large span I beams with restricted depth

We’re having an extension done which involves a 7m beam going into the ceiling to hold up 2 rooms. Due to the layout of the extension, the total depth of the beam needs to fit within the floor void which is 240mm deep. Currently the structural engineer has specced a 254mm beam (which is going to weigh almost a tonne), which will (just) show through the ceiling.

We’re trying to avoid this, and one option is to re-plasterboard the existing ceiling so it’s lowered a little bit to hide the beam – but looking to figure out if there’s any other solutions? It seems like there are “twinned beams” you can get but our builder (haven’t spoken to the engineer yet) thinks you don’t gain anything due to the width – I don’t really understand the physics of that, but wondering if anyone has any other suggestions that might help?

when you briefed the structural engineer did you say you wanted the beams hidden?  they would be the first person I’d ask, given as you’ve spent many £££ on them

Changing the depth is going to change all the calcs and they will need redoing.

Dropping the ceiling is an obvious option.

Unless its going to interfere with your lighting  have a 14cm visiable bar could be made into a feature.  Our neightbours painted it bright yellow and it actually looks pretty good in real life

I’d just drop the ceiling to below the beam if it is just a 14mm difference. You’ll get the added benefit of being able to do any wiring and plumbing in the existing part without disturbing upstairs too

Standard beams come in particular depths, from 254mm you jump down to 203mm as the next section – and the stiffness and strength is reduced a _lot_.  I’d doubt you can reduce down that far, so you may be a little stuck without getting custom sections fabricated (not as expensive as you might think).

Better ask the engineer though…

That beam is going to deflect as well 🙂

Two beams is a potential option but depends on how the load is to be supported.  Talk to your SE about this they are best placed to advise on this as they will know how the loads are acting and need to be supported.

Resi stuff is always a nightmare, with unrealistic proposals being put in front of clients then they expect miracles to happen!

I suggest a pole to support a thinner beam. Added advantage you can take up a new dancing hobby?

Note, I’m not a structural engineer 😉

I’d just embrace it….. As long as the rooms on either side are big enough.

Are the heights of the two ceilings (either side of the beam) the same?

If not (and there’s a good chance of this) you could use the beam to disguise the difference.

That must be some thickness of beam to be weighing almost a tonne, is it one beam to carry two skin of brick ?

The beams in our kitchen needed two layers of plasterboard to cover them, related to fire safety (this was 1997). Possibly because it was a kitchen, but you should check the requirements for boxing the beam in.

thanks for the suggestions..

The room is going to be one large (~7m by 7m) kitchen/diner/lounge thing, the ceiling in the existing bit can be lowered if needed, and the ceiling in the new bit can be either the height of the old ceiling or a different height. The thickness (and weight) is all about the width of the beam, its not directly supporting any load bearing wwalls, but the whole of the floor in the room above rests on it, and there’s a beam from there up to the ceiling (hidden in the walls of the room above) which will help hold up the roof.

Losing 2cm from our ceiling won’t be the end of the world – its not a massively high ceiling today but its alright, but not ideal – it’d be nice if there was a decent (and not *too* bad from a cost perspective) way of resolving it, but outside of custom job (which I’d assume for a beam this big wouldn’t be cheap) we might have some difficulty.

I’ll have a chat to the SE over the next couple of days and see what they say – I just read up on the plasterboard thing and it seems like 1 layer is good enough for a 2 story house, 2 layers is needed for >3 floors or flats

Does SE say which 254 section it is. Judging by the length and you saying it is 1ton in weight then it must be a 254 x 254 UC. The 254 is just a serial size , The actual depth depend on its weight per metre.  There are 5 beams in that serial size , smallest  is 254 deep, largest 289 deep. Is it a 73, 89, 107,132 or 167kg per metre. My money is on the 132kg to get to a ton. That is 276 deep.  Are you able to install it in one piece ? If you are splicing you may have to allow for a splice plate on the top and bottom too which could be up to 20mm each

Here’s hoping whoever is installing the beam knows what they are doing, trying to get that thing off a lorry, up the garden path, in between two rows of acrows and flush internal walls so no piers. Splicing it into 3 if allowed would add 40mm +bolt head and nuts

I just relooked at the sheet, it’s a 254 254 89, so actually significantly under a tonne (that weight was from the builder complaining about how to get it to the back of the house). What’s interesting (and I hadn’t realised) is that’s actually more than 254mm deep -its 260.4mm deep, on top of which we need to put some joist hangers(2-3mm) so we are a total of 23mm of height too much.

