Beam & Block Overkill?

[andy]

Hi all

 

We're slowing getting there with our design (slow being the operative word here!) but I've had the final construction drawings from our structural engineer now for our foundations that I am concerned by (we are about to get a firm quote from our groundworker who wants to actually start soon!).

 

The house is mostly single story with a mezzanine area on the top right corner of the image.  It's timber frame with walls filled with warmcell (including internal walls) yet our SE wants dwarf walls either coming through or built on the beam and block floor (and then the 300mm EPS100 and 125mm slab) which the internal walls are supported by.  I am at a loss to understand why this is a requirement as the house is circa 5.5m wide on the left section and 8m on the right section (as you look at the picture). I'm concerned that the thermal efficiency is compromised as a result.

 

Does what the SE designed seem appropriate?

 

P.S. I have no idea what the red dots are signifying!

[Declan52]

Why beam and block, is it poor ground conditions??? Can you not fill the founds with hardcore then put in your concrete floor with mesh and have it polished up. Then you have a smooth platform to set your timber frame on.

Can you put up the first floor plan so we can see what the purpose of the walls that are in red and yellow are carrying above.

[ProDave]

I set my designers the brief "to simplify the foundations" by that I meant remove unecessary sleeper walls.  I got the impression designers do that as an easy way to reduce beam sizes.  My reasoning was the upstairs joists are going to span that gap, so the downstairs ones are damned well going to span the same gap without the aid of sleeper walls.

[andy]

At pub, will respond with floor plan details tomorrow!

 

We're on chalk but some disturbed ground but nowt too crazy (trees right next to previous house and general disturbed ground over past 100 years!).  The whole left section is ground floor only apart from some loft space in half of it.

[Simplysimon]

just done my b&b floor, wouldn't do it again. expensive and quite a bit of deflection, live and learn.

section IE has thermaliteblocks and IF has marmox. TBH i wouldn't bother with either as @ProDave says, make it simple - lay a slab.

[Declan52]

1 hour ago, andy said:

At pub, will respond with floor plan details tomorrow!

 

We're on chalk but some disturbed ground but nowt too crazy (trees right next to previous house and general disturbed ground over past 100 years!).  The whole left section is ground floor only apart from some loft space in half of it.

So if the left side is only ground floor why is he making you build walls beneath what I presume are going to be internal timber walls. The beam and block can easily carry that small weight never mind 125mm of reinforced concrete.

[Jeremy Harris]

I'm afraid I really struggle to see any advantage in using beam and block.  Thermally, it's poor, as no matter how much insulation you install, there is always going to be a higher heat loss than a ground bearing slab with the same insulation, just because the air in the undercroft will be cold in cold weather, colder than the ground by a fair amount.

 

What's the rational behind anyone using beam and block, as I'm genuinely puzzled?

[Roundtuit]

10 minutes ago, JSHarris said:

 

What's the rational behind anyone using beam and block, as I'm genuinely puzzled?

 

In our case, unstable ground conditions (fenland) coupled with a flood risk assessment requiring elevated finished floor levels.

[Jeremy Harris]

1 minute ago, Roundtuit said:

 

In our case, unstable ground conditions (fenland) coupled with a flood risk assessment requiring elevated finished floor levels.

 

Could the space under the floor not be filled with insulation, with the vents closed off?

 

The other thing that puzzles me is that a ground bearing slab places far less load on the underlying ground than strip foundations, so on weak ground I can't quite understand why something that's the foundation equivalent of stiletto heels, in terms of the point loads applied to the underlying ground, can be viewed as being better.

 

I can understand the flood risk concern, but that's just a matter of setting the finished floor level above the flood risk level (something we were obliged to do as a planning condition, with our slab).

[Roundtuit]

Building regs require the area under the block and beam to be ventilated I believe.  I'm sure there are other options but anything 'non-standard' is hard work locally in terms of expertise and willingness to break the mould, so we're piled with a ring beam.  We're probably about 500mm out of the ground for FFL (300mm above adjacent road). On the plus side, no muck-away, and room for 175mm of PIR on top of the block and beam.

[Temp]

I haven't really looked at the drawing (it's 2am) but has he added walls to reduce the span of the beams?

[Jeremy Harris]

9 hours ago, Roundtuit said:

Building regs require the area under the block and beam to be ventilated I believe.  I'm sure there are other options but anything 'non-standard' is hard work locally in terms of expertise and willingness to break the mould, so we're piled with a ring beam.  We're probably about 500mm out of the ground for FFL (300mm above adjacent road). On the plus side, no muck-away, and room for 175mm of PIR on top of the block and beam.

