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Truss Loading Charts with Multiple Motors

Discussion in 'Scenery, Props, and Rigging' started by bshock84, Feb 7, 2019.

  1. bshock84

    bshock84 Member

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    Hey everyone. This seems like kind of stupid question, and one I should certainly know by now, so I apologize if it is, but I just want to be sure how these things work, with as little assumption as possible.

    When reading truss loading charts, I assume this is based on hanging a particular span of truss with two points, is that correct?

    If that's the case, what happens to the loading chart when you add a third motor, say a center point? Does that then cut that span in half, and you then have two separate spans?

    Say I have a load chart that lists a 48' span of truss with a UDL of 46lbs per foot. And also lists a 24' span of the same truss with a UDL of 204lbs per foot.

    If i hang a 48' span of this truss, but then add a center point motor, do I then have 2 - 24' spans with UDLs of 204lbs per foot, or is it still counted as a 48' span?

    Seems to me that I would treat it as 2 spans of 24'? Is this correct at all? Can anyone confirm or set me in the right direction on this?

    Thanks!
     
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  2. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    More or less yes, but the middle motor wiil see on the order of twice the load if balanced but don't think you can have them fairly share the load without load cells on each line. You'll never do it by eye.
     
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  3. bshock84

    bshock84 Member

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    Thanks!

    Yes, I understand what you're saying about the center motor taking twice the load.
    I am more just trying to calculate loads on the truss span for a design I'm working on, to make sure it's inline with the load ratings from the manufacturer.
    I will have load cells inline on the motors to verify those calculations as well however.
     
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  4. danTt

    danTt Well-Known Member

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  5. DanH

    DanH Member

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    And remember that load cells are just another tool, and computers and sensors can be EXTREMELY accurate if you tell them to be.
    (from a recent show where every time someone breathed on the truss, the load cell readings shifted noticeably, causing much unnecessary excitement)
    It's similar to the time I had a guy at a festival tell me we had BIG power issues, because on his ten-dollar multimeter, one leg was reading 119v, one was reading 120v, and one was REALLY HOT at 122v! (numbers for illustration, don't recall the actual ones)
     
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  6. porkchop

    porkchop Well-Known Member

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    If you can get a hold of Harry Donovan's book Entertainment Rigging: A Practical Guide he goes over these kinds of loading situations pretty thoroughly. The book still isn't cheap to buy (although worth it every penny), but ask around it's common and someone might loan you a copy.
     
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  7. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @DanH While we're veering off into multimeters, there are folks lugging expensive Flukes not realizing their meters are reading high due to their batteries being low and their internal reference voltages falling below their internal regulators. More than enough said.
    Toodleoo!
    Ron Hebbard
     
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  8. Ancient Engineer

    Ancient Engineer Well-Known Member

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    Firstly:
    There is no such thing...

    Secondly:

    I had a seasoned veteran tell me that he could "by eye" balance a 3-way load like the OP described... So we had him do it (while we watched the scales) and he was pretty dumbfounded that he NEVER could get it even close... with like five tries.

    Thirdish:
    No, no, no... GET THIS BOOK. A super-duper valuable reference worth its weight in osmium.

    Fourthly:
    Dear (insert name of favorite diety here), how many times has there been an argument in the venue beacuse two meters (or more) didn't match.

    Prior to each gig I test my OG Fluke 87 against my Tek DM5010 bench meter to be sure that it is accurate across the expected voltages for that rig.

    If there is an error I note it on the Met/Cal card in between the meter and the case. (It is rarely more than .02VDC and .05VAC and is subject to battery and the whims of an old meter...)

    My co-horts know my procedure so I have been called in to settle some pretty heated debates.

    (BTW I send the Tek out for Met/Cal annually. It has never been out of spec... (amazingly!))

    P.S. Where I work currently the average voltage for 1P 120VAC 60Hz varies by location spanning 131V to 111V. I use a lot of power conditioning... Now back to your regularly scheduled post.
     
