About Freestanding Aerial Structures

in Aerial Equipment, Rigging on .

About Freestanding Aerial Structures

Freestanding aerial structures are a very unique item from the Circus Industry, and very peculiar.

They are usually found in 3 different types.

  • A-Frames
  • Quadripods (very similar to A-Frames)
  • Tripods

They have advantages and inconveniences to each one of them. In the next lines I will do my best to explain the Pro's and Con's, and the things to look for in these kind of rigs.

One very important point that is often forgotten, is that using a pulley at the top of the structure with a cable going down one leg will divide by half the allowable load on your structure! More explanations on this phenomenon below.

General recommendations on structures.

If you do not know what to buy, here is a short guide:

1. Decide what are you needs.

Most rigs can be categorized as: Less than 15', or 16' or more. 15' or less rigs are more affordable, but often less stable. On the other hand, they are lightweight, transportable and often can be used at lower settings indoors.

15' or less

If you plan to train at home, do simple drops, keep in shape, your probably fine with lower rigs. Go measure on your house 15', you will know quickly if it can be good enough for you. Tripods this height are not great, since often you'll want to use 2 points to maximise the available height, and might want to do figures very high up. The legs which are close together at the top might become cumbersome.

16' or more

If you plan to train higher skills, higher load tricks, want to perform with it, or do straps (which usually require more height), then this type of structure is for you.

Then you are left with the choice of Tripods or Quadripods. If you do not need lots of height at the top, or do high load props, I'd recommend the Tripods. Most tripods also do not accommodate Trapezes, except CircusConcepts' ACHILLE using the Trapeze Spread bar. They are also lighter and more portable.

Quadripods on the other hand offer more space at the top, and often many will be tempted to use it with 2 different props on each end of the bar.

Although, Quadripods are usually less stable by design, and not designed nor tested for such loads. If you plan to do this, ask the manufacturer!!

Also, even if they are often engineer rated for higher loads, the reality is that they aren't in normal use necessarily stronger. Why? Because they stand on 4 legs. Just like any chair or table with 4 legs, unless you have a perfectly even space of 20 x 25 foot, the weight distribution on the legs will be uneven. One leg will end up receiving more load than the other, and this one will buckle first. Tripods do not have this problem, as loads always equalize on 3 legs.

They are often tested on large, perfectly flat industrial floors, and often not tested, only load calculated. The reality is that most of us will not have such a large space, perfectly even, so we will hinder the capacity of the structure.

Here is more detailed info on the available type of structures:

A Frames

Pro's

· You can find some of them very affordable.

Cons

· Often not load rated (not appropriate for Aerials use)

· Usually not stable sideways

· Usually quite low

What to watch for:

Vertical legs on each side - they will be very unstable and move sideways a lot, and force the joint. Not a good design

Side legs should be at an angle, but not too much, unless they are attached together (from one side to each other)

Are all the legs attached together with cables? If they are all the rig will be a lot more solid. If not, unless the angle of the legs is small, they won't be able to resist high load that dynamic tricks can cause.

Quadripods :

Pros

· More space at the top, enough space to rig trapezes and in some cases 2 different props at the same time (*not recommended unless manufacturer approved, as most are not designed for this!!)

Cons

· Less stable than tripods

· Will lose lots of their rated strength if not on perfectly even floor

· Take more floor space, very weak if not well designed.

· Heavier than other structures.

What to watch for:

· All legs should be linked together with cables

· Joint between legs and top bar should be very solid, and ideally with diagonal bars

· Large top bar (otherwise, get a tripod!)

· Legs at not more than 25 degrees angle. Otherwise they will be very long for the height, and their weight will worsen the buckling, which is what limits the load of such structures. An effect often not considered by 'classic' engineering, so unless the rig was really break tested, will result in lower than expected breaking load.

· Not too heavy.

· Limited play in joints.

· Not too many joints. The more joints, the lower the load and the less stability.

Tripods :

Pros

· Most stable structures.

Cons

· Limited space available on top

· Only 1 top point, unless a Trapeze Spread bar is available

What to watch for:

· Tripods are the easiest to use and most stable structure. It's hard to design them wrong.

· Legs at not more than 25 degrees angle. Otherwise they will be very long for the height, and their weight will worsen the buckling, which is what limits the load of such structures. An effect often not considered by 'classic' engineering, so unless the rig was really break tested, will result in lower than expected breaking load.

