regarding use to High tensile cable/rod for supporting Canopy 2

[Arbu]

Dear All,


I am having a cantilever canopy long span > 5 m. Design concept is a tube with Pretension cable (30 kN) rod as shown below image.

There are lot of high tensile cables available in market, but I want some economical solution.I Was thinking to use 10.9 grade rod as pretension rod (Preferably -10 mm or 12 mm diameter). Is it advisable to use? there will be 30kN pretension in rod/cable. The length of rod and canopy is around 5 m.

Please guide.

 

[dik]

for that cantilever distance, I'd likely change the 105mm for something like 1 or 2m... for a start...specially if your cantilever is 5m.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[r13]

Same here.

 

[JLNJ]

Check your canopy for uplift. Your tension-only rod won't help whith that.

The steep angle of your rod will mean some very high forces. High forces in a small rod will bring with it a lot of elongation. Your canopy may be so flexible that it will have too much "bounce" in it. A 300 pound guy standing at the end will increase the rod force by 10,000#.

It's one of those things that you need to run some numbers on and see what jumps out. It's a rather simple problem. Perform some engineering. Get some results. Repeat.

 

[MegaStructures]

Can you extend the column a little higher , so you can have a bit more efficient geometry for your tension rod? You have to really pull on the rod at that low of an angle to react a vertical force at the end.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[MIStructE_IRE]

There’s no way this works.. If it does, you’re a magician, not an engineer!

Also, uplift as others have mentioned, is often a governing factor in canopy designs.

In addition, check local stresses on your column connecton. You’ll want some stiffeners in there too.

 

[Arbu]

Dear All,

there is no chance to increase the height of column. There is height restraint, that's why I am trying like a prestress steel beam. Also one correction I will make that there is no roof covering its a pergola. As each section of steel beam+ pretension cable is cladded with 60(Horizontal) x220 mm Aluminium profile. The suction will be on the top of profile will be minimum and the height from ground is 3 m only. Every member of pergola is having tension rod. I will check again wind suction on profile if need I will increase self weight to counter Wind load. Even I will check by putting a 0.9 kN as live load at the tip of member (free end).

Also all the pergolas are braced to each other by welded tube at certain distance (2 m C/C).
My doubt is - can I use Grade 10.9 Rod as a pretension rod. Or I need some special High tensile cable to use.

 

[Arbu]

Dear JLNJ,


Forces or not much its only carrying self weight and I already done Nonlinear cable analysis for full model. hardly total Dead load is 17 kg/m (Including weight of beam) and live load will be 12 kg/m each pergola.

 

[Arbu]

Dear All,


Please check the below concept and let me know it will work or not. I just checked manually to get some confidence.

of course some dead load is already balanced by Pretension, I will check again for suction as per your valuable suggestion. But I think for Gravity load this system will work. If need even I will reduce pretension and increase dead load to manage with suction. What you think if its open type pergola it will be affected byu wind much? As wind will be passing in between members suction wont be any major issue. Please suggest.

 

[MIStructE_IRE]

Can we see it drawn to scale? 120mm rise on a 5000mm long cantilever?

 

[1503-44]

Yeah. Might just eliminate the cable and slope and increase the top flange thickness of a variable sectioned beam.

 

[Blackstar123]

Is the column also a cantilever as shown in your sketches?

If so, then increasing the angle of bracing rod might also not be effective. Because in the end the column will be rotating due to the loads and so is your canopy.

Similarly in case of a cantilever column, a pre tensioned cable instead of solving your problem, will compound it. You will be exerting an additional load on column which will deflect the column in the same direction as the canopy.

 

[Arbu]

its already there in first question, it is drawing by scale.


I tried this its increasing the beam weight and increasing deflection


yes it is cantilever, but I made it strong compare to beam so its deflection horizontal around 5 mm only.

My concept is like pretension beam, Instead of concrete flange I am using steel beam, its like a steel prestresses beam. because I will keep connector through the length of cable . Cable will pass through the connector.

 

[1503-44]

Tough one.

 

[dik]

You would have difficulty doing that with a steel beam and moment connection.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[JLNJ]

You are focusing on only the cable pretension. It's an interesting problem, but only a small part. You will have a long skinny element in compression - will it be stable out-of-plane? Why are you so intent on pretensioning the cable? Just to reduce the sag? I'm not sure it's worth messing with on such a small project. Contractors I deal with who put up pergolas likely don't know much about pretensioning cables. Heck, I run into "steel erectors" who know nothing about pretensioning high strength bolts.

Other considerations:

The post will be flexible, both in its connection to the foundation and due to its height. Make sure you consider this flexibility and commensurate deflection.
I don't know what code you are bound to, I'm guessing just your conscience, but, if you design for such a small live load (12kg/m is just 8 plf), one person hanging from the end doubles, or triples, or quadrupoles the force in your tension cable. I would not allow this in a design I was responsible for. I find this to be dangerous.
One day, the vines and vegetation will cover the pergola and it will catch a fair amount of wind, rain, ice, and snow. I would not turn a blind eye to this either.

 

[dik]

Tell your client it simply cannot be done... in its present form.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik