Difference in structural engineer opinions - how to proceed?

boots_babe

vacheron said:

I'd ask both engineers if either of them had had a structure collapse or partially collapse in their respective 30 year careers.

If the answer from both is no, go with whichever you prefer. 

I'd also keep Building Control updated with any changes in the plans and provide them with updated drawings as soon as possible. They will confirm if the solution is acceptable as part of their signoff.  

Both are saying never had any issues. Both are saying they are right. Both can't be right though, so we need to understand how best to navigate our decision.

BC know about this, and once we make our own decision on which we are going with, they know we will be sending them the proposed plan and calcs. 

ThisIsWeird

Blimey!

Ok, is your builder actually refusing to carry out your own SE's instructions?!

"Option 1 - knock down the wall, build a pillar at each and end insert an RSJ. The engineer is telling us that the RSJ is actually over spec'd for what is needed as he was being cautious. He has also said that once the wall is down, we must examine the foundations to see what is there, and consult with him, to check his solution is going to still be valid based upon what we find." I'm guessing that this is your own SE's plan, and it is what I would expect to see; ie, a 'this will work, provided the founds are what I expect them to be. This can only be determined on site once they have been exposed. Changes may be required...' sort of thing. And, BuildControl have accepted this.

In which case, this SE may determine that substantial foundation reinforcement is required - which makes sense, as the two pillars will now be holding the focused load of what a whole wall used to. What's going above this wall - a house storey?

So, the main difference seems to be the size of RSJ. One ('your' SE) reckons they've over-spec'd it already, but the builder's SE says it's not big enough? That's a bit alarming!

What to do? I'd only worry about the RSJ sizing - keep it simple! I'd either go back to your own SE and ask them respectfully to confirm their calcs, or just pass it on to the BCO to do so. Once you are reassured that the RSJ size is fine, I'd let them get on - once the founds are exposed, the BCO will want a gander, and they will likely state if reinforcement is required. 

Although prob not a qualified SE in themselves, your BCO will almost certainly know these other two SE's (for all good and bad reasons!), and will be able to have a chat with yours, certainly, and will know their reputation.

boots_babe

ThisIsWeird said:

Blimey!

Ok, is your builder actually refusing to carry out your own SE's instructions?!

"Option 1 - knock down the wall, build a pillar at each and end insert an RSJ. The engineer is telling us that the RSJ is actually over spec'd for what is needed as he was being cautious. He has also said that once the wall is down, we must examine the foundations to see what is there, and consult with him, to check his solution is going to still be valid based upon what we find." I'm guessing that this is your own SE's plan, and it is what I would expect to see; ie, a 'this will work, provided the founds are what I expect them to be. This can only be determined on site once they have been exposed. Changes may be required...' sort of thing. And, BuildControl have accepted this.

In which case, this SE may determine that substantial foundation reinforcement is required - which makes sense, as the two pillars will now be holding the focused load of what a whole wall used to. What's going above this wall - a house storey?

So, the main difference seems to be the size of RSJ. One ('your' SE) reckons they've over-spec'd it already, but the builder's SE says it's not big enough? That's a bit alarming!

What to do? I'd only worry about the RSJ sizing - keep it simple! I'd either go back to your own SE and ask them respectfully to confirm their calcs, or just pass it on to the BCO to do so. Once you are reassured that the RSJ size is fine, I'd let them get on - once the founds are exposed, the BCO will want a gander, and they will likely state if reinforcement is required. 

Although prob not a qualified SE in themselves, your BCO will almost certainly know these other two SE's (for all good and bad reasons!), and will be able to have a chat with yours, certainly, and will know their reputation.

Builder isn't refusing at all. He has just raised it as a concern, he will happily do whichever option we ask him to - provided BC sign off of course.

Yes option 1 is 'our' structural engineer. My mind is in the same place as yours, re the point about let's see the foundations first before we decide. 

Then re the RSJ size difference, that seems a big worry, as how can a mathematical calculation throw up totally different answers? It can't. 

The wall in question holds up the second floor of the house. The extension area will adjoin where the wall was/is, and will be single storey.

Your advice regarding asking BC to give their view is really helpful. We had not considered that, we were thinking we needed to make a choice, then go to BC to ask them to review/approve. But if we can engage with them now and ask their input, that would be a really helpful neutral (and informed) view. Thank you.

FreeBear

Option 1 is saying dig down to the foundation to see what is there. If it is determined to be insufficient, reinforce with a concrete pad.

Option 2 ignores the dig down & investigate, and goes straight to a reinforced concrete pad. Quite possible he is familiar with the type of foundation after having worked on similar properties in the area.

ThisIsWeird

Our BCO was fab, very approachable, very fair, but didn't take prisoners, and knew all the builders he'd need to keep a firm eye on!
You have a genuine dilemma, B-B, for which there should be a simple answer. Your BCO cannot just ignore this should you request a quiet word.
Are both SEs suggesting a similar type of RSJ? Is it a proper I-beam type, or a Catnic? It's not down to different types, is it? 

Section62

boots_babe said:

Annemos said:

 I'm not sure sorry, as thankfully haven't had to ever deal with subsidence. But they are proposing a certain size of trench being dug under where the RSJ will be inserted, which will be filled with reinforced concrete, so that the weight of the RSJ is not just on the 2 pillars at either end, but spread across the reinforced concrete area too.

That on its own might imply there was a question about the bearing capacity of the ground, or the capacity of the structure being built onto.... but that isn't entirely consistent with needing to use a larger beam (RSJ) - the beam is sized to support the loads imposed on it whilst keeping deflection (movement) within the design allowances.  Is there more than one problem with the design they are trying to fix?

boots_babe said:

Both are saying never had any issues. Both are saying they are right. Both can't be right though, so we need to understand how best to navigate our decision.

