Multipoint Suspension – It’s not as easy as it seems
Picture an average gig. The truck doors open, a bustle of activity explodes around the truck as stage hands push boxes to where they need to be and the riggers are in the air rigging chain hoists for the truss. It’s a simple show, 1 span of truss upstage and downstage with moving lights suspended by two 500kg hoists each.
Except, today, they added 2 LED screens to the upstage truss. Nobody mentioned to the head rigger that this was happening and the schedule is tight already. He eyeballs the screens, figures that his truss might be overloaded with that load on such a long span and so he adds a third hoist to the centre of the span to take up the extra load.The truss gets hoisted into the air and the centre hoist promptly fails. When the centre hoist fails there is too much load on the outside hoists and they fail also. The truss hits the ground damaging expensive equipment and potentially taking lives.
Why? Was the hoist faulty? Were the screens heavier than he thought? On any other day this could have been the case, but not today. The centre hoist was overloaded. Let’s look at the original loads first.
12m span of truss – 75kg
20 x moving light fixtures (evenly distributed on the truss) – 35kg each – 700kg
Total load on the truss – 775kg
Obviously, two 500kg hoists will hold that easily.
Next we look at the loads after the LED screens were added.
12m span of truss – 75kg
20 x moving light fixtures (evenly distributed on the truss) – 35kg each – 700kg
2 x LED Screens – 300kg each – 600kg
Total load on the truss – 1375kg
Here, it’s equally obvious that the 2 hoists don’t have enough capacity to handle the required load. Adding a third 500kg hoist should give us a third of the load on each hoist, or, about 345kg, right? Adding that extra hoist should be more than enough to support the additional weight and get the show on the road.
It isn’t.
The answer is in the load calculation for multi-point beam loads.
In Figure 1 we see the truss span with a hoist at each end. In this case, intuition tells us that each hoist must be supporting half the weight of the truss span.
Figure 1 - 2 Hoist Suspension
Figure 2 shows three hoists supporting the same piece of truss. Some would say that you have added a third hoist and so the resulting load on each supporting hoist is one third of the total weight. This is not the case however.
Figure 2 - 3 Hoist Suspension
To determine the resulting loads on support points on a beam or truss supported by more than two hoists we go to the Three Moment Theorem. The maths involved are quite complex and beyond the scope of this article but Figure 3 shows a chart of the resultant loads for multiple suspension points.
Fig 3 – Mathematical Prediction of Multipoint Suspension
As we can see, adding a third hoist to the truss means that the load on the centre hoist is now 63% of the total load! If we look at the loading figures again we see that with a total load of 1375kg the centre hoist is now holding 866kg.
In this case, the rigger was correct that his two hoists were not capable of suspending the load, but, adding a third hoist did not solve that problem, it just made it worse. What other options were available to solve this problem?
Below are just some of the options available.
- Add 2 more hoists for a total of 4. Then the heaviest loaded hoists would be seeing 36% of the load or 495kg, just within the working limits of the hoists. This option leaves very little safety margin to account for dynamic loads of up to 1.4 times the load when chain hoists start or stop.
- Change the 500kg hoists for 1000kg hoists and still use only 2 hoists. Each hoist sees 50% of the load or 688kg. Provided the truss is rated for the load over the 12m span then this is the ideal situation.
- Add a load cell system to measure the individual hoist loads to ensure that the system is not overloaded.
- As a last resort, find somewhere else to rig the screens so that they aren’t overloading the hoists on the truss.
This is just one example of a situation that could happen and does happen on a regular basis. Knowing these load charts or having a reference with you on the job can make your rigging safer, and faster. These load charts, however, are only a prediction tool. They make assumptions such as the load being perfectly uniform or that the hoists are always synchronised with each other.
If the load is not uniform then the loads on each hoist will vary from the prediction. If one hoist runs faster or slower than the others the same case would apply. Picture a situation where the outer hoists run significantly slower than the centre hoist. In that case, 100% of the load could be transferred to the centre hoist (see Figure 4). The reverse is also true where the centre hoist is slow then the load would be transferred to the outside hoists (see Figure 5). Both situations will overload the hoists and cause potential hazards.
Fig 4 – Centre Hoist is overloaded and the outside hoists are unloaded
Fig 5 – Outside hoists take the entire load while the centre hoist is unloaded
Predictions and mathematics can go a long way to helping us understand the load distribution but in the real world it doesn’t always happen as we predicted. To know what is happening with our loads we really need to add a load measuring system. By inserting a load cell into the system at each suspension point, we can track in real time the actual loads on the system. If a hoist is running slow or fast it will be seen on the load cells long before it actually causes a serious problem.
The combination of proper calculation and planning before rigging the load combined with load cell systems to monitor the loads as it actually happens, means that you can ensure that the system is safe at all times.
Remember, just adding an extra hoist doesn’t always mean it is going to do the job.
For more information on safe rigging practices in the entertainment industry we recommend:-
Entertainment Rigging— Harry Donovan (http://www.riggingbooksandprograms.com/)
Maths for Riggers—More Skills Australia (http://www.moreskills.com.au)
Prolyte Black Book—Prolyte (http://www.prolyte.com/brochures for a PDF or contact SSE for a Hardcopy)