It is coming in a single beam and current plan is to manhandle it into the house, there’s a pier at one end and it can poke out further at the other end (difficult to describe), but yeah getting it in doesn’t sound like a DIY job

2x C section ‘channel’ bolted back to back?

Check with someone other than a ‘rack of eye, twist of gob’ type engineer like me.

Note, I’m not a structural engineer 😉

But you are a dancer? Is that what your username stands for, dancing boy!!

We have a beam that runs right through the garage and then the ceiling of our bedroom. It was there when the house was built obviously (mid 90s) and I think it’s basically a necessity due to the size of the spaces involved a double garage and then an adjacent bedroom about the same size as a single garage, with only one wall in between to support all the roof loads.

It’s a noticeable boxed in chunk on the ceiling in our bedroom, but not an eyesore, we’re used to it though I suppose. Would I choose it? No but I think without it I wouldn’t get such a monstrous man cave so on balance it’s not so bad. Paint it magnolia, make jokes about adding sex swings and poles, it’s ok.

“I just read up on the plasterboard thing and it seems like 1 layer is good enough for a 2 story house, 2 layers is needed for >3 floors or flats”

We are only two stories, but we got 2 layers, and the builder used a consulting structural engineer* to specify the beams and what not, so I am assuming it was necessary. May be some other thing, or the rules changed.

* we have 2 rsjs about 3.5m long which hold up a masonry internal wall and the top half of a chimney breast. Big bastards they are.

Ignore all comments on this thread other than the ones that say speak to your engineer………

I just relooked at the sheet, it’s a 254 254 89, so actually significantly under a tonne (that weight was from the builder complaining about how to get it to the back of the house). What’s interesting (and I hadn’t realised) is that’s actually more than 254mm deep -its 260.4mm deep, on top of which we need to put some joist hangers(2-3mm) so we are a total of 23mm of height too much

It is quite likely that the limiting factor is deflection (but several factors need to be considered):

The relevant 2nd moment of inertia (Iyy) of that section is 14,300 cm^4

A 203 x 203 x 127 UC has Iyy of 15,400 cm^4 and is ~241 deep, so that *may* be a potential option, but it depends on what other loads and structural details are required (and it will be nearly a tonne: ~900kg).

Speak (nicely) to your SE. You may have to wait longer / pay more for the steel if it isn’t a preferred section, so be clear whether this is a compromise you can accept.

When I worked in our modern open-plan office I could feel the Golightly’s (the pair of them were heading on 30st a piece, but both well over 6″) walk past on their way to their 2nd breakfast, rather disconcerting,,,

The desks were laid out in long rows with the ‘corridor’ smack bang in the middle of the supporting columns – about 10m across and the beams looked a good 600mm deep.

Re fireproof plasterboard, if you don’t know, its the pink stuff.  Make sure you take a picture when it is installed as even if your builder is ‘dealing with building control’ they may well forget to take photos and then when the inspector comes round and its all plastered it might cause for some ackward conversations.  Luckily I had some pictures I’d taken of the kitchen that happened to show the beams in pink board

I also think it doesn’t have to be covered, as I’ve seen some designs with the beams showing.  I presume these have some sort of special fire retarding paint on them.

It is quite likely that the limiting factor is deflection (but several factors need to be considered):

The relevant 2nd moment of inertia (Iyy) of that section is 14,300 cm^4

A 203 x 203 x 127 UC has Iyy of 15,400 cm^4 and is ~241 deep, so that *may* be a potential option, but it depends on what other loads and structural details are required (and it will be nearly a tonne: ~900kg).

