Misc. Is My Steel Square Tubing Strong Enough?

  • Thread starter djosephm
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Summary
Trying to see if the steel tubing I bought is strong enough to carry load.
Summary: Trying to see if the steel tubing I bought is strong enough to carry load.

Hello all I bought some square steel tubing today in hopes of putting up sail shades in my back yard, I am concerned now that I did not get big enough tubing.

I have a triangular sail shade 20’ x 20’ x 20’ and a rectangular shade 20’ x 16’.

I bought 3”x3” 11 gauge (.120) steel tubing. 2 pieces at 14’ each and 1 piece at 10’.

I plan on concreting the steel tubing in the ground, 10 ft piece about 3.5’ in ground and then 14 ft pieces 4’ in concrete so they all form a triangle.

I will put up triangle shade, then with the rectangle I will attach between house brick and the tall poles.

Did I get too small of steel for this application? I already bought the tubing and hauled it home.

I guess I figured the sail would rip before the steel failed.
 

marcusl

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A quick check online (you could have done the same) shows that a 60 mph wind exerts 15-20 pounds per square foot, for a force of 4800- 6400 lbs on just your rectangular sail. Much more if a hurricane blows through. I’d want a professional structural engineer’s opinion.
 
A quick check online (you could have done the same) shows that a 60 mph wind exerts 15-20 pounds per square foot, for a force of 4800- 6400 lbs on just your rectangular sail. Much more if a hurricane blows through. I’d want a professional structural engineer’s opinion.
Thank you for your quick and witty reply, but by your logic one could attach a large bucket to the sail and fill it with 20 grown men and watch them fly. There is more to it and I was simply asking for some advice. I’m not going to pay a structural engineer to calculate the best way to put an awning up over a pool. I will angle the 3 tubes away from each other. They will all share load. I have a feeling if something fails it’s going to be a fastener not a steel pole. Thanks anyway.
 
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berkeman

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Welcome to the PF.
Summary: Trying to see if the steel tubing I bought is strong enough to carry load.

I have a triangular sail shade 20’ x 20’ x 20’ and a rectangular shade 20’ x 16’.
I'm having trouble picturing how this all goes together. Can you attach a sketch?
a 60 mph wind exerts 15-20 pounds per square foot, for a force of 4800- 6400 lbs on just your rectangular sail.
Sounds like you should consider making it easy to drop the shade pieces when the wind picks up. Do you usually get strong winds on very sunny, hot days? Alternately, you could use a mesh fabric that lets a portion of the wind through, but still blocks the sun well. Although, I guess if the shade fabrics are mostly horizontal, you may not get much wind loading after all...


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ChemAir

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I guess I figured the sail would rip before the steel failed.
I don't know if that design basis is valid, or in accordance with your local codes or rules. Hypothetically, if it was, you'd first need to know when the sail would rip. If nylon has a tensile strength of 70 MPa (10,150 psi), then a 2" wide,1/16" thick piece should yield at 1250-1300 lbf. Realize 1) that fabrics will have different tensile strength than the parent material, and 2) Nylon stretches before it breaks, and it can be quite a lot. I'm mentioning this to point out the fabric may be quite strong, but you should find out for sure.

Given that, and assuming you have a properly designed fixed footing, no, I don't believe a 3"x3"x0.120" post extending 10' above grade will reliably hold a sail until it tears without excessive bending and fatigue. That's half a ton on a 10' cantilever. That post is probably only good for maybe 250 lbf lateral at the top without bending enough to get fatigued.

I was simply asking for some advice.
Talk to someone that has performance information on the fabrics and their connection methodology, knows building codes for the area you live in, and can make proper recommendations for this. The seller of the awnings probably has some guidelines to start with, or has someone they know to talk to that does. There may be a break away fastener in the system that minimizes wind loading, so all this speculation/concern may be unwarranted.

I have a feeling if something fails it’s going to be a fastener not a steel pole.
If you know the specifications for both, the first point of failure should be no surprise. With that much lever arm, you can permanently bend that 3x3x1/8 post with a 3/8" eye bolt connection at the top without breaking the eye bolt.

If it was mine, I'd expect it to be at least 6" sch 40 structural pipe, which will tolerate 1000 lbf at the top, and work out the foundations when I had some expected load information (lateral and uplift) from the manufacturer. What you have is effectively a staked tent, and you don't know how big the stakes need to be. You don't want the tent to blow away with the stakes.

If you absolutely, positively, can't change materials for your posts, you still need to know the load information. You may be able to use them with guy wires or anchors if the load is too great unsupported, in the same way power poles are braced.
 

