Structural calculations – is one set definitive?

Long story short, we’re planning on knocking the wall down between kitchen and dining room in our old cottage. It’s an external wall, very little foundations, design for the steels for that bit is fine.
The small kitchen is at right angles to the main house, and we wanted to increase the feeling of space by vaulting the ceiling. We’ve had calculations for 2 options – 1 with collar ties that would involve lifting the roof, and the second with a cranked beam coming off the steel we’ll put in for the wall removal at right angles. All that is fine, however the SE has specified that the rafters need replacing as well, and their specification seems way OTT according to two separate builders we’ve consulted.
There is no hint of flexibility in the communications we’ve had with them

Do we just accept this as gospel (their requirements would basically render the walls useless for kitchen units) or so we cut out losses and try consulting a different SE?

And no, we haven’t got an architect involved, as multiple professionals advised it wasn’t a big enough project, but that’s starting to feel like a big mistake now!

You need calcs for a solution that solves your problem. So you are going to need an SE that gets what you are trying to do.

This might not be the one you have at moment

Just remember , a builder is not a Structural Engineer, and what they think is acceptable may not be on paper. You are paying for SE to design you something that’s not going to fail under load , and it sounds like thats what they have given you.

Probably worth a second opinion. I would compile a few photos , sketches etc of what you want to achieve, and consult another SE. The less leg work they have to do, the cheaper it will be. Show them the calcs if you have them

Good advice, thanks both!
Good point about the builder – we’re conscious that builders are absolutely not the same, but it’s that thing where when multiple people raise something, it starts to raise doubts in my mind, IYSWIM?

We’ve got all the calcs for both approaches so may be an idea for us to talk to another SE with them I guess.

Ultimately, both designs are unworkable, but we’d always said that when we do this we don’t want to end up regretting not having done that ‘extra bit’, as moving from here is not something we plan on doing for a long time!

Ask builders and SE how much back up you have if you go with their proposed solution and the whole thing starts shifting in a few years.

Option 1 if I am reading correctly you will be stripping the original roof and putting in new collar tie trusses for the roof to raise it? Option 2 you’re raising it via steelwork and new purlins etc.

How much are the rafters increasing by? What’s the age of the existing ones and conditions etc? It’s usually easier and better to rip out the old rafters and just put new in. You’re raising it so will be easier to put in new wall plates, straps , rafters etc than mess about re-using. Surprised the builders are querying.

Always have a chuckle when builders say SE over design stuff and go OTT and then you go on site and stuff has been missed out, or just not installed correctly. Should have got an architect involved FWIW as it does sound large enough. They don’t just do the plans, but they would act as lead designer and organise all of this.

Well worth a second opinion.  I have seen a structural engineer  stating a wall was load bearing when it was not.  We took the ceiling down to show there was nothing above it to which he went “oops sorry – my mistake you were right”  If he had been listened to there would have been many thousands of pounds worth of unneeded structural steel put in.

I have no faith in them at all – thats not the only time I have seen similar

Is it not that the builders are probably right but any come back following an issue would fail because they aren’t SEs?
And an SE is working to avoid any future issues but must follow current best practice to avoid any come back on themselves?

Essentially you don’t have a solution because of the wall units. So the starting point should be the wall units fitting and the solution is what makes that work. Seems like you need a better SE….?
(Very easy to say that I know)

The SE should have agreed a conceptual solution with you and then done calculations to substantiate it. They will need to be happy that the solution is stable but also that the calculations will be accepted by Building Control; these should be the same thing, but Building Control can be inflexible and ask for things to be done their way, even thought the Building Regulations allow different solutions.

I don’t quite follow the interaction between the vaulted roof and the wall units?

Thanks again all. Despite being in my early 40s this is the first structural work we’ve ever been involved with, so are pretty green with it all. To be clear, I’m absolutely not saying that a builder knows more than an SE, it’s entirely possible I’ve asked the wrong questions of them. The builder wasn’t saying they were wrong either, we were just chatting and trying to figure out why the sizes have been specced. We were trying to avoid lifting the roof due to cost, and that is only half the extension. There’s a bedroom on the other end that would then come into scope if we started lifting the roof.

Absolutely happy to get an architect involved if needs be., Essentially we’ve paid for some calculations and if we end up having them redone then it’s something we’ll have to put down to experience.

We’re not considering doing anything contrary to the calcs, and it’s my understanding that Building Control will need to see them to sign it off anyways.

For context, the roof space is circa 1.2m above existing ceiling. Width is just over 2m, length just over 2. It’s a single story extension that joins the house with a small hip. Roof is currently in good repair, estimated to be 70s maybe.
Current apex/ridge is 25×125, rafters 75×50 at 500 centres.
SE has specified a 152×152 RSJ for the ridge, existing rafters overboard with 11mm OSB, then 50×150 rafters at 400 centres.

@tjagain I suspect you have more faith in structural engineers than you think. How many defective buildings have you worked and lived in over your lifetime?  Just because one lazy sod hasn’t done his job properly doesn’t mean you can write off an entire discipline.

I’d also point out dealing with small resi schemes is an absolute pain in the arse, no one wants to pay for the service then moan when conservatism creeps into the design as we’re not provided with the correct info and don’t have the fee to chase. Personally I and my company steer clear and only do house extensions for friends.

And yep, builders don’t always know best, I’ve been in to rectify a few interesting jobs where no engineer has been employed, typically lack of movement joints or wall ties in large masonry panels, but also cutting chunks out of beams for joists or pipes etc…

Sorry, @greybeard – r wall units in the kitchen are currently right up against the ceiling (which is pretty low)
If we are reading the SE diagrams right, there’s circa 250mm (by the time you add on insulation, board etc) added to the inside of the roof ‘triangle’ – the builder was trying to figure out how that is supposed to attach to the wall on hangers, and not end up below the level of the existing ceiling, IYSWIM?

@twistedpencil I very much get where you’re coming from, likely we’re one of those nightmare residentials as we don’t know what we should be asking 😀
In our defense I was very specific when the SE originally came round to measure up. That’s how we ended up with the design for the main steels in the exterior wall. However, he wsn’t the one that did the calcs, that appears to have been delegated down.

Its not just one occurrence – that was the most obvious.  I have seen others.  But point taken – as with most things you only see the cockups.

Its just well worth getting a second opinion.  the guy the OP engaged might be right or might be wrong.  He won’t know until he gets a second opinion.  I’ve seen all sorts of stupidity and illegal stuff from both architects and structural engineers.  Every time I have seen them at work its basic errors that I could spot.

Seen builder eff things up as well

I’d also point out dealing with small resi schemes is an absolute pain in the arse, no one wants to pay for the service then moan when conservatism creeps into the design as we’re not provided with the correct info and don’t have the fee to chase. Personally I and my company steer clear and only do house extensions for friends.

Does this mean the small residential jobs only get the numpties?  all the ones I am thinking of are small residential jobs

IAASE – I’m also married to one who deals almost exclusively in these kind of projects. There’s plenty of very skilled engineers working on domestic stuff, but they’re all busy and finding one is not so easy. Good architects and contractors will have a working relationship with a preferred engineer – most of my wife’s work is repeat business.
Smaller projects can be a massive pain in the arse – there’s usually quite a difference between what the client wishes for, what the builder can build, what’s possible in engineering terms and the budget everyone is willing to work to for design, materials and labour. The skill of the engineer is finding the best intersection of that particular Venn diagram.

I don’t know what the fees the OP has paid and the terms of engagement, but my wife usually prices fairly high – for that she offers a good level of support and engagement throughout the project to optimise the solution for both client and builder and deal with issues along the way. It can be a struggle to convince clients it is worth it from the outset (it’s still a very small percentage of the overall cost, and peanuts compared to Architectural fees) – but she’s taken on enough projects involving recovering other engineers and builders work to have been proven right. If she’s been beaten down on fees then it’s likely to be a one-off non-negotiable design and Building Regs submission. I know what some others charge for domestic work, and the fees available mean you’d have to churn out a dozen simple calc projects a week to make a living on it at that end of the market. That leaves next to no time in a project for anything a bit bespoke or rework on something after the fact. I’m not making excuses for the situation the OP has found themselves in – but it’s a simple fact of economics.

