an extension and the need for underpinning?

[^woody^]

Footings in pre war properties might only be 12 inches deep by 16 inches wide.

What war would this be .... Crimea, Napoleonic?

The OP has a 1930s semi, there will be adequate foundations for a beam, otherwise the house would be a post war pile of rubble

Also, what you tend to see when an over overenthusiastic engineer gets involved is lots of fancy masonry and concrete calculations, but not a single soil test

As for returns, a 2.1m high solid or cavity wall will be adequately braced by the first floor or the return wall above the opening. Goalposts, smoleposts

 

[tony1851]

There are two issues here;
1. the suitability of the existing foundations and
2. overall lateral stability.

FWIW, I think Woody has a point. Although a pier may be relatively short, there will be some load transfer into the adjacent wall (either gable wall or party wall) which will help to spread the load over a greater length of foundation.

With regard to lateral stability, this seems to have become an issue in recent years when doing rear breakthroughs.
For some SEs, using a spreadsheet to design a moment frame and connections can be effective in terms of the time it takes. However, with some additional time spent on calculations, I have found that in most two-storey houses, the combined moment of resistance of the remaining ground floor walls is often sufficient to resist lateral loading due to wind.

I agree it's best to be cautious, but I feel the OP is being spooked a little too much, and the problem might not be as serious as he assumes.

 

[RonnyRaygun]

What war would this be .... Crimea, Napoleonic?

The OP has a 1930s semi, there will be adequate foundations for a beam, otherwise the house would be a post war pile of rubble.

Woody, I thought you were supposed to be an expert on all things building related. Obviously you are not else you would know that many pre *second world war* houses have tiny foundations. But I suspect you design beams all the time with no consideration of loads to the masonry or foundations.
I don't understand why a house on small foundations would be a pile of rubble post 1945 (assuming you are talking about the second world war.) As we both know traditional building techniques are more forgiving to movement than modern due to the use of lime mortars, so foundation depth was less critical with regard to frost action and building near trees. It doesn't make these foundations more capable of carrying high loads to the soil.

Also, what you tend to see when an over overenthusiastic engineer gets involved is lots of fancy masonry and concrete calculations, but not a single soil test

I've honestly never known a soil test be carried out on a small domestic foundation. Building control generally know the bearing capacity in a given area. 100kN/m2 is pretty standard for clay in London at least

As for returns, a 2.1m high solid or cavity wall will be adequately braced by the first floor or the return wall above the opening. Goalposts, smoleposts

2.1m high is one thing, the width of the remaining masonry is quite another. In this case, 665mm is probably OK for lateral stability, but much less wouldn't be acceptable by engineers or BC.
The first floor might adequately brace the building if it is sufficiently tied in and able to act as a diaphragm. No engineer I've ever met would rely on that from an old property when removing a huge chunk of wall and significantly reducing structural integrity.

Hey Woody, don't envy you when Birmingham is hit by a repeat of the 1987 gales

 

[RonnyRaygun]

I agree it's best to be cautious, but I feel the OP is being spooked a little too much, and the problem might not be as serious as he assumes.

I'm not trying to spook the OP. I was generalising and not talking about this job per se. I agree that 665mm is probably OK for stability at least.
I can't see much load being carried into the gable wall foundation, especially if it's a shallow footing.

The answer is simple regardless. Dig a trial pit and calculate the loads onto the footing.
The OP needs a professional to give their opinion having seen the property.

 

[RonnyRaygun]

Perhaps I'd better go and see my BCO and see what they say.

Exactly, that's the best place to begin

 

[jeds]

Removing nearly 80% of one elevation isn't the same thing as knocking through a couple of meters and bunging in a beam. Many 1930s houses have perfectly good brickwork and foundations but I've seen plenty that were very poor. Frankly it's a bit naive to suggest you will be ok to just get on with it without knowing either way. You are proposing a fairly substantial project, why would a bit of investigation and a couple of hundred quid for a decent surveyor or engineer to check your design worry you. As I said, you are buttressed by the adjoining property so you will probably be ok. But if it were me I would want to know for sure and you'll probably have to provide calcs for BC anyway so what do you lose?

 

[DIYedboy]

Jeds, my initial post was a fact finding exercise based on my ignorance and the expertise of you lot.

I have learnt a lot from everyone's advice, I assumed the only consideration relating to a lintel was it's ability to carry the load above it

Naturally I will have to get an architect to produce plans and get building regs to OK the project in the usual way. But it is a wise man who tries to understand the complications of a project before he embarks on something that could spiral out of control, don't you agree.

From what has been said already, I think I should reduce the size of the opening in order to reduce the load on the foundations and therefore the possibility of underpinning (subject to digging a couple of trial holes)

 

[tony1851]

[quote="jeds";p="2735383" As I said, you are buttressed by the adjoining property so you will probably be ok.[/quote]

jeds;
I agree with your point about support by an adjoining house and have had a difference with an LABC engineer on this precise issue recently.

The engineer was insisting that the semi we were altering was calculated as a free-standing building. He would not accept the principle of lateral support from the adjoining house, on the grounds that it could be demolished.

The likelihood of the neighbours waking up one morning and deciding to demolish their house is rather small - probably smaller than an earthquake or alien spacecraft hitting the house.

In this instance, I was working for a developer-client who wanted to sell as quickly as possible, so I had to put a moment-frame in to get on with the job. I would have loved the chance of doing a formal appeal on this point.

 

[^woody^]

Foundations can take the load that a domestic beam will impose, and a house works as a single unit with everything working together in terms of stability

A typically semi, can have the back ripped out at ground floor and a suitably sized beam put in, and it will not fail

 

[RonnyRaygun]

The likelihood of the neighbours waking up one morning and deciding to demolish their house is rather small - probably smaller than an earthquake or alien spacecraft hitting the house.

True Tony, but if relying on support from an adjoining house you are also relying on them never doing the same thing as you have done. Of course a decent engineer would consider this and design accordingly.

I mean the engineer responsible for the adjoining property by the way, not trying to say you're not a decent engineer

 

[RonnyRaygun]

Foundations can take the load that a domestic beam will impose, and a house works as a single unit with everything working together in terms of stability

A typically semi, can have the back ripped out at ground floor and a suitably sized beam put in, and it will not fail

Hey Woody, don't envy you when Birmingham is hit by a repeat of the 1987 gales

 

[^woody^]

So by the same token, then engineer should design suitable foundations in case the neighbours decides to dig a big pool or a basement?

Design for what is, not what may never happen

In Tony's case, it is the neighbour who should do the extra design work when and if he did the same as the first

 

[RonnyRaygun]

So by the same token, then engineer should design suitable foundations in case the neighbours decides to dig a big pool or a basement?

Design for what is, not what may never happen

In Tony's case, it is the neighbour who should do the extra design work when and if he did the same as the first

That's what I said!

 

[^woody^]

Yes, but I said it more clearly