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Thread: Overhead rigging - Multiple points of failure vs safety factor of fewer

  1. #1
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    Overhead rigging - Multiple points of failure vs safety factor of fewer

    Hello all,


    I am currently running the numbers for overhead rigging of some speakers for an installed PA system. The current point of contention is the WLL of some forged shoulder eye bolts and the angle at which they'll be loaded, which leads into a more general issue of a 3-point rig versus 2-point.


    The speakers each weigh 40 lbs, and will be flown individually. Each of them came with 3 machinery-type forged shoulder eye bolts; two of them 3/8"-16 G5 for primary support, a third 1/4"-20 G5 for use in angle adjustment. The current plan is to use three slings of 1/8" 7x7 aircraft cable, with thimbles and wire clips at each end, which will then be anchored with similar eye bolts to an adjustable bracket, that will in turn be fastened with 3/8"x2" lag screws to the ceiling rafters.


    My numbers on the slings are a worst-case breaking strength of 1500lbs for the cable itself, times .2 for a 5:1 safety factor, times .95^2 for use of two mechanical splices per sling, times .85^2 for use of two 8:1 D/d thimbles per sling, times cos(30)=.866 allowing a basket angle up to 30* from parallel to gravity, giving a max safe working load per cable of 169 lbs. That provides a safety factor of better than 4:1 against the WLL, over 20:1 against breaking strength. I think we're fine here.


    Now, the original plan was to suspend the cabinet primarily from two provided anchor points at the top of the speaker, which are in a plane with the CG of the speakers parallel to the face; the cabinet would hang vertically when suspended by these anchors. The angle adjustment cable will be tied to the provided anchor point provided low on the rear face of the cabinet, forming a tripod basket sling. Each sling will then have its own anchor in the mounting bracket, providing multiple points of failure all the way through the rig.


    The issue with this is the low center of gravity caused by the woofer's voice coil, basically a big lump of iron and ferrite magnet positioned low in the cabinet. Using these three anchor points, the angle adjustment cable would end up picking up most of the load (and at an angle somewhere significantly over 45*), and also shifting the CG of the entire rig backwards.


    The primary anchor point on the speaker that provides the closest natural suspension angle to the angle we'll need is a single anchor at the top of the rear face of the cabinet. My colleague suggests using this one instead of the two top anchors, but I'm apprehensive. The natural hanging angle of the cabinet from this single anchor point is 39*, making the load angle on the eye bolt 51*, exceeding most manufacturer recommendations for load angle on a machinery eye bolt, this time on the primary load anchor and not an adjustment line.


    I did see one reference saying machinery eye bolts can be used with a shear load, assuming that the load is still exerted in the plane of the eye and that a WLL of 25% of the eye bolt's straightline WLL is used for any angle over 45*. The nominal WLL rating (again with a 5:1 safety factor or better versus breaking strength) for a 3/8" machinery FSEB is 1000 lbs, giving this anchor point a max WLL of 250 lbs, still five times actual load. A 1/4" machinery eye bolt has a WLL of 500 lbs in straightline, so the worst-case shear WLL would be 125 lbs, only 3 times actual load (but with an overall safety factor of 15:1).


    Even though the math seems to add up, I'm still apprehensive about trusting human safety to only two points of failure, and about subjecting a single primary anchor to essentially the full weight of the speaker at a high shear angle. Two points of failure is still better than one, but based on these calculations I would still prefer to use the two anchors on the top, because in that situation the most likely point of failure is the angle adjustment, and if it fails it would simply leave the speaker hanging vertically from its two primary anchors, each picking up a paltry 20 lbs against a 160lb max WLL for the cables (and not even close to the 1200lb straightline WLL of the eye bolts). Using the rear primary anchor to pick up almost all of the load, that cable and eye bolts now become the most likely point of failure, and if that fails, the only remaining tether is the angle adjustment cable and its anchors, which have the lowest overall worst-case safety factor. Such a failure would also be more violent, causing increased cantilevering of the lag screws holding the entire mount to the rafter.


    Am I right to be apprehensive, or should I trust the relatively high safety factor inherent in the rigging calculations and go with the 2-point flying rig?


    A third option would be to have an emergency tether, perhaps as beefy as 1/4", anchored to an unused primary anchor point and to the center of the mounting bracket (or, even better, independently attached to the rafter). It would calm my fears but might detract aesthetically, which is a concern as it usually is when trying to minimize the visual impact of big black boxes hanging in midair.

  2. #2
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    Quote Originally Posted by KShannon View Post
    Hello all,

    <snip of a pretty detailed question, which shows a logical thought process>

    Am I right to be apprehensive...?

    The short answer: yes.
    But not for the reasons you cite.
    I will provide a long, detailed answer after the game.

    dale

  3. #3
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    I'm working with Kelly to get some pictures uploaded.
    Is the OP still around?