So to answer the OP – engineering calcs are definitive by nature (it either works or it doesn’t) but design solutions are infinite – if you can’t get one out of your current engineer, find another.

A structural engineer has very little incentive to look for a material or space efficient solution. They’re making sure it’s as safe as possible without significant consideration of other factors. That’s why architects are often a better solution to the overall problem.

A structural engineer has very little incentive to look for a material or space efficient solution. They’re making sure it’s as safe as possible without significant consideration of other factors. That’s why architects are often a better solution to the overall problem.

Bollocks. A structural engineer works to the guidance that

A. Sets up how strong it needs to be for a given situation
B. Provides the testing calcs to ensure any solution can be proved mathematically

They also have to maintain CPD as part of their chartership that includes things like material efficiency. And largely they like to solve problems which includes cost considerations etc.

It’s nothing to do with as safe as possible.

But while we are bashing all throwing random stereotypes about, builders are **** useless and architects can’t even join the lines at the corners of the their drawings.

I thought our SE was very good. had a good conversation about out requirements (primarily no internal posts). and he came up with a solution with BC reviewed and signed off the calcs

every builder we had look at the job said it was massive overkill (9 steels) and said they would have done it different. I asked the first few if they knew how to do the calcs to do suggest different, and they didn’t so I stuck with the expert!

initially I fancied my chances understanding the calcs (studied advanced mechanics many moons ago) but no chance all that knowledge is long gone!

have a convo with the SE about your concerns, after all you are the client it maybe there is a different approach they ruled out as it was more expensive / timely but if they know your requirements better it maybe viable. you may of course have to pay for a second lot of calcs to be run

Its the Structural Engineers PI that you be claiming against if something fails, not the builders. That’s one thing you are paying him for, to cover that liability.

But while we are bashing all throwing random stereotypes about, builders are **** useless and architects can’t even join the lines at the corners of the their drawings.

yup 🙂

And don’t even get me started on Nurses TJ 😁

DT78. You probably would be okay from the model perspective, load paths and distributions etc that’s basic stuff (but picking the method of modelling and solving is trickier.) You might struggle from scratch.

The rest you need the eurocodes and steelwork charts to understand the geometries of the sections and the notations. It’s really formulaic but not particularly taxing.

Prestressed concrete however is wizardry. I got 41percent largely because there was no danger I was going to ever want to do that in the real world so gave it precisely no effort. See… Efficient engineer here.

And don’t even get me started on Nurses TJ 😁

Fat and lazy?  🙂

Well this was a fun read this morning LOL. Meanwhile I have had a developer as me if I can get rid of movement joints in a 40m long wall as it’s too expensive. Aye mate, you crack on…………..

Aye mate, you crack on…………..

Lolz – that deserves recognition 🙂

Clearly my experiences are an outlier and twistedpencil above gave a good reason – these small domestic jobs are not worth the effort for the big companies – so do the big companies have all the competent engineers leaving those of us that need a structural engineer for wee jobs with only the choice of the folk that cannot make it in the big firms so we are likely to come across the less able folk?

Clearly tower blocks and so do not fall down all the time

apologies for casting aspersions on an entire profession

A lot of the smaller resi jobs are done by one man bands or small consultants. The issue there is to make ends meet they have a lot on. Resi jobs are usually a pain in the arse as they usually have smaller contractors on or just one man builders on who want to crack on and get their payment. There’s a process that usually happens and with smaller jobs, that’s happening less and less. There’s so much pressure from people who want it now. My usual saying is, do you want it now or want it right? You issue your stuff and you may not hear anything for weeks. Meanwhile you’ve cracked on with other projects, then you get a call, client has changed their mind, they want XYZ and can we have your updated design and drawings by yesterday as to meet budget and meet deadlines we need it.

Majority of young un’s coming into the industry want to work on the big stuff. I have worked on sports stadia, hospitals, high rise etc. But I now work for a smallish consultancy and love it as you get more involved, but I really enjoyed my time working on the big stuff. Problem with smallish consultancy is you get young uns in, train em and a lot want to leave after to work on the big stuff, totally understandable.

