cliftyhanger
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| posted on 7/10/20 at 07:27 PM |
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Strength of wood versus steel joists (carport)
After the good advice I was given re planning and a carport, here is the next question.
When I build the carport, it will have a side-to-side span of 5.5m which I really do not want to have a central support.
I plan to use 0.7 steel roof panels, nice and light. But the almost 6m span will need something strong to support the roof, and allow for snow (ha, if
ever) and the dead load of the roof structure.
Cladco reckon their 170mm (i.6mm thick) Z purlins will span 6m. And I have seen plans that suggest 150x50mm timber would also be OK, though looks a
little small to me.
However, I am wondering about 48mm scaffold poles. Possibly 2 welded together in an 8 as that would keep the profile low, and be affordable. I realise
I beams would be ideal, but costwise would be rather hefty 
Any other ideas/advice would be very welcome
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Shooter63
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| posted on 7/10/20 at 07:45 PM |
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A big no no on the scaffold tubes, just buy the correct kit as recommended
Shooter
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cliftyhanger
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| posted on 7/10/20 at 07:57 PM |
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I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large. And 150x50 timber is again flimsy at that
length. Or maybe I am paranoid!
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mcerd1
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| posted on 7/10/20 at 08:23 PM |
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quote:
Originally posted by cliftyhanger
I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large.!
The right word is 'efficient' 
Assuming the calcs are right then z purlins can hold a lot of roof for very little cost (way cheaper than rolled steel sections)
Or you could look for 'C' purlins to the same strengh if they'd suit you any better...
-
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SteveWalker
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| posted on 7/10/20 at 08:36 PM |
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If you take a look at Building Regulations Approved Document A
1991, you will find a set of span tables at the back (later versions omit the tables, but I don't know when they dropped them).
A quick glance suggest that you are looking at SC4 timber, 220mm x 63mm, with 400mm spacing, for a 5.5 span - which coincidentally is the same as our
conservatory and the same timbers that I used for a flat-roofed section of that.
[Edited on 7/10/20 by SteveWalker]
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ReMan
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| posted on 7/10/20 at 09:41 PM |
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Thats a big carport, but they are great places. Good luck with the build
Why a steel roof though? Would you not benefit from the daylight of a clear one?
And I am interested why scaffolds are such a no, even though I agree they are not the right thing
www.plusnine.co.uk
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cliftyhanger
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| posted on 8/10/20 at 06:39 AM |
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Clear roof aesthetics are not great. Corrugated plastic looks very 1960's. and polycarb is no favorite of mine.
But the biggest issue is the "geography". The driveway at the pavement end drops buy almost 1/2m over its width, and although on the upper
side it only drops 70mm from front of garage to the pavement, the other side the drop is again 1/2m. Upshot is the roof will be very visible to
pedestrians etc as it will have to slope down from garage to pavement (well, 1m back from pavement)
It also means that to keep to no-planning rules, 2.5m on the downhill side means only 2m on the top. Take off 200mm for the structure and I (being a
shortarse) will just about be able to clear my head.
But all that is moot. The wife likes the look of the sheet metal roofs, especially when edged with the large trims. However, tomorrow she is popping
in to the neighbours for a cuppa, be interesting to see how that goes. The other option is OSB and fibreglassing.
Looks like z purlins are the way to go according to https://steadmans.co.uk/wp-content/uploads/2019/10/Purlin%20load%20tables.pdf
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pigeondave
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| posted on 8/10/20 at 09:22 AM |
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quote:
Originally posted by SteveWalker
If you take a look at Building Regulations Approved Document A
1991, you will find a set of span tables at the back (later versions omit the tables, but I don't know when they dropped them).
A quick glance suggest that you are looking at SC4 timber, 220mm x 63mm, with 400mm spacing, for a 5.5 span - which coincidentally is the same as our
conservatory and the same timbers that I used for a flat-roofed section of that.
[Edited on 7/10/20 by SteveWalker]
Dude at least link to the latest one. [Edit] To be fair the older ones do have the load span tables for timber in them. Good point.
https://www.planningportal.co.uk/info/200135/approved_documents/62/part_a_-_structure
direct link
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/429060/BR_PDF_AD_A_2013.pdf
Also I'm not sure that SC4 is still a thing. I think it was renamed back in '97?
@cliftyhanger I'd be more worried about the wind, rather than snow loading. You want to make sure you got enough fixings in the sheet
material so that it doesn't pull off.
[Edited on 8/10/20 by pigeondave]
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bi22le
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| posted on 8/10/20 at 11:37 AM |
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quote:
Originally posted by ReMan
Thats a big carport, but they are great places. Good luck with the build
Why a steel roof though? Would you not benefit from the daylight of a clear one?
And I am interested why scaffolds are such a no, even though I agree they are not the right thing
I would speculate that the comment was based on a few points.
- The 6m span gap on 50mm dia tubes is a lot to ask, even if they dont fail, they would displace a fair amount causing failure at the connected
ends
- They are design to work in compress \ tension and not so much bending (they are used this way, but with reduced distances) and as such a re made to
that level. They may be seamed and of low grade.
As an additional to this though, I had my loft extended last year and they put a "tin top" on so they could work at a faster rate. This
spanned my complete house as was obviously built to withstand wind \ rain \ snow etc. It had ALOT of support using special I beam style sections.
Imagine two parallel poles approx 400mm apart with zip zag stays between them.
So the original suggestion is not mad, but you need depth to your support. look at 2nd moment of area equation to understand that depth is the most
important thing for rigidity.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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cliftyhanger
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| posted on 8/10/20 at 12:42 PM |
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quote:
Originally posted by bi22le
quote:
Originally posted by ReMan
Thats a big carport, but they are great places. Good luck with the build
Why a steel roof though? Would you not benefit from the daylight of a clear one?
