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Designing Pipe Trusses

03/01/2011 1:49 AM

I'm looking to build a pipe truss to support the front of a basic farm shed. Clear span 9m or 30'.

I've built a few of these over the years. Basic principle is two lengths of pipe and a heap of 1/2" steel rod. Cut rod into 900mm lengths and bend each length in the middle at 90 degrees to form an "L". Position pipes about a foot apart and then weld the "L's" in a zigzag between the pipes to form a basic truss. I've never had one fail but then I might just have been lucky! Either that or I've over designed them.

This time I'd like to do it properly. Can anyone help with directing me to somewhere where I can get some ideas on designing this truss correctly. My skills lie in other directions (gas), so I'm not too excited about going back to first principles and trying to work out all the loads. A simple truss ain't so simple when you do that! There must be some broad principles or calculations that can be applied to this sort of problem

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#1

Re: Designing pipe trusses

03/01/2011 2:26 AM

°A picture would be nice. I cannot visualize how any 90° bent members would help better than some other choices, such as a 60° zigzag.

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#2
In reply to #1

Re: Designing pipe trusses

03/01/2011 2:51 AM

Can't help with a picture just now but I'll try to describe. You take a piece of steel rod, bent at 90 degrees, position it to form a "V" and then place this on the lower pipe. Weld in place. Weld the ends of the rod (or top of the "V") to the upper pipe. Position another rod section so you now have a "W". Weld in place. Repeat process.

You end up with two pieces of pipe running parallel with steel rod zigzagging in between at 45 degrees to vertical.

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#3
In reply to #2

Re: Designing pipe trusses

03/01/2011 3:27 AM

That paints a good picture. I don't know the whole answer to your situation. Searching on "method of moments", "Bow's notation", "Warren truss", etc., might turn up some useful information. Compression members should be kept short if possible; length does not matter much in tension members. My apologies if that seems vague.

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#4
In reply to #3

Re: Designing pipe trusses

03/01/2011 5:22 AM

Thanks Tornado, but please, stop it, stop it, stop it! That's just the answer I don't need. It makes my head hurt. Yes, I know I could do it if I really, really had to, but I'd rather cuddle crocodiles than have to nut out the intricacies of this sort of truss.

Really there's not many options in member length. Two pipes, one top, one bottom. Top's compression, bottom's tension. Both the same length. I've always made the top pipe one size larger than the bottom but my pipe sizing has always been based on gut feeling, plus what's handy. I'm hoping this thread might bring some logic to the whole exercise.

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#7
In reply to #4

Re: Designing pipe trusses

03/01/2011 10:07 AM

It sounds like a homemade bar joist? Thus

If yes, a little help is here at Wiki. Usually fabricated with steel angle in lieu of pipe.

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#9
In reply to #7

Re: Designing pipe trusses

03/01/2011 6:10 PM

Yes, that's the sort of thing I'm talking about. Thanks for that Doorman. I haven't an image of one and you've saved me having to go and take one.

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#14
In reply to #1

Re: Designing pipe trusses

03/02/2011 12:22 AM

Greetings Tornado,

You are right, the angle between the web diagonals or verticals is not as important as the properties of chords and diagonals (struts (in compression) and ties (in tension)) together with panel lengths to take dead and live loads, including stress reversal due to wind load.

Let's not forget the welds...

Cheers

Vince

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#5

Re: Designing pipe trusses

03/01/2011 7:47 AM

I assume your 2 pipes are one above the other?

First I would estimate the applied load, presumably a UDL from the brickwork etc above the truss.

Then decide whether the beam is built into the walls at the end or in effect simply supported. Work out the maximum bending moment.

Make a prelim choice of pipe size and separation. Work our 2nd moment of area and section modulus based on the pipe cross-section area and separation. Work out material stress, and go round the loop again till it looks right.

