Overengineering a Pergola

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Molynoox

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Apologies if the following is just ridiculous over engineering for something simple, a slightly bad habit of mine unfortunately, but I can't decide what is best and looking for your thoughts.

So a friend has spotted my pergola lurking in my garden which I built last year and she has decided that she wants one too and has asked me to build one for her. Because of the specifics of her site, I have an interesting engineering problem, which I would like your thoughts on. It is concerning the methods I will use to hold the structure in place.


Proposed pergola design
Here are some pics of mine, the new one will be identical but only 4 metres wide, with 6 support posts, so kind of what you can see in the picture. Front and back posts are spaced 350mm apart. Sideways spacing 2 metres between posts


martin pergola 1.jpg

Spars in the picture are 2.1m, the spars on the new pergola will only be 1.7m. Probably Cedar, but may need to go for C24, which is a bit heavier.


Site
The site for the pergola is half onto a patio and half onto grass, so I will need to use a mixture of foundations to secure it in place.
Picture of the site below, support posts will be next to the fence with the spars overhanging the chairs. Posts shown in yellow, you can ignore the numbers for now, I explain that in a minute..

The paving slabs have been professionally laid and have what look like at least 50mm mortar underneath, and I am told there was some 'rubble' placed under there too. We might get more details as she is going to contact the guy that laid it, but for now we can assume 50mm mortar under the paving slabs, and some sort of MOT type 1 base.
site and posts.jpg

So what's the big problem?
The added complication comes from the fact this is a 'cantilever pergola' (if such a term exists) and the rear support posts will be in tension and the front support posts will be in compression. So, combined with the mixture of patio and grass for ground, this actually gives me 4 types of fixing requirement:


  1. Front post (in compression), into grass
  2. Front post (in compressio), into patio / concrete
  3. Rear Post (in tension) into grass
  4. Rear Post (in tension) into patio / concrete

The numbers 1,2,3,4 in the picture are the fixing types listed above

Proposed fixings
So, here are my thoughts on best method for each type:


For the type 1 and type 3 I am proposing to use 800mm self install groundscrews - I used those on my pergola and they are rock solid and perfect when going into soil. See pictures. Other than the cost I see no reason not to use these again (delivery is £45 and because I only want 2 groundscrews this becomes quite significant).
LINK

For type 2, the weight of the structure does most of the work and the fixings are mostly to stop sideways movement from the wind. I am proposing using concealed post bases from Simpson, combined with 175mm resin anchors, see pictures.

Concealed post fixing
concealed post anchors.jpg

Resin

For type 4, this is where it gets interesting, because the fixing has to resist the 'toppling' motion of the pergola, but I can't use a groundscrew because it is going into patio / concrete. This is where I am most keen to get your thoughts. There are 3 key options I can think of - and they may all be terrible ideas?


Option 4A, post base and resin anchor: same as with type2: concealed post base + resin anchors
Thoughts: would this be strong enough to resist the natural 'toppling' motion of the pergola? It wouldn't be taking all the force as the two posts on the far right would be fastened with groundscrews which I know wont move as they can cope with several hundred kilos of pull out force.


Option 4B, threaded bar, threaded insert: long (600mm?) M20 threaded bar, half of bar in post and half of bar in ground. Threaded inserts to attach bar to post. Resin to attach bar into ground.
Thoughts: my thinking here is that the long bar will resist the structure toppling because the natural rotation will be constrained by the long bar in the ground - the tip of the bar will move in an arc, but its a vertical hole, so it wont be able to. Does that make sense?

Dodgy sketch showing 4B and 4C options:
threaded bar options.jpg


Option 4C, threaded bar, square flange: same as 4B but use a square metal plate / flange fastened to bottom of each post, with a M20 thread in the middle - the threaded bar would screw into the thread in the flange. Post would need boring out to accept the bar.

Thoughts: does such a plate exist - I think it probably does but I can t find it by searching?

I would appreciate any wisdom and opinions on this please:)

Martin
 

Fanous

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Hello mate, if you want the extra mile, and fancy something unusual, Samurai Carpenter (youtuber) did the base of the timber pillars on top of boulder, and it looked ace!



Not quite answering your question, but I think I will do just that when doing my pergola eventually.

The important bit is to get water away from the wood, let it drip and dry after the rain, so prevent sitting water at the bottom of the timber if you can.
 