· All legs should be linked together with cables

· Limited play in joints.

· Not too many joints. The more joints, the lower the load and the less stability.

Using pulleys and mechanical advantages, and it's result on the structures

UNDERSTANDING Load differences between using a Pulley or attaching directly to the top point(s)

Using a pulley at the top of your structure will double the load that you and your props apply to the structure. The effect is well described below. It does so even more from the fact that on Freestanding Structures, the rope going through the pulley is usually attached to one of the legs of the structure (ideally, if you use 2 points and you can, attach it to 2 different legs!)

Buckling is the strength limitation of those structures, and the rope attached to the leg pulls 'up' the leg, while the loads 'push it down', thus doubling also the force on the leg itself, weakening it. Luckily, often in Aerials the dynamic loads are very short lived, so the 'bounce' of the structure absorbs some of it to dissipate it, before the leg has time to buckle.

Here is an extract to the ACHILLES Freestanding Aerial Structure's Use Manual, which explains well the difference.

This Tripod has a Minimum Breaking Load of 5500 Lbs / 2500 Kg / 24.5 kN at full height. Safety standards limit the actual load to a 5:1 safety factor, meaning that you can only apply a maximum load of 1100 Lbs / 500 Kgf / 4.9 kN to your tripod's attachment point(s).

When fixing your props directly to the Eye Bolts, 4.9 kN is the total maximum Force that the artist, multiplied by a dynamic factor, plus the weight of the props can generate. See example below

When fixing your props with a rope attached at the bottom of the tripod, through a pulley to your props, 2.45 kN is the total maximum Force that the artist, multiplied by a dynamic factor, plus the weight of the props can generate. See example below.

Here is a Schema of the forces to help you better understand this principle:


Examples:

*Note that in my example, I do not multiply the weight of the prop with the dynamic factor - why? Because in almost all cases, the prop is static when the artist loads it, thus the weight of the prop is not multiplied.

#1 - Situation: One artist weighing 68 kg - 150 Lbs attached to the rig, using a 50 Lbs Trapeze, and doing very hard dynamic moves.

The prop is attached directly to the Eyebolts

Load:

Artist Weight * Dynamic Factor + Weight of Trapeze = Total Load

150 Lbs * 3 + 50 = 500 Lbs force Total Load to the Tripod.

Load is OK for the Tripod


#2 - Situation: One artist weighing 68 kg - 150 Lbs attached to the rig, using a 50 Lbs Trapeze, and doing very hard dynamic moves.

The prop is attached trough a pulley to the bottom of the Tripod.

Load:

(Artist Weight * Dynamic Factor + Weight of Trapeze) * 2 (we double the load due to the Reaction force at the bottom = Total Load

(150 Lbs * 3 + 50 ) * 2 = 1000 Lbs force Total Load

Load is OK for the Tripod but near the recommended limit

One has to understand that a dynamic factor of 3 is very high. Often, dynamic forces will be lower, but as a rule of thumb, 3 is a safe factor to use.

To determine the load applied by the Artist and the props, you should always test it using a Load Cell.

The next 2 situations show the difference it can make very well. Both are the same artists and props, but rigged differently.


#3 - Situation- Duo Working trapeze on the Tripod, each artist weighing 68 kg - 150 Lbs

NEVER will both artists do dynamic moves at the same time (one is the porter (fixed weight) and the flyer creates forces by swinging) Only one of them will do a Dynamic move while the other artist is 'stable'.

The prop is attached directly to the Eyebolts

Load:

Dynamic Artist Weight * Dynamic Factor + Fixed Artist Weight + Weight of Trapeze = Total Load

150 Lbs * 3 + 150 + 50 Lbs = 650 Lbs force Total

Load is OK for the Tripod


#4 - Situation -Duo Working trapeze on the Tripod, each artist weighing 68 kg - 150 Lbs

NEVER will both artists do dynamic moves at the same time (one is the porter (fixed weight) and the flyer creates forces by swinging) Only one of them will do a Dynamic move while the other artist is 'stable'.

