We are looking to use 4" square tubing to hold up some temporary light fixtures 'till we get our main poles up. How high can I lift these fixtures before I risk failure of the tubing (i.e. bending / collapse).
Assumptions / Data:
I'm told the tubing walls are 1/4" thick & the tubing is mild steel. The light fixtures weigh 50 pounds each. At least one light fixture per pole, but prefer three. The surface area of the fixture for catching wind is about 12 ft^2. Assume 40 mph wind maximum. Assume also that the 50 pounds would be distributed about 1 foot away from the pole.
Any mechanical engineers out there got a short cut for doing these calculations?
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07:07 PM
PFF
System Bot
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
Oops... Better add that for calculating the bending, assume that the 50 pound fixtures will be held 1 foot away from the pole. Of course the poles are standing vertically.
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07:09 PM
84fiero123 Member
Posts: 29950 From: farmington, maine usa Registered: Oct 2004
You got lucky, I just had to call for steel prices for a couple of projects I have in the works. Engine crane and bumpers. 4” square tubing goes for $196.86 for a 24’ length. Delivery to my place on Monday or Wednesday.
How you are anchoring these can affect the stability of the posts. Welding a plate on the base then bolting to concrete. Burying in the ground 4’. Also you have to take into account bracing on the top below the weight point if you are coming out a ft. away from the poles.
If these are just temps for telephone poles that will replace them unless you are a steel contractor and have these on hand I would just wait. You are spending a lot of money for a temporary pole.
As far as how high you can go before they will bend there are a lot more factors than just weight that you didn’t mention or allow for.
The size of the light area that will catch wind. The pole may hold at 12’ in a 20 MHP wind, but not 50 MPH wind.
Sorry I’m not an engineer so I can’t give you a real answer other than to say, wait for the light poles themselves.
Edit to add sorry I didn’t see the area in the original ------------------ Technology is great when it works, and one big pain in the ass when it doesn't. Detroit iron rules all the rest are just toys.
[This message has been edited by 84fiero123 (edited 01-29-2008).]
Steve makes some very valid points, as we should expect!! He knows steel inside and out I have a couple of questions, over and above those Steve asks: How high do you NEED them to be? Why square tubing? Round is a better bet, in most instances. What WILL the final supports actually be?? What is the interference footprint of these poles? In other words, would it not be possible to use guy cables, to give better stability? And (three x50lbs) on 4x4 inch 1/4 inch profile is not going to be safe at any kind of height. Give us some more information!! Nick
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09:30 PM
84fiero123 Member
Posts: 29950 From: farmington, maine usa Registered: Oct 2004
Hey Nick thanks for catching what I forgot. I think my memory is getting worse, instead of better.
I can’t remember did you give me any input on my air tank bumper idea?
I was beginning to think you got lost or something. You got to stop letting Ellie leave you alone. Hey I mean Zwerver and you alone just get into to much trouble.
Steve
------------------ Technology is great when it works, and one big pain in the ass when it doesn't. Detroit iron rules all the rest are just toys.
[This message has been edited by 84fiero123 (edited 01-29-2008).]
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11:27 PM
Jan 30th, 2008
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
4” square tubing goes for $196.86 for a 24’ length. You are spending a lot of money for a temporary pole.
Fortunately, I've got 3 x 15+ feet sections being donated that were headed for the scrap yard.
quote
Originally posted by 84fiero123:
How you are anchoring these can affect the stability of the posts.
Current plan is to bury in the ground (rule of thumb about 10% of the height) & cement them in.
quote
Originally posted by 84fiero123:
Also you have to take into account bracing on the top below the weight point if you are coming out a ft. away from the poles.
Weight per fixture counts the bracing as well. The main weight is the integral balast which is fairly tight to the pole. Estimating the weight at 1 foot out is safe.
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12:05 AM
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
These are 1500 watt, integral balast fixtures. A mixture between Musco & Hubbel.
Right now, I've got four 50' class 3, creasol treated poles erected holding up 8 fixtures each running down the center of 10-12 acres of land. Can't afford the copper to hook them up right now (about 16 grand counting the breaker boxes, contactors, etc), so we have 3 fixtures temporarily hooked up on the top of a hill that is already about 12' above the surface level pointing at about 1/8 of this land. I have several more poles, but it will be a month or more before we erect these. (We'll put about 35' poles on this hill.)
