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  1. #1
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Bearing in mind I'm neither an engineer or machinist but do have a bit of an epoxy fetish ;) I've been going through some designs of gantry and I'm a bit unsure about the feasibility of constructing something.

    This is my inspiration and I'm aiming to end up with something similar.

    Click image for larger version. 

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    This is the basic gantry concept without any of the supporting struts in place:

    Click image for larger version. 

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    It is 60x40x4mm steel box, 1.4m long.

    For the struts I can either use solid steel rods or steel angle, the problem I have is that using steel rod will mean welding the struts in place but if I use steel angle I will have to cut and bend the ends of it very precisely to get sufficent surface area for the epoxy joint to hold properly.

    Question 1 - is the basic concept a good design of gantry?

    Question 2 - is welding the steel rods in place really going to distort the steel, (they will only be small welds, not much more heat than a tack)

    Question 3 - Do I have a snowball's chance in hell of bashing bits of angle iron into the right shape using only a few bits of wood and a big hammer?

    Any thoughts?

  2. #2
    Quote Originally Posted by D.C. View Post
    Bearing in mind I'm neither an engineer or machinist but do have a bit of an epoxy fetish ;) I've been going through some designs of gantry and I'm a bit unsure about the feasibility of constructing something.

    This is my inspiration and I'm aiming to end up with something similar.

    Click image for larger version. 

Name:	Spigot-Triangular-Truss.jpg 
Views:	2875 
Size:	58.3 KB 
ID:	7604

    Any thoughts?

    Truss is strong with things hanging from it, not so sure about applying pressure other way round.

  3. #3
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    If you think about the design of that particular truss, it should deal well with forces in all sorts of directions I was hpoing it should perform better than an unsupported L shape gantry, big hollow box shape or a couple of pieces of extrusion bolted together.

    Or am I missing something blindingly obvious?

  4. #4
    Lighting truss is designed to be like a hollow box with most of the weight taken out, the chords are thin walled typically 2mm, it`s very strong in compression. As shown apex up, lights would be hung below and the forces compress the top chord.

    But as enough recent stage collapses have shown, push it in the middle and lose most of its strength.

    That`s my very basic understanding, which may not be entirely accurate, not a rigger nor do play one on T.V. ;-)

  5. #5
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    As I understand it a truss works efficently because as force is transmitted through it one part is in compression and another in tension which leads to a stable shape. A triangle is a stable shape but a square isn't. When you diagonally brace a square you are in effect creating two triangles that form a compound truss which is why diagonal bracing works so effectively.

    In the arrangement I posted, the struts should be able to cope with forces in any direction encountered by a cnc machine a lot more effectively than a design based on one or two beams that are fixed to a thick plate either end, or so I thought.

    I've just had a quick look around the web for stage collapses and I can't find any where the investigation blamed a failure of the truss system for the accident, do you have any links to demonstrate what you mean?

    For example if you read

    Indiana State Fair stage collapse - Wikipedia, the free encyclopedia

    The failure ... was due to the inadequate capacity of the lateral load resisting system, which was comprised of guy lines connected to concrete "Jersey barrier" ballast. The concrete barriers used as anchors for the guy lines were not fixed in place; they resisted loading only by friction with the ground and through their own weight—about 4,200 pounds (1,900 kg).[6] Just before the collapse, wind loading caused several of the barriers to slide or pivot from their original positions, allowing the top of the truss structure to lean toward the crowd. The subsequent bending forces within the support columns were too large, and the structure collapsed under its own weight.[6] Measurements indicated that the total weight was 70,000 pounds (32,000 kg).

  6. #6
    Quote Originally Posted by D.C. View Post
    I can't find any where the investigation blamed a failure of the truss system for the accident, do you have any links to demonstrate what you mean?

    ****The subsequent bending forces within the support columns were too large, ****and the structure collapsed under its own weight.[6] Measurements indicated that the total weight was 70,000 pounds (32,000 kg).
    Toronto the week before, there has been a few others.

    The legs on self raising stage roof systems are truss on end, even with thin wall its very strong in compression, but tilted off axis , its a thin tube getting bent.

    The saving is in weight, early lighting truss was steel antenna tri truss, its a lot of weight to manhandle and to hoist.

    Mebbe 2D truss shape , as used by Steeldeck staging

    Middlesex University steeldeck 8x4 by Rory - 3D Warehouse

    again its used to reduce weight for a given strength, if weight is less of a concern apart from moving the mass of the gantry and possibly damping resonance?

  7. #7
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Quote Originally Posted by Musht View Post
    Toronto the week before, there has been a few others.
    If you look at pictures of the Toronto collapse it is very clearly a scaffold structure that has collapsed and no report has been released so far, let alone one claiming truss beam failure. Just because stages collapse and truss systems are often to hang lights above a stage doesn't mean that all truss designs are fundamentaly flawed.

    Anyway, given that the thread is about a cnc gantry built from multiple 4mm thick pieces of steel box section I doubt very much that any force is going to be generated that will come close to causing structural failure, death and destruction just so long as an invading column of tanks don't decide to take a short cut through my garage. :)

    I was more interested in a comparison with the usual designs of gantry, like for example a couple pieces of alu extrusion bolted together or the unsupported L shape, bolted to plate supports at either end.

  8. #8
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    This is the gantry with the supports added, it makes it a little easier to understand what I'm suggesting (the view is from the rear side):

    The red box section will be mounted to some SBR rails, the orange box section forms the main parts of the gantry, the yellow bits are struts made from steel angle and the grey bits are 10mm steel bar.

    Click image for larger version. 

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    Edit to add, click the piccy for a larger view.

  9. #9
    Quote Originally Posted by D.C. View Post
    If you look at pictures of the Toronto collapse it is very clearly a scaffold structure that has collapsed and no report has been released so far, let alone one claiming truss beam failure. Just because stages collapse and truss systems are often to hang lights above a stage doesn't mean that all truss designs are fundamentaly flawed.
    Sorry , where did you draw that i said all truss designs are fundamentally flawed?

    As said not a rigger, not playing one on T.V. not playing one on the internet, haven`t said anything about the inherent safety or otherwise of trussing, of which many miles are hung above peoples heads very safely every day and night of the week.

    Eaxmple of stage collapses, of which there has been multiple incidents in last 5 years, was used as an example of how aluminium truss or indeed steel tubular scaffolding will NOT:

    "deal well with forces in all sorts of directions"

    It is immensley strong for it`s weight but will fold up like a wet newspaper if forces are applied in the wrong direction.

    Sorry you couldn`t read to end of the post before having to reply

    " cnc gantry built from multiple 4mm thick pieces of steel box section"

    a link was provided to a 3d model of a staging system that uses steel box section with triangulated reinforcement, more similar to your requirement than 3D alloy truss.

    "comparison with the usual designs of gantry"

    Cause you missed it last time:

    "weight is less of a concern apart from moving the mass of the gantry and possibly damping resonance?"

    Not questions can answer afraid.

  10. #10
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Sorry about this Musht when threads get like this I normally apologise and walk but as this is my thread and I was actually asking for help and so far all I have had is an argument about stage collapses that is completely and utterly unhelpful to me.

    Would you mind walking away from the thread please so that someone else could try to help me without derailing the thread with an argument?

    I am really not interested in having an argument but I would like some relevant and helpful feedback from people to assist me.

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