The prop is attached directly to THROUGH PULLEYS

Load:

Dynamic Artist Weight * Dynamic Factor + Fixed Artist Weight + Weight of Trapeze + Reaction Force

= Total Load

(150 Lbs * 3 + 150 + 50 Lbs) * 2 = 1300Lbs force Total

Load is NOT OK for the Tripod - It exceeds the recommended safety factor. The trapeze should be attached directly to the top points, not trough a pulley.


TYPICAL SET UP of Rigging for Freestanding Structures

Note that the first 2 setups will double the force on the Tripod, while the 3rd one will not.

 

If you wish to use mechanical advantage (pulley block) WITHOUT compromising much the full load bearing capacity, refer to the sketch below for 'Minimal Set-Up'.

Setup with Mechanical advantage with little compromise on strength .


Extra factors

When approving loads for structures, we are always putting a 5:1 minimum safety factor on the working load. This will account for Metal Fatigue, impurities in materials, outside factors (like uneven ground) and human factor (basic misuse)

There is still a lot of research also to be done on dynamic loading on freestanding structures. We are currently working on this subject, and already made so Research and Development programs on this subject. More data to come in the future on this.

Conclusions:

The use of freestanding aerial structure has many factors to it. The most important is to asses your needs and the types of loads you will be applying, and then getting a structure that is appropriate for this use.

Especially for any high structures, use only rated structures by known and reliable manufacturers, where they provide a certificate with the item, without having to request it.

A 'shaky' structure, even if rated a high load, might not be as strong as advertised. 'Shakiness' is instability, instability will result in uneven loading, thus a load concentration on some parts of the structure which will fail prematurely. We are aware of structures on the market with advertised load which seem, and probably are, impossible.

Engineering of Freestanding Aerial Structures is not rocket science. As long as the welds are done by a professional (a certified one ideally), and unless the structure is quite badly designed, the buckling effect is usually the weaker factor of a structure, and this strength is quite easy to calculate, and even to guess. The larger diameter the leg, the stronger. That's it… Then, the more joints, the more 'play', and the play will increase the buckling effect. There are other factors to consider, but this is mainly it. Joint will less play/movement will offer better strength, and the opposite way too.

Investing a little more in your structure is always a good investment. Remember that these are small amounts compared to others. If you are a professional, you can pay back your investment in a few contracts.

A truck driver or a building contractor will need years to recoup his tooling or truck investment. In the circus we are lucky to be able to use affordable items.

But, don't be afraid to invest a bit, and maintain or change your items as per manufacturer's recommendation. It is your life and well being at the end of the line.

Why do you have to change the brake pads on your car every so often? Because they use, and need to be changed for your safety. Aerial props are used and abused, and since they are affordable, it's often best to change the whole item after it's service life.

And finally, go have fun and be safe. Circus is made for everyone, and there is a discipline for every person standing, or not, in the circus to keep you having fun and enjoying the freedom of creating.



Photo Credit : Erin Ball, workshop for disabled persons, Kingston Circus Arts.

Last update: Nov 02, 2020

Hugo Noel

Name: Hugo Noël President of CircusConcepts and internationally recognised Artist Grew up in: St-Antoine-de-Tilly, Québec, Canada Raised part-time on a sailboat sailing around the Atlantic Ocean, Hugo is a very friendly person who likes to spend quality time with friends. Since he discovered circus in 2003, he has been consumed by his passion for creation and physical accomplishments. He first started by learning the basics at the Québec Circus School, and at the same time started competing in trampoline. Just in time for him to finish his studies in mechanical engineering, he was hired by Cirque Starlight, in Switzerland, where he made his debut as a professional circus performer. He since then worked for various companies in touring or fixed show in Europe and America. After leaving the “LOVE” show in Las Vegas, he went back to Canada to perform his many skills in corporate events. His acts have had since a great success, having been presented in over 25 countries around the world, more than 15 festivals and numerous awards, including 4 at the most recognised circus festival of Monte-Carlo, and his act was again presented in 2016 for the "Best of last 40 years" in Monte-Carlo. There are now an average of 15 artists working for Circus Concepts for galas and shows, and also have 3 persons working with us in the office engineering side of the company, 4 in the manufacture, creating the dream apparatus of the artists. Using the skills he has been taught by his father since very young, and other knowledge he acquired in a work and study education in Mechanical Engineering and while he “ran away with the circus”, Hugo also draws and shapes metal to help circus artist achieve success – making people believe in dreams. See shows.circusconcepts.com for more info on shows.

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