Can someone explain to me what I am missing?

[SugarPlum]

Hello, First time posting, so be gentle, and sorry it's a bit long!

 

We have an extension project in planning/just started building stage. For various reasons which I won't go into, I have sacked the structural engineers that were working on our project and am now left with an almost complete foundation design along with some part calculated beams where the extension will join the existing house.

 

I tried, without much luck, to get the original and a possible replacement structural engineer to explain why the foundation design is as it is, and thought I would ask on here to see if anyone can explain to me why I appear to be being dense... (I am a Chartered Engineer by the way so I reckon I have a fairly decent brain, just not a structural engineer).

 

The situation: we have a house that has been extended at various points in the past. The original is mid to late 1700s, followed by a 2 storey extension built we think sometime between 1900 and 1930 (single skin construction, slate dpc and no dpc under the quarry tiles bedded on thin concrete). Tacked onto the side of this part is a single storey extension that I am guessing is 1950s or 1960s - it appears to have cavity walling and a concrete floor, along with modern footings. Summarising slightly, but we are joining a 2 storey prefabricated timber frame extension to the side and above this single storey extension - the ground floor of which will be open plan - hence the current exterior wall will be demolished. The initial engineers designed a block and beam ground floor (clay soil, with trees around 15m away). We had soil analysis done and this recommended depths of 1.95 metres in one corner away from the current house, and 1.4m adjacent to the 2 storey part of the current house  (i.e. a spot that is currently inside the house). The foundation design calls for the total removal of the concrete slab and the footings that make up the existing single storey part of the house. As the new footings would then join onto footings that will no way be to modern standards, let alone meeting the 1.4m depth specified by the soil analysis, they have said the existing house will require underpinning, but only where the new footings join the old.

 

I don't understand why we can't keep the current footings and flooring and join onto them. Differential movement has been muttered by both the sacked and the potential new structural engineer, but again I don't understand how the design mitigates this - in fact to my mind it would exacerbate it...? In that if we carried out the design, one wall of our current house would be tied to the new, much deeper footings and at the join points would be rock solid, but surely that same wall would now be at risk of cracking because the footings are not even? And the other walls of the house that range from early 1900s footings (i.e. some, but not deep) to mid 1700s footings (I.e. almost none) could now move at a vastly different rate to the extension and that one wall of the existing house. Surely this will just introduce problems away from the extension? The prospective engineer suggested it is an indemnity issue - is that really it? Does that mean crack in new extension = insurance payout but crack in existing part of house that isn't the new extension = no pay out? Who's risk is actually being protected here??? Or am I missing something??

 

I look forward to any replies, hopefully someone will help me from tearing my hair out!

[DevilDamo]

I have read it all but did get a little lost halfway through. So what is the issue that needs resolving or clarifying?

[SugarPlum]

I don't understand why the foundation design calls for the removal of the existing ground floor and footings, which then necessitates underpinning of the older wall rather than reusing the more modern slab and footings. The only explanation I have had so far is that it is something to do with insurance which makes no sense to me...

[Temp]

44 minutes ago, SugarPlum said:

now left with an almost complete foundation design along with some part calculated beams where the extension will join the existing house.

 

Are you removing the wall or creating a significant opening between the existing and new? This would change how the weight of the wall above the opening is distributed on the foundations. Typically the load becomes more concentrated. If they are marginal then that might explain why they need upgrading.

Edited July 21, 2020 by Temp

[SugarPlum]

I don't think so. In terms of the join from extension to 2 storey part of the house, we are opening up a doorway to make the existing room more open plan which happens to be where the timber frame will join. It is a 7kN point load apparently, at first floor joist level. The load point would be above where the foundations would be underpinned, but given that this load point will be in the middle of another beam approx 1.5m wide, surely any load distributed to the foundations would not benefit from the underpinning? In terms of the existing footings, very little of the extension will rest on these, 95% of the frame is outside the footprint of the existing house. Hard to describe really, I could upload a sketch or two if it helps?

[Temp]

I think sketches would certainly help.