    Bolt location in a rafter.jpg
    Last edited by Kelly Bramble; 02-04-2014 at 07:44 AM. Reason: Attach image

  4. #4
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    Quote Originally Posted by dalecyr View Post
    I'm working with Kelly to get some pictures uploaded.
    Is the OP still around?

    Bolt location in a rafter.jpg
    I am indeed, looking forward to additional educated opinions on this subject.

  5. #5
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    Let's address each of the issues, but re-arranging them a bit...

    Quote Originally Posted by KShannon View Post
    The speakers each weigh 40 lbs, and will be flown individually. Each of them came with 3 machinery-type forged shoulder eye bolts; two of them 3/8"-16 G5 for primary support, a third 1/4"-20 G5 for use in angle adjustment. The current plan is to use three slings of 1/8" 7x7 aircraft cable, with thimbles and wire clips at each end,
    I'd rather you call them cables, unless you intend to make a circle by attaching the two eyes together with a shackle.
    If you are using 1/8" cable, 7x19 Aircraft cable would be a more 'standard' choice. It is stronger and more flexible than 7x7. But 7x7 will work.

    Quote Originally Posted by KShannon View Post
    which will then be anchored with similar eye bolts to an adjustable bracket, that will in turn be fastened with 3/8"x2" lag screws to the ceiling rafters.
    lag screws... yuck.
    If you intend to lag into the bottom of the rafter with a lag screw, be aware that the lag screw will have no load rating at all.
    The only thing holding that lag screw in place is the wood surrounding the threads.
    If there is any wood there; what if there is a hidden knot right there where you decided to put the lag screw, and / or the
    wood is rotten?
    You are betting your life on the integrity of the wood in the rafter, which is well... "iffy" at best.
    I suggest that you do not rig your speakers this way.
    I never allow installations on my rigs to use lag screws.
    Further, you should be hanging the rig from the center of the rafter,
    half way between the top side and the bottom side of the rafter. See the drawing.

    Quote Originally Posted by KShannon View Post
    My numbers on the slings are a worst-case breaking strength of 1500lbs for the cable itself, times .2 for a 5:1 safety factor, times .95^2 for use of two mechanical splices per sling, times .85^2 for use of two 8:1 D/d thimbles per sling, times cos(30)=.866 allowing a basket angle up to 30* from parallel to gravity, giving a max safe working load per cable of 169 lbs. That provides a safety factor of better than 4:1 against the WLL, over 20:1 against breaking strength. I think we're fine here.
    It sounds like you intend to use cable clips.
    A better solution would be to use Nicopress sleeves.
    Cable clips have to be installed correctly, then inspected / maintained / retightened occasionally.
    The crimp sleeves also have to be installed correctly, but then are maintenance free.
    Quote Originally Posted by KShannon View Post
    Now, the original plan was to suspend the cabinet primarily from two provided anchor points at the top of the speaker, which are in a plane with the CG of the speakers parallel to the face; the cabinet would hang vertically when suspended by these anchors. The angle adjustment cable will be tied to the provided anchor point provided low on the rear face of the cabinet, forming a tripod basket sling. Each sling will then have its own anchor in the mounting bracket, providing multiple points of failure all the way through the rig.

    The issue with this is the low center of gravity caused by the woofer's voice coil, basically a big lump of iron and ferrite magnet positioned low in the cabinet. Using these three anchor points, the angle adjustment cable would end up picking up most of the load (and at an angle somewhere significantly over 45*), and also shifting the CG of the entire rig backwards.
    If I were doing this rig, I would purchase some hardware, probably from ATM Flyware, to connect to the speaker, and bring the free hanging CG closer to where you want it. see drawing.
    Also... if the speaker is really hanging more horizontal than vertical, I would consider moving the speaker to a location that allows a not-so-drastic pull back.
    Quote Originally Posted by KShannon View Post
    The primary anchor point on the speaker that provides the closest natural suspension angle to the angle we'll need is a single anchor at the top of the rear face of the cabinet. My colleague suggests using this one instead of the two top anchors, but I'm apprehensive. The natural hanging angle of the cabinet from this single anchor point is 39*, making the load angle on the eye bolt 51*, exceeding most manufacturer recommendations for load angle on a machinery eye bolt, this time on the primary load anchor and not an adjustment line.

