Post and steel rope framework for grape vine

Question

I am planning to build a framework to support a grape vine above a patio. Ideally I would like to use steel rope to provide this framework, rather than a full timber frame.

Diagram attached.

There is a wall at one end of the framework, and I would hope to use wall anchors (diagram - triangle symbols) instead of posts at that end.

A 'circumference' steel rope would start near a wall anchor, go around the full outside edge of the framework, and join to itself via a tensioner. The cross pieces would be individual ropes, joined with untensioned thimble eyes.

At the other end, are two posts, both in concrete (depth ~7-800mm). Post B is freestanding. Post A can be bolted to wall at up to 900 mm high (above that is just windows).

Overall dimensions are 3000 x 5500 mm. Wire rope (4mm) will weigh ~ 3 kg. Not sure on weight of vine - maybe as much as 50kg when in full leaf?!

My question is - how can I construct this to avoid post B and to some extent post A bowing, as the steel ropes will be tensioned to prevent the ropes sagging, and the vine will also exert force. Ideally I could have something between post B along the 5500mm length in compression, but at that length it would have to be strong enough to support itself and not sag. Sort of defeats the purpose of not building a full timber pergola as well.

Also to add, my initial thought was that the posts should be 150mm timber. Could look at steel box section, but very much depends on advice I receive here

Thank you for your help

Answer 1

You could always do a type of over load test on the posts to see if they bend. To my knowledge timber carries a lot of variance. You would also want to know if the structure can survive windy conditions especially with all the leaves they might be subjected to a lot of load.

You could have them laying flat anchored on one side and add weight on the other and let gravity do it's work.

Edit 1: I am having difficulty understanding how tall these posts are at least the freestanding one, and also do you have any particular type of wood in mind? Am I right to say that it is 800mm in height? As far as I understand that is not enough height for it to be described as a pergola. If you would like to perform the calculations yourself omnitools can be a good sanity check. You have to make sure that the bending stress [https://www.omnicalculator.com/construction/bending-stress] is less than the fiber elastic limit for the particular wood type [https://www.engineeringtoolbox.com/timber-mechanical-properties-d_1789.html]. Also as far as deflection goes you can have a look at using this tool. [https://www.omnicalculator.com/construction/beam-deflection]. I can help some more if you answer the questions above.

Answer 2

Two points:
First:
The wires will bend, no matter how much tension you put on them. That’s just the nature of the beast. But the pattern will still be attractive.
You’ll want to use wedge anchors in the supporting wall, these have the greatest tensile strength. Make sure the wall is solid!

Second :
For the beams, if you use 150mm wood beam in a concrete base you will have no problem. Choose treated beams that have a straight grain, no knots. Hard to find at the length you need, about 4m.
Except that, over time the beams will tend to warp in the direction of the stress. (That’s how they make curved beams, with heat, humidity, pressure and time.)
You could minimize this by having the beams lean away from vertical a couple of cm diagonally, so that when tensioned by the cables they become vertical. Then they will have some stress in the opposite direction. Or you could install crossmembers from about halfway up the beams at a 60 degree base angle. These could be 15mm by 5mm.