[SugarPlum]

A very quick sketch... hopefully makes some sense... I have tried to show two beams in a T shape, one is designed by the timber frame company as the mechanism to join to the existing house. The other is to remove a doorway and open up the stairs to be open plan to the room. The suggested underpinning would be directly under the beam designed by the timber frame folks, as would new footings.  The dotted line by 1950s/60s and 1998 etc is the current external wall ( the house has a very complicated extension history...)

[SugarPlum]

Another sketch from an elevation perspective. The more I draw, the more it makes no sense.... surely this shows that all the new weight will bear on the original foundations? And the underpinning is doing nothing (other than introducing differential movement)?

Edited July 21, 2020 by SugarPlum

[Gus Potter]

Interesting job this. A few initial thoughts, you have maybe covered this already though.

The differential settlement could well be an issue. Worth looking to see (you may already have) if you can decouple / simplify the problem to avoid underpinning, creating a stiff point in the old house and so on. If you can you could allow the new extension to move independantly from the older structure and detail for this - tricky though!

The  7.0 kN point load is not too onerous a reaction from the beam framing into the beam over the slapping. It may be that this is just carrying some floor load rather than floor and roof load etc. You may be able to under sling this beam from the roof above and transfer this load somewhere else where it is more easily dealt with.

This now leaves you with just the slapping. What might be able to do here is to create a steel box frame. Beam over the top, two columns each side and a heavier beam at the bottom. The bottom beam goes just below ground floor level. The load comes down the columns and the bottom beam spreads the load more evenly over the whole length of the existing found which you don’t touch. Simplfying - the existing found still experiences roughly the same loading regime.

You’ll lose a bit on the width of the opening but this is less disruptive. I saw a chimney breast in there. Worth thinking about but if you are using an old chimney to support say a pad stone / beam. Consider the integrity of the masonry. It may have some form of sulphate attack from soot so it looks ok on the outside but less so on the in. The box frame column can resolve any potential issues here.

You can also use the columns of the steel box to provide lateral restraint to the walls where you cut an opening. Can come in handy.

With a box frame you can then get really keen if you have to with the analysis by changing the stiffness of the connections, you can shift the load about this way a little to fine tune. There seems to be a wall at the back of the stair to come down. This will unload the found so maybe opportunity to add a bit of load back in here.

Temp makes a good point about the existing single storey found in that the extra storey may overload them.

Sugar plum – post some plan view drawings if you want, floor and roof joist layout.

Need to nip off now. The trees, foundation depths are probably worth a later look.

[Jilly]

Not sure if this helps, but I took two years of procrastination because of differing structural opinions, but I didn't break ground until it was resolved as it was so confusing. I found an engineer who would provide calculations to show that the existing structure was fine to reuse (stable conversion), so you may need another opinion... and the BC SE may have his own ideas too, so you need an SE that can argue their case well.

 

Not sure if I've missed something but not joining the building and extension foundations can be done, to allow differential movement with some flexible joining of the building which I don't know the name of. 

 

Padstones foundations can can be one solution. 

 

I've been warned that some SEs over spec to work defensively. 

 

[SugarPlum]

Thanks Gus and Jilly. I have been thinking that decoupling the structures seemed like a logical thing to do (and indeed seems to be implied if you look sideways at the NHBC guidance on building a new property adjoining an old one as a proxy for extension guidance - the original Structural Engineers openly admitted they followed the NHBC guidance for new buildings, and when I queried that they - incorrectly, I believe - stated it would not be compliant from a BR perspective to do anything else). I have today got in contact with some Structural Engineers who specialise in conservation work, hoping that they might take into account the effects on the entire building.

Gus, you have me (more) worried about a ridge beam that the original structural engineers designed now - we converted the very low ceilings in the original cottage to vaulted ceilings. This necessitated a roughly 8m long steel beam, which rests on both ends on the masonry that is the interior face of a chimney breast.... I was already concerned that they had assumed these walls are cavity walls when in fact they are at least 250 year old single skin walls, now that's another thing to add to my worry list.... 

 

Gus, I will see what actual plans I can upload - slightly wary due to copyright and also the fact that we're likely to take legal action against the original structural engineers (for breach of contract law) so don't wan't to be plastering anything identifiable over the internet...

