Insulating twin beams...

[matthyde83]

Some advice required... I have a lintel across some sliding doors (4.3m).  SE has specified 2x 203 133 UB 30 beams bolted at 600 centres. 

I'm having blown in EPS beads so its occurred to me that they will probably get in the gap (150mm cavity). 

How do you prevent thermal bridging here though, any better suggestions than letting the beads in!  Surely the bolts and spacers will create a cold bridge? 

[markc]

Difficult to fix multiple beams together without thermal bridges so the beams are usually treated as one and the insulation is placed around the unit rather than splitting the two.

but it is not Impossible to insulate the two with a bit of ingenuity and insulating web members instead of simple steel plates and/or bolts

[PeterW]

What is the outer treatment of the beams ..? Are you using render board or are you hoping to brick the outer beam in ..? You can sandwich EPS between them but usually it is just hollow and you insulate the inside of the flange. 

[MikeSharp01]

Get some insulation on the outside of the beam, as PeterW says, even a thin layer of PU will break the cold bridge relatively effectively.

[cwr]

Or you could ask the SE if heavier sections could be used to avoid bolting them together. Would of course be more expensive, if even possible/ feasible.

 

My SE originally spec'd a single very heavy beam with a flat plate welded on top for a 5.6m opening. I asked for there to be two separate beams which he did. Interestingly he then spec'd two heavy UC's, I asked him why not a deeper and lighter UB, he said because of torsional buckling.

 

No harm in asking.

[matthyde83]

8 hours ago, cwr said:

Or you could ask the SE if heavier sections could be used to avoid bolting them together. Would of course be more expensive, if even possible/ feasible.

 

My SE originally spec'd a single very heavy beam with a flat plate welded on top for a 5.6m opening. I asked for there to be two separate beams which he did. Interestingly he then spec'd two heavy UC's, I asked him why not a deeper and lighter UB, he said because of torsional buckling.

 

No harm in asking.

My structural engineer isn’t the most helpful / available so I’d prefer to avoid asking him anything tbh!

[matthyde83]

9 hours ago, PeterW said:

What is the outer treatment of the beams ..? Are you using render board or are you hoping to brick the outer beam in ..? You can sandwich EPS between them but usually it is just hollow and you insulate the inside of the flange. 

That’s a thought.  We actually have a balcony and are taking the doors up to ceiling level so the outside will be hidden by a joist running in front of it.  Could therefore easily sandwhich something in there.  
 

Although it’s a 133 beam in a 100 wall…

[PeterW]

58 minutes ago, matthyde83 said:

Although it’s a 133 beam in a 100 wall…

But it’s two side by side so ~270mm in a cavity wall, which should be a minimum 300mm wide…

[matthyde83]

4 hours ago, PeterW said:

But it’s two side by side so ~270mm in a cavity wall, which should be a minimum 300mm wide…

Yep 350mm will just mean 33mm is unsupported on the inside of the cavity rather than the beam being central that’s all. Not a problem I suppose. 

[PeterW]

13 minutes ago, matthyde83 said:

Yep 350mm will just mean 33mm is unsupported on the inside of the cavity rather than the beam being central that’s all. Not a problem I suppose. 

But you don’t make the beams central or they would overhang - the whole point of bolting them together is that you create a single beam and both webs will be on brickwork. 

[SuperPav]

I'm not sure I understand the section detail here of 2x133 beams under the 350mm wall. The indiviual leaves should really be centred over the web of each beam. Which would mean an akward 15mm protrusion of the chords either side of the wall.

 

In any case, I'd fill the inside web (facing the room) with 60mm PIR, and then cover over the beam on the interior side with whatever thickness of insulated render board (marmox/wedi) that brings you flush with the plasterboard. Board the underside of the beams with the same insulated board, and then pump the cavity full of beads which will also fill the space between the two beams.

 

Also I'm not a structural engineer but I've yet to be convinced that bolting two beams at 600mm centres achieves anything when used in a cavity wall installation. (We have independent beams in our build for the two leaves).

[PeterW]

2 hours ago, SuperPav said:

Also I'm not a structural engineer but I've yet to be convinced that bolting two beams at 600mm centres achieves anything when used in a cavity wall installation. (We have independent beams in our build for the two leaves).

It stops rotation and allows you to use the torsional strength of the thinner profile. See it quite a lot and they usually also specify whether spacer tubes are required or if the flanges need to touch. In reality they are acting as Channel sections with an overlap into the cavity which won’t cause any issues.