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carlyai

Structural Engineer Comments Please

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Beautiful wifey wanted to make a structure for Jasmine and vines to cling to and pots to hang from. So my response was, (from past experience) ' no plan, no structure'. So when the structure started without a plan I said 'OK, no 45 degree stays to support the roof' as they always build them too low, just at head-hitting-height.

So the structure was built with lots of bracing and reinforcing between the concrete piers, which the guys insisted would support the roof in strong winds and the roof wouldn't blow away. 

So does the cantilever type back structure really support the roof structure against strong winds? 

You can't see it from the pics but the 2 posts are tied into the fence posts.

20200521_071437.jpg

20200521_071418.jpg

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1 hour ago, RangerP703 said:

how is it anchored to the concrete posts ?

The concrete posts have 2 rebar out the top that is welded to the beam between the 2 posts. The structure under that beam is also welded to the rebar inside the posts thru holes drilled in the concrete.

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The only way I can see to rescue it, given that the cantilevered structure can’t have vertical support on to the tile, will be a couple of diagonal supports between the inner rail and the base of your garden wall. That would take most of the tension stress off the concrete posts and you can probably sell it as extra places to hang plants.

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That sucker really should be bolted with 1” all thread rods and backing plates/doubler plates instead of/in addition to welding to the rebar. 
 

I normally don’t have an opinion on stuff like this as it’s usually “ok” but this one is a little sketchy. 
 

Really shouldn’t have used what looks like precast posts either. Should have formed your own casting the holes in the post at the top for all thread and plates. 
 

Id at the very least add a brace in the back with bolts and plates. 

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If you want to balance the loads, the back projection has to equal the front projection. Plus you would need to put weights on the back to counter the weights attached at the front. If the forces arent balanced it will tip unless  it has a rigid (resist turning moment) connection to the post. If the post can take this turning moment, then the foundation needs to be big enough to avoid pulling out and toppling.  That's the structural engineering perspective. However, the forces are small, and it looks well built, so it might be okay. Wind should not be a problem, because it is an open structure.

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1 minute ago, Bramley said:

If you want to balance the loads, the back projection has to equal the front projection. Plus you would need to put weights on the back to counter the weights attached at the front. If the forces arent balanced it will tip unless  it has a rigid (resist turning moment) connection to the post. If the post can take this turning moment, then the foundation needs to be big enough to avoid pulling out and toppling.  That's the structural engineering perspective. However, the forces are small, and it looks well built, so it might be okay. Wind should not be a problem, because it is an open structure.

Just one word of warning, if it does topple, it will rotate forward. Will it hit the roof of the house? If so, you might want to trim it back a bit. Another option is to fix guy wires at the back to concrete blocks embedded in the soil.

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42 minutes ago, carlyai said:

The concrete posts have 2 rebar out the top that is welded to the beam between the 2 posts. The structure under that beam is also welded to the rebar inside the posts thru holes drilled in the concrete.

Hmm ok, So basically it is only the rebar holding the thing up, I would not be putting too much load on it.

 

I personally would have built it some what like this.

 

pergola.jpg.cee0afc532bed74d3158bd5bd88698a4.jpg

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49 minutes ago, carlyai said:

The concrete posts have 2 rebar out the top that is welded to the beam between the 2 posts. The structure under that beam is also welded to the rebar inside the posts thru holes drilled in the concrete.

That will provide a route for water to get into the concrete so you will have a rusting problem within a few years. If I were you, I would wire brush all the exposed steel and coat it with zinc rich paint and then seal the entry points with bitumen sealer. Then keep an eye on it.

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Looks quite impressive albeit very front heavy, an option would be to install two steel posts to the front of the structure, easy to do and making it as strong as the rock of Gibralter.

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9 minutes ago, RangerP703 said:

Hmm ok, So basically it is only the rebar holding the thing up, I would not be putting too much load on it.

 

I personally would have built it some what like this.

 

pergola.jpg.cee0afc532bed74d3158bd5bd88698a4.jpg

That design has the same issues - out of balance forces, so it will tip forward. The stays at the back  take all the rotational forces down to the foundation, but if the foundation is big enough (massive) it will be okay. The key to cantilever strucures is to balance the forces back and front.

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