Triangular truss

[lightsource]

Hi all,

 

This maybe a stupid question, but here goes ;)

 

Why is triangular truss based on an Isocolese triangle, therefore requiring apex up and apex down corners, whereas an equilateral triangle truss would have 'universal' corners.

[csg]

depends on the truss - im using some slick triangle at the moment that has an equilateral section

[paulears]

Which triangular truss system is NOT equilateral in cross section?

[J Pearce]

I don't get the equilateral idea, even for equilateral triangle you will need at least 4 lots of every corner, apex on the inside of the corner, apex on the outside, apex on the top, apex on the bottom.

[Tom]

Which triangular truss system is NOT equilateral in cross section?

Opti Trilite I believe

[paulears]

The Trilite I have is 12" max, with 10" centres tube to tube - each is the same as far as I can tell. So apex up or down is pretty important, but there are no problems joining tube - 120 degrees one way or the other. I'm beginning to doubt myself here? What point would there be in using 3 chord truss that only went together one way?

[johnhuson]

Hi all,

 

This maybe a stupid question, but here goes ;)

 

Why is triangular truss based on an Isocolese triangle, therefore requiring apex up and apex down corners, whereas an equilateral triangle truss would have 'universal' corners.

Whilst there are triangular truss systems based around isosceles triangles there are a also a lot of systems that are equilateral, either way I must be miss understanding what you mean by 'universal' corners? You still need different configurations to achieve apex up or down.

[lightsource]

This maybe a stupid question, but here goes ;)

 

Well, it was pretty much a stupid question, sorry. There was a product I had a brief look at that had Apex up left and right, and Apex down left and right, basically meaning 4 possible corner combinations.

 

Ignore me......

 

The idiot leaves the building

[mac.calder]

I must admit I have never seen tri-truss which was not equilateral - The only thing that stops you assembling our truss with the apex in a number of directions is how the bolt holes line up (one bolt either side of the apex, and two bolt holes on the side opposite the apex)

[benash]

What point would there be in using 3 chord truss that only went together one way?

 

An isosceles triangle can be stronger than an equilateral triangle on a weight basis.

 

An explanation for those who care: Any time you have a beam that spans a certain distance, the strength of the beam is determined by a number of factors. If it was a solid rectangular section the strength (all other things being equal) is proportional to BH^3 where B is the distance across the base (the width) and H is the height of it. As you can see if you increase the value of B the strength goes up proportionally (double B and you double the strength). If you increase the value of H the strength goes up by the cube (Double H = 8x Strength).

 

In general the overall performance of a truss is related to the same BH^3. So if you have an equilateral truss, for a given value of B you get a certain H. The material that you use creating that B is working for you, but not a whole lot. If you were to use an isosceles triangle, for a given value of B you can have a much larger H (and hence a disproportionately higher strength) at the cost of very little material. So in general, an isosceles triangle should be stronger for a given weight.

[librarian28]

I'm sure I've read somewhere that you're 'not allowed' to hang a truss apex down?

An isosceles triangle can be stronger than an equilateral triangle on a weight basis.

An explanation for those who care: Any time you have a beam that spans a certain distance, the strength of the beam is determined by a number of factors. If it was a solid rectangular section the strength (all other things being equal) is proportional to BH^3 where B is the distance across the base (the width) and H is the height of it. As you can see if you increase the value of B the strength goes up proportionally (double B and you double the strength). If you increase the value of H the strength goes up by the cube (Double H = 8x Strength).

In general the overall performance of a truss is related to the same BH^3. So if you have an equilateral truss, for a given value of B you get a certain H. The material that you use creating that B is working for you, but not a whole lot. If you were to use an isosceles triangle, for a given value of B you can have a much larger H (and hence a disproportionately higher strength) at the cost of very little material. So in general, an isosceles triangle should be stronger for a given weight.

I'm not good at formulas, but - if apex down - the distance B is very small giving a much smaller 'strength' than when used apex up.

 

The only thing that stops you assembling our truss with the apex in a number of directions is how the bolt holes line up (one bolt either side of the apex, and two bolt holes on the side opposite the apex)

This surely suggests that the manufacturer had a plan in mind - that the side opposite the apex should be either at the bottom (in tension) or - maybe- at the top (in compression). But I've still got a niggle that the manufacturer intends it to be used apex up.

[tokm]

I've always taken it that you can use tri-lite, litebeam, etc (triangular truss in general) either way up and to just be aware that it has different load ratings depending on which way round its being used..

 

Many people have said that it is indeed stronger apex up, just most often used apex down to allow easy cabling. This load sheet from Total Fabs seems to back all this up >> Litebeam Load Chart.

 

As you'll see on there, apex up @ 6m you get 71kg UDL, but apex down @ 6m, the rating drops to only 31KG.

 

Hopefully someone with a little more authority/experience than me will be able to explain/confirm if this difference in rating is to do with the shape of the triangle or if its purely based on the fact you have two cords taking the downwards force rather than just one..

 

T

[paulears]

I think it's that reason - two chords in tension vs one in compression or one in tension, two in compression. With thin walled tube, the spigots would seem the weak link at the holes in the tensioned chords, and the one in compression could be weak - especially if dented. I've not seen any data on it, but I wonder if bare end truss like Trilite suffers more than something like Astralite that has solid flanges on the end of each chord? Quite interesting, though.

[benash]

I'm not good at formulas, but - if apex down - the distance B is very small giving a much smaller 'strength' than when used apex up.

The 'B' value is an overall width. That equation isn't quite so directly usable for a triangular section (or for a truss for that matter). The point of that equation was to demonstrate that you get the best strength by making a beam (or truss or whatever) deeper rather than wider and that by increasing the depth you get a strength increase that is massively larger than if you increase the width. For such a general analysis it doesn't take into account the triangular form or whether the truss is apex up or down.

 

I think it's that reason - two chords in tension vs one in compression or one in tension, two in compression. With thin walled tube, the spigots would seem the weak link at the holes in the tensioned chords, and the one in compression could be weak - especially if dented. I've not seen any data on it, but I wonder if bare end truss like Trilite suffers more than something like Astralite that has solid flanges on the end of each chord? Quite interesting, though.

 

My initial thought was that the limiting condition would be the chords in compression buckling. If that is indeed the case I would have expected that a simply supported (as in, one support at each end, think goal post truss) truss should be hung apex down. I'd be interested to know if that matches with manufacturer's instructions.

 

As far as the loads on the end connections go, for a properly designed truss (that isn't deflecting substantially) at the end points you shouldn't get massive tensile forces axially along the chord. The chord(s) in tension should balance the chord(s) in compression so at your end connections you end up with a straight vertical load (i.e. shearing off the end of the truss rather than pulling it out lengthways). All this only counts for truss used horizontally of course.

 

If someone has a piece of tri truss around that they don't want any more, would you mind loading it up until it breaks and show us the failure mode? Just kidding, but if there are any manufacturers around I'd be really interested to hear what the limiting factor (buckling/tensile failure/end connectors/weld strength) is.