Slab edge thickening

[willbish]

For those with an insulated raft foundation, what was your ground bearing capacity figure? And was your raft designed with extra thickening?

 

My SE had made noises that my ground bearing figure of 50kN/M2 is a bit low but workable. His latest design of the raft has 25% of the area thickened to 400mm and in these areas insulation reduced from 300mm to 150mm.

 

This feels like a big step down in thermal performance to me, but perhaps I shouldn't be so concerned.

 

During the ground investigation I was told that I had ideal ground for a raft so not sure how a 50kN/M2 figure has been derived.

 

I've asked the GI chap if he could revisit his Standard Penetration test results with hope to increasing the ground bearing figure; no response yet.

 

I also feel the SE is being over cautious; he has the slab edge thickened where a single story porch will be. That seems really unnecessary.

 

[Jeremy Harris]

Our slab has 300mm of EPS under most of it, a 400mm high x 200mm wide upstand around the edges and the edge ring beam is 200mm x 200mm.  The slab is 100mm thick across most of the area.  There's a layer of steel fabric in the centre of the 100mm deep section and four tied rebars running around the 200mm x 200mm outer edge ring beam.

 

Soil max allowable bearing load is way over 100 kN/m², so didn't I bother measuring it accurately (simple 2 x 2 test with a lump hammer confirmed it was way over 100 kN/m²).  House actual bearing load on the soil beneath, allowing for 45 deg outward load spreading from 150mm deep packed crushed stone bedding, works out to be just over 4 kN/m².

 

In other words, the reserve between the minimum possible allowable soil bearing load of 100 kN/m² and the actual house and foundation applied bearing load is a factor of around 25, which is extremely conservative, even for the typical ranges of reserve normally allowed in civil engineering.

 

I'd question why you have such a massively thick slab.  250mm seem way OTT for the non-bearing load area to be.  Our reinforced 100mm thick slab is plenty thick enough, with it's 200mm x 200mm reinforced ring beam.  Here's a sketch showing a section through our house and slab (it's one drawn up as a part of my building regs submission):

 

Structure - Detail for BC.pdf

[PeterW]

35 minutes ago, JSHarris said:

 250mm seem way OTT for the non-bearing load area to be.  

 

@willbish are you having UFH in that too as it’s going to be a serious heat sink if you are ! As @JSHarris has said, 250mm is thick and even if the SE is looking at uneven stress loading, putting 2 or 3 beams at 250mm with the remainder at 100mm would be ample - unless you’re building a car park ..??!

[jack]

Our foundation (see attachment below) is similar to Jeremy's, but also has two deeper parallel spines extending the length of the house, similar to the ringbeam in Jeremy's attachment (ie, they only have 100mm insulation underneath them).

 

There are also 150mm wide ribs extending between the spines and ringbeams every 1200. The ribs extend 100mm lower than the 100mm thick slab.

 

The house is 289m². I don't recall the ground bearing numbers, but we did have some made up ground to deal with (we're right at the end of what was a sand and gravel quarry many years ago).

 

 

Foundation details.pdf

[Jeremy Harris]

10 minutes ago, PeterW said:

 

@willbish are you having UFH in that too as it’s going to be a serious heat sink if you are ! As @JSHarris has said, 250mm is thick and even if the SE is looking at uneven stress loading, putting 2 or 3 beams at 250mm with the remainder at 100mm would be ample - unless you’re building a car park ..??!

 

Ours has two cross beams that are under two load-bearing internal walls, and those cross beams are the same spec as the ring beam, 200mm x 200mm.  There are some photos of our slab being built in this blog entry: Slab being built and poured  If you look closely you can see the two additional cross beams; one has a dog leg in it.  They support the walls either side of our hall, which goes right up to the big gluelam ridge beam around 6.5m above.  Those two internal walls help support the first floor as well as the centre part of the ridge beam (our roof uses ridge-hung rafters, so most of the roof loads are taken by the ridge beam)

 

 

[jack]

Just now, JSHarris said:

Ours has two cross beams that are under two load-bearing internal walls, and those cross beams are the same spec as the ring beam, 200mm x 200mm.  There are some photos of our slab being built in this blog entry: Slab being built and poured  If you look closely you can see the two additional cross beams; one has a dog leg in it.  They support the walls either side of our hall, which goes right up to the big gluelam ridge beam around 6.5m above.  Those two internal walls help support the first floor as well as the centre part of the ridge beam (our roof uses ridge-hung rafters, so most of the roof loads are taken by the ridge beam)

 

Ah, very similar to ours then, even down to having a dogleg in one of your cross beams!

[willbish]

Thanks @JSHarris your figures and dimensions are much more in the region of what I was expecting.

 

I'm not too fussed about the 250mm slab thickness over non loading bearing areas, it is the common design with Isoquick. My understanding of the Isoquick system was the thicker 250mm slab throughout would compensate the need for a ring beam/ edge thickening therefore allowing 300mm insulation throughout.

 

I have already asked the SE if anything could be done to reduce the slab thickening, his reply was no. I asked him what ground bearing he would need to scrap the slab thickening. His response 'somewhere in the region of 100kN/m2' Hence returning to the ground investigation chap.

 

Just to add to the pressure, I've being told if I don’t approve the draft plans on Monday the SE will not be able to finalise next week before going on holiday for two weeks!

 

 

@PeterW Yes to UFH. The SE has specified thickened strips as well much like @jack so the thickened area is now 25% of total slab area

 

[Gone West]

1 hour ago, willbish said:

For those with an insulated raft foundation, what was your ground bearing capacity figure? And was your raft designed with extra thickening?

