# Forum Home Renovation Pergolas, Gazebos, Strombellas & Rotundas  Span tables for posts?

## nomisg

Howdy all, 
I'm looking to install a 7.5m x 4.5 Arbor using timber square profile posts (e.g. 150x150) instead of using beams.  
Something like  https://media.architecturaldigest.co...gardens-10.jpg 
The problem is, I need spans upto 4.5m and I just can't seam to find any span tables for square posts, used as beams. Was hoping to use cypress pine (f7) but may have a to go another stronger wood e.g. Ironbark or even seasoned hardwords.

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## nomisg

Crude outline of the arbor I have planned, no roof, rather will grow vines through it

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## Gooner

Looking at the span tables I have on hand for cypress pine, I donÂt see anything for 150x150. However, it looks like you may be marginal at best. 
Perhaps something to consider, when I built my pergola I needed a 5.5m span. The beam size I wanted to use was just under spec for the span. To get it to work, I used some diagonal cypress pine supports as per the photo.

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## Marc

Considering that your arbor has zero load, plus some vines, the span tables are irrelevant. Go with the looks and common sense. Place the rafter between two supports and step on it. If it does not break it is Ok for your pergola/ arbor/ thingy.  :Smilie:

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## Moondog55

How heavy are the vines going to be when mature?
How aggressively do they/will they penetrate cracks in the timber joins?
Vertically laminated 140*45 are solid looking or you could assemble* "T"* beams which can look even larger or you could just use MGP10-H3 timber as per the span tables
You have drawn it with three posts on the 7500mm length; I would be using four at a minimum and preferably five. 
You also need to think about the plant forcing the sides apart so you need some way of stopping that, heavy stainless steel cable and large eyebolts work for that as would galvanised steel reinforcing mesh

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## nomisg

> How heavy are the vines going to be when mature?
> How aggressively do they/will they penetrate cracks in the timber joins?
> Vertically laminated 140*45 are solid looking or you could assemble* "T"* beams which can look even larger or you could just use MGP10-H3 timber as per the span tables
> You have drawn it with three posts on the 7500mm length; I would be using four at a minimum and preferably five. 
> You also need to think about the plant forcing the sides apart so you need some way of stopping that, heavy stainless steel cable and large eyebolts work for that as would galvanised steel reinforcing mesh

  Don't see an issue with the weight on the vines on the top (they will get prune to stop them getting to big/woody) nor will they damage the wood. 
LVL's are not an option, as I want it to naturally weather. 
"You have drawn it with three posts on the 7500mm length; I would be using four at a minimum and preferably five." Any science to your thinking?

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## nomisg

Thanks for all your thoughts. 
But still find it so weird that span table's don't exist for square profiled timber. Considering people certainly make pergolas/arbors out of them.  
I'm 100% that seasoned hardwood like iron bark would work, but its gonna cost me at least 4x times more in timber and hard to track down in Adelaide.  
I have considered using slim profile steel brackets, that extend say 50cm at the joins. Achieving the same thing, Gooner suggested diagonal cypress pine supports.

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## Moondog55

> Don't see an issue with the weight on the vines on the top (they will get prune to stop them getting to big/woody) nor will they damage the wood.  
> "You have drawn it with three posts on the 7500mm length; I would be using four at a minimum and preferably five." Any science to your thinking?

  Yes the wider the spacing of the posts the bigger the timber size/ section depth needs to be, just look at the span table to tell you how fast they get huge, no span tables exist because square timber in beams is a waste of wood, the strength across a gap comes from the depth not the width, if you use square timber for the look of the thing I'd guess that you would simply use the span tables for depth and then add wood until it is as wide as it is deep, you are looking at bridge and wharf timbers, plenty of that around as S/H but hugely expensive, a hundred dollars a metre or so. 
Steel SHS would be cheaper.
The tendrils on vines will get into every nook, crack and cranny in any construction and try to force them apart, I am just about to remove a strong and healthy Monstera for that very reason, before it destroys my house

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## nomisg

> Yes the wider the spacing of the posts the bigger the timber size/ section depth needs to be, just look at the span table to tell you how fast they get huge, no span tables exist because square timber in beams is a waste of wood, the strength across a gap comes from the depth not the width, if you use square timber for the look of the thing I'd guess that you would simply use the span tables for depth and then add wood until it is as wide as it is deep, you are looking at bridge and wharf timbers, plenty of that around as S/H but hugely expensive, a hundred dollars a metre or so. 
> Steel SHS would be cheaper.
> The tendrils on vines will get into every nook, crack and cranny in any construction and try to force them apart, I am just about to remove a strong and healthy Monstera for that very reason, before it destroys my house

  Cheers, 
The depth makes sense in regards to span strength, but then I also can't find span tables for arbors (e.g. no roof load). 
Funny you say Steel SHS, one of the other options I'm looking at,  is doing it in cortens steel SHS. I have found the product I need, but can't find anyone in Adelaide that has worked with it. E.g. I ask them, so how do you weld them together and maintain the cortens look, I get blank faces. Install/metal work won't be cheap, but then again, it will then likely outlast my house and myself.

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## Marc

Nomi, You need to rephrase your question. 
    You will not find "span tables for post" simply because post do not span. They stand vertical. Beams on the other hand do span between two supports or more. 
I am sure that a proper search will uncover span tables for square sections, and to Moondog ... the load capacity of a timber member is the sum of all the wood mass in the cross section, not only the first 2 inches, otherwise floor beams would not be paired to increase span. 
    If you really must know how much you can load your pergola beams, in case you decide to park your car under it and lift your engine out of the car using a chain block attached to the beam ... check the span section for beams, where it says 2x Notice that 140x45 is not the same as 2x140x45, in fact it spans twice as much not surprisingly so it stands to reason that 3x140x45 would span 3 times as much or close enough to that figure. 
    However as I stated previously this is all academic and for entertaining purposes only, since a wooden structure with no load or almost zero added load only needs to support itself and withstand the wind, and occasional seismic motions of hypothetical magnitude.  
The beams you intend to use at 150x150 will surely support themselves plus vines and if in doubt, all you need to do is place them on a couple of bricks at each end and stand on it. That will put your mind at rest. 
Jump up and down if you must for good measure.  :Smilie:

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