Motor load limits and other rigging...

[JBKC21]

Searched and couldn't find what I was looking for.

Quick background info: My Venue has a main grid that's flown from the high steel. This rood structure is a one off, custom designed system and not your traditntal steel roof structure. Because of this, the main truss grid is currently hung from 4 2-ton motors. Each point on the high steel is a welded D-Ring rated at 2-tons.

Here's the question. What's everyones comfort zone in regards to nearing maximum weight on those points/motors? Because my main cable drop happens on my USR side, that motor has about 700lbs more weight than another motor. On shows where the act wants to fly additional lighting or like this show next week, fly their own video wall... I'm always up against my weight restrictions. I've had a few shows where I'm riding 99% of the limit.

I'm not a rigger, so that's out of my comfort zone! I'm looking for general guidance on what's ideal, what's safe and what's realistic.

Thank you!

EDIT: Crap, I just realized there is a rigging section on the forum. Feel free to move.

 

[hobbsies]

Do you actually fly this grid in and out? If not, why not install more permanent lines that can withstand higher weight limits.

 

[danTt]

A motor is rated for what a motor is rated for. That being said, Do you know how much your structure weighs? When you start lifting on more than two points, the loads on any individual motor get tricky to calculate, motors do not run at the same speed and as the grid gets out of level different motors will take different amounts of weight. This sounds like a wise place to use load cells. If you do not trust the rig to hold the weight you are putting on it (and making this post suggests to me that you might not be) I'd recommend consulting with local engineers and riggers on current loads, and alternative methods of distributing the weight. When you're looking at that much weight, guessing is a bad idea.

 

[porkchop]

If you're running that close to the limit very often I would make sure that you install load cells so that you actually know how much the weight is on each point rather than knowing how much weight should be on each point. Decent systems can be had for reasonable prices, especially for only 4 points.
As far as how to handle shows that want to hang heavy objects in your space, is it reasonable to build motor cradles that can sit on top of your grid so that these shows can have their own points and not weigh down your grid? Depending on the original design you might need to bring in an engineer to certify that the grid can handle the additional points, but it could give you more flexibility to offer to the customer while also making the venue safer. That combination isn't that hard to sell for capital investment.

 

[Amiers]

WL and rigging aren't much discussed on these boards cause of the uncertainty of most rigs.

If you feel it can't hold the weight talk to the people that built it.

Nobody here can tell you what you have or what it is rated for and hold it operates.

That being said.

Be safe and don't let people bully you to do something you are not comfortable doing. Saying No sucks but sometimes it's safer.

 

[soundlight]

I would highly suggest a cable pick motor to take the weight of your cable drop.

 

[JBKC21]

Thanks for the responses. I do have load cells on all four points and never have exceeded the limits of the points/motors. I know how much weight the roof structure can support and I'm thousands and thousands of pounds below that.

I guess my original question is more about what other people are comfortable with. We all know that if a cable, motor, point etc is rated for "x" it likely can handle far more. While I'd never exceed the "limit" and I've had a few heated arguments with visiting tour/production managers About that, I'm wondering what the comfort zone is? I've had one show where my load cell was at 3998lbs all day on the USR motor. Made me nervous all day despite this being within the limit. That's the top of my comfort zone I guess!

My total rigs weight is 6,790lbs. It went up very slightly this year with the install of the new D&B PA.

 

[porkchop]

Thanks for the responses. I do have load cells on all four points and never have exceeded the limits of the points/motors. I know how much weight the roof structure can support and I'm thousands and thousands of pounds below that.

I guess my original question is more about what other people are comfortable with. We all know that if a cable, motor, point etc is rated for "x" it likely can handle far more. While I'd never exceed the "limit" and I've had a few heated arguments with visiting tour/production managers About that, I'm wondering what the comfort zone is? I've had one show where my load cell was at 3998lbs all day on the USR motor. Made me nervous all day despite this being within the limit. That's the top of my comfort zone I guess!

My total rigs weight is 6,790lbs. It went up very slightly this year with the install of the new D&B PA.

