Span calculation

[AlwaysBestToAsk]

Hi all,

I'm currently putting together a SIP panel room and due to an error on my part i've mis-calculated something. To try and keep this simple i've got 2 walls parallel with each other and the INSIDE gap is 3m (so including the 12cm thick wall on each side the total outside to outside is about 3.24m), now i bought 3m long SIP panels to go across the top of the walls (pretty much flat but with a 1:40 rise) but these will now just fall as the gap they needed to "overhang" is the same at 3m so i'm looking to ideally put wooden splines in the 3m rebates of these SIP panels using C24 timber which would be 100mm x 100mm and then be a length of 3.24m (or ideally slightly longer) but given the width of the SIP panels these would be at a spacing of 1.2m apart ? There is nothing to sit (weight-wise) on top of the flat roof but i need to be able to walk on it to fit the rubber roof and any maintenance and snow/rain etc. I've looked at the C24 span tables and there are no entries (obviously) for exactly 100mm x 100mm so how do i work out if the timber is ok to support this ? The nearest size i can see is 72 x 120 which says the span can be 3.2 so i'm thinking not but if this is the case then i need to find another option such as putting a timber "ledge" along each wall and sitting the end of the SIP panels onto them but then they eat into the interior space.....any advice would be great
I've also noticed that C24 span tables do give different variations on the max span so another reason i thought i'd check
Hope this makes sense

 

[tony1851]

It depends on the load you want to support (live load + dead load) and what maximum deflection (bend) you can put up with.

 

[AlwaysBestToAsk]

So i know i'm 15 stone and then SIP panels are 56KG (for the whole of the 1.2m x 3m SIP panel) so about 20kg per m2 and then i "think" about 2kg per m2 for the rubber roof ? The maximum deflection i'm not sure on.
I know there is no deflection in the SIP panels themselves at 3m lengths so i guess that would assist in minimizing the bend ?
So if i'm working this out right (bearing in mind i measured the width wrong) then i'd say the dead load would be about 22kg per m2 (so about 0.2kN/m2) and i've no idea how to work out the live load ?

 

[tony1851]

Roof live load - typically snow - is usually taken as 0.75 kN/m².

So with your dead load, say 1.0 kN/m². Area of panel supported by each 100x100 beam = 1.2 x 3 = 3.6m² therefore total distributed load on 1 beam = 3.6kN.

Reaction each end (ie at the bearing) = 1.8 kN. A 100 x 100 C24 with full bearing will easily take both the shear force along the gain and compressive force perpendicular to the grain.

What you won't know is the deflection, because the panel each side will have its' own stiffness and will contribute to reducing deflection of the beam.

 

[AlwaysBestToAsk]

lol.... never surprises me how much there is too learn about things you always thought were simple

So..... reading from what you've put i'm ok to use 100 x 100 C24 from the perspective of strength (i.e actually holding the weight) but we can't confirm what deflection (if any) there will be ?

 

[tony1851]

If each 100 x 100 beam was supporting all the weight of half a panel from each side, it would deflect approx 20mm. As there will be stiffness in the SIPs themselves, they will work together with the beam and the deflection will be reduced, but by how much....?

 

[^woody^]

Those timbers may end up twisting more than Chubby Checker. And what about the thermal bridge of using such a chunky piece of timber between the insulation - normally if anything its a thinner section which is bonded to the foam core.

Can't you just ask the manufacturer for advice?

 

[tony1851]

........the party's over now.............

 

[^woody^]

Just beginning, come out of the kitchen

 

[AlwaysBestToAsk]

Thanks tony1851, as long as it'll support the weight of me walking on it then that's most important and i'll think about the deflection, possibly put a false ceiling with soft insulation in to act as a sort of "buffer" for any bend on it but i'll have a think about that