Your 1/2" rods need to keep the pipes apart. If you can arrange for the load to be on the bottom pipe the rods are in tension so easier to take the load (and to calculate), but sounds unlikely with your setup. Estimate the load per rod from the UDL, assuming all of it goes on the rods, for safety (in theory 1/2 the load should be taken by each pipe, so only 1/2 on the rods). Don't forget to allow for the rods not being vertical. Might be better to have them at say 30° from vertical as this reduces the load, and there is very little tendency for the pipes to move parallel to each other. Work out rod diameter needed to avoid buckling.

You might need to go go round the whole loop again to get something acceptable.

Also keep in mind that the beam must not go over sideways as it would take much less load in that condition. Try to keep the 2 ends fixed to something to constrain them vertical, and don't make the pipe separation too many times the diameter, I'd say max about 5 x as a guess.

Hope this is some help.........Codey

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#10
In reply to #5

Re: Designing pipe trusses

03/01/2011 6:29 PM

Thanks codey, that is of some use. I used to know how to work out bending moments, but I'm not so sure on the next bit regarding the "2nd moment of area and section modulus". I suppose that's why I need to be a structural engineer, according to the bloke who posted below!

When I've built these trusses in the past, they've generally been bolted between two uprights. Either timber poles or steel pipe. You just weld a piece of 75 x 10mm flat bar across the ends of the two pipes and bolt this to the supporting upright. Simple and effective.

I've never seen a failure in this type of truss, there's heaps holding up farm sheds, usually built by the farmer. My suspicion is that they are generally way over designed. I built one once for lifting engines out of vehicles. It was just a quickie construction to solve an immediate problem but as these things go, it was still there fifteen years later, rusting away. It never failed, even when lifting a big old diesel engine that was well over a ton. Admittedly it was only a 3m span but you can see why there'd be no issues holding up a tin roof!

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#11
In reply to #10

Re: Designing pipe trusses

03/01/2011 9:47 PM

"2nd moment of area and section modulus" are only the beginning for calculating the chord sizes, another consideration is lateral stability, what stops them buckling sideways, the codes calls for bridging, In the absence of this kind of knowledge, you need to be way overdesigned; that is why I refused to give you any simple way to cut down on your sizes, they only work if buckling is prevented and you need to understand it to prevent it.

I would not expect you to see any failures around, they would be cleared out and replaced till they found one that works.

And you didn't give me a couple of quick and dirty rules to allow me to cobble a gas plant or two together.

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#15
In reply to #11

Re: Designing pipe trusses

03/02/2011 2:54 AM

Why do you bother to reply if you aren't interested in helping? You haven't asked about lateral stability, I'd be happy to give more information if asked. I'm not interested in cutting down my sizes, the pipes there, just waiting to be assembled. I was simply interested to know how the design scrubbed up.

Thanks for nothing.

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#26
In reply to #15

Re: Designing pipe trusses

03/04/2011 11:31 AM

Sorry if this explanation sounds like a lecture but I don't know how else to say it.

Historically, people built on experience, trial and error. You don't see many of the errors because they get rebuilt, they do show up on some of the Gothic cathedrals where the buttresses have been modified a number of times. People like me, looked at things and asked, Why? they analysed, and came up with theories, they tested the theories, and developed design protocols. We who were trained in these procedures have no background in trial and error work, we don't think that way, our work is governed by building codes and discipline codes and specifications, authorities revue our calculations, they don't accept "rules of thumb".

You work on the trial and error model, and you asked us to bring you across to an inexact simplification of our exact way of working, but you provided almost no information on what you do know and the parameters of what you wanted.

Assuming the loading is evenly spread, the bending moment is M = WL/8 where W is total load and L is the span. Divide M by the center to center distance between the pipes and you get the force in the pipe. Divide that by the cross section of the pipe and you have the stress in the pipe at the center of the span. Working out the allowable stress in the compression chord is complex and varies with the distances between laterally stabilized points, so you are probably better of with some arbitrary limit but I haven't a clue as to what it should be. The material probably yields at a stress above 30,000lbs/square inch but you should be only a fairly small proportion that.

