How would this be constructed?

[ChrisZx]

We've got plans for a rear extension, which includes a steel ring for knocking through the rear wall. I will be DIYing most of it, but most probably not this beam. Nonetheless I want to get an idea of how it needs to be installed and at what stage of construction.

Was hoping we could build the extension before knocking through and installing this beam (well maybe not the roof lantern), but I'm not sure it's possible.

What do you guys think? Does the bottom beam need to go in before/at the same time as the new foundations? Could the other beam/pillars be left until a later date? The pillar on the party wall is shown as being part of the new wall that side, but what if we got it redesigned so that it stuck out as a small pillar instead, would that make it easier?

As an aside I'm not even sure this beam is wide enough, pretty sure we have cavity walls. But I'm at work and can't measure right now!

 

[^woody^]

Can't understand what the massive 9" beam in the gound is supposed to be doing.

 

[ChrisZx]

I guess the idea is to spread the load over the length of the existing foundation rather than concentrate it under the two pillars. But I'm sure you knew that! House is 50's so I think that means likely to have shallow strip foundations?

Builder that did our loft thought it was overkill compared to what he's seen before. Would you/anyone else be inclined to agree? I've been thinking about getting a second structural engineer to have a look but no point spending the money if there's no realistic prospect of it changing.

 

[John D v2.0]

Surely if the foundation is suspect, it would be far cheaper to dig a hole and mix up a new one? 1m3 of concrete and a couple of holes to go with it would surely be fine.
Edit - I only know that because that's exactly what our SE specified, the original Foundation turned out not to exist as the house it's built directly on the ground

 

[^woody^]

But I'm sure you knew that!

No I didn't.

There are walls and foundations there already, I don't know what a beam with a column on the end of it set into the ground is supposed to be doing.

A point load on the end of the beam is not going ro be spead along such a narrow beam. That's why I dont know what it is supposed to be doing.

 

[ChrisZx]

No I didn't.

For clarity I don't know that's the reason/idea, it's just the only thing I could think of. But now I think of it, maybe my loft builder said he reckoned it was about stopping the pillars spreading...

There are walls and foundations there already, I don't know what a beam with a column on the end of it set into the ground is supposed to be doing.

A point load on the end of the beam is not going ro be spead along such a narrow beam. That's why I dont know what it is supposed to be doing.

So you're saying you think the foundation would spread the load anyway, and the beam is unnecessary? Would that not depend on the depth of the foundation? It is two point loads of course, and the new beam is also encased in a massive block of concrete so my gut feeling would be it would be pretty sturdy...

Surely if the foundation is suspect, it would be far cheaper to dig a hole and mix up a new one? 1m3 of concrete and a couple of holes to go with it would surely be fine.
Edit - I only know that because that's exactly what our SE specified, the original Foundation turned out not to exist as the house it's built directly on the ground

So you've got a block of concrete under each pillar? Again, our loft builder mentioned something like that as well, but we only discussed the whole subject in passing

 

[^woody^]

That's about what a 3.5m opening?

That's nothing and it looks like the engineer has way over designed this without actually designing a thing ie thinking.

I've routinely done greater spans than that, with similar loading and similar layouts and never seen such a thing in the ground. Everything is braced so it's looking like a bit of a nonsense with not just that ground beam but those posts.

 

[^woody^]

Back to the question though, yes you can build the extension and knock though afterwards.

 

[tony1851]

As above, you SE has designed a ridiculously expensive and completely unnecessary 'moment frame' (aka goalpost).
The trouble nowadays is that too many young engineers design supporting structures completely independent of the houses' own structure, and that way is invariably more expensive.

You just don't need a ground beam and two steel columns in that position - only the one beam supporting the wall over the breakthrough. The reaction from the beam will not be acting as a point load but will be dispersed through the adjoining masonry until it gets down to the foundation, where it will be a minimal increase.

Take the advice you've already been given, otherwise your SE will be laughing all the way to the bank; he's taken your money for 25 seconds-worth of work to pull up a spread sheet, tap in a few numbers and given you a thick (impressive but meaningless) wad of calculations without actually doing the job he was supposed to be trained for.

 

[John D v2.0]

So you've got a block of concrete under each pillar? Again, our loft builder mentioned something like that as well, but we only discussed the whole subject in passing

Yes well actually the pillar in the middle of the room has a block of concrete, but the other end goes onto a padstone cast into the existing wall as the se said it would be easier to do that and it would be strong enough because the weight is spread over the existing lack of foundations. Pretty much what Tony has said above.

 

[ChrisZx]

Thanks guys, second opinion it is then