I have worked for several large consultants and there is some right dross that works in them. But they can hide, be pushed onto stuff that’s not client facing etc. It’s the client facing ones that polish the turds and get the wrap for it. But smaller consultants they can’t carry people like that. I am not saying it doesn’t happen, but it’s less so.

The industry is a bit of a mess IMO. We all want the same thing. Make some brass and deliver what the client wants. But it’s become finger pointy, not working as a team as such. E.g I am working on a retail refurb. Part of the building will need tanking. That’s usually the Architect’s remit / specialists. We gave some input at a meeting yesterday. Got an email from the Architect today and he’s pretty much wiped his hands of it and pointed the finger at us saying we have specified it. Errrr no pal, you’re lead designer.

Anyways, I waffle.

Ta – its always interesting to learn about other stuff

Bollocks. A structural engineer works to the guidance that

A. Sets up how strong it needs to be for a given situation
B. Provides the testing calcs to ensure any solution can be proved mathematically

They also have to maintain CPD as part of their chartership that includes things like material efficiency. And largely they like to solve problems which includes cost considerations etc.

Rubbish – there are myriad ways to accomplish the same structural capability, the simplest of which involve varying materials and cost to achieve changes in size and shape.

I am a structural engineer, but in a different field. There will be conservatism in the calculation due to the risk, there will be an assumption of cost based on perceived value and from what the OP has said, their decision and willingness to review it are rather telling.

Just to add, I had a hernia op a month ago and the nurse was brilliant.  Therefore I am now of the opinion all nurses are awesome 😀

The doctors on the other hand…

I am a structural engineer, but in a different field. There will be conservatism in the calculation due to the risk, there will be an assumption of cost based on perceived value and from what the OP has said, their decision and willingness to review it are rather telling.

We apply factors of safety. If that’s what you mean?

Any SE should think about simple solutions. Based on cost, buildability etc.

If I am reading correctly by the OP they have been told about two solutions. The SE has given them the sizing of items and the OP has been told by the builder(s) that they think there is no need to replace the rafters. At the end of the day, like what’s been said the PI for the design is on the SE, not the builder. The cost is probably not a lot too for the calcs and design. So why should they go back and redesign because Barry the builder said they’re over designed?

Ask the builder to take design responsibility on and see what the builder says, I bet I know the answer straight away.

have a convo with the SE about your concerns, after all you are the client it maybe there is a different approach they ruled out as it was more expensive / timely but if they know your requirements better it maybe viable.

^This.

Any set of calculations will be based on explicit and implicit assumptions. Explain what you wish to achieve, why the current scheme doesn’t allow it, and ask if there is an alternative.

Things like beam sections will usually default to what is commonly available / used (because this makes it easier(=cheaper) for you and the builder). The SE will calculate whether they are adequate for the envisaged loadings. If they are, it’ll be a tick in the box – the chosen scheme will ‘do the job’. I wouldn’t usually expect a SE (without specific instruction) to try and optimise the design to (say) minimise the depth of the beams/joists – possibly at the cost of more of them being required – since there are an almost infinite number of possible answers. However, if thay are asked to do so, or are given (say) a height constraint that wasn’t part of the original brief, then thay may well come up with a different answer.

I’d also recommend checking letters and qualifications of engineers. It’s not a requirement to be qualified to submit calls to building control, and having the letters doesn’t automatically make a decent engineer but it does go some way. Also some companies use the Institution of Civil or Structural Engineers when they shouldn’t.

Timp CEng MIStructE 😉

I know many chartered engineers I wouldn’t trust to design a tent and one of the best engineers I worked with isn’t chartered.

Agreed but it’s a starting point if wondering about the capabilities of an engineer.
There reaches a point where on the job experience is more important than letters but as the title is not protected there is no other way to identify. No doubt your unchartered friend would be able to pass if it was a requirement for the job.

Timp CEng MIStructE (below average tent designer)

Yeh agree.

Let me write a bit more about my experience. We undertook a major remodelling of our house. The concepts by the architect were good but the downstands and pillars in my view were detrimental to the feel we wanted.

We found an SE who got the concept and designed to meet the brief. Yes it was harder to build. Yes it causes more compromise elsewhere. Yes it was worth the effort.

Find the right SE the problem can be solved