Overengineering a Pergola

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Jameshow

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How about drilling a 25mm hole through the slap and then drilling a 12mm drill at a number of different angles so that your in effect making a cone of resin below the slab, which due to its shape will be impossible to pull out???
 

Molynoox

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How about drilling a 25mm hole through the slap and then drilling a 12mm drill at a number of different angles so that your in effect making a cone of resin below the slab, which due to its shape will be impossible to pull out???
That's an interesting idea. Might take a lot of resin though 😃
 

Phill05

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Valhalla:
Have you given through to Saddle stone?

Do you mean Staddle Stones?


Yes sorry getting my spelling checher sorted is next weeks job, should be "Thought" and "Staddle Stones"
 

Molynoox

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Well, I've come up with a new idea; calculate the centre of gravity of the structure and then put the front posts there.

Why...
This allows me to retain the same 'cantilever' design concept, which my friend is so keen on, but it also addresses the main 'flaw' in the design as it eliminates the tipping force in either direction. In theory I would then only need to secure it for wind, which is a smaller problem to deal with.

I'm still interested in maybe using those staddle stones, and haven't decided on fixings yet - either way, by placing the posts in a central position I can minimise the problem, and the risk.

So for those interested, I used the principal of leverage and 'moment arms' to find the CoG. And because I don't trust my calculations, I then checked the total moment on either side of the new calculated CoG to verify it was exactly equal on each side. To my amazement it was 😅

Short version
So to cut a long story short, using spars of 1600mm in length, the CoG is 372mm from the rear post. If I drop from 13 spars down to 12 it shifts it even further back, to 357mm.

Here is the resulting design, dotted line the CoG at 372mm:
pergola 400mm.jpg

pergola 400mm wide view.jpg


Long version
I've no idea if anybody finds this interesting or useful, but I thought it was a good thought experiment, if slightly ridiculous, and it had pragmatic application for me as it pretty much solved my problem (I think), so it might be handy for others too. So here are the details.

There are three components
1. Spars (100 x 50 x 1600), qty 13
2. Support rails (150 x 50 x 2000), qty 2
3. Posts (100 x 100 x 2300), qty 3

I didn't bother including density in the calculations as all components are the same material, and therefore I just worked from volume - the ratios are all the same and it eliminates one variable to mess around with, winner! So essentially I have set density at '1' to get mass. Although I suppose it might be interesting to put in the correct density and get the weight of the structure at some point. Anyway...

For the initial calculation of 'X' I set the arbitrary datum as the centre of the rear post, this eliminated a lot of messing around because it sets the moment arm at zero for two of the three components (because they are lying on the datum and hence moment is zero).

When calculating CoG I temporarily 'removed' the front posts - these will get added in later once I know where to put them (i.e. the position of the CoG). Essentially, this simplified structure with just the 3 rear posts is what I am trying to 'balance'.

Initial calcs
initial calcs.jpg


result and calc check
pergola calc check.jpg


Latest plan with the pergola
I plan to put the posts at 400mm from rear posts, not 372mm, this puts a little more weight on the rear posts and makes it more stable. I also have the diagonal braces which I deliberately didn't include in the CoG calcs, this adds a little more weight to the back again so makes things even more stable. Finally, the spars hang out 100mm at the back, which I also eliminated from the calcs. Because there are 13 spars this is the equivelent of 1.3m of additional timber extra at the very back of the structure, again moving CoG further backwards than calculated.

I will still have two groundscrews and a mixture of staddle stones and/or resin anchors on the other posts. So I'm pretty comfortable with this now, as I say, it only has to resist wind movement now so the fixings are doing less of a critical job than before.

Martin
 

Dabop

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Here's an idea to get extra weight onto the rear- add another beam on top of the 'rear' ie right hand side posts but on top of the cross beams (the cantalevered ones)
Use the heaviest/densest wood you can, that would not look out of place, and increase the downforce on the rear posts ie effectively moving the center of gravity towards the rear posts
Screenshot from 2022-06-05 19-34-11.png

(I just got 6 new posts for my carport to replace the rotted out ones- they are 'ironbark' cut locally here in Australia- and those are damn heavy- a single 3"x3" 3.2m long I can't lift by myself lol)- stick something like that on top, and you have a lot of extra downforce on those rear posts lol)
 

Stevekane

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Youve done an impressive amount of work thinking it through and working it out but as somone who tends to overbuild nearly everything I make my concern is not your CofG but your inherent instability in having a very small footprint, and Im quite likely wrong but is it enough to have the structure “balanced” whereby a small additional loading could alter your CofG significantly?
Steve.
 

Molynoox

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Here's an idea to get extra weight onto the rear- add another beam on top of the 'rear' ie right hand side posts but on top of the cross beams (the cantalevered ones)
Use the heaviest/densest wood you can, that would not look out of place, and increase the downforce on the rear posts ie effectively moving the center of gravity towards the rear postsView attachment 137038
(I just got 6 new posts for my carport to replace the rotted out ones- they are 'ironbark' cut locally here in Australia- and those are damn heavy- a single 3"x3" 3.2m long I can't lift by myself lol)- stick something like that on top, and you have a lot of extra downforce on those rear posts lol)
Cool idea. The additional timber might be best at the base where it is less visible and might also be a slightly better distribution of the weight.
 

Molynoox

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Youve done an impressive amount of work thinking it through and working it out but as somone who tends to overbuild nearly everything I make my concern is not your CofG but your inherent instability in having a very small footprint, and Im quite likely wrong but is it enough to have the structure “balanced” whereby a small additional loading could alter your CofG significantly?
Steve.
Yep, a very valid line of thought.
 

Jameshow

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I'd consider wind loading to be as great if not of greater effect than the general mass of the structure?

How exposed is the site??
 

Molynoox

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I'd consider wind loading to be as great if not of greater effect than the general mass of the structure?

How exposed is the site??
I've modelled a few bits of the surroundings in the screenshot, it's adjacent to the house and surrounded by 1.8m fence, I would say fairly well shielded.
It's not much of a sail, most of the wind will escape through the gaps but there will be some loading I agree.
I've got some quotes coming for the staddle stones for the rear posts, don't know how much they weigh but they look very heavy.
 

Molynoox

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Essentially making my own staddle stones. Nice idea, not sure how it would come out or if there would be a saving over buying them.
 
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