The force in the diagonal, and its weld, at the end of the truss is the reaction force multiplied by 1.414 if it is at 45 degrees.

You now have only a small part of the knowledge needed to properly design your truss, just remember that a little knowledge can be very dangerous.

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#27
In reply to #26

Re: Designing pipe trusses

03/05/2011 4:38 AM

Thank you for a good answer ptg. I'm sorry if my replies to your previous contributions have been a tad grumpy, but I confess to being a bit disappointed with this thread.

I'm interested in the point you raise about the trial and error method. My background is farming, I did uni in electrical engineering which is the one field I've never worked in. I've done just about everything else though, always around the primary industries; farming forestry, mining, fishing.

I can see that you take offence at the trial and error philosophy, but I think you are too harsh. When I was building, we had a job where the spans had been designed by an structural engineer. We started putting the timbers up and I looked at them and thought "they're not right". It wasn't a matter of wondering, it was obvious that the beam height versus span was simply wrong. Sure enough, engineer had pressed the wrong button on the calculator. No big deal, the beams became joists and some new, bigger, beams were ordered. Thing is, it would have been a big deal if it had been left to those who every thought is governed by design protocols.

My query regarding my hay shed truss was prompted by a desire for structural theory to meet real life "down on the farm". The farm's a hobby for me, which provides some tax advantages, but it's where I come from and where I came from, people built things with bits of steel pipe and nothing ever fell down. Bit like the pyramids, still there, but where's the approved plans?!

I'm simply interested. I've built several sheds of this nature before. To the best of my knowledge they're still there. I have found a photo of one I did in 1982. I'm trying to work out how to post it (advice appreciated, anyone who knows!). I suspect that the pipe (versus the more professional angle iron version) gives a huge amount of surplus strength and hides a multitude of sins.

It's an interesting field of discussion, which I'd hoped that this thread would stimulate, between those who know that this will work and those who have calculated this will work. I once had an trained and experienced fitter working for me. One of the hardest things I had to do was to train him to estimate the centre of something. "Drill a hole in the middle of that beam".... "NO!! put away the calipers, drill a hole in the middle" "but boss, where's the middle?" Satisfying part about it is that he went on to bigger and better things and still quotes learning to estimate the centre as a milestone in his career.

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#28
In reply to #27

Re: Designing pipe trusses

03/05/2011 6:07 AM

What anonymous chicken 'heart', had the temerity to vote the OP Off Topic on his own thread!

Especially as the reply was not only apologetic, but conciliatory in nature AND included 'useful life experience' on who saves your butt when you 'hit the wrong button'.

And don't tell me you haven't - given you've been practicing more than 5 minutes.

This is ridiculous and very poor form.

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#29
In reply to #28

Re: Designing pipe trusses

03/05/2011 11:22 AM

I agree, I just countered it.

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#33
In reply to #28

Re: Designing pipe trusses

03/05/2011 8:29 PM

In case you want to know, it was I who voted it OT. Whether the OP is able or willing to use technical comments about truss design, he is certainly NOT entitled to be disappointed in such input. And hence he was OT in his own thread. It's just as simple as that.

Now please correct your dopey neutralizing vote.

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#34
In reply to #33

Re: Designing pipe trusses

03/06/2011 1:38 AM

Seems like we disagree on interpretation of the intent of the OP's comment.

Afraid I have to stand by my reading of it as; 'I was' (but am not now) - verses, as you apparently read it; 'I am' (and remain so).

Vive la différence

& &

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#35
In reply to #33

Re: Designing pipe trusses

03/06/2011 5:38 PM

Sorry to be pedantic Tornado, but the thread is the topic and any comment that I care to make on it is surely "on topic"?

Actually I'm sorry that you appear to have taken offence at my disappointment. You were the first to respond and as such I'm grateful. The disappointment I was refering to was at the path the thread was taking, not at any of the individual contributions. With the possible exception of one post, all were helpful relevant answers, including your own.

