Black Pipe / Flange Strength

Howdy, All:

I'm new here but thought you guys/gals in the trade might be able to help me out with a non-plumbing project I am working on.

I am building a back yard horizontal bar (think one gymnastics bar) for my daughter to break her neck on. I'm wondering about the strength of my design and thought I'd ask for your input.

My design: I will cement two ten foot 4"x6" PT posts in the ground 4 feet deep (I live in Maine and this is a good frost depth). I will then secure an 8 foot section of 1 1/2inch black pipe (think steel pipe) to these posts by attaching a flange to each threaded end of the pipe. The flange has four holes for screws. Instead of using screws, I will drill through the 4"x6" and use carraige bolts to through-bolt the flange/pipe to the posts at either end of the pipe.

My concerns: The flange only seems to thread onto the black pipe about 1/2 way through the flange (approximately 1/4" - 3/8" into the fitting). Is this enough to hold over the long haul? No matter how much tension I put on the fitting to get it to screw on further, it will not budge. Also, the four holes in the flange that I will attach by through-bolting into the 4"x6" posts will only accept a carraige bolt that is 5/16". I've chosen galvanized carraige bolts in this diameter. Will four at either end have sufficient shear strenght to keep the pipe from breaking free?

Thanks much for any insight or additional ideas you can offer -- either about the design or the strength of the attachments.

Tim

For your kid, sure it will work. Would I do the same for my daughter not a chance.

Seriously, overbuild the thing, don't use parts that 'should' or 'might' work. 8' spread for serious flex and tensions at all angles with 1/4" bar set into an endcap held in place with 5/16" carriage bolts. Sounds like a disaster waiting to happen. Hope the health insurance is up to date.

Why black and not galvanized for outdoor?

I would overbuild at no extra cost by drilling holes through the posts and using flanges on the outside, not the inside. Gives virtually no shear load on the 4 flange bolts/screws at each end; the only shear force s/b rotational from gripping the bar to start/stop rotating around it.

If your daughter is the size of most gymnasts, the forces will not be too great.

Thanks for the replies.

I too thought it would be good to use the flanges on the outside of the posts -- drill through the post and attach flanges to outside. Unfortunately, the only flanges available screw on so the flat edge is facing outward (i.e. can only attach to the inside of post, not outside). Are there others available that could screw on to the pipe and lay flat against the outside of the post if the pipe is let through the post?

I have come up with an alternate idea -- I will have the flanges or a 1/4" steel plate welded on the end of the pipe, then will bolt through the post with a heavier, stainless steel carraige bolt. That way there shouldn't be any post twisting and the shear strength issue is eliminated.

What are your thoughts on this re-design?

Thanks again, Tim

Iron pipe threads are tapered, so you would not expect it to bottom out like a bolt into a nut.

Black iron pipe will rust, and galvanized isn't too smooth, so neither will be great long-term. I'd prefer maybe SS.

Once you have the flange bolted down, you may be able to get another turn or two past hand tight. But, using a pipe wrench will score the black pipe, and could compromise the galvanized layer in the process.

Depending on your soil, swinging back and forth will put a lot of stress on those poles and even with the concrete, you may have the assembly moving. You may need guy lines, or diagonal supports. If you used something like that, you might not need to cement the poles in, which may allow them to last longer.

Thanks, jadnashua.

I went with iron/black steel pipe for the smoothness. I have primed it and put about 8 coats of a really good exterior industrial grade spray paint on it. It's really smooth and should hold up long enough for her to get some good use out of it before she outgrow's it.

I'll see how the cementing in works before I go with a guy wire system. I've done some research and it shouldn't be too difficult to retrofit.

Thanks again for everyone's advice.

Tim

You could forget the flanges, and drill through the posts & pipe and run a carriage bolt through each end.

Or tap out the flange.

Or countersink the outside of the post to accept the protrusion of the "top" of the flange.

pipe

Use a stainless steel closet hanger rod. Drill it through the posts, (If you want to you can thread it and put pipe caps to seal the ends. Then drill through the post and pipe laterally and use bolts to keep it from rotating.

