Lintels in foundation for building over

[sambotc]

I've got to build over the set of drains for the rear single story extension I'm building. Been a really tough job getting to good ground (2 metre deep) and the build over paperwork took ages too.

So I've got concrete poured and now need to bridge where the drains are. They are about 1000mm wide so need so lintels approx 1300mm. The LABC guy said any would do as they are designed to span this distance?

I see some are 65mm high x 100mm wide, others are 140mm high x 100mm wide etc?

They have different loading capability though, so is there a number I should be looking to achieve for foundation strength or is it just a case of using whatever as the guy seemed to make out?

Thanks in advance

Sam

 

[John D v2.0]

Your SE should have specified, but at a guess the 65 would be fine, it's hardly wider than a doorway. Although I'm surprised you needed 1000 gaps for the drains, I'm assuming they're not the thames water tunnel going through your foundation.

 

[nickjb]

There is a school of thought that if it is long enough its strong enough which it sounds like your bco is saying. That said I put 140mm lintels in mine. Probably overkill but they weren't much more expensive but they were pretty heavy.

 

[cjard]

Conceptually, lintels only really need to support the triangle of masonry units above the opening (i.e. Whatever would fall out if you built the wall then removed the lintel..). For a 1000mm opening, that's about 10 bricks

 

[sambotc]

Thanks for the replies.

We have no SE as it's being done through a self build notice with BC.

It is a pair of drains, soil and rain water, hence the 1000mm trench they dug to lay both in.

Long enough is strong enough sounds just like what he was saying!

Thanks

Sam

 

[Ian H]

I'd have got them in a 250mm trench

 

[^woody^]

A 1000mm gap between two strips is a big enough distance to get differential movement of the foundations. I'm surprised that you was not asked to place a small section of concrete between the pipes

 

[sambotc]

Hi Woody,

That sounds alarming! I did question the fact that they weren't really 'attached' together with much.

The foundations are about 2 metres deeps and have been poured into shellet, pretty solid! The drains are dug into this too, ex council job.

Do you think there will be issues?

Not sure what else can be done?

Thanks in advance

Sam

 

[^woody^]

How deep are the pipes?

 

[sambotc]

About 1.5 at a guess, the footings went lower to prevent any potential loading being applied onto the drains, that's what B.C wanted anyway, plus the ground above that was all made up and no good apparently anyway.

 

[^woody^]

Ideally, the concrete should have gone under the pipes to form a continuous strip, and this makes the extension load bear on the ground evenly.

This should have been designed by your designer or engineer, not just left for the building control person as he is not there to design.

There is not a lot you can do about it now. Its not ideal to have done it like that. It's a potential for uneven settlement or movement but not a certainty.

 

[cjard]

I'd be less inclined to worry at the depths being discussed; there's a lot of wall above to spread out the load and I can't see (lack of) a strip of concrete under the pipes changing things significantly in that regard