As the OP, I have to carry responsibility for the thread; the quality of the answers depends on the quality of the question. My intent was to provoke general discussion on the design of such trusses. I'd hoped to be directed to some interesting resources, perhaps even hear about some spectacular failures, bit of anecdotal information as how home made trusses stacked up when subjected to a full analysis, possibly resulting from the aforementioned spectacular failure!

I intentionally posted in General Discussion, but still it didn't quite work out. Surely I'm entitled to feel disappointed? Actually, we've ended up with a thread with some quite good stuff in it so I'm quite happy. The disapointment stage was back when I was being rude to ptg. The comment you found off topic was simply my apology for that discourtesy.

Back on topic, I had hoped to be posting an image of the completed truss today but it was such a lovely weekend here, I confess that I spent it on the water, not welding. This indolence may delay things for several weeks as I'm booked up for a while. Never mind, I'll still try to remember to post an image.

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#36
In reply to #35

Re: Designing pipe trusses

03/06/2011 8:27 PM

No, being a thread's originator most emphatically does not guarantee that one's further posts are on topic. For instance, what if the OP just changes the subject altogether? (This has sometimes happened.)

There are other things that repliers might have contributed but didn't, such as whole sections of steel design manuals. However, what was contributed (for a time, anyway) seemed to lead positively. Up to the point in question, you were not entitled to be disappointed about anything. That was just poor sportsmanship. (Though not as bad as kastrupsky, MS Divekar, and other such whiners.)

Your post 4, replying to my 3, said "stop, stop, stop." You might not want to read about moments, Bow's notation, or other structural design concepts. But somebody in the picture needs to know about such things. To evade that is pure negligence. Your truss might be conservatively rule-of-thumb designed, and may hold up. But if you or someone else doesn't know how or why, it may fail. If so, I hope it doesn't fall on you, your family, your livestock, or your machinery.

Now get real, dammit.

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#38
In reply to #36

Re: Designing pipe trusses

03/07/2011 5:42 AM

Oh dear. I'm sorry Tornado. I don't know your cultural background and I'm wondering if I've stuffed up by assuming that you look at life the same way as me.

Please trust me, when I replied to your comment "stop, stop, stop", I was not meaning any offence to you. In my culture, my tongue was firmly in cheek. Essentially I was not serious. I was valuing your contribution but I was hoping to steer the thread down another path.

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#37
In reply to #35

Re: Designing pipe trusses

03/06/2011 9:08 PM

I think you are entitled to have an opinion about the direction of the thread

You as OP don't have any responsibility for anybody's responses but your own...

nor can you direct which direction a thread will take

You have done a much better than average job of providing a good OP & following up

There is a diversity of opinion about what constitutes a GA/Off Topic & what if any meaning to assign them

There are many times various "professionals" will deem you too dim to understand the answer because you asked the question. Additionally there is a reluctance to give an estimate or ballpark answer, for fear of death or dismemberment.

Of course we are all responsible to preform our own due diligence & assess the quality of any information, no matter the source

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#39
In reply to #37

Re: Designing pipe trusses

03/07/2011 6:08 AM

Thank you Garth. That's exactly how I feel about it. You cast your thread upon the waters and see where it ends up.

I feel that I didn't phrase my original post as well as I should but I'm not sure where I went wrong. There's lot's I didn't put in but common sense says that you shouldn't have to present a peer reviewed essay as a subject for general discussion. I can see that I hit some raw nerves with the concept that one of the uninitiated should even express interest in design fundamentals, but that was entirely an unintended result.

At the end of the thread, one thing has stood out for me, and that is that no professional (or informed amateur!) in the field has come forward and said " your design sucks", or alternatively, "that's so far over designed, you wouldn't believe". My conclusion from this is that the experts in this field, that are viewing this thread, really have no gut feeling for this sort of thing. Obviously I wouldn't expect anyone to sit down and do all the sums for me, but I'm surprised that no one has offered a subjective opinion on anything other than construction details.

