Proposed workshop build

[MikeJhn]

Oh dear, you always read what you want instead of what is posted, you obviously know a little about this, but using type 1 under a floor slab shows your lack of real knowledge, and off at a tangent with 15mm gaps, the fabric should be sufficiently supported to prevent sagging between chairs, I said aggregate is generally 20mm the depth of the reinforcement in the concrete structure is designed as the d1 depth of the moment arm and is not to prevent rusting even though if is important for the cover to be consistent, concrete cancer is usually due to bad workmanship, the relationship of reinforcing cover to concrete strength is a new one on this Ove Arup trained ex Chartered Structural Engineer, cover is always related to size of aggregate, the designed depth required by the calculations taking into account any shear links, main bars and the tension or compression laps.

 

[DomP]

So can someone clarify for me then what spec I need for my slab?

I want a slab roughly 7m x 5m for a framed workshop clad in either feather edge or corrugated sheet with corrugated roof on 3 courses of engineering bricks.

The ground is pretty level so I intend digging out to 200-250mm to that my type 1, blinding sand and slab bring me to 50mm above ground.

 

[Skydivermel]

Why don't you go for ground screws. Simple, effective and relatively cheap. See here or here

 

[DomP]

Why don't you go for ground screws. Simple, effective and relatively cheap. See here or here

Because that means a floor level well above ground which will lose me internal height or increase overall height of the build. I'd also like a very firm base which a suspended floor can't offer

There is also several very huge stones below topsoil level one of which is in my veg patch that as much as we've tried we can't find an edge to get under it. Another was removed when we had our soakaway dug which is now on the end of my parents lane 4ft high bt 3ft diameter.

 

[johna.clements]

Oh dear, you always read what you want instead of what is posted, you obviously know a little about this, but using type 1 under a floor slab shows your lack of real knowledge, and off at a tangent with 15mm gaps, the fabric should be sufficiently supported to prevent sagging between chairs, I said aggregate is generally 20mm the depth of the reinforcement in the concrete structure is designed as the d1 depth of the moment arm and is not to prevent rusting even though if is important for the cover to be consistent, concrete cancer is usually due to bad workmanship, the relationship of reinforcing cover to concrete strength is a new one on this Ove Arup trained ex Chartered Structural Engineer, cover is always related to size of aggregate, the designed depth required by the calculations taking into account any shear links, main bars and the tension or compression laps.

If you read the thread you will see that people before me talked about placing type 1 under the slab. It seems to be a common recommendation / practice on this forum. I would assume it is recommended because it is free draining, so will not be susceptible to frost heave which is why it is used under roads. It is also widely available in small loads unlike something like 6N which is tested for sulfate as well as being free draining. If people are going to lay concrete on a granular material or the soil with some dpm over it you have to counter the unevenness of the surface.

You proposed 20mm cover. The tolerance for cover is -5mm therefore with your proposed 20mm cover you are allowed 15mm cover before it is out of tolerance. If you want a minimum cover of 20mm you need to specify a nominal cover of 25mm to allow for deflection.

I think you may have forgotten how a simple reinforced concrete beam is designed. In a simply supported beam that is acting like a floor joist spanning between two walls the top of the beam is in compression and the bottom is in tension. We are assuming that the slab in the shed will act in the same way with the top of the slab, where you stand in compression and the bottom in tension (this may not be the case but most likely is).



You may want to watch the video but there is a helpful sketch at 10 seconds which is all you need to watch. In the sketch the cover is the gap between the steel (sown as As Area of steel) and the bottom of the beam (or slab). The lever arm from the steel up to the center of the concrete which is in compression is marked as z on the sketch rather than d1. If you increase the cover to and do not also increase the overall depth of the slab the lever arm will be reduced and so the slab will be weaker in tension.




I am sure you have worked with BS8110 which was replaced by eurocode 2 in 2004 but they are similar. There are tens of thousands of structures built to these standards in the UK.