Right now, these three fixtures are about at chest level. It would be desireable to get them about 12-15 foot higher if possible. With the availability of the steel at no cost, it is tempting to lift them up a bit more but not at the cost of safety. That's why I'm looking for other opinions on this thought.
It is also tempting to use just basic cantilever calculations, but since most of the force is really going to be in compression I don't think they will apply. Only the moment of weight along with the wind drag can be applied using this technique. My gut instinct is that if they have any tendency to bend at all, they are going to fold over like a piece of paper - especially since it is square tubing.
Obviously, the force is going to be directly proportional to the square of the height (at least I think). I'm a ChE by training & a research chemist / mechanical hack by trade. That's why I'm checking to see if we have any ME's on this forum that can lend some guidance.
[This message has been edited by RTNmsds (edited 01-30-2008).]
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12:18 AM
84fiero123 Member
Posts: 29950 From: farmington, maine usa Registered: Oct 2004
As Nick asked can you guy wire the posts to the ground in 3 directions? That would give you a extra level of safety.
I was going to ask how you are going to dig holes in these temps then I saw you are in South Carolina, I thought was South Dakota for some reason.
If the steel posts are freebies and you can get away with just one place, mounting post. Have you considered a triangle mount of all three posts for the lights?
How do I explain this, putting all three posts in the ground near each other and mounting the lights on a bar mounted to all three. That way you could go higher without guy wiring each post. Then the 10’ height would be manageable/safe.
Sorry I can’t be of more help.
Where the hell is Wade when we need him?
------------------ Technology is great when it works, and one big pain in the ass when it doesn't. Detroit iron rules all the rest are just toys.
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07:15 AM
cliffw Member
Posts: 37837 From: Bandera, Texas, USA Registered: Jun 2003
I am no mechanical engineer but the below calculation is dead wrong.
quote
Originally posted by RTNmsds: Current plan is to bury in the ground (rule of thumb about 10% of the height) & cement them in.
You expect to bury a 15 foot pole with weight at the top just a foot and a half deep, . I bury a six foot fence post a foot and a half to two foot deep. Even if it was a chain link fence with no wind load. I also cement it in the ground. I do not build fences either. I drill oil wells and we have many such lights mounted on 2"square metal tubing ten foot tall. No more than 1/4" thick walls if that (we do tend to overbuild though). They are not buried in the ground but inserted into sockets which are welded in place. The sockets are less than a foot deep. Your concern is with the anchoring. My guestimate is bury the pole at least three foot into the ground in a large diameter hole filled with cement. You will be good to go.
quote
Originally posted by RTNmsds: Fortunately, I've got 3 x 15+ feet sections being donated that were headed for the scrap yard.
, who scraps such valuable metal ? I can often get stuff like that given to me (oil field perks) but it is not headed to the scrap yard.
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07:20 AM
84fiero123 Member
Posts: 29950 From: farmington, maine usa Registered: Oct 2004
Originally posted by cliffw: You expect to bury a 15 foot pole with weight at the top just a foot and a half deep, . I bury a six foot fence post a foot and a half to two foot deep. Even if it was a chain link fence with no wind load. I also cement it in the ground. I do not build fences either .
I miss or forget things to easy. I got to admit that I missed the depth being only a 1.5’ deep. You are right Cliff, I bury my fence posts 3’ deep for line posts and 4’ deep for corner posts on 8’ posts and all I’m putting up is cattle fencing.
If you have the equipment to go that deep, go at least a third of the height of the posts and that will give you a lot more stability.
------------------ Technology is great when it works, and one big pain in the ass when it doesn't. Detroit iron rules all the rest are just toys.
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09:29 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
yes, for the base - I assumed the base was already installed - being these are temporary - until the "right" ones arrive. and, most light poles I see, the base is poured, and a bolt plate is mounted on top. the 1/4" 4x4 square is fine for holding up the lights @ 15'
worry about your base - the 4x4 steel is just fine. in fact - it's probably stronger than the actual light pole.
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09:41 AM
PFF
System Bot
84fiero123 Member
Posts: 29950 From: farmington, maine usa Registered: Oct 2004
yes, for the base - I assumed the base was already installed - being these are temporary - until the "right" ones arrive. and, most light poles I see, the base is poured, and a bolt plate is mounted on top. the 1/4" 4x4 square is fine for holding up the lights @ 15'
worry about your base - the 4x4 steel is just fine. in fact - it's probably stronger than the actual light pole.
