Forces Acting on a Pipe

[Bill002]

Hello,

I have come by a problem that seems simple enough but has me stumped. Please see attached image since it is easier to understand. I am trying to use a 8" DIA Steel pipe as a transition point. I have rebar in a U shaped fashion wrapping the pipe in either direction and evenly distributing the forces. How can I tell if the pipe is strong enough to not buckle under these forces?

Thanks!

 

[JAE]

Fill the pipe with concrete?

 

[Bill002]

Yes JAE, I was thinking that too. But how can I check it mathematically and approve the design?

 

[dhengr]

You need to give us a few more clues if we are going to play this guessing game successfully. Yes, fill the 8" pipe with conc., but it will bend, as a beam hung from 6 spring supports unless it is also encased in a large block of conc. called a dead man. If the pipe can bend, the center two 11k hanger rods will actually carry 14k or some such, the next two will carry 12k and the end two will carry 7k each, or some such. These values are a function of the bending stiffness of the pipe vs. the elongation stiffness of the 11k rods. One name for this problem is an elastic matching problem.

You will also have some very high bearing stresses btwn. the rod surfaces and the outer surface of the pipe, so you may at least want to grind off the rebar deformations in that bearing area, and/or weld the rods to the pipe so you provide some real bearing area btwn. the two. You might be better off using square bars instead of rounds, and welding anchorage plates to the ends of the bars. You might also be better off putting two of the 11k hanger rods btwn. the two 33k loaded rods and moving them all much closer together on a piece of pipe or solid round bar. Then splay the hanger and load rods out into their members over some distance.

 

[Bill002]

I am sorry, I haven't provided too much information. This is a open question and I am looking at different options of best achieving this. I think it is a great idea to have a concrete dead man type of situation. And it also prevents the steel from corroding in the long term.

So you think it is a better idea to move the cables closer together? I was thinking in the contrary, if I had it spread apart and forces distributed along the pipe it would be a better idea.

I think having the pipe filled with concrete and a concrete dead man around it covering the pipe and cable is a safe way to do this.

 

[Bill002]

I am still sort of confused on how to calculate the capacity of the pipe. Could I consider this as a Round HSS filled with Concrete (AISC Table 4-17)?

 

[paddingtongreen]

Before you consider the pipe for strength, you have to figure out how you can load each support bar to 11k. In real life, all or almost all of it will go to the first bar

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

Kidrah:

What kind of an engineer are you, what is your position or experience level? Have you read the FAQ’s and participation rules and requirements to play here on E-Tips? Is this part of the design of the suspension bridge that you were talking about a week or so ago? Are we designing this suspension bridge for you here? We don’t do design on these forums, we expect the engineer doing the design to have a vague idea what he/she is doing, and the concepts behind what they are designing. You seem to have kinda missed the drift of part of my earlier post, from the engineering standpoint. There is a drastic difference btwn. rebars and cables and you’ve used the two terms interchangeable here, which is it? Do you think your sketch will distribute the 66k load to the six 11k hanger rods equally? Do you want it to?

 

[Bill002]

I am a recent graduate engineer. Sorry if I broke the terms of the message board. I think I have an idea of what I am doing but am soliciting opinions on the design and trying to see if there might be a better way to do it. I think the feedback I got from here (inserting concrete in and around the pipe) was great. I analyzed it on RISA by inserting 6 springs at the reaction points as you had mentioned. By no means am I asking you to design this. Thanks for your support.

 

[dhengr]

Kidrah:
Fair enough. But you still need to dig out your Statics, Strength of Materials, and first couple Structural Engineering text books if you are going to do this kind of work and want to look at all credible. You should also look for a local mentor, maybe your boss or a senior engineer who you can do sketches with and get an immediate critique from or redirection; see the same projects specs. and plans, and go over them at the same time for some preliminary ideas, etc. On this significant a design problem, your boss should know what you know and what you don’t know so he/she can guide you and keep you and the company out of trouble. He/she has a vested interest in seeing you do well and be successful. If you don’t have that kind of help within the company, look for it outside the company, but locally, so you can have a beer and just hash some of this stuff over, on a regular basis. Then come here for additional ideas, confirmation of your general approach, or a variety of opinions or refinements, etc.