    Last edited: Feb 13, 2019
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  9. DavidJones

    DavidJones Active Member

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    While you will never be able to accurately balance multi-point truss loads by eye. In most cases, it's largely irrelevant, and load cells are not used. Why? How? In our planning we make sure we are well under our limitations, of the truss, the motors and the point its hung from. A 40'-60' truss on 3 or 4 points with a typical rock and roll setup, is usually no big deal. You do the load calculations out of due diligence, but once you are doing it for a while you can pretty well guess what the loads will be just from experience.
    now when you get into HEAVY loads over a short span, line Video Walls, load cells become imperative, and you must rebalance the system as you build; with fixed speed hoists, anything over 3 motors, it's not possible to take a little more weight on one without making drastic shifts on other ones.
     
  10. Ancient Engineer

    Ancient Engineer Well-Known Member

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    Is that 3 or 4 motors on single truss points or 4 truss points on 2 motors?

    Yeah... that kind of complacency is actually dangerous Mr. Jones.
    I suspect that the tour manager and insurance company have strong opinions about those kinds of lazy attitudes towards guessing and irrellevance.
    As a potential audience member who might be sitting under your guesswork... I have strong negative opinions.

    At least there is some truth in this post...
     
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  11. egilson1

    egilson1 Well-Known Member Premium Member

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    So what we are talking about is what is known as a statically indeterminate structure. Due to the number of variables it is impossible to calculate the actual load on 3 or more suspensions. Now that's not to say you can't get close, but the slightest thing can cause the real world loads to be drastically different. I'll make the argument that even a 2 suspension system is only as accurate as the precision of your calculations AND the actual hang of the equipment. move a fixture 6 inches and the loads are different.

    On truss load charts the span is defined as the distance between suspensions. so you are correct that a 48' truss run suspended on three motors with 2 at either end and one in the middle would create 2 truss spans of 24' long. you would then use the loading data for a truss span of 25'. Keep in mind that truss is NOT a lifting device, and hence is not made with the same design factor as other rigging hardware, usually at least 5:1. Truss can have a DF of anywhere from 1.2 - 2. So in fact it is VERY easy to overload a truss, which is why you need to do at least simple load calculations for EVERY hang.

    Below is the mathematical load distribution on a beam with multiple suspensions. This uses what is known as the Three Moment Theorem. Not very practical when your on site trying to load in the show. Harry Donovan's book as referenced before has a great rule of thumb based on this that is much more usable by riggers.

    Beam suspension.JPG
     
  12. DavidJones

    DavidJones Active Member

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    Perhaps you misread my post. The VAST MAJORITY of truss flown in the US is flown without loadcells. This is 100% true. Load cells are still a rarity in the business; ask any rigger. I am one of the people who advocate for them, but in many cases, they are unnecessary, in other cases that are mandatory.

    I posted nothing about being complacent. If I hung a 40' truss with 12 fixtures yesterday, and I and I know I was at 1000 lbs per point, which was fine because I had a 1500lb per point limit. Next week I plan to same the same truss from the same points, but with only 10 fixtures, I know it will also be fine because it's LESS than I hung last time, but as I said you still do the load calculations to be diligent.

    I said Diligent, that's actually the OPPOSITE of complacent.

    Edit: There is no need to be rude.
     
    Last edited: Mar 2, 2019
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  13. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    DavidJones - for this forum - predominantly high school and some college and community thetra level - i agree wirth ArchitectEngineer that your attitude and approach is far to casual and cavalier. If it works for you, fine, but not what should be taught to first timers.
     
  14. DavidJones

    DavidJones Active Member

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    Well, I guess we can disagree here. Everything I said was true and correct to how things are done in the industry. I won't tell lies to "protect" the kids. Why pretend something is done differently when it's not? Who are you helping by giving a false representation?

    While I don't encourage untrained people to take on rigging tasks, I also won't tell them that if they hang a truss with not load monitoring a monster will eat them in their sleep. But hey if that's your thing Bill, have at it.
     
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