Structural Engineer Comments Please

[carlyai]

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.

[RangerP703]

how is it anchored to the concrete posts ?

[worgeordie]

The weight of the structure,plants when they grow,then high wind,

if you get it where you are,only time will tell,I have posts like that

supporting a place to hang orchids, after time the posts start to

crack,due to iron wire rusting and expanding,so keep an eye on

them.

regards Worgeordie

[RangerP703]

2 minutes ago, worgeordie said:

The weight of the structure,plants when they grow,then high wind,

if you get it where you are,only time will tell,I have posts like that

supporting a place to hang orchids, after time the posts start to

crack,due to iron wire rusting and expanding,so keep an eye on

them.

regards Worgeordie

Thats why I asked how it is anchored to the posts, to me it looks like once weight is ontop it will break the anchor points and pivot forward on the posts.

[steve187]

i think the weak point is the concrete posts

[sometimewoodworker]

The huge problem with that structure is that it will break sooner rather than later. Concrete is very strong in compression but weak on a stretching load. You are totally reliant on the reinforcement in the concrete and unless made for you it will be very thin.

 

the posts are designed to take weight on them not pulling On them them They are not designed for a cantilever weight 

[carlyai]

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.

[sometimewoodworker]

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.

[Mama Noodle]

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. 

[Bramley]

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.

[Bramley]

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.

[RangerP703]

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.

 

[Bramley]

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.

[vogie]

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.

[Bramley]

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.

 

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.

[Bramley]

Just now, vogie said:

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.

That would do the trick.

[4reaL]

17 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.

 

 

Engineer here but not structural.

 

This pic is correct and you will need to add some buttresses to your structure which could be welded to your current steel frame and then bolted into the cement.

 

Doing this would enhance the integrity of the design you have and still give it the open airy feeling that seemed was intended. 

[RangerP703]

6 minutes ago, Bramley said:

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.

Would need to calculate the bending moment due to the weight of the load to determine if the foundations are suitable or not.

[carlyai]

1 hour ago, sometimewoodworker said:

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.

Yes, that was my suggestion just before they finished it.

So at the moment it all depends on the structural strength of those two concrete posts and if they have enough rebar and concrete mix strength to flex without cracking or breaking. 

I didn't know the structure was going that high and i was the one who said 'no diagonal beams'.  Catch 22. 

And the strength of the foundations. 

 

Edited May 24, 2020 by carlyai

[Crossy]

Too late now I know, but I would have used steel verticals. No issue with bending moments causing tension cracks then.

 

Two U sections welded back-to-back so there's no hollow space to fill with water and rust out.

 

[DaLa]

Just had a 6000 square foot 2 storey shop built here and they went down 16 metres. That's half the depth of the the foundation of the World Trade Centre in New York . I was in construction back in the UK and you could build most domestic structures off a 1 metre foundation. That is where I would be focussing my attention.

 

The cantilever looks reasonable to me, however in my opinion the main forces will always be taken by the braces and they are on the wrong side of the posts if they were designed to work in either tensile or compressive mode. If it's welded then I don't see the wind loading tearing it off the posts, I'd be more concerned about the posts not being secure in the ground if the loading from the beams is off centre.

[sometimewoodworker]

14 minutes ago, carlyai said:

I didn't know the structure was going that high and i was the one who said 'no diagonal beams'.  Catch 22. 

This is what I mean either blue or red.

 

11 minutes ago, Crossy said:

Two U sections welded back-to-back so there's no hollow space to fill with water and rust out.

 

Actually I think that Crossy has your answer or inspired it.

 

add a c section to the back of each concrete post with the back to the concrete. Weld it at the top and bolt it in 3 or 4 places in the length. That will sufficiently reinforce the concrete and will not make any significant change to the look

Edited May 24, 2020 by sometimewoodworker

[Crossy]

Just now, sometimewoodworker said:

add a c section to the back of each concrete post with the back to the concrete. Weld it at the top and bolt it in 3 or 4 places in the length. That will sufficiently reinforce the concrete and will not male any significant change to the look

 

Now that works for me, but I'd still worry about the bending moment at ground level, I'd want the steel going into the concrete foundation.

 

If that's a non-starter then a nice 5mm plate welded and braced at the bottom of the C then anchor bolted into the concrete would work.

[Mama Noodle]

If that was mine, I would bury a small concrete block behind each post, then use 1/2“ stainless steel wire rope, bulldog clamps, and a turnbuckle to make an aesthetic tie back system behind each post.   

 

[Crossy]

2 minutes ago, Mama Noodle said:

If that was mine, I would bury a small concrete block behind each post, then use 1/2“ stainless steel wire rope, bulldog clamps, and a turnbuckle to make an aesthetic tie back system behind each post.   

 

A couple of climbing plants and nobody would know it's there ????

 

[Kwasaki]

4 hours ago, carlyai said:

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. 

That's a fine structure nothing much for winds to do any harm.

As long as the concrete post footings were concreted in correctly it's OK that would be the only thing to be concerned about.

You don't say how that was done and how deep they were so I cannot comment further.                

[bankruatsteve]

Without material specs and dimensions, just a guess: It looks fine to me.  There should be little if any lateral force on the columns.  ????

[sometimewoodworker]

54 minutes ago, Kwasaki said:

That's a fine structure nothing much for winds to do any harm.

It’s not the winds, they won’t make any difference to it and haven’t been a concern. It’s the plants that will be hanging at the end of the cantilevered section.

[Metropolitian]

How about using counter weights method to even the forces. (only Z forces and no F's)

 

Strong plant pots filled with layer of concrete or lead hung up with chains or steel wires.

 

[Kwasaki]

7 hours ago, sometimewoodworker said:

It’s not the winds, they won’t make any difference to it and haven’t been a concern. It’s the plants that will be hanging at the end of the cantilevered section.

Fair call but that would not be my concern, you pointed out quite rightly before if i recall correctly that the concrete post foundation is the whole key to a project like this, and I agree.