I understand the litigation bit, but really, subjective opinion is subjective opinion. It's not hard to qualify comment, in order to avoid death or dismemberment.

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#40
In reply to #39

Re: Designing pipe trusses

03/07/2011 7:47 AM

Now really - you're actually going full victim?

Come on - get over it - a bit of push and shove and you get 'biblical'.

It's a truss for "F's" sake! - not the end of the world as we know it.

"no gut feeling for this sort of thing" Of course we - well at least I - do. Then you/I do the sums and see if they agree. If they don't - then generally, you/I have hit the wrong button.

But: At the end of the day - pass or fail - it's your truss.

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#44
In reply to #40

Re: Designing pipe trusses

03/07/2011 5:15 PM

What on earth's a full victim? Sorry about being biblical. To me it meant that half the fun of a thread is seeing where it goes, not that this is something to be lamented. The other interpretation didn't occur to me till I read your comment.

My comment re gut feeling was probably not really tactful, written at the end of a long day dealing with people who want to blow themselves up (bit like people who want to drop sheds on their heads LOL) but it is quite honest. I am surprised. The basic information on the design was right there at the start and I expanded on it along the way. Top chord 2"NB, bottom 1.5"NB, 300mm apart, with 1/2" bar zig zaging at 45 degrees.

I confess to not having given loads though. No snow, non-cyclonic, 5 degree pitch, not enclosed (might be in the future) apart from one side wall, total roof mass around 450kg, rear wall supported at five points and braced. The truss supports the front opening and will actually effectively span 8m as I'll be putting 45 degree braces 500mm in from the ends. Truss supported on 150mm CCA treated posts, unless I'm successful in scoring some 6" steel pipe that I've got my eye on! Posts concreted in as deep as my post drill will go. 1200mm, if I don't hit a rock.

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#41
In reply to #39

Re: Designing pipe trusses

03/07/2011 7:59 AM

One of the reasons that no one has done this is the lack of real engineering information from the very beginning. What are the dead/live/wind/snow loads on the surface that this truss supports? If this truss is on one edge, what supports the opposite edge, and how far away is it? What slope?

Earlier I hoped that this truss would not collapse on you and yours, but now I am beginning to hope that it does collapse, on you only. Jeez, if anyone ever deserved a Darwin award....

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#43
In reply to #41

Re: Designing pipe trusses

03/07/2011 3:43 PM

Tornado, for the most part your comments have been correct. I hope we all can learn from the conversation.

Add one more line to this crazy conversation. We are here to help if we can, when asking for someone help on a subject such as this. One should understand there is only so much help that can be given across the internet via a type message. I would love to be able to help design the worlds problems away but is just not possible. I am sure others feel the same and words get typed that maybe we should not have. But it is easy to type out your feelings without really saying what you mean.

Good luck with our trusses. I would for safety's sake, look at the more conventional designs that have a proven track record around the word. That way your not reinventing the wheel.

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#42
In reply to #39

Re: Designing pipe trusses

03/07/2011 1:55 PM

Hey nutwood, I've got this piece of string, I'm going to tie one end to a high branch and hang the baby's cradle from it. When the wind blows, will it hold or will the baby fall down, cradle and all?

You didn't even give us the spacing between the chords, roofing load, snow load, or wind uplift, whether the shed is enclosed or not (makes a huge difference in wind uplift), that's why I gave you the method I did, in #26, above.

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#45
In reply to #42

Re: Designing pipe trusses

03/07/2011 5:25 PM

I'm sorry ptg, you'll have to give more information for a detailed answer but my gut feeling is that the success, or otherwise, of your baby dangling will rest on how well you tie your knots. Well tethered, with good strong string I'd say you'll be OK.

I qualify the above by saying that you should only attempt this if baby dangling is allowed in your district and you've submitted full plans to the baby dangling department.

