Should I use C16 or C24 timber?

[msw109]

I am building a pergola. I have 6"x 6" 9ft posts set in concrete with decking built around. I wish to suspend a swing seat and hang 2 hammock seats form the finished structure. My timber between the posts will be 6" x 2" attached at the top by 12" coach bolts. These timbers will be double to support the joists (which will be notched into each other by around 1.5"

My question is.... what grade timber should I use for the finished joists on top of the support timbers? Bearing in mind they will be supporting possibly 4 people..I was thinking 6" x 2" but not sure whether C16 would be enough

Ta in advance

 

[tony1851]

You don't say what the span is (that's critical), and I can't quite picture how your structure is arranged, but perhaps that's me missing something.

 

[msw109]

DOH rushed off........ the span is 11 feet......design is typical pergola joists laid on joists basically

 

[msw109]

picture of how it will be laid out .........roughly same design

Untitled

• msw109
• 1 Mar 2012
• 2

 

[tony1851]

Two 6" x 2" C24 timbers securely fixed together, used externally, over a span of 11ft will support a permanent, static, uniformly distributed load of approx 600kg at the maximum allowable bending stress of the timber.(it will deflect noticeably under this load).C16 timber would be about 30% less.
But this needs to be reduced if the load is moving, which it would be in your case, possibly by 20 - 30%.
The maximum load allowable will also be affected by where along the beam you fix the suspension points (obviously worse nearer the middle).
Also, you need to be careful of the bolting - make sure the bolts are not too near the edge or end of the timbers,
To be a bit safer, why not consider two 8" x 2" C24 instead?
PS just seen your pic; the main bearer on that looks a lot deper than 6".

 

[msw109]

That illustration is just for explanation purposes....i might use 8x2 the for the joists .......will 6x2 (one inside the post and one outside) do for the carrier timber on the outside?

 

[msw109]

I am wanting to secure the swing and hammocks to the joists that run across the centre of the structure(not the outside timbers)

whats your thoughts on that?

 

[msw109]

the outside "bearing" timbers will be fixed to the posts with 2x C10 12" coach bolts on each side, then the joists will be notched into this

 

[tony1851]

If one timber is inside the post and the other on the outside, they will not work together as one, even if you join them with blocks. To achieve the load capacity quoted, they need to be fixed tight together along the whole length.

 

[msw109]

OK. i want to go inside/outside with the bearer timbers for sure, then lay the joist timbers across......in effect the joists are picked up 4 times over the span. In this case should I use 8x2 C24 for the whole of the rest of my structure just to be safe?

 

[tony1851]

whether it's worth using 8 x 2 for the joists resting on the main bearer I wouldn't know as it depends partly on the span of those joists; but I would certainly consider using two 8 x 2s for the main bearer.

 

[msw109]

have ordered 8x2 C24 for everything......i think this will do what i need within my tolerances.....the span i am concerned about is about ten feet