Wall position on foundations

[plantyroo]

Hi folks

We're having an extension built at the rear of our house (3m deep x 4m wide) and the builder has dug 2/3rd's of the foundations so far. The extension is being done under PD and the contract with the builder is one for design and build.

Having looked at the trench's yesterday when I got home from work, I noticed that the outer edge had only dug to 3.05m. I had expected it to be 3.15m to allow the standard brick and block construction to sit in the middle of the 600mm wide foundation strip.

I spoke to the builder last night about this and he has said that he usually builds towards the outer edge of the foundation strip (leaving a 50mm gap to the edge), as the internal side of the foundations will be used to sit the floor slab on.

I'm not sure about this, so wondered if you would mind sharing your views? The concrete has not been poured yet, so it's an ideal time to correct any mistakes (even if it will mean a bit more digging for them!!).

I'd welcome any comments.

Many thanks

 

[noseall]

All masonry should sit centrally onto the foundation concrete.

The floor slab sits upon varying materials, one being hard core another being insulation. At no point does the floor slab sit directly upon the foundation concrete nor does it gain any particular benefit from it.

You are being conned.

Building control will want to see the building sitting centrally on the concrete. You may have to move the building in to do so.

 

[plantyroo]

Thanks Nose

I suspected that to be the case!

I will ask them to dig out the extra 10cm's when the arrive on Monday. Doesn't sound like much but I'd prefer not to compromise the internal space when there is an opportunity to put it right.

I'm presuming that there will be no detrimental impact on the foundation strip being wider than 600mm?

Many thanks again for the reply.

 

[wavetrain]

I agree with noseall that you are being conned. If I was you I would ring BC 8am Monday morning. You are paying BC after all. You are building this under BC aren't you?

Can I ask how deep are the foottings going down? You also mentioned they are strip rather than trench. That is a mighty deep floor slab to be sitting on a strip foundation unless the footings are very shallow. Please let us know the outcome of this next week.

 

[plantyroo]

It certainly is being done under BC (builder has arranged inspection for Monday afternoon).

Sorry I probably used the wrong terminology earlier. They have dug trench's 600mm wide and 1200mm deep so far - with any adjustments recommended by BC to be dug after their visit.

The builders have been very good so far so I'm sure they won't have a problem with adjusting the footings. Apart from the additional concrete required I'm presuming the additional width of the trench won't have an impact on the foundations?

 

[noseall]

I'm presuming that there will be no detrimental impact on the foundation strip being wider than 600mm?

No problem with the trench being wider.

We have ended up with some enormous trenches particularly when digging along side an old drain run and the sides have collapsed!

I once built a 600mm high, 3m long wall upon three cubic metres of concrete simply because of the trench arrangement meant filling with concrete was cheaper and less hassle than shuttering.

 

[RedHerring2]

They have dug trench's 600mm wide and 1200mm deep so far - with any adjustments recommended by BC to be dug after their visit.

The extra 100mm of trench width doesn't have to extend all the way up the trench side wall, just the depth of the concrete footings, and perhaps a touch more. Although there is always a danger of overhang collapsing into the trench.
It would reduce working space for any brick/blockwork in the ground, but brickies will manage with a 50mm working space.

 

[geraint]

All masonry should sit centrally onto the foundation concrete.

The floor slab sits upon varying materials, one being hard core another being insulation. At no point does the floor slab sit directly upon the foundation concrete nor does it gain any particular benefit from it.

You are being conned.

Building control will want to see the building sitting centrally on the concrete. You may have to move the building in to do so.

i disagree... you can build on the edge of the footing if need be.. and it will have no detyriment effect on the stability...

 

[wavetrain]

All masonry should sit centrally onto the foundation concrete.

The floor slab sits upon varying materials, one being hard core another being insulation. At no point does the floor slab sit directly upon the foundation concrete nor does it gain any particular benefit from it.

You are being conned.

Building control will want to see the building sitting centrally on the concrete. You may have to move the building in to do so.

i disagree... you can build on the edge of the footing if need be.. and it will have no detyriment effect on the stability...

Are you saying a strip foundation cannot tilt over time even though it will now have a turning momemnt on it?

 

[RedHerring2]

As the BCO will decide the quality of the footings, is it worth discussing the theory of turning movements of the footings?

 

[RonnyRaygun]

I've never had to design an eccentrically loaded footing, but building on the edge of a narrow footing of, say, 600mm will have a significant effect on its stability.
Bearing pressures will be massively increased on one side, and possibly end up negative on the other, and tilting is a real issue.

The way to avoid this is to ensure the eccentricity remains in the middle third of the footing, and that means either building concentrically or making the footing wider.

I'm sure active and passive pressures can also be taken into account, but having never done a design, don't know for sure...maybe Static does?

Building Control shouldn't sign off a footing with the masonry on its edge without seeing some calcs to prove it works.

 

[noseall]

i disagree... you can build on the edge of the footing if need be.. and it will have no detyriment effect on the stability...

Only if they have been designed to do so. Cannock and Stafford authorities even insist on visiting the site to check the masonry is central as part of their remit.

 

[wavetrain]

As the BCO will decide the quality of the footings, is it worth discussing the theory of turning movements of the footings?

Is this supposed to be a redherring? BCOs are not infallable.

 

[RedHerring2]

As the BCO will decide the quality of the footings, is it worth discussing the theory of turning movements of the footings?

Is this supposed to be a redherring? BCOs are not infallable.

Not on this occasion. (You just wanted the word association, didn't you? )
I'm simply saying the BCO will decide if the footing and placement of the wall is to their satisfaction. It's immaterial and irrelevant what other conversations and discussions are going on around the subject.
If the OP is 'advised' that the wall can be built off-center, any discussions, arguments, calculations will be more problematic than simply complying with the BCO's requirements.
Unless there are other complications that we're not aware of.

 

[wavetrain]

The OP should have the structual plans his builder is working to. I have found this link of interest for those who like to read about this subject: http://www.eng-tips.com/viewthread.cfm?qid=195857&page=39

It will be interesting to learn from the OP if in fact the excentric load had been calculated for by the SE and the builder was following plans as required, or as we first thought the builder was conning the OP.