In event rigging, the standard safety factor is actually 7:1. So if a motor is rated for 1 ton, it can be expected to actually hold 7 tons. The extra safety factor is mostly because of shock loads, where the rig weighs a lot more when it bounces. Like if you’re lifting your entire rig off the ground and stop the motors, it will tend to bounce slightly as the motors all click off at the same time. And that bounce causes the rig to exert a lot more force than a simple static load. Dynamic load probably isn’t a major concern for permanent structures, (probably why you can get away with only 20% as standard) but it’s a big concern for motors and truss that move around a lot.
Especially in indoor events (where wind is a negligible factor), the vast majority of failures are caused by dynamic loads or by one motor being overloaded compared to the rest. When that overloaded motor fails, that can cascade to other motors as they are suddenly holding more weight than expected. Imagine lifting a uniform load using rigid box truss, but one motor on the end of the truss is slightly higher than the rest. That one motor will end up holding a lot more weight than the rest. And if that motor on the end fails, you have just caused a shock load on the next motor, as the rig settles onto it. And if you’re already nearing your weight limits, that single failure can quickly cascade into the entire rig collapsing.
Better rigging systems actually have weight sensors on each motor, so you can know exactly when one motor is overloaded. But the vast majority of event rigging isn’t done using the nice stuff. It’s usually done using the chain motors that have been rebuilt 3 times in the past decade.
As an engineer, what the fuck‽ Who doesn’t double check the weights then add a safety margin (20% is traditional for us)‽
In event rigging, the standard safety factor is actually 7:1. So if a motor is rated for 1 ton, it can be expected to actually hold 7 tons. The extra safety factor is mostly because of shock loads, where the rig weighs a lot more when it bounces. Like if you’re lifting your entire rig off the ground and stop the motors, it will tend to bounce slightly as the motors all click off at the same time. And that bounce causes the rig to exert a lot more force than a simple static load. Dynamic load probably isn’t a major concern for permanent structures, (probably why you can get away with only 20% as standard) but it’s a big concern for motors and truss that move around a lot.
Especially in indoor events (where wind is a negligible factor), the vast majority of failures are caused by dynamic loads or by one motor being overloaded compared to the rest. When that overloaded motor fails, that can cascade to other motors as they are suddenly holding more weight than expected. Imagine lifting a uniform load using rigid box truss, but one motor on the end of the truss is slightly higher than the rest. That one motor will end up holding a lot more weight than the rest. And if that motor on the end fails, you have just caused a shock load on the next motor, as the rig settles onto it. And if you’re already nearing your weight limits, that single failure can quickly cascade into the entire rig collapsing.
Better rigging systems actually have weight sensors on each motor, so you can know exactly when one motor is overloaded. But the vast majority of event rigging isn’t done using the nice stuff. It’s usually done using the chain motors that have been rebuilt 3 times in the past decade.