Cain Posted July 19, 2019 Share Posted July 19, 2019 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.pdf 1 Link to comment Share on other sites More sharing options...
Triassic Posted July 19, 2019 Share Posted July 19, 2019 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/ 1 Link to comment Share on other sites More sharing options...
Cain Posted July 19, 2019 Author Share Posted July 19, 2019 Fantastic, will do, thanks Link to comment Share on other sites More sharing options...
Conor Posted July 19, 2019 Share Posted July 19, 2019 Speak to Kore. Quote came out competative and quote for engineering calcs was £600 for 115m² slab. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 19, 2019 Share Posted July 19, 2019 Hilliard certified our Kore passive slab, too. This is a section through it: Structure - Detail for BC.pdf and this is the report Hilliard wrote on the Kore passive slab system: Kore_Insulated_Foundations_Report.pdf Link to comment Share on other sites More sharing options...
Russell griffiths Posted July 20, 2019 Share Posted July 20, 2019 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. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 20, 2019 Share Posted July 20, 2019 Could always transfer the bearing load from the frame into padstones on the slab to spread the load. Link to comment Share on other sites More sharing options...
scottishjohn Posted July 20, 2019 Share Posted July 20, 2019 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 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 20, 2019 Share Posted July 20, 2019 2 minutes ago, 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 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. Link to comment Share on other sites More sharing options...
scottishjohn Posted July 20, 2019 Share Posted July 20, 2019 15 minutes ago, 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. 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 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 20, 2019 Share Posted July 20, 2019 2 hours ago, 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 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. 1 Link to comment Share on other sites More sharing options...
Cain Posted July 25, 2019 Author Share Posted July 25, 2019 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. Link to comment Share on other sites More sharing options...
scottishjohn Posted July 25, 2019 Share Posted July 25, 2019 1 hour ago, 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. 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 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 26, 2019 Share Posted July 26, 2019 Looks just like our passive slab, and pretty much every one I've seen, with reinforced beams for the load bearing areas. Link to comment Share on other sites More sharing options...
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