Connecting layers of steel mesh in foundations

[phatboy]

I am beginning a self build journey early next year (Project thread to come, no doubt plenty of mistakes to learn from!)

I have our engineer drawings back now, and the 300mm deep foundations need 2 layers of A252 steel. How is best to connect these, mainly keeping the upper one in place?

I've seen 'top hats', 'deck chairs' and 'hi-chairs'. What's the best way to do it? Engineer hasn't detailed anything, just that 2 layers are required, 50mm from top and 50mm from bottom.

If I use the hi-chairs, do I need to run them continuously or can they be chopped up and tied along the foundation? I understand they are only there to keep the mesh in place while the concrete is poured and set. Foundations are mostly 600mm wide

And photos of steel mesh installation in progress would be much appreciated!

Thanks

 

[^woody^]

Half a job engineer.

He should specify how the reinforcement is placed. What if there is a failure and you blame the engineer and he blames your method and advice from DIYnot?

Get him to detail it or refund you.

 

[RonnyRaygun]

Is this a strip footing? Sounds to me like it is and the mesh is just to help resist differential movement.

We wouldn’t usually specify on the drawings how to support the reinforcement, but it is mentioned in our construction notes - have you checked the ones I assume your engineers provided?

Anyway, the high chairs come in long lengths as they are usually used for slabs , so cut them to around 500mm wide and put them at regular enough centres to prevent the mesh sagging significantly.

You could support the bottom layer on pieces of concrete brick or block paver.

 

[phatboy]

He should specify how the reinforcement is placed. What if there is a failure and you blame the engineer and he blames your method and advice from DIYnot?

I'd assumed decisions on how to connect them were to be made by the builder, and the engineer just specified where they were to go.

Is this a strip footing? Sounds to me like it is and the mesh is just to help resist differential movement.

Yes it is

We wouldn’t usually specify on the drawings how to support the reinforcement, but it is mentioned in our construction notes - have you checked the ones I assume your engineers provided?

Their 'General design notes' covers a lot of things, but not this!

Anyway, the high chairs come in long lengths as they are usually used for slabs , so cut them to around 500mm wide and put them at regular enough centres to prevent the mesh sagging significantly.

You could support the bottom layer on pieces of concrete brick or block paver.

Fantastic, I've been looking online for a couple of weeks and not found anything near to a robust answer like that!

I'm thinking that every 1m, with the high chairs fixed to the meshes would be fine? I'll decide on the day, depending how much sag there is (Or get the engineer to spell it out).

Dare I ask, where there is a stepping in the foundation, should I be bending some 8mm bars and fixing them between the differing height bottom layers, and top layers?

My only concern going back to the engineer, is they seem to over specify..... neighbours who have managed to not need an engineer have built without any mesh and years on it 'seems' to be fine.

 

[RonnyRaygun]

I'm thinking that every 1m, with the high chairs fixed to the meshes would be fine? I'll decide on the day, depending how much sag there is (Or get the engineer to spell it out).

1m sounds fine but yeah, eye it in, and wouldn’t hurt to get your engineer to confirm.

Dare I ask, where there is a stepping in the foundation, should I be bending some 8mm bars and fixing them between the differing height bottom layers, and top layers?

That’s a question for your engineer as they should definitely provide a detail for this, but yes, you need to maintain continuity at the step so tying the bars together is essential.

My only concern going back to the engineer, is they seem to over specify..... neighbours who have managed to not need an engineer have built without any mesh and years on it 'seems' to be fine.

Have you had a ground investigation carried out? There may be something in the report that guides the engineer towards specifying reinforcement in the strips.
Reinforcement in strip foundations is usually found where there is a risk of differential settlement, or where the foundations are likely to bear onto particularly weak or loose ground.
It often seems like engineers over-specify, but usually it’s working to within the constraints of the British Standards or NHBC Standards (which most engineers refer to even when there is no warranty provider).
Plus you only get your fingers burnt once or twice by cutting a design too close to the bone and after that you add a little extra comfort factor

 

[phatboy]

That’s a question for your engineer as they should definitely provide a detail for this, but yes, you need to maintain continuity at the step so tying the bars together is essential.

I may have been unfair to the engineer..... I spotted on the plan now (Designed for A1, printed in A3 by me!) that 'refer to steel bending schedule is mentioned', but the schedule isn't provided, so that needs chasing up. Hopefully it will have this, and maybe a method for the steel layer spacing.

Have you had a ground investigation carried out? There may be something in the report that guides the engineer towards specifying reinforcement in the strips.
Reinforcement in strip foundations is usually found where there is a risk of differential settlement, or where the foundations are likely to bear onto particularly weak or loose ground.

When he visited to inspect the trial holes, he felt the soil. It's basically sand, used to be dunes until 70 years ago. Maybe this is the reason?

 

[RonnyRaygun]

I may have been unfair to the engineer..... I spotted on the plan now (Designed for A1, printed in A3 by me!)

Funny - I’ve had the debate with colleagues many times that we are much better putting drawings on A3 where possible as your average builder / homeowner doesn’t own an A1 printer!

When he visited to inspect the trial holes, he felt the soil. It's basically sand, used to be dunes until 70 years ago. Maybe this is the reason?

Sounds like a good reason to put mesh in.