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| Antenna mast made from SCH40 steel pipe | |||
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| Posted by: rtell ® 03/14/2007, 13:28:44 Author Profile eMail author Edit |
I am trying to analyze the forces on a simple pipe mast for holding a ham radio antenna. The mast will be made from 2.5" SCH40 steel pipe. My problem is as follows: 30 foot long pipe is oriented vertically and fixed at base and with bracket to side of house at 7 feet above base (23 ft extends above bracket with no guying). Load on mast is due to force of wind distributed uniformly along mast, assume 70 mph max wind, and lateral load of wind on antenna computed to be 30 pounds in 70 mph wind located at very top of mast. Question: After I calculate the bending moment at the house bracket, what is correct way to estimate the "pull out" force at the bracket? I simply took the total moment at the bracket and divided it by 7 ft to get what I think is the force that would be exerted away from the bracket if the wind is blowing in the right direction for maximum force. My concern is bending of the pipe mast as well as pull-out strength of the lag bolts that secure the bracket to the house. Based on info found in the Wood Engineering Handbook, I calculated the pull out strength of 3"x3/8" lag screws with 2.25" of penetration into Doug Fir (specific gravity = 0.46) as 2725 pounds. Does this sound reasonable or do I have a fatal flaw in my analysis? Finally, since one cannot get 30 foot lengths of SCH40 pipe, how can I calculate the reduction in maximum bending moment for 2.5" pipe if I insert a threaded coupler on the end of a 21 foot joint and add 9 more feet? I assume that the insertion of the coupler will decrease the maximum bending moment of the total assembly but I can't find any guidance on how to calculate or estimate this reduction. Or, is there any reduction in reality for SCH40 type couplings? Thanks for any critique. My background is in electromagnetics, not mechanical engineering. ...Ric |
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| Re: Antenna mast made from SCH40 steel pipe | |||
| Re: Antenna mast made from SCH40 steel pipe -- rtell | Post Reply | Top of thread | Forum |
| Posted by: Rosen1 ® 04/03/2007, 14:06:07 Author Profile eMail author Edit |
Ric, Imagine as you will, a pencil clamped in a bench vice. If you push on the free end, where will it break? Yes, at the bottom. I note that you plan to support the pipe at the 7' mark. If the anchor and the ground and at the 7' mark is strong enough to hold while the pipe bends, then the effective length of the pipe is 23' for bending moment, and the coupling would receive 9/23rds of what is present at the 7' point. I don't know about your coupling, but I believe your weakest point will still be at 7'. Now with regard to anchoring the 7' mark with your house, it is not just a pull that you should be concerned with (which would be as you calculated: 1/7th of the moment), but consider that it must support that pole all 360 degrees. Therefore, if this anchor is in the middle of a room wall, consider the amount of flex that will be induced; it could easily crack your sheetrock, or if construction did not anticipate these new forces, it could rip out studs and siding with it. If you can spread the impact on your wall across many studs, then the load becomes more distributed.
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| Posted by: swearingen ® 03/16/2007, 10:05:51 Author Profile eMail author Edit |
You are correct in saying that the bending moment at the house bracket is the maximum the pipe will see. Combine that with your pipe section modulus to see where your stresses are. It needs to be quite a bit lower than yield because gust dynamics will probably control the design. As for your house support, the way to calculate the loads is to sum moments about the base: Sum of Moments = 0 = (30lb)(30ft) + (mast force)(23/2 + 7) - R(7ft) Where R is the reaction at the house and mast force is the force of the wind on the mast on the upper 23ft (assuming the lower 7' is shielded). Also, I think your calculated load on the antenna looks a little low. If it's one of those fold out rib-looking antennas, I'd say you're looking at 50lbs or so at that height for 70mph. Now, about your lag screws: it's great that the screws can hold that much load (it seems reasonable), but can what you're attaching them to hold the load? Are you lagging to the soffet? Where are the nearest nails? Essentially, you must chase the load into the ground. The load goes from the lag screws to the wood, to the nails nailing the wood to the rafters/header/soffet, to the rafters/header/soffet, to the nails holding those down, and so on down through the walls into the slab. Keep in mind the load will follow the stiffest path and every link in that chain must be strong enough. Don't forget that in a strong storm this thing will be whipping around, so the support system needs to be VERY strong to deal with the dynamic issues. I really wouldn't worry about your splice, but considering your setup, the best place to put the splice would be high up, i.e., the 21' section is fixed to the ground and the house and the 9' section is on top holding the antenna. The moment is much lower than than if you flip it the other way around. |
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| Posted by: rtell ® 03/19/2007, 13:06:13 Author Profile eMail author Edit |
Thank you very much for your reply to my questions. The house bracket would be lag screwed into 2x6 studs that from the outer wall of the house though cedar siding. The bracket uses two lag screws into each of two studs and is made from heavy gauge steel (1/2 inch with big gussets to the arm that extends from the base plate of the bracket). I think I follow your explanation now. Thanks for the help. ...Ric |
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