Maximum Horizontal tension on a 10’ steel post

[Brian1025]

I’m helping my dad to put up a metal post that needs to be a minimum of 10’ high. The post will have two wires (thin cables) attached to the top at 90 degrees from each other. The wires will stretch approximately 27’ where they will be anchored. He’d like them relatively tight but a slight sag is ok. We can’t find a long enough steel or galvanized pipe that can reach 10’ high and have about 4’cemented in a form tube. Can we use a 1.5”W plumbers pipe or similar galvanized pipe that is 10’ long and connected to each other for a total of 20’ which will be cut down to size accordingly? Will the horizontal tension cause the pipe to fail? Any suggestions are greatly appreciated.

 

[r13]

That's the benefit of asking engineers, instead of the engineer.

 

[dhengr]

Brian1025:
Just a few quick thoughts on your problem…,
1.) If digging the hole was the easy part, that probably means you don’t have very good soil, near grade, for resisting lateral loads on your pole foundation. Therefore, dig deeper and for a larger sono-tube. If the soil will stand on its own and you pour the conc. right away, you might be able to forget the sono-tube and just carefully fill the hole with conc., without mixing a lot of falling soil in with it. Puddle the conc. while you are filling the hole, to remove voids. If you need the sono-tube because of loose soil, then you want to compact well around it, after filling, because your found. depends on being able to push against firm, well compacted, soil top & bot. to prevent it from leaning too much. Draw a side view of this and you’ll imagine that the conc. pushes on one side at the top and the opposite side of the hole at the bot. as the pole tries to lean over. You might actually set the sono-tube and your pole a few degrees out of plumb, away from the direction it will be pulled by the cables, because it will likely move laterally some amount. This is your judgement call, with the little info. you’ve given about the whole problem. Generally, a 4’ sono-tube will work in a 6’ deep hole. Fill conc. 2’ deep and lift the tube a foot; fill another 18” and lift the tube to a few inches above grade, and finish filling. Slope the top to drain away from the pipe.
2.) You could just as likely pull the facia trim off of your house if you are not careful about how you do your attachments at the house.
3.) The real problem is that stretched cables like these exert a very high tension force (horiz. force) at their attachments, for very little gravity load on the cable, or as you try to take/stretch all of the sag out of the cable. Google “cable stresses” or “catenary cable” to learn more. Under its own weight or a flexible uniform load the cable will take the catenary shape. Concentrated gravity loads cause a different cable shape, forces and stresses, but your sting of lights will cause a catenary. You can test this high horiz. force issue by stretching a rope horizontally from two fixed points, with a spring scale at one end. While you may only weigh 200lbs., when you hang on the rope, the spring scale will show a force many times your 200lbs. You can pull a car up an incline with the same type of rope application. With the rope fixed to a tree and the car bumper, a relatively small lateral load on the rope, and some lateral movement of the rope (the sag) you will pull the car. Otherwise, we really can’t answer your question, except in generalities, since you’ve given so little engineering info. needed to really address the whole problem.
4.) We do generally try to be helpful. If you had read carefully when you signed into E-Tips, I think they explain that this is a site for engineers to interact with other engineers. It is not a site for free engineering on DIY’er. projects. You can probably imagine much of the above, you just don’t know how to put real numbers on it. And, we can’t see it from here, nor can we design it for you, from here. BA’s red cable configuration is intended to remove the concentrated load from the 45’ long cable, it is a real killer, see 3.) above. More sag in your light strings means less lateral load at the top of your pipe pole.

 

[BAretired]

Scheme 1 and 2 are shown below:

The load w in both schemes was assumed to be 0.4 plf for the weight of wire plus lights. No consideration was given to ice loading or wind loading. All tension values are approximate.

In Scheme 1, wire BD and BE are attached to the midpoint of wire ABC. Tension values are shown for all wires assuming a sag of 6". Several values are shown for wire ABC depending on the horizontal offset of Point B.

In Scheme 2, all tension values are shown assuming a sag of 6". Wire ABC supports only its own weight. The length of wires AE and CD are estimated.

BA

 

[Brian1025]

BA..thank you that makes sense
Oldestguy... I hope to avoid needing a brace but if I have to I will.
Retired13... this is for all intents permanent. The horizontal line is 45’ not 90.
PhamEng....I expect a little bit of sag more than 6” as I earlier stated and no trapeze artists or knuckles head friends will be swinging from this.
Rabbit12... most definitely no life threatening situation here. I weeded out my Darwin buddies a long time ago. Plus with all this added expertise I should not need to replace this pole. I’ll do it right the first time.. one and done.
Dhengr... thank you for your well thought out response! I seriously think I got lucky digging this hole. Only one candlepin bowling ball sized rock an no roots from my neighbors large tree. Still I will go the extra foot deep I’m almost at the point I can’t pinch the posthole digger’s handles apart anymore. Yes the facia is a concern. I’m confident that I will be mounting an eye bolt anchor into some very sturdy thick wood. This house was built in 1880 and they spared nothing when it comes to construction. I’ve got lots of YouTube and reading RE: catenary wire. A lot of good material just came up. I can visualize this and correct, I don’t know how to apply the exact math but I have a MUCH better idea now.
BA...my takeaway from the 2 schemes you were kind enough to lay out is that scheme 1 line ABC needs to have a lot more sag in the center to reduce the tension enough so that wires won’t snap or pull the post over.
Thank you all again

 

[IFRs]

There may be an advantage to a system that is not super strong - if a random person gets hung up on the wire while (drunk and) running through your yard avoiding a dog (etc) they will get less hurt if the system collapses instead of killing him. Not sure where you are but even if the wires are up high, a combination of a tall person that can jump, sag over time, perhaps snow buildup on the ground, etc... Also, your local authorities, insurance (especially insurance), neighbors, etc may have some reasonable objection or suggestions to this construction. I presume you are using outdoor certified lights and wire, GFCI, etc.

 

[BAretired]

BA...my takeaway from the 2 schemes you were kind enough to lay out is that scheme 1 line ABC needs to have a lot more sag in the center to reduce the tension enough so that wires won’t snap or pull the post over.

Just to be clear, sag implies downward motion. I suspect you knew that. Point B needs to move horizontally in order to resist the two 50# forces from wires BD and BE. Because of the upward slope of those wires, Point B actually moves up slightly.

BA