Steel beam queries

[TobyLeRone]

Hi everyone, I'm in the process of starting out with an extension to our property that will involve knocking through the rear wall, with the intent on creating a large open kitchen/diner/living area.

Architect is on with the plans now, and we'll be getting an SE in to calculate steel requirements and foundations to make sure it's done properly. However, something as important as my home, and as it's a structural change, I always want to understand what is being done, and more importantly why, before the experts get started. Just so I have at least some idea of what they're talking about.

The span when we knock through is going to be 6 metres, and with a large opening like this, isn't there a risk of the beam buckling? One other concern I have is that the house is quite high up in the pennines and is exposed at the back. I'd expect this will be taken into account, but to what extent will this impact any design?

I'm sure a bigger UB would help to prevent this, but as the blocks in the wall that will be supporting the beams are only 100 mm, doesn't that limit the size of UB we can spec? Or is it acceptable to have part of the flange 'overhanging' the block?

Alternatively, will the two beams (for inner and outer cavity wall) be connected together to offer additional strength against the buckling? If so, what is the best way to do this?

I know a lot of questions, but it'll really help me get my head around what's being done and, I hope, limit some of my anxiety.

Cheers

 

[tony1851]

The SE will specify the beam(s) to prevent buckling. You can either do it one wide-flange beam, or two separate beams.
There should be no need to bolt two separate beams together; in fact, the design Code specifically prohibits bolting two parallel beams together if the intention is solely to prevent buckling. Yet we still see builders, architects and inspectors asking for beams to be bolted together...?

 

[domdee]

i think the SE will have seen a lot more complex set ups than this. he will have cals to factor in wind, snow load etc.

If your happy to over spec it with a bigger steel then i dont see hwy he wouldnt do that, but the price will go up quite a bit id expect.

He might ask for some extra pillars to be build or opt for a goal post arrangement if he thinks your current brick/block work isnt sufficient to take the load. and there will be pads added underneath the beam to prevent crush.

 

[paulrockliffe]

Why is bolting the beams together prohibited? Genuine question, I presume that means that bolting together is worse than now, but can't work out why. Thanks!

 

[TobyLeRone]

Thanks for the replies. All helping me get a better idea of what's involved. I don't doubt that I'm over analysing this and thinking that it's a really ultra-complex job with unique characteristics, when in fact the SE, Architect and Builder will be doing loads of very similar jobs all over the place, and as you say, some far more complex.

So, regarding the bolting together, is it that it's not that bolting together is bad, but if the aim is solely to prevent buckling, then it's not the going to work.

 

[John D v2.0]

specifically prohibits bolting two parallel beams together if the intention is solely to prevent buckling.

I presume that means that bolting together is worse than no

that's not what he said, to me it sounds like, if the sutionation is that if they are not bolted together then they would fail on buckling, if they are bolted together, that is not an allowed solution. But if they wouldn't fail on buckling then feel free to bolt them together.

 

[tony1851]

Steel beams can be subject to buckling over long spans, particularly those with relatively narrow flanges. In normal bending, the top flange is subject to compression and can fail by buckling sideways long before the steel reaches its maximum allowable stress.

The way to stop it is to rigidly hold the top flange in position, eg by mechanical fixing of floor beams on to the flange. It's not allowed to restrain two slender beams by bolting them together through the webs with spacer tubes - it's been described as like two drunks trying to support each other from falling over.

If the SE designs two beams, one for each skin, each beam should be capable of resisting buckling forces on its own (if they are welded together by top and bottom plates at intervals, that can be acceptable as long as the welds are proved strong enough, but site welding is not recommended because of quality- and safety issues.)

 

[tony1851]

tthe sutionation is that if they are not bolted together then they would fail on buckling, if they are bolted together, that is not an allowed solution. But if they wouldn't fail on buckling then feel free to bolt them together.

That's it.

 

[tony1851]

https://www.labc.co.uk/news/beware-don’t-just-bolt-your-beams

 

[Notch7]

https://www.labc.co.uk/news/beware-don’t-just-bolt-your-beams

interesting

Ive done a few jobs where the SE has specified 2 beams with spacers -I couldnt say though that each beam was strong enough to take the deflection on its own, perhaps that was the case.

I did one job where the SE specified different height beams, which was a pain.

 

[tony1851]

interesting

Ive done a few jobs where the SE has specified 2 beams with spacers -I couldnt say though that each beam was strong enough to take the deflection on its own, perhaps that was the case.

I did one job where the SE specified different height beams, which was a pain.

If the SE was hoping that by bolting the beams together through the webs he would somehow even out deflection between them, he should go back to college.

 

[Leofric]

You are paying an architect and structural engineer to work on the design of your extension ,so the best thing is to discuss your concerns with them. They are working for you and should have no problem discussing your project with you.

 

[TobyLeRone]

You are paying an architect and structural engineer to work on the design of your extension ,so the best thing is to discuss your concerns with them. They are working for you and should have no problem discussing your project with you.

Of course, but then it wouldn't be impossible to imagine that some professionals may not like being questioned, some may even be a little patronising. Even in the few messages on this post there are instances highlighted where SE's, builders and BCO's have ignored (or don't know) the Code. From my point of view, I previously didn't know that bolting steel beams together wouldn't even out deflection. If anyone had told me that's why they do it, I'd have believed them. If they tell me that now, I know they're not correct. All thanks to this forum.

As this is an advice forum, that's all I'm doing, looking for some core advice so that I can at least have a basic understanding of what the design 'may' look like. Surely that's a good thing isn't it?

 

[Leofric]

but then it wouldn't be impossible to imagine that some professionals may not like being questioned, some may even be a little patronising.

Then you are employing the wrong architect and structural engineer

Even in the few messages on this post there are instances highlighted where SE's, builders and BCO's have ignored (or don't know) the Code.

and there are instances on here where people don't know what they are talking about - but ask away ,as you say that is what the forum is for.
I just wonder how you have chosen your architect and SE when you seem reluctant to even talk to them.

 

[TobyLeRone]

Then you are employing the wrong architect and structural engineer
and there are instances on here where people don't know what they are talking about - but ask away ,as you say that is what the forum is for.
I just wonder how you have chosen your architect and SE when you seem reluctant to even talk to them.

I'm not reluctant to talk to them at all (when have I actually said that)? We're in the very early stages and haven't yet spoken with the SE, I'm just getting as much information as I can before we talk to the SE about this and other elements of the project (why would I speak to an architect about steel beam lateral restraints and flange width? I've had plenty of conversations with him about placement of skylights and how to ensure the room doesn't overheat in the summer though).

Call this research, this is one (of many) resources I'm using so that I at least know a little about what I am being told. If I do employ the wrong architect and SE, then by doing my research in advance I will at least know that this is the case. If I don't do the research, if I don't ask the questions, then how would I know either way?

It's a bit odd that you're coming on to a help forum, to effectively say, "don't go asking questions on this forum and asking for help or advice".