 

I have an Isoquick insulated raft foundation with 300mm insulation under the raft and 200mm thick upstand. The raft is constant 200mm thick RC35 reinforced with two layers of A393. The maximum GBP is 50 kN/m2. There is 200mm of type 1 compacted in 50mm layers and 50mm granite fines under the insulation extending out 700mm beyond the edge of the insulation. I could PM you a copy of my structural calculations if that would help.

[willbish]

@PeterStarck that would be very helpful thank you. What you've described is exactly what I want/think is appropriate

[Gone West]

@willbish okay I'll dig them out (it was just over eight years ago) and PM you later.

[Russell griffiths]

Can I add a non educated but common sense thought to this. 

 

My feelings are that adding thickness to the concrete does hardly anything without the correct reinforcement 

 

your drawing shows extra extra thickness of concrete but no additional reinforcement in that area, if you maintain the 250mm thickness all over but add extra reo in the ring beam area I’m convinced that would improve things over just thickening the concrete. 

 

 

I have been installing my ringbeam over piles this week and all the high load areas have 8x16mm bars and the low load ares only have 6 x16mm bars I questioned this and was told it was the beams that were carrying the floor load as well as the wall load that have the additional steel. 

[Alexphd1]

@willbish Do you have different specs of insulation under your slab? We have eps300 under the ring beam and eps100 under the main slab (150mm concrete)?  

We had a big difference between the two engineers (different companies) in the width of the ring beam dimemsions with the 2nd engineer who has more experience with a raft foundation designed a smaller ring beam. 

[mvincentd]

Pure chalk base excavated to between 1m and 4m below topsoil...chalk rated by SE at nominally 125kpa so he was thrilled when i proposed 'mega' 300kpa insulation;

 

[willbish]

15 hours ago, Russell griffiths said:

My feelings are that adding thickness to the concrete does hardly anything without the correct reinforcement 

Agreed, will raise this point this morning.

 

 

15 hours ago, Alexphd1 said:

Do you have different specs of insulation under your slab? We have eps300 under the ring beam and eps100 under the main slab (150mm concrete)? 

Isoquick uses the same grade of insulation throughout

[willbish]

15 hours ago, Alexphd1 said:

We had a big difference between the two engineers (different companies) 

I don't have the luxury of employing two engineers. I'm working with a recommendation direct from Isoquick so should be getting the most appropriate design.

[Triassic]

16 hours ago, Russell griffiths said:

feelings are that adding thickness to the concrete does hardly anything without the correct reinforcement

That was my initial thought when I saw your drawings, where’s the steel reinforcing in the ring beam?

[Pete]

Not sure if this helps but this is our passive slab detail. 

Foundation section A Peter Vasey.pdf

[MikeSharp01]

I notice no reinforcement along the ring beam in your section drawing. I am no structural engineer but I would have thought this was a requirement probably both above and below the neutral axis.

On 22/07/2018 at 09:00, willbish said:

unnecessary.

 



As you say without good reinforcement the concrete will need to be much greater volume. If it's an ISOQUIK design they must be carrying the structural risk if the thing fails.

[Jeremy Harris]

IIRC, our ring beam had four rebars that were around 10mm to 12mm dia running around it, all set inside the 200mm x 200mm ring beam section.  These were tied to the fabric running across the centre of the slab with loads of 10mm to 12mm diameter bent bars, as in the photo below (not all the steels are tied in yet in this photo - there's loads more wire tying to be done before the pour):

 

[jack]

Our ringbeam reinforcement before being dropped into position:

 

[willbish]

We have reached a workable solution today. No ring beam or slab thickening in any areas, which means I get 300mm of insulation under the whole slab. But as a compromise a 300mm slab throughout with, as yet unspecified but, 'significant' amount of reinforcement!

 

What did @PeterW say about a serious heat sink! How much heat is this 30m3 slab going to hold ?

 

The extra 50mm shouldn't affect my levels too much, everything so far has been buried deeper than necessary. I will have one run of 110mm storm pipe in the bottom 50mm of type1 layer however, assuming that can be left as is?

Edited July 23, 2018 by willbish

[A_L]

12 minutes ago, willbish said:

How much heat is this 30m3 slab going to hold ?

 

 

@willbish, about 18.3kWh/°C

[jack]

14 minutes ago, willbish said:

But as a compromise a 300mm slab throughout with, as yet unspecified but, 'significant' amount of reinforcement!

 

Wow, mega! 

 

One thing I wouldn't worry about is heat capacity. If anything it'll be harder to overshoot heating/cooling.

 

Speaking of which, our slab cooling experiment is going exceptionally well. It was just short of 30 degrees here today, but it's been extremely comfortable inside (at least downstairs). Everyone who walks in is gobsmacked at the temperature difference. The concrete floors are if anything a little too cool to be pleasant unless you've been out in the heat, but it's extremely pleasant not to be trying to work in a hot study. I've been running the water temp at 16 degrees and that seems to be fine - just need to get a cooling thermostat set up. 

[willbish]

8 minutes ago, jack said:

Speaking of which, our slab cooling experiment is going exceptionally well.

Great to hear. Cant wait to do the same in our new place, my current flat is shocking. Only 5 years old, adequate standard of insulation but the decrement delay is terrible. The natural ventilation is quoted to be 5.0m3/h.m2 but I think its much lower than that. We cant fully open the windows until the 2 year old is in bed because he'll climb out! Mechanical ventilation needs to be mandatory on new builds IMO.

 

Anyway here's to a chunky slab!

[Russell griffiths]

Not sure who is doing your slab or how you are going to buy your reo but I have just had 3 tonne of cut and bent steel all prefabricated from a company called 

total building supplies, I was shocked how cheap it was and had to question if they had missed something out as it was £2000 cheaper than a quote I had already had .