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#18
In reply to #10

Re: Designing pipe trusses

03/02/2011 10:24 AM

Hello nutwood

I like the "I used to know how to work out bending moments"

To the sort of accuracy you're working to, can take 2nd moment of area I = 2*A*(h/2)2 = A*h2/2 and section modulus Z = I/(h/2) = 2*A*(h/2) = A*h

where A = cross-section area of each pipe, h = c/l distance between. This is for equal pipe sizes, it's more complicated if they're different.

Stress = bending moment/Z. You need I to calculate deflection, which should also be checked, but you need the formula.

Cheers.........Codey

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#23
In reply to #18

Re: Designing pipe trusses

03/02/2011 6:20 PM

Thanks again codey. It's nice to get a answer to my question rather than a lecture.

I know people mean well but it get's frustrating to be treated like an idiot. Before I got into my current line of work, I was a builder for fifteen years so I do know something about structures. I even did a bit of structural stuff back in uni days but whilst I remember the principles, the nitty gritty is long gone. Use it or lose it!

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#6

Re: Designing Pipe Trusses

03/01/2011 8:37 AM

"This time I'd like to do it properly."

"so I'm not too excited about going back to first principles and trying to work out all the loads"

???????

Why do you think that we structural engineers design these things if just anybody could do it based on a basic rule or two? In fact, it is not a lot of work to perform a proper design as long as you know what you are doing.

Can you give me a couple of basic principles that would allow me to design a gas processing and delivery system?

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#8
In reply to #6

Re: Designing Pipe Trusses

03/01/2011 6:06 PM

Well, that's not very helpful. I appreciate that this is how structural engineers earn their bread but this is quite a simple question. These trusses are out there in their millions, usually built by amateurs, and I've always wondered how they scrub from a design point of view.

I can build one tomorrow, using 2" (3.6mm wall) pipe for the top and 11/2" (3.2mm wall) pipe for the bottom and I know it'll do the job, but I know that from experience, not calculation. I thought it'd be interesting to ask the question of those who are wiser than me in these matters. To me, that's what this forum is about.

Regarding your query on a gas processing plant; no I can't help with a couple of basic principles. Equally so though, I'm not asking for basic principles to construct a skyscraper, just a simple truss. If you would like some basic principles to help you with a simple gas issue, I'd be more than happy to help.

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#12

Re: Designing Pipe Trusses

03/01/2011 11:26 PM

Nutwood -- Forget about playing structural engineer unless you have that kind of experience. Even though I had the training necessary to design one of these beams (50 years ago) and still have my old textbook I would first find someone who makes and sells such things for a living.

I would outline my job and as best I could and approach a fabricator for a quote. In doing so you may run into someone whose answer is "show me a detailed drawing with all dimensions, etc. and I'll give you a quote". This is clearly a supplier that wants nothing to do with designing the beam. Say thanks and try the next guy.

Perhaps he will be making lots of these things and already has a computer database that will pick a beam design if you provide the span and the live and dead loadings. The latter should be in the local building codes although you may have to add static loads if you plan to hang anything heavier than yourself from it. So it will be easy for this guy to give you a preliminary quote for estimating. At this point it is reasonable for you to inquire into the depth of the beam, what it is made out of in terms of steel shapes, how heavy it will be and what is the nature of the end construction for positioning on the building walls and the upper web to which a floor or roof will be attached. This is all valuable information from which with a little logical arithmetic you can back calculate and then compare your results with the way you have built these things in the past.

It may well turn out that somebody else's professionally designed beam may well be in the range of the cost of your labor and materials for the way you build. Close enough that you may well become a customer. Were I in the biz I would readily entertain an inquiry like yours even if I knew where he was coming from. Sure the customer might just be picking my brains, but if I have confidence in my business I would welcome the opportunity to sell the job. Even if the prospect goes and builds it himself I know all the while he will be comparing his own progress with the early quote or estimate. And he may just be back the next time he faces such a project.