From EC2
""4.4.1.2 Minimum cover, Cmin
(1)P Minimum concrete cover, Cmin, shall be provided in order to ensure:
the safe transmission of bond forces (see also Sections 7 and 8)
the protection of the steel against corrosion (durability)
an adequate fire resistance (see EN 1992-1-2""
https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.1.2004.pdf
What do you think corrosion of steel is!

from EC2""
) For concrete cast against uneven surfaces, the nominal cover should generally be
increased by allowing larger deviations in design. Tile increase should comply with the
~difference caused by the unevenness, but the nonlinal cover should be at least k1 nlm for@il
concrete cast against prepared ground (including blinding) and k2 mm for concrete cast directly
against soil. The cover to the reinforcement for any surface feature, such as ribbed finishes or
exposed aggregate, should also be increased to take account of the uneven surface (see
4.4.1.2 (11)).
52
Note: The values of k1 and k2 for use in a Country may be found in its National Annex. The recommended
values are 40 mm and 75 mm""

BS8110 gives 40mm nominal cover for concrete cast on blinding (thin layer of concrete to work off) and 75mm nominal cover when cast against the earth. This is why I suggested 40 to 50mm nominal cover.

 

[Molynoox]

Are you trying to keep it under 2.5m in height?
If so, how will you span 4.8m?

Martin

 

[DomP]

Are you trying to keep it under 2.5m in height?
If so, how will you span 4.8m?

Martin

2.5 at eaves

 

[Molynoox]

Because that means a floor level well above ground which will lose me internal height or increase overall height of the build. I'd also like a very firm base which a suspended floor can't offer

There is also several very huge stones below topsoil level one of which is in my veg patch that as much as we've tried we can't find an edge to get under it. Another was removed when we had our soakaway dug which is now on the end of my parents lane 4ft high bt 3ft diameter.

I think there is a perception that concrete pads sit flush and groundscrews sit on stilts.

With screws you set the height you want them. Yes you need 50mm air gap, but if that's an issue you can dig down 50mm beforehand. The only way to get a concrete pad flush is to dig down by the depth of the pad, which is more than 50mm anyway.

So from height perspective they are pretty similar aren't they?

Martin

 

[Molynoox]

2.5 at eaves

Ah ok. Within permitted development?

 

[macca]

In regards to building height I'm guessing you are restricting yourself to 2.5m for permitted development, last i checked you can go significantly higher if you were to have a dual pitch roof. This would also reduced the depth of timber needed to span your workshop which will also reduce internal height, and we haven't even gone into warm roof/cold roof. its worth noting the height at eaves is also considered at the highest point of natural ground level directly under the eaves of the building. If your new site is within 1M of your property boundary your going to need fire proof wall on those sides of the build (corrugated metal roof sheets usually get around this) and building regs will kick in at 15m2.

You can get treated cls but at 38mm i'd advise your go for the industry standard treated '2 x' material which comes in around 45-47mm, you'd thank me when you're bonding 2 sheets on the uprights! go for 4.8M lengths with doubled plates and stagger the joins, timber scarcely comes straight and the longer it is the more issues you'll get and pay more for the privilege.
be aware your courses of brick will soak in water like nothing else, the simplest way around this would be bringing your cladding down over them to screen out any water but this would bring you into your proposed splash zone. I agree that large overhangs will be a key to solving many of your issues, its just good practise for any building IMO.

 

[DomP]

Yeah dual pitched which I believe allows for 4m at top of pitch and 2.5 at eaves, within 1m of boundary too but when I checked it was 30m² before regs kick in.

Regarding pad height, I would be getting it in the ground to 50mm above surrounding area, ground screws would be that plus the height of floor joists and flooring

 

[Molynoox]

Yeah dual pitched which I believe allows for 4m at top of pitch and 2.5 at eaves, within 1m of boundary too but when I checked it was 30m² before regs kick in.

Regarding pad height, I would be getting it in the ground to 50mm above surrounding area, ground screws would be that plus the height of floor joists and flooring

I recommend reading the regs, what you are proposing is not allowed.

Martin

 

[DomP]

I recommend reading the regs, what you are proposing is not allowed.

