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.

 

[jimstructures]

A sketch of the plan view and several orthogonal elevations would really help get appropriate responses.

Jim

 

[Lomarandil]

The question all hinges around what type of wires will be attached, and where you are geographically. Without running numbers, a small diameter galvanized pipe may be sufficient for something like radio antenna, but probably not an insulated power line.

As the weight of the wire increases, you will require additional tension to keep the sag to an acceptable limit. Because the two tensions are orthogonal to each other, they each pull against the post (rather than pulling against each other).

Assuming this is a "permanent" installation, the wire diameter is also important. Depending on your locale, the wire will also need to resist wind, or may accumulate ice during the winter.

The other trick with such a slender pipe will be that the high degree of flexibility may lead to wind-driven vibrations (aeolian vibration is the proper term).

Fill in some details for us. My gut feel is that 1.5" pipe isn't going to work, but that larger pipe might.

----
just call me Lo.

 

[oldestguy]

Say what this is for if you want reasonable answers How's come you can't get pipe 20 feet long? That's pretty much standard.. With any reasonable pull and a non-sag wire, you need foundation. That might be a concrete cylinder one foot in diameter, maybe 6 feet deep, depending on loads. So far it sounds like a ham radio antenna, right? By the way they don't like personal projects here.]

 

[Brian1025]

I didn’t know a personal project was discouraged here. This is NOT school work. This site came up on a search as ‘ask an engineer’ I registered as a student.
I am in Boston. This steel post will be used to string lights and will be permanent. The wire is 1/16 coated = 3/32 also rated for 480 lbs. Total weight of all lights 16 lbs. I have no way to transport a 20’ pipe. The pipe will be 4 feet in the ground using a form tube, either 6” or 8” wide. I am totally open to changing pipe size, pipe type, form size and form depth. I can also change wire easily if that is needed.
Thank you Jim , Lo and oldestguy! I think I’ve addressed each concern. I did my best best with the drawings.

 

[Brian1025]

Another drawing

 

[phamENG]

Brian - still not clear what this is doing.

Personal projects/DIYs/etc. are discouraged. This site is for asking engineers - but more specifically it's for engineers to ask other engineers and to hold professional, technical discussions. If this is a project that might need an engineer, you need to hire one to look at your project. If this is just a post to string some garden lights or something similar - you're probably ok, but put some guy wires back down into the ground on the opposite side of the lines going out to support the lights.

 

[BAretired]

Just a preliminary thought, would you consider eliminating the 45' line and using the dashed red lines instead? I think they would be a little easier to control the sag.

BA

 

[BAretired]

Or alternatively, include the 45' wire but do not support wires at midpoint.

BA

 

[oldestguy]

It all depends on not allowing any sag. If none, very unlikely. I'd go for a 3" I.D. standard weight pipe filled with concrete mortar and set in a one foot diameter hole filled with concrete. Depth at least 4 feet, prefer more. Some sag is bound to occur, but live with it. For wire get wire for a cloths line so it can carry some load. Allow at least 6" for sag. No design needed.

 

[Brian1025]

Yes this is a personal project. I thought I explained that. If you want me to leave I will. If I get my help I will thank the group and be gone. I arrived here via google search for ‘ask an engineer. I bragged to my dad that I could get the answer.

I am open to changing the design to the red dotted line. Not clear how that would help with sag or reduce tension on the free standing post which is what this is all about. The post is on the property line so guidelines in the opposite direction are not an option.

 

[Brian1025]

3” pipe filled with concrete sounds ideal. I thought a 1.5” pipe was too weak.
Thank you

 

[phamENG]

My biggest concern is with the foundation. What are you doing with these wires? What are they supporting? If this was explained and I missed it, I apologize.

If this needs to be tight with relatively no sag, or sagging some with any kind of vertical load, your post is going to tip over. Even with 4' in the ground, with a lateral load at 10' it's not going to perform very well. You should definitely consider guy wires for the other side of your post.

 

[oldestguy]

As to posting guide, I'd stay here. Just remember next time. The reason fr the concrete is twofold. It increases the bending strength. It also keeps rain pretty much out so interior rust is less likely. A mix of mortar fr brick work will do. Just get all the air out in the process of filling. Try to vibrate it in and don't use any more water than necessary. A lot of water in the mix will cause shrinkage later.

 

[Brian1025]

PhamEng... the wires will be used to support string lights. Total weight of lights no more than 16 lbs. Unfortunately guidelines are not an option since this is located on the property line. A bit of sag is expected I’m thinking it won’t be more than 6” in the center. Also it will be pulled sideways at the center where another wire will be connected to the house in a V form. A fair amount of sag is ok here since the points on the house are 18’ high.
Oldest guy .. Thanks I won’t delete this. Most definitely a cement filled 3” galvanized pipe sunk 4’ in the ground with a form tube is the best. I can even go 5’ into the ground. Digging the hole was the easiest part. I can’t imagine that it could fail or the entire thing tip over but that’s precisely why I’m seeking help from experts.

 

[BAretired]

The force applied to each wire varies with weight of wire plus weight of lights and permitted sag. In the case of the 45' length, assuming a total weight of 0.4 plf and a sag of 6", the tension will be approximately 0.4*45^2/(8*0.5) = 203# (assuming no point loads at midspan). The vertical load at each end is only 9#.

If the original scheme is used, there would be two point loads acting at midspan, each approximately 50#. Assuming they are at 45 degrees to the 45' long wire (in plan), the perpendicular point load is about 70# horizontal. If the horizontal deflection is limited to 6", same as the vertical deflection, this would add a horizontal component of about 70*45/(4*.5) = 1575# which, together with the value above, would put a tension of about 1590# in the 45' long wire which would need to be anchored at the top of the post and corner of the garage.

BA

 

[oldestguy]

If for some reason the support pipe leans, you can go inside with a knee brace, founded on a concrete block, say about the size of a 5 gal bucket. .Secure a bracket to the pipe with a few hose clamps. A weld might be better.

 

[r13]

Is this a permanent setup, or seasonal? Is the horizontal line 45' long, or 90'? I think two posts setup is more doable.

 

[phamENG]

Ok. Thought so. Just realize that the more you pull the wire tight, the more the horizontal load you'll have (centenary statics aren't always intuitive the first time you look at it) and the more your post will want to lean over on you. Keep a little sag in it, don't invite any trapeze artists to your backyard barbecues, and you should fine.

 

[Rabbit12]

What's the worst case here? The pole bends and the wires/lights sag a little more than the OP likes? He then goes back and puts in a heavier duty pole. I don't see a serious life/safety risk here using these as the OP is proposing. Could someone get seriously hurt, I suppose, and they'd also likely win a Darwin.

When you go to tension your cable you'll know when there is too much as you'll start to deflect your poles.