HELP! Been let down by my SE at the eleventh hour...

[Munzy]

Hi all,

I have some steel work scheduled for Saturday and the one remaining piece of the jigsaw hasn't arrived from our SE who has gone AWOL!

I wonder if anyone here can help so that I can buy material tomorrow and not delay the fabricators.

We are removing an exterior wall and fabricating a steel frame on site which will house an aluminium-framed glazed wall and also carry a roof purlin. We have the design and calcs for the steel frame itself, no problem there, the issue is the calcs for the purlin have not arrived.

If I show you a photo with dimensions it might make more sense:



So the wall being removed is triangular in shape. The steel frame consists of two upright steel columns (one tall on the right and one short on the left both 100x100x5.0 SHS) and an inclined beam (178x102x19 UB) which runs from one column to the other on a slope. Halfway along this inclined beam sits a purlin (labelled g) which will need to be carried by the steel via some brackets either side of the timber and bolted through. All good with this part.

The purlin is currently timber and measures 180x65x6500mm. It WAS supported halfway along it's length by a block wall which no longer exists (it's currently propped). You can probably see that the roof runs up at one angle to first floor height and then the angle increases up to the ridge. I mention this because it is important to know the upper roof is supported on a wall plate, I assume that the purlin is doing nothing to support the upper slope and is simply stopping the lower (shallower) section from spreading.

The question I need an answer to is whether the current timber purlin is strong enough to support the roof along the full 6.5m span. If it is NOT (which I assume is the case) then I need to know:

• What size replacement I should get... or...
• How I can beef this one up... or...
• What size steel I could replace it with which can sit within a brick wall at the far end and be welded directly onto the new steel framework at the window end

If anyone can help me out of a rather frustrating hole I would be eternally grateful and will send beer tokens immediately!

Dom

 

[tony1851]

The timber beam would not do on that span.
In steel, you would be looking at something like either a 203 x 102, or a 152 x 152 x 30. You would bolt a 100 x 50 timber
plate to the top flange for nailing the rafters.
The steel purlin could be fixed to the 178 x 102 by welding a vertical plate to the web of the latter, and with a couple
of 20mm bolts through the web of the purlin.

 

[Munzy]

The timber beam would not do on that span.
In steel, you would be looking at something like either a 203 x 102, or a 152 x 152 x 30. You would bolt a 100 x 50 timber
plate to the top flange for nailing the rafters.
The steel purlin could be fixed to the 178 x 102 by welding a vertical plate to the web of the latter, and with a couple
of 20mm bolts through the web of the purlin.

Tony, that’s great, thanks for clarifying, so helpful.

If I was to look at replacing the timber what sort of dimensions should I be looking at. If I doubled-up with the same size as the existing, bolted together would this work?

 

[tony1851]

Doubling-up might work (subject to number-crunching) but deflection might be an issue.

 

[Munzy]

Doubling-up might work (subject to number-crunching) but deflection might be an issue.

Will get some steel prices tomorrow.

Thanks again Tony!