's nothing to do with being in the ground, it's to do with the base of the timber being kept cool, because of the thermal bridge to the ground that the concrete pad would provide.  The effect of this for an indoor structure would be to create a condensation locus at the base of the timber.

 

Our insulated slab has a load of point loads from the whole mass of the house into timbers with a 89mm x 38mm section applied to a timber sole plate that works just like a padstone.  Structurally all that is required is to reduce the point bearing stress to a value that is well within the allowable bearing stress for that area of the slab.  This is dead easy, and doesn't take much at all.  For example, let's assume that the slab is the same as ours (which it is, it looks near-identical AFAICS). 

 

Say the outer reinforced concrete ring beam has a max allowable bearing stress of 5 N/mm² (that's a very conservative figure, as after 30 days cure plain RC35 concrete should be able to take at least 10N/mm², without allowing for all the steel reinforcement).  For a post bearing area of 150mm x 150mm (22,500mm²) the max allowable load is about 112.5 kN, or around 11.5 tonnes.   Just adding a modest padstone would increase that in direct proportion to the ratio of bearing areas, assuming sufficient thickness to allow for the lower edge perimeter to remain with the 45° line.

[Cain]

Thanks Gents, I will keep you posted on the solution.  But this may be one for the 2 off internal posts - especially in conjunction with the padsone if by themselves it wont suffice.

[scottishjohn]

  On 25/07/2019 at 20:52, Cain said:

Thanks Gents, I will keep you posted on the solution.  But this may be one for the 2 off internal posts - especially in conjunction with the padsone if by themselves it wont suffice.

Expand  

yes that looks like i was meaning  thicker concrete where posts will sit 

I would expect that to be the case where ever the cook frame touchs foundations

[Jeremy Harris]

Looks just like our passive slab, and pretty much every one I've seen, with reinforced beams for the load bearing areas.