[SugarPlum]

This is the loads from the timber frame, which I have (roughly!) annotated with existing walls/foundations- does that help? The timber frame company have said that the frame will not load the existing house in any other way, which leads me to believe that the floor joists and roof joists run parallel to the 1900s wall with chimney breast.

I've already had a conversation with the frame manufacturer about the possibility of shifting the loading points if it makes the foundation/floor design easier, but they were reluctant.

I think we could live with not opening up over the stairs if it mean the structure of the extension was able to be independent. 

[Gus Potter]

I have chucked in some other general stuff here but for SugarPlum it’s worth a closer look at the decoupling. The timber frame will also shrink a bit so you have that movement to account for too. Keep it simple – hence the attraction of separating (decoupling) the old and the new.

Interesting point about the NHBC guidance. It is guidance though. Once you get a handle on the particular features and behaviour/ nuances of the existing structure you can apply first principles of design and adapt the guidance to fit the particular scheme. You need to put a bit more detailed design work in and think laterally, but it should save you money in the long run. We know every building is different in its own way so one solution fits all. That’s why we love them.

Plum, you may well still be able to have the slapping so don’t throw in towel just yet. You could save a lot of money not underpinning so the additional cost of a box frame may not be significant.

Had a look a Sugarplums floor plan. Point worth clarifying with the timber frames designer is how the lateral stability works, this is the bit that stops the building moving sideways when say the wind blows. There is a fair bit of glass in plums ground floor so less wall panels to resist the sideways movement.  This can start to crop up later especially if you live in a windy spot so best to get this out the way and clarified early as it can transfer down / impact on the foundation design.

Aside,  if you have a drawing showing loads, if possible find out if they are “unfactored” or “factored loads”. An “unfactored load” has no safety factors applied. The difference between the two is significant.

Plums’ ridge beam is worth a look at in general terms. Plum – if you are unlucky and things start to go pear shaped with this then you have a few options so perhaps don’t worry too much.

For the curious - If you have a basic roof then roughly the two rafters are tied together at the bottom. This tie forms the ceiling and the whole lot often sits on the outside walls. The rafters want to push the walls out, the ceiling ties hold the two walls together.

When you vault the ceiling the tie is often cut out unless you leave them in and make a feature of them. You can solve wall spread often by putting in a beam up at ridge height (as Plum mentions) that say spans gable to gable.

But you have now transferred roof weight to the gable. You may have a chimney flue or maybe or small opening, right under where you want to put the end of the ridge beam. Old gable walls can be a bit less stable at times. One way of getting round this is to put in an A frame on the inside of the gable. This supports the end of the ridge beam.

The bottom of the A frame extends down to the attic floor level / main wall head  level and you transfer the load back into the wall here. What you have done is take the load away - down and sideways from say the flue. Here there is a bit more compression in the wall and this can help too, the gable wall is perhaps less weathered etc so the wall can be more sound at this point. This method introduces some horizontal forces (due to what is called an eccentricity) which you need to deal with but it’s all doable.

I'll leave the discussion on building regs, the NHBC etc for another time..

[SugarPlum]

Thanks Gus. I think decoupling seems to be the right approach here, just need to find a Structural Engineer in my neck of the woods that agrees now! The concept of underpinning just doesn't sit right with me at all, partially because it means we can't afford to complete the extension, but mostly because I am trying to preserve the old parts of the house as much as possible. Whilst it's not listed, we're doing as much 'like for like' as we can in the old bits, and adding back in the original features that have been mishandled in the past and I'd personally be devastated if something we did caused damage....

 

53 minutes ago, Gus Potter said:

. Once you get a handle on the particular features and behaviour/ nuances of the existing structure you can apply first principles of design and adapt the guidance to fit the particular scheme. You need to put a bit more detailed design work in and think laterally,

 

This I totally get... I'm a Systems Engineer by trade, which means I spend my days designing technology and then making sure that the different engineering teams build their bits so they all work together. Decisions made in one area impact the other areas and need creative solutions to enable each team to deliver, whilst the whole still achieves its goal - lateral thinking is my bread and butter...and probably why I am not satisfied with the apparent 'design by numbers' approach that the original Engineers seemed to have... they were not that impressed that I kept asking questions...