I just came back from my buddies after giving him a spare respirator. He is an electrician who has done stadium lights before and mentioned something none of us have thought of. Most stadium lights are only 4” aluminum and they are 15’ tall.
When I was driving home I looked and noticed just what he said. All the parking lots I drove by had lights, some just 1 some with 2 opposite each other.
Most just with a base anchored in 3’ above ground concrete post.
So maybe in retrospect, maybe we are being over cautious of the bending factor.
Where the hell is Wade when you need him?
So why could you burry a 12” or better sauna tube, fill it witrh concreat and a colple of rods going up into the square tube a 1’,
------------------ Technology is great when it works, and one big pain in the ass when it doesn't. Detroit iron rules all the rest are just toys.
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11:41 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
This is probably overkill, but I was thinking about a four or five foot ground penetration with about six inches of concrete around the sides. You're on the West end of the state with more rock than we have here at the beach. We have sand and pine cones to build our stuff with. A fifty pound weight on the top of a fifteen foot pole will create a good bit of torque all on it's own if the lights kinda hang out to the side. Add wind to that and it just makes the torque greater. If the pole fails at all, it's bend it the base or try to tip the pole and concrete over together if the ground around it's not compacted right. That's the only place where you'll have any opposing forces.
------------------ Whade' "The Duck Formerly Known As Wade" Duck '87 GT Auto '88 Ferrario '84 Indy
I didn´t respond any more, because the thought of 150 lbs being supported on 4 inch square tubing is asking for trouble. Even ONE 50lb'er is too much, IMHO. Rule of theumb should be 35% buried in AT LEAST third cu yard of cement, with cross-members welded to the bit that is encased in the cement., in firm ground. That is for 1 50 lb'er. Just imagine this...somebody weighing 150 lbs, swinging on the top of a tube 15 ft high..it would bend, and crease, as Whade says, at a point some 18 inches above the ground surface Nick
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04:17 PM
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
As Nick asked can you guy wire the posts to the ground in 3 directions? That would give you a extra level of safety.
Guide wires could present a trip hazard, but....
quote
Originally posted by 84fiero123:
If the steel posts are freebies and you can get away with just one place, mounting post. Have you considered a triangle mount of all three posts for the lights?
How do I explain this, putting all three posts in the ground near each other and mounting the lights on a bar mounted to all three. That way you could go higher without guy wiring each post. Then the 10’ height would be manageable/safe.
Simple & elequent. I could even mount them like a highway sign & use one the main light brackets across the top to hold the bank of lights.
Good idea!! Thanks.
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04:32 PM
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
, who scraps such valuable metal ? I can often get stuff like that given to me (oil field perks) but it is not headed to the scrap yard.
Businesses that make good money. Serving kids & having a 501(c)3 registration often makes the decision easier for these guys to not think too hard about recovering 4-5 hundred dollars versus just writing it off.
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04:38 PM
RTNmsds Member
Posts: 1104 From: Woodruff, SC Registered: Oct 2003
Rule of thumb should be 35% buried in AT LEAST third cu yard of cement....
I think you might be shocked to know how "deep" (or not) a 40' telephone pole is buried in the ground. I'll make some quick measures when I'm at the complex next time, but I'm pretty sure my salvaged 40-50 footers (which are now 35-40' because the tops got cut off) were only about 5 feet in the ground originally (without being cemented).
Wade is also very correct with respect to the soil here (or lack thereof). Took a looong time to get through the rock for the last four poles with a commercial pole truck / auger. That thing was smoking.
Thanks for everyone's comments. Helps to have a few friends to bounce opinions back & forth with. I've got a solid plan now. We'll be safe & I'm confident on the success we'll have.
Now if I can just get that friend out with his truck again... I can by-pass all this temporary stuff
I have a couple of questions, over and above those Steve asks:
. I bury a six foot fence post a foot and a half to two foot deep. Even if it was a chain link fence with no wind load. I also cement it in the ground.
, who scraps such valuable metal ? I can often get stuff like that given to me (oil field perks) but it is not headed to the scrap yard.
A fifty pound weight on the top of a fifteen foot pole will create a good bit of torque all on it's own if the lights kinda hang out to the side. Add wind to that and it just makes the torque greater. If the pole fails at all, it's bend it the base or try to tip the pole and concrete over together if the ground around it's not compacted right. That's the only place where you'll have any opposing forces. 