HOJ

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Martin, I wouldn't normally discourage people from doing things, my advice is, please consider your liability & risk in all this, I carry £5mill PL, I wouldn't do it.
 

Molynoox

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Thanks fanous, I will watch that later tonight, looks interesting. I will defo be keeping it all off the ground and away from water, good advice 👍
Martin
 

Jameshow

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Martin, I wouldn't normally discourage people from doing things, my advice is, please consider your liability & risk in all this, I carry £5mill PL, I wouldn't do it.
Is this a commercial operation or for a freind. If former get insured / structural calcs. If latter use your noggins!
 

Molynoox

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It's for a friend but she will pay, so it's a bit of both. I want it to be right though.
I have mentioned to her already that the cantilever design may not be possible if I can't find a robust solution for the foundations and that we may need to have a traditional design with posts on the corners.
 

Jake

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Resin won't work, it needs a solid to fix into and the only bit that is the slab. Can't you lift a slab for each post, clear mortar and mot or whatever in the middle and put a ground screw in, and find a way of connecting under slab ground screw to over slab Simpson stand off thingy
 

sawtooth-9

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Great design, looks great - and no, it's not "over engineered "
There is not a great distance between front and rear upright posts, and the "overhang " is not large- meaning the upward force on the rear post and the downward force on the front post is not large compared to the overall downward force due to the weight of the timber itself.
I would lay a set of concrete "sleepers" running from front to rear of each set of posts.
I would do the front of the sleeper about a 18 inches wide and at least 8 inches thick - laid on a nice deep and wide gravel bed. That way you can dyna bolt brackets to hold the posts
And, by the way, screw the insurance rubbish !
Good luck
 

Jameshow

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Resin won't work, it needs a solid to fix into and the only bit that is the slab. Can't you lift a slab for each post, clear mortar and mot or whatever in the middle and put a ground screw in, and find a way of connecting under slab ground screw to over slab Simpson stand off thingy
Can you core drill a 100mm hole in the slab and put a ground screw in and fit the core drill waste back in place?
 

Molynoox

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Can you core drill a 100mm hole in the slab and put a ground screw in and fit the core drill waste back in place?
Ooo, that's not a bad idea! Core drill into stone... I will need to look into that.
I also wondered about using some sort of ballast on the rear legs, instead of an anchor.
Martin
 

Jones

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A paving slab is not a foundation. You'll need to go through the slab somehow, the bar hammered through a hole idea might work depending on ground conditions. I built a rose arch using that ,idea 22 mm round bar was hammered in up to 1 m then cut off at required level and a washer welded on 200 mm down. The posts had a 22mm hole drilled centrally and where then knocked down onto the steel bar. The above ground bit was painted and I hope below ground will take a while to rust away.
 

sawtooth-9

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That's a lovely piece of work.
Great for a compression load ( the front of the pergola ) Guess the effectiveness ( to some extent ) will depend on the soil characteristics.
The rear of the pergola has an ( upward force ) so need to be "tied down" ). There are several simple ways, depending on the soil characteristics.
 

sawtooth-9

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That's a naive statement, Martin does what ever he wants in his own back yard but take that out to a 3rd party site, irrespective if its for gain or reward your into liability and risk territory, I'll stop now & leave that here..
If you build it well, and take reasonable care ( to avoid acts or omissions which you could reasonably foresee would injure your neighbour ), it wont fall down or injure others

You may think the comment is naive, but perhaps YOU should recall the FULL definition of negligence by Lord Atkin. If you build well, where are ALL the necessary elements for an action in tort (negligence )?

In that case why pay for public liability insurance ?
You only do that where you want someone else to cover for your incompetence !

Legal studies do come in use sometimes !
I will leave this now but would be happy to advocate
 
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TheTiddles

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…and when you’re done being infallible at making pergolas, pick the winning lottery numbers and retire. Occasionally things go wrong, that’s what insurance is for, one could reasonably foresee that, as someone once said.
 

Stevekane

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It looks to me as though your real problem is having the slabs already laid.
Its a super looking design that plays on the death defying feat of the great big flying beams, all supported by a couple of posts just 12 ins apart,,,and as your well aware its the bit you cannot see thats the key to it all,,,is it possible to lift the slabs where the posts want to be, get in decent fixings and re-lay your slabs. Bear in mind that they are probably only laying on a weak grit cement base.
Steve.
 
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