 

The ventilation requirement was intended for timber suspended floors, and I have heard of one case where it's been lifted for a concrete suspended floor, with the undercroft being filled with loose insulation (IIRC I think it was Leca).

 

175mm of PIR will give just over double the downward heat loss when it's 0°C outside as the 300mm of EPS under a passive slab laid on to ground at around 8°C (there's no particular need for passive slab insulation to be below ground level).

[andy]

10 hours ago, Declan52 said:

So if the left side is only ground floor why is he making you build walls beneath what I presume are going to be internal timber walls. The beam and block can easily carry that small weight never mind 125mm of reinforced concrete.

 

That's what I don't get - it seems seriously over-engineered.

[andy]

So here's the layouts - I've cleaned them up to remove a lot of "noise" on the pics but hopefully this gives you an idea.

 

Main house section is 8.5m x 11.2m (at widest, measured top to bottom x left to right), bedrooms is 5.1m x 16.3m.  There's a step up in level of 1m just to the right of the steps in the bedroom area due to slope of site.

 

I can understand the need for dwarf walls across some of the beam spans but not for any walls that penetrate the 300mm EPS100.

[Randomiser]

On clay don't you need to allow for expansion and contraction of the ground under the slab?  So pile down to something solid, ring beam with something like Cellcore under it, the beam and block with space below for expansion.  You could put a slab on Cellcore, but then it isn't really ground bearing.  I am only a beginner on all this, but that is what my internet research has suggested to me about our site with clay and trees in close proximity.

[andy]

Dig down 1m and we’re on solid chalk here...

[Jeremy Harris]

We have a passive slab on clay.  We dug out ~200mm below the levelled area where our house sits and filled it with compacted MOT3.  I did get the clay assessed, and it turned out to be hard gault, with a near-zero risk of movement with moisture change, so not all clay presents a heave risk.

 

Chalk is damned good in terms of both max allowable bearing stress and drainage, so would seem ideal for a pretty simple passive slab foundation.

[andy]

Exactly, it’s complicated by the slope and large trees that were right next to old house.  We’ve really struggled to get sense out of SE and are tempted to revisit quotes from insulated raft companies, cutting our loses with SE.

[Declan52]

17 minutes ago, andy said:

Exactly, it’s complicated by the slope and large trees that were right next to old house.  We’ve really struggled to get sense out of SE and are tempted to revisit quotes from insulated raft companies, cutting our loses with SE.

How big of a slope are you taking about??

Can the trees be cut down???

If your on chalk at 1m deep why are you even looking at beam and block???

[Vijay]

I was told because of radon gas, I had to use B&B, maybe yours is the same?

[andy]

4 minutes ago, Declan52 said:

How big of a slope are you taking about??

Can the trees be cut down???

If your on chalk at 1m deep why are you even looking at beam and block???

 

Trees were by old house and gone along with house, possibility of shrinkage due to that over time I think.

 

Site is about 2m higher at back of house which we’ve terraced roughly into the required 2 levels with 1m difference, rest taking up by small walls/sloping banks.

[andy]

Just now, Vijay said:

I was told because of radon gas, I had to use B&B, maybe yours is the same?

 

No radon here, it’s all based on SE “comfort” basically.

 

He even asked where the insulation went on and insulated raft foundations, alarm bells went off then but the company was the on our architects used...

[Jeremy Harris]

9 minutes ago, Vijay said:

I was told because of radon gas, I had to use B&B, maybe yours is the same?

 

 

Passive slabs are often installed with a Radon sump and barrier.  Our guys came with all the parts to install the Radon sump etc, but didn't fit them because I told them there was no need here, as we're on sedimentary rock.  A Radon barrier and sump is only usually needed if the ground has igneous rock beneath, most commonly granite, which will usually contain fairly high levels of radioactive material.

[scottishjohn]

16 minutes ago, JSHarris said:

 

 

Passive slabs are often installed with a Radon sump and barrier.  Our guys came with all the parts to install the Radon sump etc, but didn't fit them because I told them there was no need here, as we're on sedimentary rock.  A Radon barrier and sump is only usually needed if the ground has igneous rock beneath, most commonly granite, which will usually contain fairly high levels of radioactive material.

is there not a radon barrier material you can fit under slab?

[Jeremy Harris]

1 minute ago, scottishjohn said:

is there not a radon barrier material you can fit under slab?

 

Yes, as I wrote, they fit a Radon barrier and sump as standard when installing a slab in a Radon area.