Martin

What aspect of my proposal? Everything I've read on the planning portal suggests it is

 

[Molynoox]

You said within 1m of boundary, therefore max height is 2.5m.
Regardless of single or dual pitch

 

[Molynoox]

What aspect of my proposal? Everything I've read on the planning portal suggests it is

Also your comment about building regs applying to 30m2 only is incorrect

 

[DomP]

That's not what the planning portal suggests, I have no dwellings or domestic properties on any of my boundaries if that makes any difference

 

[DomP]

Also your comment about building regs applying to 30m2 only is incorrect

As long as 1m from boundary upto 30m² doesn't require regs is what I've just read

 

[macca]

haha i really should stop handing out advice whist watching tele, you are entirely correct, but moving away from the boundary it certainly does make a difference.

You said within 1m of boundary, therefore max height is 2.5m.
Regardless of single or dual pitch

Regs is a different ball game and apply based on varying factors not just the internal footprint of the build, once you actually decide on a design i would just call a local building inspector and get their opinion

 

[MikeJhn]

If you read the thread you will see that people before me talked about placing type 1 under the slab. It seems to be a common recommendation / practice on this forum. I would assume it is recommended because it is free draining, so will not be susceptible to frost heave which is why it is used under roads. It is also widely available in small loads unlike something like 6N which is tested for sulfate as well as being free draining. If people are going to lay concrete on a granular material or the soil with some dpm over it you have to counter the unevenness of the surface.

You proposed 20mm cover. The tolerance for cover is -5mm therefore with your proposed 20mm cover you are allowed 15mm cover before it is out of tolerance. If you want a minimum cover of 20mm you need to specify a nominal cover of 25mm to allow for deflection.

I think you may have forgotten how a simple reinforced concrete beam is designed. In a simply supported beam that is acting like a floor joist spanning between two walls the top of the beam is in compression and the bottom is in tension. We are assuming that the slab in the shed will act in the same way with the top of the slab, where you stand in compression and the bottom in tension (this may not be the case but most likely is).



You may want to watch the video but there is a helpful sketch at 10 seconds which is all you need to watch. In the sketch the cover is the gap between the steel (sown as As Area of steel) and the bottom of the beam (or slab). The lever arm from the steel up to the center of the concrete which is in compression is marked as z on the sketch rather than d1. If you increase the cover to and do not also increase the overall depth of the slab the lever arm will be reduced and so the slab will be weaker in tension.




I am sure you have worked with BS8110 which was replaced by eurocode 2 in 2004 but they are similar. There are tens of thousands of structures built to these standards in the UK.

From EC2
""4.4.1.2 Minimum cover, Cmin
(1)P Minimum concrete cover, Cmin, shall be provided in order to ensure:
the safe transmission of bond forces (see also Sections 7 and 8)
the protection of the steel against corrosion (durability)
an adequate fire resistance (see EN 1992-1-2""
https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.1.2004.pdf
What do you think corrosion of steel is!

from EC2""
) For concrete cast against uneven surfaces, the nominal cover should generally be
increased by allowing larger deviations in design. Tile increase should comply with the
~difference caused by the unevenness, but the nonlinal cover should be at least k1 nlm for@il
concrete cast against prepared ground (including blinding) and k2 mm for concrete cast directly
against soil. The cover to the reinforcement for any surface feature, such as ribbed finishes or
exposed aggregate, should also be increased to take account of the uneven surface (see
4.4.1.2 (11)).
52
Note: The values of k1 and k2 for use in a Country may be found in its National Annex. The recommended
values are 40 mm and 75 mm""

BS8110 gives 40mm nominal cover for concrete cast on blinding (thin layer of concrete to work off) and 75mm nominal cover when cast against the earth. This is why I suggested 40 to 50mm nominal cover.

And the point of this diatribe is?

 

[rs6mra]

As long as 1m from boundary upto 30m² doesn't require regs is what I've just read

The last time I checked building regs which was ages ago if you are within 1m of your boundary as you are proposing, the max is 2.5m. If you were to exceed 15m² then the material must be non-combustible and it was suggested earlier that you can get away with corrugated metal sheets; otherwise building regs kick in.......................