    I did see one reference saying machinery eye bolts can be used with a shear load, assuming that the load is still exerted in the plane of the eye and that a WLL of 25% of the eye bolt's straightline WLL is used for any angle over 45*. The nominal WLL rating (again with a 5:1 safety factor or better versus breaking strength) for a 3/8" machinery FSEB is 1000 lbs, giving this anchor point a max WLL of 250 lbs, still five times actual load. A 1/4" machinery eye bolt has a WLL of 500 lbs in straightline, so the worst-case shear WLL would be 125 lbs, only 3 times actual load (but with an overall safety factor of 15:1).
    You understand this correctly. I do occasionally use eye bolts in shear, with the proper de-rating.
    Quote Originally Posted by KShannon View Post
    Even though the math seems to add up, I'm still apprehensive about trusting human safety to only two points of failure, and about subjecting a single primary anchor to essentially the full weight of the speaker at a high shear angle. Two points of failure is still better than one, but based on these calculations I would still prefer to use the two anchors on the top, because in that situation the most likely point of failure is the angle adjustment, and if it fails it would simply leave the speaker hanging vertically from its two primary anchors, each picking up a paltry 20 lbs against a 160lb max WLL for the cables (and not even close to the 1200lb straightline WLL of the eye bolts). Using the rear primary anchor to pick up almost all of the load, that cable and eye bolts now become the most likely point of failure, and if that fails, the only remaining tether is the angle adjustment cable and its anchors, which have the lowest overall worst-case safety factor. Such a failure would also be more violent, causing increased cantilevering of the lag screws holding the entire mount to the rafter.
    You also understand this correctly. The shock load of a falling 40 lb. speaker, suspended by wire rope cable, is very, very high. I would expect that the cable would readily fail catastrophically.
    See this paper from one of my colleagues: http://tdt.usitt.org/GetPDF.aspx?PDF=49-2shockloads

    However, there is a more practical point; using the top points of attachment on the top allows you to 'steer' the speaker, i.e., keeping it lined up in the proper left/right direction.
    You are on the right track.

    To sum up: use a through bolt technique on the rafters;
    use a crimp style termination on your cables, with a thimble in each of the eyes;
    use a bracket to change the natural CG point; locate the speaker for a more vertical than horizontal hang;
    use both top points (now on the bracket) both for failure mode and steer ability.

    If you care to post (or email) a detailed drawing of the intended rig, I will review it.
    Hope that helps.

  6. #6
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    This drawing shows how not to use a lag/lag eye.

    Don't lag into the bottom of the rafter;
    Do through bolt through the center of the rafter.


    Bolt location in a rafter.jpg

    Change the CG by using a bracket rated for the purpose:
    Speaker Bracket.jpg

  7. #7
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    Thanks for the response.

    I hadn't thought about using press-fit sleeves instead of cable clips, but it would give a cleaner look, and if they are maintenance-free as you say, they do seem the obvious choice. We also hadn't considered using a bracket to move the CG back while utilizing the better anchor points on top; that's genius.

    As for the lag screws, I know better than to only trust the interface between threads and wood. The current design has 4 to 6 lag screws, somewhere between 3/8"-3" and 1/2"-4", securing a steel plate to the back side of the rafter in question. That mounting plate then supports a short steel downpipe, to provide just enough clearance below the rafter for a T-shaped mounting bracket, made of square steel structural tubing, which will have the machinery eye bolts to support the cables that actually suspend the speaker. It sounds complex but really isn't; we came up with this design out of a desire to minimize the visual impact of the speaker hanging from the rafter by keeping the heavy structural steel components up as close to the rafter as possible, while providing the necessary height and angle adjustments (in both the horizontal and vertical) to position them properly. The trick is attaching the bracket to the downpipe at its center of balance with the speaker installed, to avoid a cantilever force on the lag screws, and this entire issue of changing the anchor positions arose when my colleague started doing suspension tests to determine the CB of the cabinet at the proper angle, and found that the rear angle adjustment cable ended up supporting virtually all the weight, which would push the T of the suspension bracket forward to the extreme to balance it.

    The speakers themselves can only be flown or mounted at a vertical angle not to exceed 45*, so we already know we can't point the speakers severely downward, however we also have to keep them mounted as high in the ceiling as possible, for two main reasons, one aesthetic, one practical. The ceiling is dark-stained brown wood (basically a pile roof, relatively common for this age of building IME), so the black speakers will blend in very easily from the planned mounting point as long as we don't hang them low enough for them to have the white rear wall and stained glass windows behind the chancel as a backdrop. Also, these speakers throw out a nearly uniform omnidirectional dispersion of frequencies under 500Hz, which is par for the course for most 2-way cabinets, and they are being mounted in a center cluster above the chancel area of the sanctuary to cover the pews in the nave, so between their natural dispersion and the additional lobing underneath them that we'll get from the center cluster, we need them as far away from the lavaliere mic on the pastor standing underneath them as we can get them. That will in turn necessitate a more severe downward angle to keep the dispersion pattern over the pews and off the back wall, shifting the center of balance rearward, precipitating the entire discussion.
    Last edited by KShannon; 02-05-2014 at 12:19 PM.

  8. #8
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    You may want to consider adding a turnbuckle on one side of the rig so that you can level the speaker.
    Attempting to make two custom length cables exactly the same length is... "difficult".

    If you use the press fit sleeves, be sure to use a "go / no-go" gauge to test each crimp.
    No exceptions on this one. Test each, and every, crimp.

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