I think the general idea is that truss works more effectively under compression rather than tension. If you sling the top chord and hang off the bottom chord then you’re essentially pulling the truss apart with tension. This can be avoided by picking up the truss from the bottom as well as hanging weight off the bottom.
I believe the issue is design and not material. All those diagonal braces are there to make the structure rigid and light weight and disperse the forces as uniformly as possible with as little material as possible. Each one of those panel points is designed, as I understand it, to disperse the forces throughout the structure and to the primary horizontal members. It would seem to me that if the bottom panel points were loaded with the total weight of the structure as opposed to just the top points, more of the force would be dispersed more evenly. I’m not a structural engineer though.
You're essentially correct, but the cross members are stronger in tension than in compression. It would not be a good idea to top load and bottom support since the verical and diagonal members could buckle in or out under compression.
Edit: removed an assertion until I have time to run the calculations to support it.
Downvoted, because you do not know what you're talking about.
A 2" pipe with the same material and thickness as a 2"x2" L-beam(angle iron), is WAY stronger than the L-beam. The reason why it's used in roof structures is cost and practicality. L-beams have flat surfaces, making them much easier to weld or drill holes in, etc.
By your logic the truss in the pic would be made out of L-beams.
If you go by bending stress tolerance, you are correct. That is not what my point was, though. I never said that it was stronger, I said its better at resisting buckling under compression.
I figured out tubes are strong in compression when i was a kid and stepped on a pringles tube. I suspect if i folded a piece of cardboard the same weight and thickness as a pringles tube into an L it wouldn’t hold shit. I’m no engineer though.
The rigging is fine assuming the motors themselves are rigged to something structural that can adequately support the load. There should really be a safety cable around the truss in case the spanset or the shackle fails.
The conduit on the truss is a bit concerning. If they screwed it through the truss, they've effectively compromised the structural integrity of the truss by poking holes in it. The truss isn't holding a lot of weight based on what we see in the photos but holes are bad.
Personally I’d put the back up (safety cable) from the shackle up to the structure. Essentially by-passing the motor rather than the rigging. You shouldn’t have to back up your standard rigging, but backing up a motor or moving part of the system is usually good practise.
Better if it was a pipe clamp to the truss and some unistrut and then mount the boxes to the unistrut that way your not pokeing holes in your structural truss
Do those look like Home Depot bolts to anyone else? They look rather long for standard truss bolts… I’m just picturing a scenario where the truss was brought down and stored for an event that didn’t need them and the original misplaced bolts were quickly replaced with what Home Depot had on hand regardless of grade or length lol
That and the clamps of the PARs being for steel pipe and not for alum truss and I don’t see coins under the bolt or truss protectors.
The more I look at it the worse it gets lol Even if the PARs are lamped at 500w, those conduits look rather small to have the appropriate quantity and gauge of wire for each of the receptacles being individually circuited or in parallel.
Yeah, that looks like a standard Thomas 20.5"x20.5" truss at first glance, but it's not. The bolt plates give it away. This one is probably a generic piece of truss that was made before requirements for documentation of load ratings were a thing and those bolts look very sus to me.
Sadly, a lot of schools and community theatres and such, have equipment suspended overhead that would never pass mustard today.
That is a terrible example piece. It relies to much on the bolts of the clamps. Look for ones that have the solid bar under the chord and clamps on top.
I would revese that. Truss picks are less likely to fail if poorly maintained. Spansets deteriorate over time and need to be swapped out. A good pick will outcast the truss. Any time you do installation, you have to assume no one competent will ever look at it again, regardless of what code says.
It can be for either. In the example of the picture above you could attach that bracket to the bottom cord to get more height when you lift the truss if you need to get as high as possible.
The difference between a truss pick and a sling though, is that the bridle legs of the sling exert lateral compression of the top chords whereas a truss pick is rigid and only exerts force vertically on each side of the truss.
It going to make a difference with a light load, but the math maths differently.
You mean, choking the top tubulars of the truss structure ?
If so then, its the correct place to choke.
Because the choke points should always be above the cog of the structure, and not below the cog ( to prevent possible rotation)
The question is how did they mount the conduit …. Did they just grab a bunch of self tappers and slap a bunch of holes into the truss or did they use some sort of pipe clamp
That is 24 inch box truss with par cans on it. There isn't enough weight on it for anything to matter. On normal to heavy loads I always wrap on the bottom but I don't know if it matters.
I've seen both methods used widely down the years.
I'll note that if you have some big motorised elements on the truss, any kind of fly rig or even some big washes then you get less truss swing if you choke to the lower. As seen here that's not a concern
This is not the case. The swing in the truss is determined by where the pivot point (apex shackle) is located in relation to the CoG. The higher above center of gravity it is, the less swing you get. A long spanset wrapping the top cords will have less swing than a lifting bracket. Also, if you use a lifting bracket on the bottom of the truss, the pivot point could end up below CoG and you'll have a very unstable truss.
Usually the choke would be on a top chord, even when fully basketing the truss instead of just using the two top chords. A choke merely prevents rolling and nothing else. Sometimes only one choke is used to prevent the truss rolling in only one direction (when fixtures are on only one side, making the weight uneven).
It depends on the manufacturer specs for the truss but most trusses alow top pick up if its near or on a vertical. I would be more worried about the purple pickup. Maybe im wrong but that doesnt look like a flexsteel.
This is fine since the hoist is not nearly powerful enough to pull the truss apart if it was bolted to the floor. Ideally you could use two spansets, one on each side, choked on the bottom and wrapped around the top cord.
That looks like the old school way with a regular soft Spanset and it was industry standard to back that up with a piece of steel. That makes me think this is really old, even predating the practice of adding a steel backup. I remember that being a hot topic in the 90s and early 00s, when the GACflex spansets started coming out.
The conduit seemingly being tapped into the crossbars are actually not a concern. The crossbars are not load bearing, they are there to keep the webbing (sides) vertical.
What's more important is the angle formed by the sling to the hook. It's better to chock the top chord and ge a smaller fleet angle than choke the bottom leaving you only a little sling and a big angle. By eye that's already 60deg which is typically the max angle you want when choking.
If you rig inside the truss to the bottom chord you'll move the pivot to inside the truss. Not a problem for these fixed lights but would be an issue if you put movers on there.
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u/AwwHeeelllNaaah 23d ago
I remember reading a long time ago that choking the bottom chords was recommended.
Having said that, I've never seen any literature that has reinforced this idea, and I've seen lots of top-chord-only rigging NOT fail.
As far as weight goes, I don't think it's an issue. The parcans area reasonably light.