It is possible they could both be right.  Structural engineering is about finding a solution to a problem, and usually there is more than one solution that works.  Arriving at the best solution means looking at other factors such as cost, time to implement, availability of materials and labour etc.  Structural engineering is more art than science - there are factors you can include or exclude depending on the exact details of the situation. One SE might work to one set of assumptions, another might work to another. So long as they are following the correct method and applying the right assumptions there is not necessarily anything 'wrong' (in absolute terms) with arriving at two different answers.

boots_babe said:

Then re the RSJ size difference, that seems a big worry, as how can a mathematical calculation throw up totally different answers? It can't. 

As above, it is much more than a mathematical calculation...  the concern I'd have is why the beam size is different in what sounds like a bearing/support issue.

But ultimately the advice give already is right - you are paying Building Control to do he worrying about this.... if they are happy with a proposed solution and the builder is willing to build it, then that should give you sufficient comfort.

vacheron

ThisIsWeird said:

Our BCO was fab, very approachable, very fair, but didn't take prisoners, and knew all the builders he'd need to keep a firm eye on!
You have a genuine dilemma, B-B, for which there should be a simple answer. Your BCO cannot just ignore this should you request a quiet word.
Are both SEs suggesting a similar type of RSJ? Is it a proper I-beam type, or a Catnic? It's not down to different types, is it? 

Same here. Our BCO was a great resource to have on side to keep out Architecht and SE in check. Our builder was great, but the aforementioned pair less so! 

I originally thought an independent Architecht's drawing would have given us exactly what we wanted and a frame of reference in case the builder tried to cut any costs / corners, but in reality most of the basic design and appearance was designed and 3D modelled by me anyway, with only minimal technical content added by the Architecht, much of which was identified as questionable / incorrect and/or improved by our excellent builder who pre-empted all of the BCO's findings in relation to the errors and oversights of our Architect and Structural Engineer's designs! 

In hindsight we should have just gone with our design and used an architechtural technologist to make our own ideas "realisable", then left it to the builder and his recommended SE which would have saved weeks of delays and been cheaper.

Still, you live and learn. 

ThisIsWeird

Section62 said:

As above, it is much more than a mathematical calculation...  the concern I'd have is why the beam size is different in what sounds like a bearing/support issue.

My wee concern is that the 'undersized' (original) SE also reckons their RSJ is already 'oversized'! So the builder's SE suggests oversized-oversized...

There always remains the possibility that, like some surgeons, SEs are toooo precious to admit any error.

Hopefully the BCO will sober up anyone who requires it. Ultimately, you live and die by your calcs; if the house falls down, and you survive, you can sue...

casper_gutman

boots_babe said:

Then re the RSJ size difference, that seems a big worry, as how can a mathematical calculation throw up totally different answers? It can't. 

They won't both be plugging the same numbers into their calculators and getting different results. There are different approaches that can be adopted at every stage of a design calculation.

First you have to assess the load the beam will have to carry, and there are various approaches here - it has to support so many square metres of brickwork, such and such an area of the first floor, some part of the loft. But do you know what area of the first floor is supported on that wall? Which way do the joists run? One engineer may make a pessimistic assumption, the other may check and find things are better than that. What load is on the floor in the loft? Next to nothing now, but maybe one engineer allows for a possible future loft conversion.

The approach taken when assessing how well the beam will be restrained against twisting and deflecting sideways can make a difference to the answer they get in the end.

If the steel is supporting a cavity wall you will typically use one piece of steel under each leaf of the wall. One of these will carry more load that the other. An engineer may design for the 'worst case' and then just use two of those. Or they may find a clever way of connecting the two steels together so the load is shared between them.

There are lots of different things like this that could affect the outcome. It's not just a question of "Length x width x number of floors = size of beam needed".

Oh, and to sound a note of optimism: it is very unlikely that the size of the beam proposed by even a semi-competent engineer will make the difference between the house standing up and the house falling down. The design of steelwork for supporting openings like this is almost certainly governed by limiting deflections to avoid excessive cracking of the walls above, or just by making the top of the beam wide enough to support the bricks. If it was going to fall down, this would more likely be because the ends weren't properly supported, or due to an unsafe methodology being adopted during construction.

Doozergirl

FreeBear said:

Option 1 is saying dig down to the foundation to see what is there. If it is determined to be insufficient, reinforce with a concrete pad.

Option 2 ignores the dig down & investigate, and goes straight to a reinforced concrete pad. Quite possible he is familiar with the type of foundation after having worked on similar properties in the area.

If I were a layperson, this is the post I would read first.  

There's a couple of sensible following posts as well, but I don't read the SEs approaches as being entirely different.  

If number 1 is building piers, they need to be supported by decent foundations.  They're just saying to check first. If you go go to dig to number 2s plan and there's already substantial footings in place, you won't be building more.  

However, years of experience tells me that it's almost never better than you expect.  

Personally, I would go with the builder's choice because that builder has years of experience with structural engineers and having to trust them to keep them safe whilst working too.   We have been presented with SE calcs in the past where the SE hasn't even been to site and have based their calcs off incorrect architects drawings - that confrims my own idea about the basic level of service SEs should provide and therefore we trust our own experience and they guy we've used for over a decade (who is always prepared to discuss the different options available - there is always more than one)

i've also seen some really dangerous work that has apparently been designed by an SE and subsequently passed by BC, so that also doesn't fill me with confidence either.  

SEs are more qualified than BCOs and BCOs aren't allowed to offer design advice, so I'm not sure what help they would be in the situation 

number 1 is giving you an open ended question that may well end like number 2's solution.