That is the type of specific rigging question we try to avoid answering because we can't possibly know all of the details of your situation. The best advise I am personally willing to give you is to make sure your equipment is well maintained and ask the manufacture if you have any questions.

As a side note, many CM chain hoists are rated in metric tons (2200 lbs.). Perhaps that will help ease your concerns.

 

[Chris15]

If the system as an engineered whole has a safe working load of 2 tonnes (metric), then you can load it to 2.0 tonnes.
The engineering will be designed with a relevant safety factor which means minimum breaking strengths are several times more than the SWL, that safety factor being dependent on the application and industry best practices...

 

[Footer]

Agreed, a chain motor already has the design factor figured in. Also, good on you for having load cells already in place. That at least gives you something to sleep by... though load cells are never perfect. For the odd ball video wall, that should be going to its own points/motors/truss. Video walls are actually a very dynamic load when loading them in.... lots of starting and stopping and they can get rather heavy fast. Also, the people carrying video walls without rigging are the worst. I also agree with @soundlight, get something for cable management to get that extra weight off the grid.

 

[JBKC21]

Agreed, a chain motor already has the design factor figured in. Also, good on you for having load cells already in place. That at least gives you something to sleep by... though load cells are never perfect. For the odd ball video wall, that should be going to its own points/motors/truss. Video walls are actually a very dynamic load when loading them in.... lots of starting and stopping and they can get rather heavy fast. Also, the people carrying video walls without rigging are the worst. I also agree with @soundlight, get something for cable management to get that extra weight off the grid.

I've struggled for a few years now to come up with a clean and useful solution to getting the weight off. I have a spare 1-ton motor meant to be used as a cable pick but lack a easy way to hang it. I'll post some pictures and see if anyone has some ideas to offer.

 

[Van]

.... I'll post some pictures and see if anyone has some ideas to offer.

Cool, 'cause I'd love to see it.
What venue? if I may ask, or if you're allowed to say.

 

[Moose Hatrack]

Based on images posted earlier, he's 4 blocks North of me in a VERY popular outdoor venue. I often stroll through it during lunch.
EDIT: typo

 

[kicknargel]

As stated, the motors and other manufactured rigging gear have already applied a design factor (or safety factor) before giving you the working load limit. The actual breaking strength is usually several times higher than the published rating. So loading to the given limit under the intended usage application is OK. But, what about those more custom D-rings on your high steel? Who determined their rating and what design factor did they use? How scientific was it?

 

[JBKC21]

As stated, the motors and other manufactured rigging gear have already applied a design factor (or safety factor) before giving you the working load limit. The actual breaking strength is usually several times higher than the published rating. So loading to the given limit under the intended usage application is OK. But, what about those more custom D-rings on your high steel? Who determined their rating and what design factor did they use? How scientific was it?

Thanks for the reply. The D-Rings were designed and engineered by StructureFlex who designed and engineered the roof structure. I have drawings, blueprints etc that verify their capacity etc.

They somewhat weird me out, but mainly because I come from more traditional non-install style of production. You know typical burlap and wire rope/steel over beams etc. That method just looks really strong compared to a welded ring!

 

[JBKC21]

Based on images posted earlier, he's 4 blocks North of me in a VERY popular outdoor venue. I often stroll through it during lunch.
EDIT: typo

You're probably correct! Here's a few recent photos.

 

[Moose Hatrack]

JBKC21, I applaud your humility, instincts and vigilance. And I thank you for it. Kansas City does not want another tragedy like a skywalk collapse to grieve over. Nor do we want to be thankful that nobody was in the arena when the roof collapsed. Keep thinking like a detective!

 

[TimMc]

JBKC21, I applaud your humility, instincts and vigilance. And I thank you for it. Kansas City does not want another tragedy like a skywalk collapse to grieve over. Nor do we want to be thankful that nobody was in the arena when the roof collapsed. Keep thinking like a detective!

Satire warning-

What was the hit song by the Hyatt Regency Skywalkers? Bridge Over Troubled Waiters! I'll be all night, folks...

No humor for the Kemper Arena roof...