DaveC426913

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I will angle the 3 tubes away from each other.
Then run a guy wire down from the top of the tube to a point on the ground. That will go a ways toward discouraging deflection.
tarp.png
 

Spinnor

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I would think the posts would fail by buckling at the ground where they were fixed in concrete. If they were filled with concrete that should increase their strength? I like the guy wire idea, if done the bending moment of the post at the ground could be almost entirely eliminated and then the posts are in compression where they are doing what they are good at. Enjoy a dip in the pool for me!
 
Thanks for all of your suggestions. No turning back now. Part of me wishes I went with a 4 inch maybe a little heavier but let’s see how it reacts in a real world application. I may just need to monitor weather and pull sails down when it gets windy or try to find a fabric that will put less stress on poles.
 

Attachments

Then run a guy wire down from the top of the tube to a point on the ground. That will go a ways toward discouraging deflection.
View attachment 245795
I may very well do this!!! Since I have all 5 poles angled backwards I could install guy wires a foot and a half back from the top of each pole that would stabilize poles I would think.
 

DaveC426913

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I may very well do this!!! Since I have all 5 poles angled backwards I could install guy wires a foot and a half back from the top of each pole that would stabilize poles I would think.
You might consider two guy wires from the poles that join the square and triangle pieces. You want the guy wires to - as directly as possible - oppose the pull from each sail, otherwise they're useless.

sails.png



Also, you said foot and a half from the top - the farther down from the top you attach them, the more deflection you will be allowing - and the greater transverse forces. As Spinnor points out, you want the pole to experience only compressive forces - that's what they're good at - not torsional forces.
 
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berkeman

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I may very well do this!!! Since I have all 5 poles angled backwards I could install guy wires a foot and a half back from the top of each pole that would stabilize poles I would think.
I spend a fair amount of time around antennas (and around tents when camping), and it's usually a good idea to somehow make the guy wires stand out visually whenever people can be walking around them. In your case, you would want to do something that makes them pretty easy to see, but not an eyesore (since the aesthetics of your pool shade covering is important). Maybe something as simple as painting them white will make them stand out (in a subtle way) from the darker grass and fence backgrounds.

With antenna guy wires, we often tie ribbons at leg height and head height, to help folks who are walking near the structures from tripping on the wires (or worse). With some tent guy wires in areas where there are a lot of kids playing, we sometimes use the colored pool flotation noodle things strung onto the wires. That's probably not going to work for you, unless maybe you have a big pool party with lots of kids running around. For a party like that, it might be good to add the noodles to the wires for some extra safety.

Have fun by the pool :smile:


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Ideally you want guy-wires to be 90-degrees to posts. That would make for some long runs in this case. If the posts were 45-degrees outwards, then wires would be 45-degrees in other direction.

One calculation you need to do is measure load based upon average wind-speeds. Can probably find that from historical weather reports.
 

DaveC426913

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...it's usually a good idea to somehow make the guy wires stand out visually whenever people can be walking around them.
Yeah. I was thinking about this.

If there's a marine supply place near you, they sell sheaths for rigging on sailboats. They come in various lengths and diameters for different wire gauges. They're slotted, so you just slip them on. Your guy wires would be visible yet elegant.
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Alas, they only come in white.
 

sophiecentaur

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If there's a marine supply place near you, they sell sheaths for rigging on sailboats.
The idea is great but using a marine chandler's supplier is an expensive solution. I used lengths of polythene 'overflow' plumbing pipe on rigging that needed to be sheathed. The cladding that's sold for rigging does have a split running along it, so that it can be easily fitted to standing rigging but the proposed structure could allow the guys to be disconnected in order to pass the terminations to pass through. Save £££ or dollars and avoid the label "Marine" wherever possible.

But I have a comment about the proposed basic construction method. Ships' masts, which are subjected to enormous loads, are nearly always fitted with an 'articulated' foot, rather than being rigidly mounted in the hull. They are allowed to move to relieve stresses so that they are largely in compression. This requires a structure with stays but the spars can be pretty light. Two stays pulling the spars 'outwards' and light stays pulling the tops of the spars together for when the sails are taken off. Attaching the bottom of each spar with an eye and pin would mean that the spar could be taken down when needed. Sinking in concrete is a terribly final act! Most high transmitting masts have a pivoted base too - for good reasons.

My 10m (taller than a sunshade) mast was light enough for one person to lift it (just) and it supported 400 sqft total of sail area - trimmed for maximum power at sea and the sails were very tough polyester fabric.

A great idea to have sail sun blinds!
 

DaveC426913

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The idea is great but using a marine chandler's supplier is an expensive solution.
Someone once asked me if the line I used could be bought at Home Depot.
I said "No! This is marine line. Don't you know the difference?"
"No? What?"
"Marine line is three times the cost."
 
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