Lintel height limit on rear extension

[dearface]

The other slightly more expensive option is to approach the neighbour and suggest removing the beam entirely and replacing with steel higher up, your neighbour may appreciate the extra headroom too, the question is whether they want to share any of the costs. You could offer to pay for calcs, BC and insurance, if they go halves on materials and labour. Can't hurt to ask?

Neighbourly relations are poor so can't see this working!

Thanks to all feedback and suggestions. I'm now seeking second opinions.

 

[Ivor Windybottom]

The other slightly more expensive option is to approach the neighbour and suggest removing the beam entirely and replacing with steel higher up, your neighbour may appreciate the extra headroom too, the question is whether they want to share any of the costs. You could offer to pay for calcs, BC and insurance, if they go halves on materials and labour. Can't hurt to ask?

I'm assuming the beam is invisibly embedded in their back wall, so they won't have any interest in raising it unless they're looking to extend too.

 

[tony1851]

A beam continuous over two spans will have a reverse ('hogging') bending moment over the centre (party wall) support. This reverse moment reduces the bending stress in the spans each side.
Once the beam is cut at the party wall, you immediately loose that hogging moment and thereby increase the bending stress in the neighbour's span. There will be steel reinforcement there but whether it willl be suficient to take the increased tensile stress is anybody's guess. No engineer will be able to know the size and spacing of the bars present.
Also, the beam is 70 years old and the bond between the steel and concrete may have become compromised.
Any solution is likely to involve work on the neighbour's side; personally, I'd leave well alone.

 

[dearface]

A beam continuous over two spans will have a reverse ('hogging') bending moment over the centre (party wall) support. This reverse moment reduces the bending stress in the spans each side.
Once the beam is cut at the party wall, you immediately loose that hogging moment and thereby increase the bending stress in the neighbour's span. There will be steel reinforcement there but whether it willl be suficient to take the increased tensile stress is anybody's guess. No engineer will be able to know the size and spacing of the bars present.
Also, the beam is 70 years old and the bond between the steel and concrete may have become compromised.
Any solution is likely to involve work on the neighbour's side; personally, I'd leave well alone.

Thanks for this.

Accepting the beam in situ is the conclusion I've also come to

 

[Ivor Windybottom]

I think you've wimped out too easily! I do understand the argument that you'd unbalance it, but I wouldn't give up until someone had done the sums and said it can't be done.

At the worst case you could assume it's not reinforced at all and start there. These poured onsite beams are often massively oversized for the load and spans involved. It's easy enough to make it bigger to be on the safe side, and nobody ever needed to lift it. So hopefully you'd be leaving them with a beam that's still treble the size it ever needed to be anyway.

It probably won't be sufficient with no reinforcement, but hopefully you'll find it doesn't need much steel to become adequate. Then it would need a dig into your end to assess what's actually there.

But ultimately it's the height of a standard door opening. So it would look unusual but at least you won't bang your head on it, as is the case in many old cottages all over the country.

 

[Ivor Windybottom]

Actually... isn't there a risk of leaving it in? If you're removing wall from beneath it you'll be increasing its span, which is a massive risk if its strength is unknown. Or are you just leaving the original door or window spans under it?

 

[tony1851]

I think you've wimped out too easily! I do understand the argument that you'd unbalance it, but I wouldn't give up until someone had done the sums and said it can't be done.

At the worst case you could assume it's not reinforced at all and start there. These poured onsite beams are often massively oversized for the load and spans involved. It's easy enough to make it bigger to be on the safe side, and nobody ever needed to lift it. So hopefully you'd be leaving them with a beam that's still treble the size it ever needed to be anyway.

It probably won't be sufficient with no reinforcement, but hopefully you'll find it doesn't need much steel to become adequate. Then it would need a dig into your end to assess what's actually there.

But ultimately it's the height of a standard door opening. So it would look unusual but at least you won't bang your head on it, as is the case in many old cottages all over the country.

I understand what you're gettting at, but the most experienced engineer in the world wont be able to do the sums unless as a minimum they knew the size, spacing and grade of the bars. By the time they found this out by digging around, it might be too late!

 

[Ivor Windybottom]

You could just remove some bricks on the far end of it and cut/grind the concrete off its end. Find out its height, width, what bars are where and get a feel of how solid the concrete is.

Obviously still needs some intuition, but hopefully we don't (yet) live in a world where it's a choice of absolute certainty or no you can't.

My bet is it's massively oversized and just needs some common sense applying.