Beam in loft

[Worcesterman]

Hi
I was wondering if anyone could give me some advice on placing a beam in my attic. I have a 1930s house with a purlins supporting the rafters of the roof. They have suprisingly liitle supportung them. A builder has supported the two largest down onto walls belows using a couple of thick posts. Looks strong. The two smaller purlins have no wall beneath them at any point.
So I want to run a timber beam beneath them and put posts up from the beam to support the purlins. The beam needs to be 4m long.
I want to make the beam from laminating lengths of 2x12 together rather than using steel as i am working on my own. The longest length of timber i can get through the hatch is 3.6m. (Beam needs to be 4m). So my question is , is there a way of bolting say 4 lengths of 3.6 together to make a 4m length beam with comparable strength to full 4m lengths. I can make a hole in the gable end easily enough and that seems best but was just curious if bolting shorter lengths to make a longer length is possible.
Any asvice is very much appreciated.

 

[tony1851]

Use coach screws - not bolts.

 

[catlad]

Why coach screws and not bolts?

 

[Worcesterman]

Thank you. I hadn't thought of a design like that. If I may ask what is the advantage of coach screws over coach bolts. Do you know what is the ideal position of the screws in terms of their height ?
What about this design ?
"You could splice 4 together with a 400 offset on alternate beams to give you 4m. This would be fine as the bending strength you need is not at the ends."

 

[tony1851]

Bolts-v-Screws?

The fixings in the beam will be subject to shear forces as the beam bends. An 18's guage conventional wood screw (a little less than 8mm in shank diameter) will take a shear load of around 1.5kN in C16 timber. Coach screws are available in bigger diameters (10mm and 12mm being common) so they will take bigger loads.

For bolts, the permissible load in this situation for an 8mm bolt would only be 1.09kN for medium-term loading. Bolted members are also subject to joint-slip, as the holes have to be drilled slightly larger than the bolt diameter.

Toothed-plate connectors used with bolts can give higher loads, but they are not as easy to fix.

 

[tony1851]

"You could splice 4 together with a 400 offset on alternate beams to give you 4m. This would be fine as the bending strength you need is not at the ends."

As long as you've got 2 thicknesses at the middle of the span (where the bending stress will normally be greatest) it doesn't matter that there is a split in the middle section of the built-up beam.
At the ends where there is only one thickness, bending stress is insignificant - only shear stress matters there, and a 2 x12 timber will be more than adequate.

 

[Worcesterman]

Thank you. I think the beam will be good.
I am going to create a recess in the gable end wall for one end. As luck would have it there is a row of headers right where it needs to go. Bricks are bonded with 1930s lime mortar. So can be removed easily enough but I can’t think of a way to remove half a header.
Have you any experience with this ?

 

[Worcesterman]

Thats intersting. Somebody advised a very strong masonary hanger instead....That was my first thought but suspected that the load would be better onto the inner layer of bricks.

 

[tony1851]

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