Ed Weldon (retired mechanical engineer and holder of a New Jersey PE license)

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#13

Re: Designing Pipe Trusses

03/01/2011 11:40 PM

I will try to discourage you in fabricating a truss without considering the first principles of structural design. It could be a criminal act as the truss could fail in various modes of behavior injuring someone. Without those principles "on designing this truss correctly" it would be like guessing the winning lottery numbers correctly.

Before cutting or bending any steel member, I suggest you obtain either a copy of "Steel Joist Facts" by the Canadian Institute of Steel Construction, or the "Manual of Structural Design and Engineering Solutions" by Maurice E. Walmer, (Prendice-Hall) and follow the design, manufacturing, transport and erection procedures.

Good luck.

Vince

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#16

Re: Designing Pipe Trusses

03/02/2011 2:55 AM

Good grief, where's all the fun gone from CR4? I thought it'd be interesting and informative to explore the design of a simple (but not so simple) truss system knocked up by farmers all over Australia.

I've not asked for anyone to design something for me. If I wanted that information, I'd simply get my tape measure out and measure up a commercial truss. There's plenty about. I was asking for guidance as to where I could gain, or possibly refresh, my knowledge on how to do it properly.

Thank you to those who contributed in the spirit of the original question. The truss will be built and in service by Monday, but the general pool of knowledge on these trusses has not been expanded much!

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#20
In reply to #16

Re: Designing Pipe Trusses

03/02/2011 10:50 AM

You have to understand that engineers don't like to guesstimate for fear of structural failure. They are afraid to approve any design that would be considered makeshift. It has to do with liability. If an engineer approves a structural design and it fails, he is held liable for the consequences.

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#17

Re: Designing Pipe Trusses

03/02/2011 6:10 AM

With a clear span of 9m, I would advise adding some lateral bracing to the bottom chords. Look at the pics from the Photo that Doorman gave. The roof sheathing provides the bracing for the top chord, and some angle iron is added to the bottom.

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#22
In reply to #17

Re: Designing Pipe Trusses

03/02/2011 6:00 PM

Good answer Johannescnc. This truss is going across the front of a shed to provide a clear opening. It is supporting three 10" x 2" timbers that butt into it. These timbers run back to the rear wall posts. The timbers attach to the truss via a piece of 3" angle iron welded between the top and bottom pipe. The timbers bolt to the angle.

On top of the timber beams run 120mm top hat purlins to support the roof sheeting. The idea is to arrange things so that the beams actually run through the truss, between the pipes and in a "V" of the reo bar. That way I can put a bit of a front overhang on the roof to keep the weather off the truss. Also helps the timber span a bit.

The intention is to support the truss sideways and also tuck the truss up to give maximum clearance at the front.

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#19

Re: Designing Pipe Trusses

03/02/2011 10:36 AM

I've seen this type of truss used to support a floor, except it was constructed of continuous lengths of rebar bent into a zig-zag shape; the top and bottom chords of the truss were made of angle iron welded back-to-back to the sandwiched rebar. The angles were around 3"x3". Total depth of the truss was around 12". The floor was the first floor of an engineering building in a shipyard. Angle iron is better in a bending situation than pipe. (I'm guessing it was rebar, but could have been HR round bar).

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#21
In reply to #19

Re: Designing Pipe Trusses

03/02/2011 5:43 PM

Thanks ronseto.

I too have seen this type of truss with angle iron. I thionk the angle iron versions are probably manufactured trusses, kg for kg, the angle iron would be far more effective. In my experience, the pipe trusses are made of pipe because that's what farmers have kicking around. I think the pipe also makes a much more "solid" truss, better able to tolerate loading from the side, resist twisting and buckling etc. Ideally suited for holding up the front of a hay shed where you never know when some clown's going to give it a bang with a 300kg bale of hay up on the tractor forks!

Continuous zig zag makes a lot of sense when you've the faciilities to bend it accurately. A series of "V"s welded together makes for more welding but they're easier to bend up in a vice and it uses up short bits of rod.