And, still you didn’t answer half my questions, so I’ll try again. Is this the termination, dead man end, of the cables for your earlier suspension bridge? Are they cables or solid rods, they act quite differently? Your “Open questions” get you open ended, unending answers, and just tend to waste other people’s time guessing at what you want or are doing. A better problem definition in your OP, along with your sketch, would probably go a long way toward getting you better answers and ideas. Since the more complete you make your whole picture in the first place (your OP), the more closely the responses will hone in on your real problem; the more ideas, pros and cons, a better way, why this is dangerous or just won’t work, etc. These all come to mind, almost immediately, to an experienced engineer once he/she knows what you are trying to do. And, you may not even think of some of these for lack of the experience they have. The fact that you skipped right over the issue that Paddington and I picked up on, led me to more questions, specifically about your experience level. Since you say you ran this in RISA, please share with us what you got for the six spring loads, pipe deflections, along with some of the input stiffness’ and dimensions. I just picked my 14k, 12k & 7k out of the air, knowing nothing about the relative stiffness of the different elements in your system. Paddington’s guestimate was even more drastic than mine. I offered several ways to eliminate some of these problems, but you didn’t bite on those either. The transition structure from the suspension cables to the dead man or anchorage must be very stiff, so that deflection there doesn’t alter cable loadings. The cables should all elongate or be stressed at about the same level. Filling the pipe with conc. prevents pipe buckling and helps address the high bearing loads, and that’s good, and I suspect that was JAE’s first thought. Surrounding the pipe with conc. and proper reinforcing stl. (my dead man idea) would stiffen it and prevent it from deflecting, which was my intention. Or, you might eliminate the pipe bending issue and uneven spring loads by significantly reducing the length of the pipe or changing it to a much shorter solid 8" round bar. The longer pipe is probably not a good idea, and I have no idea what AISC table 4-17 says or looks like. You have to explain your reasoning better and then also listen for the potential drawbacks.

 

[BAretired]

First, resolve the statics of the problem. Then you can draw the shear force and bending moment diagram for the pipe. If you neglect the deflection of the pipe, then the hanger forces are equal. If you consider pipe deflection, the strain in the hangers must equal the deflection of the pipe.

Irrespective of the magnitude of the hanger forces, the maximum shear in the pipe appears to be 33k (you are mixing Imperial and Metric units). Also, the maximum pressure on the pipe wall is 33/BD where B is the width of contact and D is the pipe diameter.

BA

 

[saplanti]

Is this a lifting gadget or a real piping application?

If that is a lifting beam you need to consider some other things, probably the system becomes a lot easier.

Do you really need to use U-Bolts? Do U-bolts that you intended to use have some tolerance against the pipe diameter? Do the pipe diameter has tolerance on the external diameter? If your answers are "YES" you need to think about the contact stresses, and damage either on the bolt or pipe depending on the loads? Additionally if you use U-Bolts you need to use locater on both sides.

In case this is a lifting equipment I would suggest to use ring plates with shackle connection on them. You may weld the ring plates on he pipe section and secure them in location. This will prevent backling as well. If you do not weld them you can make the internal diameters close to the external diameter of the pipe section and calculate the bearing pressure for contact. Keep it under the values used in the structural code you are using. Unwelded rings have some other geometrial requirement, watch pin calculation in the structural codes.

I hope I did not confuse you on the issue you have. Good luck and regards,

Ibrahim Demir

 

[csd72]

What happens if you get oscillations in the cables and those on one side go a little slack for a split second, what stops the rod from rotating and the cables slipping off?

This is not a detail that I would feel comfortable with.

 

[chicopee]

draw shear and moment diagrams. Determine from these diagrams, max shear and max moment. Apply shear and bending stress equations to determine if you are within allowable values.
You could also do a deflection diagram.
Note that based on your free body diagram, I doubt that all support points have 11k values. I suspect that there will be forces acting downward to keep the pipe supported at all six points, so your free body diagram is in error.

 

[BAretired]

Actually, all forces will be 11k if the pipe deformations are negligible relative to the hanger strains. Otherwise, the hanger forces will will be variable.

It is a good idea to draw the shear and bending moment diagrams, but by inspection, the worst case of shear is 33k and the maximum moment occurs when all hangers carry the same load, i.e. 11k each.

BA