Remember that stainless bolts 'grades' are a step down from mild steel bolts.
For example: A grade 5 SS bolt is the same as a grade 3 mild bolt. If you want shear strength, you need to have a good alloy type bolt that will also be weatherproof.

We use cast/forged alloys in shackles when im out boilermaking.

For the 1.5†iron pile (actually steel pipe) over an 8’ span you will have a center deflection of about 0.3in in center if you hang 200lb from the center. This results in a max bending stress in the pipe of approx. 15,000 psi, which still gives you a safety factor of about 2. So I would say the 1.5†pipe is sufficient unless you daughter tips the scale at over 400lb.

5/16 grade 2 carriage bolts are more than adequate for the shear loads, one bolt will carry 1700lb in shear, so if you are using 4 you could safely step down to ¼†bolts.

You only mention 4x6s for the vertical posts, but what about the horizontal? Either way, to capture the steel pipe drill 2in holes in the center of a 2x6 or 2x8 for the pipe to go into. Then nail/bolt another 2x6 or 2x8 w/o holes on the outside of the one with holes to capture the pipe. If you do this on both sides no fasteners are needed to secure the pipe. The only down side is that the pipes can rotate in holes.

Thanks, all.

I think, with your help, I have finalized my design.

I am having the flanges welded onto the pipe after screwing them on as far as they will go. This should alleviate any twisting of the pipe once the flanges are secured to the posts.

I will then through-bolt the flanges to the posts using the largest carraige bolt that will fit through the flange openings. For added strength and safety, I will drill a hole through the post and insert the largest steel pipe I can fit through the hole and into the horizontal main pipe. This will allow the second, inserted pipe to rest on the post, hopefully preventing any catastrophic failure if the welded flange does fail.

I plan on initially just cementing the posts 4 feet in the ground. I've used this method in the past for swings, etc and it seems to hold up over the long haul with little or no wiggle. Could be the heavy clay soil I have in my yard!

Anyways, thank you all for your input. I gained some great knowledge and worked out a couple of bugs in my original design.

Tim

Depends on how fat the kid is

When the kid tires or it, the wife can always use it to hang laundry

Probably done like this in a million back yards across the country. But if you want to know the weak spot....it is the threads on the pipe. Threading creates an inherent mechanical weakness due to the pipe being thinned at that spot. Still plenty strong for residential gas or water pressure applications. But if there is any lateral leverage against that thread joint..that is the weak spot.

Thanks, Jimbo. That is why I'm having the flanges welded onto the pipe. Should eliminate the weakened area by the threads.

If you are going to have it welded, don't thread the pipe (don't worry if it already is, though), and drill out the flange so it will fit through, then weld it. Full thickness of the pipe will eliminate that potential weak point. And, since the flange would slide on the pipe, you could just have them weld it with a stub sticking out the back of the flange into your upright. Then, the flange would only be keeping it from rotating. If the pipe is already threaded, just bore out the flange to get to a point where you have enough projecting through, the threads won't do anything in that case.

While swinging on the pipe, you might get impacts at least a couple of times the body weight, depending on what was actually being done.

flanges

Most threaded flanges are cast iron, so you will NOT be able to weld them properly. And doing so will weaken them.

I would just use a length on .250 DOM tubing and secure it in bored holes say 2/3 of the depth of the posts. No need for welding. The posts would be plenty strong to support the tubing. If you wanted to really be anal about it, run carriage bolts through the post and pipe, perpendicular to the pipe.

hj said:

Most threaded flanges are cast iron, so you will NOT be able to weld them properly. And doing so will weaken them.

Click to expand...

Actually you can weld cast, it takes special training and engineering the of the welding procedure, but it can be done.

Way outside the realm of DIY kind of home projects though.

You can arc weld cast, mig weld cast, tig weld cast and gas weld cast. It just takes the right filler and knowledge.

Weld Flanges

Instead of threaded flanges go to a supply house that handles commercial pipefitting supplies and see if they have or can get you slip-on weld flanges. They will slide over the end of the pipe and are made for welded applications. That way you don't need to mess with cutting threads, you'll use plain end pipe.