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#24
In reply to #21

Re: Designing Pipe Trusses

03/02/2011 7:06 PM

Ok, I think I'm getting a clearer idea of what you're after - but bearing in mind Ronseto's very valid comment on who gets sued;

In days of old, before all scaffold builds were 'certified'; the "rule of thumb" was the span to height ratio of a truss must be greater than the pipe strength* to load ratio.

* this is the rated tensile strength of 1 'bottom or top' pipe or tube. The tube is NOT 'rated' at UTS, but something like 80% divided by the factor of safety, typically 5.

So, say you have a span of 6 meters and truss height of 0.6m - the truss ratio is 10:1

And say the 'rated tensile' of a tube is 6,000 kg, then the load must not exceed 1/10th = 600 kg

Obviously this can be worked as span vs load to find truss height and/or number of tubes required in top and bottom elements.

The lateral 'folding' is a big issue with top loads and narrow trusses, which is why 'bridges' in scaffolding are 'box truss' design. So the comments on bracing the bottom element to the roof or floor above should be taken seriously/studiously.

I've never 'sat down and worked out the real numbers' on this "rule of thumb", to evaluate 'how safe the principles are' - [read; total disclaimer] - but whenever I heard chatter on "it broke", it featured 'the idiot was way outside "the rule"'

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#30
In reply to #24

Re: Designing Pipe Trusses

03/05/2011 7:21 PM

Thank you 34point 5. Sorry I didn't acknowledge your reply earlier. I missed it on my last pass through.

You're spot on what I'm after. I'd have loved it you had also been bored one day and crunched the real numbers for comparison! There used to be lot's of these sorts of approximations floating about and whilst they were viewed for what they are, no harm was done.

At the risk of being voted "off topic" (thanks for your's, and ptg's, assistance there!) there's a rule of thumb for rafters; twice the height in inches, is the span in feet. Now this works fine for hardwood rafters, holding up a tin roof, but you should see what happens when someone applies it to pine rafters and puts concrete tiles on top. Oh dear!

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#32
In reply to #30

Re: Designing Pipe Trusses

03/05/2011 8:18 PM

It's easy enough to work it out for a given truss - 'proving the rule' is a much more involved process - so one tends to just do the former (using the established formula).

Maybe just a little bit of 'revising school' and you can pick it up again. Probably a lot easier now you have the practical experience, so 'feel', of how what works.

Another alternative is computer-modeling software - programs like Cosmos do this analysis - but there is very good 'analysis shareware' out in Ubuntu land, that you can download free and self learn via a lot of enthusiastic forum support.

Don't worry about the 'votes' too much - it's not always 'technical' adjudication that drives some folk. I was just a bit 'cross' at the 'manners'. I imagine that "the nose put out of joint" by my berate, is why I got one too. And <sigh> I will probably score again on anything I type, till they get over it.

I.e. read the comment for the content - judge not by the votes.

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#25

Re: Designing Pipe Trusses

03/03/2011 4:43 PM

What I see in this thread is you are spanning a thirty foot area with pipe, which usually comes twenty feet. This means joints, in compression and tension. If the pipe comes threaded, I would consider tees in the middle with a short nipple connecting the two spans. Presumably the ends are going to be welded to uprights of some flat nature.

If not threaded, I would recommend a reinforcing band for the pipe in tension at the weld.

For the joint in compression, lateral support is needed, so buckling doesn't happen.

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#31
In reply to #25

Re: Designing Pipe Trusses

03/05/2011 7:27 PM

Thanks Mike. Pipe was 6.5 m. It's now welded together to make 9m. Good point about the tension joint. I can't see it failing but it won't hurt to run a couple of bits of reo bar either side of the pipe over the weld.

I think the top pipe will be fine in compression. Takes a huge amount of force to buckle 2" NB pipe, joined or not. It's quite a light weight structure. Heaviest bit is the truss!

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