And I am interested why scaffolds are such a no, even though I agree they are not the right thing
I would speculate that the comment was based on a few points.
- The 6m span gap on 50mm dia tubes is a lot to ask, even if they dont fail, they would displace a fair amount causing failure at the connected
ends
- They are design to work in compress tension and not so much bending (they are used this way, but with reduced distances) and as such a re made to
that level. They may be seamed and of low grade.
As an additional to this though, I had my loft extended last year and they put a "tin top" on so they could work at a faster rate. This
spanned my complete house as was obviously built to withstand wind rain snow etc. It had ALOT of support using special I beam style sections.
Imagine two parallel poles approx 400mm apart with zip zag stays between them.
So the original suggestion is not mad, but you need depth to your support. look at 2nd moment of area equation to understand that depth is the most
important thing for rigidity.
hat is why I thought about the 8 idea, two poles welded together. I did consider the same with a flat bar between them, but then I would be up to
150mm so saving all but gone. And yes, the spanning beams with zigzags was my inspiration, they seem VERY strong.
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Charlie_Zetec
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| posted on 8/10/20 at 01:30 PM |
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quote:
Originally posted by cliftyhanger
I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large. And 150x50 timber is again flimsy at that
length. Or maybe I am paranoid!
To put it into perspective, I've just had a house extension completed - double storey on the side of my property. After discussions between my
architect and building control, it was decided that for a span of 6m between RSJ's, I would need C24 grade timber at 8' x 2" (195mm
x 45mm-ish in real terms, for you perfectionists). As this was first floor and upstairs is being used as a loft for storage, I was advised to
double-joist every other run, at 400mm intervals. I can tell you it holds perfectly well at that distance.
Artificial intelligence is no match for natural stupidity!
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pigeondave
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| posted on 8/10/20 at 02:53 PM |
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Maybe we could all pitch in an have a go at playing engineers.
If we took a load of 0.25kN/m² for the sheeting and maybe 0.6kN/m² for snow and a span of 5.1m
We could assume that its simply supported so to get the bending moment its WL/8 (thats big W) or wL²/8
so thats (0.25+0.6) x 5.1² / 8 = 2.763 kNm per 1m width and 1.1052 kNm per 0.4m or joist.
we need now to work out a Z required.
From the internet it looks like for timber Z = M/permissable bending stress parallel to grain
permissable bending II to grain = grade bending II to grain and some K factors, by the look of it.
so if we're using C24
thats
grade bending = 7.5
K2= moisture, well its out side so thats looking like class 3=0.8
K3=duration maybe 1.0 long term?
K7=depth factor (300/h)^0.11, should we guess 150 deep as its a regular size? (300/150)^0.11 = 1.07
K8=load sharing, we got a load of these at 400c/c so 1.1?
So Z = 1.1052 x 10^6 / (7.5 x 0.8 x 1 x 1.07 x 1.1) = 156499mm³ or 156x10³mm³
47x150 C24 = 176x10³mm³ so they look like they're strong enough at 400 centres
But what of the deflection and the little bit of wind that might push down on it ?????
I'm sure we can work this out.
[Edited on 8/10/20 by pigeondave]
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Charlie_Zetec
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| posted on 8/10/20 at 03:08 PM |
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quote:
Originally posted by pigeondave
Maybe we could all pitch in an have a go at playing engineers.
If we took a load of 0.25kN/m² for the sheeting and maybe 0.6kN/m² for snow and a span of 5.1m
We could assume that its simply supported so to get the bending moment its WL/8 (thats big W) or wL²/8
so thats (0.25+0.6) x 5.1² / 8 = 2.763 kNm per 1m width and 1.1052 kNm per 0.4m or joist.
we need now to work out a Z required.
From the internet it looks like for timber Z = M/permissable bending stress parallel to grain
permissable bending II to grain = grade bending II to grain and some K factors, by the look of it.
so if we're using C24
thats
grade bending = 7.5
K2= moisture, well its out side so thats looking like class 3=0.8
K3=duration maybe 1.0 long term?
K7=depth factor (300/h)^0.11, should we guess 150 deep as its a regular size? (300/150)^0.11 = 1.07
K8=load sharing, we got a load of these at 400c/c so 1.1?
So Z = 1.1052 x 10^6 / (7.5 x 0.8 x 1 x 1.07 x 1.1) = 156499mm³ or 156x10³mm³
47x150 C24 = 176x10³mm³ so they look like they're strong enough at 400 centres
But what of the deflection and the little bit of wind that might push down on it ?????
I'm sure we can work this out.
[Edited on 8/10/20 by pigeondave]
And that’s why I paid a professional to do my drawings and designs! 
Artificial intelligence is no match for natural stupidity!
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Mr Whippy
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| posted on 8/10/20 at 03:19 PM |
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I would use fully treated wood. I didn't and there was a lot of warpage even over two 2.5 metre spans and horizontal batons are a must. My span
was 5 metres using 220mm floor joists. The whole carport was 6m long. Eventually I removed the central support and it was still more than strong
enough, even jumping up and down on the roof it didn't budge.
Originally I used clear corrugated sheet for the roof but it was noisy and banged in the wind (it's always windy here), then went for a wooden
roof but the felt covering leaked. If I was to do it again I'd have used galvanised corrugated steel. I took it down after about 8 years as it
was right in front of my living room and spoilt the view.
This is the sketchup 3D model I used to work it all out before buying the wood. It worked very well and helped heaps.
[Edited on 9/10/20 by Mr Whippy]
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jacko
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| posted on 8/10/20 at 06:31 PM |
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Have a look at precast garages just to give you a idea of what holders the roof up
Jacko
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