Speak (nicely) to your SE. You may have to wait longer / pay more for the steel if it isn’t a preferred section, so be clear whether this is a compromise you can accept.

thanks – someone DM’d me to suggest that euro-dimension steel might work – and it seems like HEM220 would also fit.

the SE was worried about the deflection for a 203x203x86 (at 20mm vs 13mm for the 256 beam) but I’m not sure if that can be solved with just thicker metal or its one of those wierd things where its all about cross-section

the SE was worried about the deflection for a 203x203x86 (at 20mm vs 13mm for the 256 beam)

Stiffness / deflection scales with 2nd moment of inertia (as above)

Iyy for 203 x 203 x 86 is 9450 cm^4 so deflection would be 50% more than the original (14300/9450) – hence 20mm vs 13mm deflection (round numbers).

It’s all in the ‘Blue Book‘

the 203 x 203 x 127 would have 7% less deflection than the original (14300/15400). Whether you can find anyone who can supply (or lift it into the house!) it is a different matter.

203UC127 isn’t a standard size readily available iirc.

Your builder may struggle to procure this.  Designing resi schemes always ends up with someone saying I can’t get this section size but I can get x y and z instead.

As an alternative, if the beam isn’t supporting a wall over, use two beams to break the span up at 1/3rd points as opposed to one at the mid span of the flooring, roof over.  This will cost you more but will enable a shallower beam to be used without your SE having to spec hard to source beams.

As above, talk to your SE they are best positioned to solve this.

That’s a pretty big span and the depth of the beam may be dictated by deflection, i.e. a shallower beam may well support the load but the mid span deflection would be excessive.

Can it not go further up?

When we had plans drawn up for an extension (ended up not going forwards) which would have opened up the back of the house and knocked through the kitchen and dining room.  The solution proposed was a goalpost across the back of the house because it’s only just too wide so would need a pillar either end. But it would have gone in roughly the height of the radiators upstairs, Then the internal beam which sits within the floor would be bolted to this from underneath.

Can it not just go up one course of bricks further?

Obviously requires plastering / redecorating upstairs, but as the builder pointed out, the odds of knocking down half the back of the house and not creating cracks upstairs are slim to none anyway.  It just becomes something you budget for rather than a surprise at the end.

Can it not go further up?

its a 2 story extension so would poke into the middle of the upstairs floor, so sadly not. The beam across the new rear of the house (where we’ve a 6m wide window going in) will be much taller, as it has to support the wall above it as well

I’m just having plans drawn up for a similar lounge diner, 40m2.  The hidden bulkhead for me is a non negotiable, its a bungalow so there’s loads of space.  If a visible bulkhead was demanded, I would prefer a proper beam like proportion, not 5cm.  I ve looked at loads of pics on rightmove, as suggested make a feature of it.

If you are into podcasts, the Joe marler show with the architect was brilliant.

thanks for everyone’s help on this – I have a different question but figure it applies to the same audience

There’s a taller beam (356x171x57kg) in place above the rear windows, also approx 6m long. The builders have installed this 30cm lower than we expected (lower than what is drawn on all the diagrams). They’re telling us that this is because the ceiling joists need to sit on top of that beam, not within the web so the load of those joists is taken by the whole beam. I don’t think I really follow this (I’d assume that a beam would kinda move as one, so it doesn’t really matter where you put the load).

Either way, is there any reason that long-drop hangers can’t be used on a tall beam to hold smaller joists? We’d be using something like 200mm (possibly 250mm) joists, so if the beam was in the correct position, and hangers were used, the hanger would be 300mm long with only the bottom 200mm filled with joist. Looking at a few diagrams it seems like we might have to pack the inside of the beam with another joist to fill the space out, but I don’t see that being a massive issue.

Builders are being a bit awkward because I think they’ve realised they’ve put the beam in the wrong place by mistake, and are going to take the hit on ripping out the wall built above the beam, and the beam, and the supporting columns, and re-doing them all to the correct spec – its an expensive mistake to have made.

