conceptual: RSJ in foundation

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hi,

I just received my structural calculations and building regs documents, everything seems fine.

I'm jut curious about one concept that I can't completely understand, mainly as a learning moment. the engineer wants a box frame as we're looking to completely remove the ground floor rear wall to create an open space with our new extension. that involves installing an RSJ in the foundation. obviously the RSJ will sit right under the existing first floor wall, which rests on the original foundation of the house.

how is that even possible? in order to install something in that foundation, you will have to remove the rear part of the foundation first. won't that simply destabilise the house? how is a beam installed in such cases?
 
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They will use temporary propping to hold everything up whole they do the work. Whoever is responsible under CDM will have to make sure the design is suitable.
If you really want to be sure and it's not a standard situation, the structural designer would be the one to design the support schedule and method.
 
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Needles and props would be used but id be asking why a conventional goalpost arrangement won't work
 
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It surely means sitting above the foundation not in it. Got a drawing?
I guess my understanding of what a foundation is might be slightly off, I consider everything that's up to DPC level to be foundation. since I have a few rows of bricks below ground level, I can't see how placing the RSJ on top won't cause a massive step in the middle of the extension. it also needs to be covered in concrete (at least 75mm each side), so something will have to be removed. drawing below.

They will use temporary propping to hold everything up whole they do the work. Whoever is responsible under CDM will have to make sure the design is suitable.
If you really want to be sure and it's not a standard situation, the structural designer would be the one to design the support schedule and method.
I know about propping, that's not the issue and the structural engineer designed this after a home visit and survey. I was mainly curious about how the RSJ is placed "in the ground", what needs to be removed from the foundation to give it enough strength. I'm hoping the original foundation will hold the weight of two additional RSJs, thought the weight of the entire ground floor wall will be gone, so that should balance out.

Needles and props would be used but id be asking why a conventional goalpost arrangement won't work
there were two main reasons given for this: a goalpost frame would require some serious 1mx1m foundation pads, which would require me to excavate under the current foundations and in both neighbours' gardens. the cost of an additional RSJ is peanuts as opposed to the amount of labour and materials required for that.

the second reason given is lateral strength. despite it being a terraced house, the side walls are not "supported" by anything if I remove the rear wall. this is because there is an alleyway between me and the house on the right. the house to the left is stepped behind mine, so once again an "exposed" wall.

The note does say use existing foundation, so I guess that will have to do, the building control officer will probably advise anyway.

foundation.PNG
 
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How deep are your current foundations? If there less than 1mx600mm I'd imagine your going to have to to dig a complete new foundation the width of your house to accommodate the bottom rsj at which point it may be easier to go to a goalpost arrangement and get friendly with the neighbours
 
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Take the bricks off, install steel, cover with concrete.
 
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this is what I wanted to hear.

would this work if the entire foundation is made of bricks or if it's a strip foundation with a few rows of bricks under ground level?

I can't imagine the existing foundation not being able to hold the weight of the RSJs.
 
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We specify this detail regularly.

It does depend on the top of concrete being low enough that you can get the beam in and the cover over it. If not it's possible to surround with concrete, the alternative is to galvanise the beam and fix it to the foundation on grout and shims.
If there still isn't room you are left with having to grub out the foundation and installing a new one - depth should then be at least to current regs, deeper if there is tree influence.

The reason for this arrangement is that a goalpost needs pads for each column, which need to be concentric to the posts and end up under the neighbouring property - unless you make your opening smaller.

You could if necessary stop your slab either side of the beam and put insulation directly over the top of the beam with screed above (I would think).
 
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this is what I wanted to hear.

would this work if the entire foundation is made of bricks or if it's a strip foundation with a few rows of bricks under ground level?

I can't imagine the existing foundation not being able to hold the weight of the RSJs.

It's technically OK as long as you leave something of the existing foundation. The existing foundation was carrying the load from the existing building so if anything the new load is being reduced. The idea is that the base beam is stiff enough to spread the load on the beam through the posts and back along the existing foundation, so no new foundation required.
 
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thanks a lot, much appreciated. so if I have an old foundation, built mainly from bricks (potentially with a strip at the bottom), I can remove a few layers of bricks from it, to accommodate the depth of the beam + 75mm concrete at the top and that would be sufficient, right?
 
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thanks a lot, much appreciated. so if I have an old foundation, built mainly from bricks (potentially with a strip at the bottom), I can remove a few layers of bricks from it, to accommodate the depth of the beam + 75mm concrete at the top and that would be sufficient, right?
Yes, remove what you need to, but if you remove everything it's an entirely new foundation and then technically it should comply with current regs which might mean digging deeper and pouring new concrete.
 
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one more question, if I may. while I understand the concept of complying with building regs if something is "new", how would that work in practice? would that involve removing the entire foundation at the back of the house (under the wall to be removed) and pouring a new one? wouldn't that destabilise the walls/remaining foundations?

also, what counts as "removing" parts of the foundation? what if I only need to remove two or three rows of bricks above ground level, but below DPC, would that technically be part of the foundation?
 
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one more question, if I may. while I understand the concept of complying with building regs if something is "new", how would that work in practice? would that involve removing the entire foundation at the back of the house (under the wall to be removed) and pouring a new one? wouldn't that destabilise the walls/remaining foundations?

also, what counts as "removing" parts of the foundation? what if I only need to remove two or three rows of bricks above ground level, but below DPC, would that technically be part of the foundation?
Assuming the other foundations are perpendicular to the one being removed, it shouldn't destabilise them. Full underpinning can be done in 1m sections so it's possible to undermine locally and temporarily as long as it's done in a safe and controlled manner.

If there is a concrete foundation then that's the foundation and it's fine to remove anything above. If it's a corbelled brick foundation then you can remove most of the brickwork leaving one or, preferably, a minimum of two courses, then cast concrete encased beam on top.
 

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