RSJ Resting on timber lintel over window ?

jonCar · 17 Mar 2015

As part of a dormer conversion (in a 1927 build semi) a 3.6m RSJ will be put into the first floor to support a 50x50 steel post that the centre steel RSJ will be terminating on in the loft floor (to make way for the staircase). This first floor RSJ at one end terminates on a timber lintel of a 600mm window. Is this ok or is it best to replace the timber with steel and can the 3.6 RSJ simply rest on the replacement steel or does it need to be bolted onto it?
Thanks in advance Jon

tony1851 · 17 Mar 2015

Doubtless folk will be along to say 'no, you can't do that', but in some cases, it is perfectly OK structurally.

It depends partly on the span and cross-section size of the timber beam, partly on the load the steel beam will be putting on it, and where along the timber beam the steel rests (at the centre or near the end?)

If the steel beam is carrying floor loading only, and is itself not too heavily loaded, you could well be OK, but have a word with your SE first.

catlad · 17 Mar 2015

It may be ok structurally but I don't think building control will approve what would happen if the lintel got burned in a fire.

jonCar · 17 Mar 2015

Thanks for reply.
Timber lintel 3"x4" and the 3.6 RSJ is resting right at the edge, almost where the timber spans onto the brick. window span is 600mm.
The load will be a 6m Rsj that terminates on a 50x50 box steel post (which rests on 3.6rsj) in the centre of the loft which will bear the floor timbers.

jonCar · 17 Mar 2015

Good point Catlad. Should go for the steel then. Does it need to be bolted?

tony1851 · 17 Mar 2015

Fire is not an issue whether the supporting beam is timber or replaced by steel. They will still both need to be finished in plasterboard to 30 mins fire resistance.

If it bears right at the end of the timber lintel, then it only needs to be checked for shear stress.

It matters not what the building inspector says - if calcs can be submitted to show that it is OK structurally, the council has to accept that.

jonCar · 17 Mar 2015

Thanks for your input Tony, much appreciated.

noseall · 18 Mar 2015

It would take about an hour and £150 to replace.

How much would it cost to leave it be and prove it structurally?

I know what I would do.

tony1851 · 18 Mar 2015

noseall said:
How much would it cost to leave it be and prove it structurally?

Good point.

But presumably the steel coming onto it has been calculated by the SE. If so, the SE will already know the reaction of the steel beam onto the timber.

With that figure - say X kN - it is literally only a 10-second job using a very simple equation to check the shear stress on the timber.

jonCar · 18 Mar 2015

Thanks.
Here are the SE original drawings. The beam 'D' did not originally rest on window but because of mis-measuring I presume on his part it falls there.
I will get him to do stress check for the timber and if necessary replace. Any other input would be much appreciated.
//media.diynot.com/98000_97277_91844_60090115_thumb.jpg //media.diynot.com/98000_97277_91845_76553417_thumb.jpg

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18 Mar 2015
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jonCar · 18 Mar 2015

Just to clarify, RSJ new position is here in red. Builder thinks it will be fine and I can see his point but he is happy to change the timber for concrete lintel if i'm worried overly about it..

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Jackrae · 19 Mar 2015

Two questions :

1) Why has the beam moved south to the new position

2) What happened to the prop post "F" that the original beam location was carrying

Even in its intended location the detail of how the beam ends being supported on their "pads" should be shown, since the drawings tend to suggest it may be sitting on narrower pads (wall thickness) than the text states (300x150x100)

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RSJ Resting on timber lintel over window ?

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