I think your builder is talking bollocks and it’s not up to them to decide to stick the beam in 30cm lower than shown on the drawing. Does having it lower (as installed) affect the window it’s over?

Either way, is there any reason that long-drop hangers can’t be used on a tall beam to hold smaller joists? We’d be using something like 200mm (possibly 250mm) joists, so if the beam was in the correct position, and hangers were used, the hanger would be 300mm long with only the bottom 200mm filled with joist. Looking at a few diagrams it seems like we might have to pack the inside of the beam with another joist to fill the space out, but I don’t see that being a massive issue.

yes pretty much this.

They’re telling us that this is because the ceiling joists need to sit on top of that beam, not within the web so the load of those joists is taken by the whole beam.

they possibly have a tenuous theoretical point that either they (or you relaying it) have explained poorly.

However, assuming theres also a storey’s worth of cavity wall above this beam – so the weight of the floor and domestic live load is going to be a small proportion of the total load – I’d consider this a fairly irrelevent eccentric loading.

Does having it lower (as installed) affect the window it’s over?

yes a massive one. Our designs show floor-to-ceiling sliders across the back of the property as the open-plan space behind it is going to be 8m deep with no other windows, so we want the max light possible coming in. This is shown on every drawing we have (and planning permission) – but the builder has installed the beam as if that window will only be “normal” height (~2m) with a 30cm upstand in line with other windows around the property.

I suspect its an honest mistake, because I’d like to think that if they did have some reason why they had to reduce the size of windows by 20%, they’d have come and talked to us about it before ordering/building it and hoping we just wouldn’t notice.

it seems like the “joists lower than beam” problem is pretty common in the loft conversion space, we need something like this (which as its stolen from the screwfix site appears to be a relatively known solution)

However, assuming theres also a storey’s worth of cavity wall above this beam – so the weight of the floor and domestic live load is going to be a small proportion of the total load – I’d consider this a fairly irrelevent eccentric loading.

thanks for the info, yes a story of wall and all the loads from a fairly large pitched roof going down through the beam. good to know that physics is on our side 🙂

genuin Q: If you do as per that diagram, where does the wall the beam is supporting sit [seeing as the joist hanger is in the way]?

I assume in our case the bent-over bit of the hanger goes between 2 courses of blockwork to hold them stable, rather than nailing to some wood

I can’t remember the whole thread but if there’s drawings/architect involvement then it needs to be ‘as drawn’ and not modified to suit after an event. There will be a million knock on costly effects otherwise and Kevin will then knock your Mrs up between the adverts.

They’re telling us that this is because the ceiling joists need to sit on top of that beam, not within the web so the load of those joists is taken by the whole beam.

yeah, they have effed up. In 37 years of detailing steelwork , the joists ( be it either cold rolled steel or timber ) always go into the web of the beam. If the beam is deeper than the joists then a correct thickness timber is fixed to the top of the bottom flange ,and the timbers will sit on this . The top of the joists are usually flush to the top of the steel beam. Sometimes if the bearing on the bottom flange isn’t wide enough they shot fire / bolt a timber into the web and fix bat hangers to support the joists

As revs1972 says. See NHBC Standards

Your builders are talking BS like they always do when something is built incorrectly.

Get them to rectify their mistake & don’t compromise even though they are making things awkward – joists can be installed into the web of the beam, if not how are they going to support the joists on your 7m  beam that is going within the floor depth??
Losing 300mm on the height of your doors will make a massive difference on your new space – you will look at it every day & regret not getting it changed if you accept the mistake.

I can’t remember the whole thread but if there’s drawings/architect involvement then it needs to be ‘as drawn’ and not modified to suit after an event

Definitely this. The builders need to follow the drawings and when the current error is corrected can’t then decide to weld joist hangers on, drill through steel joists, etc, etc, because it suits them

There will be a million knock on costly effects otherwise and Kevin will then knock your Mrs up between the adverts

🙂

thanks all – you’ve confirmed my uneducated guesses about how stuff works, to the point I’ve been able to push back on the builders and this morning they agreed to move the beam back to the correct position.

In by Christmas!