This subject matter can become extremely complicated extremely quickly, but with this article, we will try our best to explain many basics principles and some more advances, so you can better assess the capacity needed for selecting and using your structures and the equipment you use.
Here at CircusConcepts we try to merge both the beautiful and creative that aerial performers need with the design and safety that riggers and engineers demand. In the words of founder and artist Hugo Noel
"My goal is to provide access to safe quality equipment to anyone, anywhere who wish to share my passion"
N.B. If you wish to jump to the conclusion: a shortlist of what is and is not recommended, you can scroll to the end of this article. But, we very highly recommend you to read this document attentively.
Introduction
First off, I would like to thank you to Sophie Latreille from the Ottawa Circus School for initiating the idea of our HOME Freestanding structures, the design of this structure gave me better understanding how users and owners of aerial studios consider forces and loads, which in turn, identified which principles had to be better explained to the users.
When I initially wrote on the HOME MAX Structure, “not recommended for high load acts or act with high dynamic factors/g-force” almost instantly every person interested in the structure was cautious and started to wonder: What can I and can’t I do on this structure !?
The statement gave the impression that the structure was not designed for most users.
I had initially put a more conservative capacity than the actual strength of the structure. This is not something most manufacturers do. I did it because I thought it would help people better assess, what to do with their structures… but turns out it did the opposite, it only seemed to make users more worried. I quickly realized that most users had no idea of the forces they create on their rigging points.
I received videos of acts and tricks pulling high loads that seemed quite standard, and that were exceeding the capacities for most structures. Many users commented: -well if I cannot pull that trick on that structure, the structure is not good enough for me.
But what most people do not understand is that it’s often the opposite… the structure’s capacity is probably good enough for their applications. But what you are doing is not ideal for you and your apparatus. Example: Doing a trick that creates 900 lb force on a pair of silk, which breaks at average 2400 lb, is not an acceptable design factor for human rigging.
Hence why I have created this article which can maybe help you understand and simplify this very complicated subject.
Goal
This document’s goal is to give you a better understanding of the basic types of forces and load you can generate with your act, how to calculate them roughly or precisely. Also give you resources to better your knowledge of the subject and at the end of day allow you to HANG IN CONFIDENCE.
Main Glossary terms for this article:
MBS( Also called MBL ) :
Minimum Breaking Strength( also sometimes referred as Minimum Breaking Load )
WLL
Working Load Limit – the maximum amount of load the items can safely handle.
DF
Design Factor / Also sometimes called Safety Factor – The factor between the MBS and WLL
.
G-Force
G-force is the Acceleration Vector - That is the multiplication of force due to acceleration or deceleration. Newton’s law – F=m*a - Force = Mass * Acceleration. It is also referred as Dynamic Loading in this article
The Basics
Minimum Breaking Strenght VS Working load Limit ( WLL )
There are two main ways to describe a load for aerial structures in the circus.
Minimum Breaking Strength (MBS ) aka Minimum Breaking Load (MBL )
This is the load at which the item will start to fail and ultimately fail catastrophically if the load is not removed.
It is usually established by the manufacturer of the equipment based on tests performed on samples of the equipment until they fail.
It should always be the LOWEST possible load measured during the testing. Say you test 3 parts of a silk, they break at 2200 lb, 2150 lb and 1900 lb, the MBL is 1900 lb.
It should be known that in almost all occurence, should you apply a load near the MBL, you will permanently damage the equipment and it will not be capable to bear that load anymore. The item must be discarded.
WLL (Working Load Limit).
On the subject of working load limits, I could write a whole book… but let’s keep it simple.
When asked about giving equipment WLL, I always say that calculating loads for circus equipment is very hard, for this essential reason: An artists’ goal is always to create something no one has thought of or imagined, while my job when assessing loads is to try to think about everything someone could possibly do on this item….
As opposed to rigging and climbing gear where the loads are something to be avoided or limited to the smallest amount possible, circus is different. Artists often will try to push the limits of their body and the items they use.
The working load limit (WLL) is the maximum force that can be safely applied to an item, continuously, for a long period of time AND/OR a high number of cycles/repetitions.
A working load limit is determined by the manufacturer, and can be subjective, although there are many regulations and standards in place to help standardize them in many industries. As the Circus world is without such standards, it must use the same methods and standards as used in the sailing, climbing, rigging, manufacturing industries.
The WLL is important to understand, it is there for your safety. It will compensate for many unknowns like uneven floor/stability, impurities in material, external factors, and accidental loads.
Recommended Max User Weight
For ease of use, at CircusConcepts we also added the Recommended Max User Weight on all our items. That makes it quite simple for someone to understand if this is OK for them or not, but there is a limitation there too… It is calculated for ‘normal use.’ Outside normal use, the artists must contact us and/or use a load cell to assess their act’s need.
Disclaimer
‘The information below will allow you to better understand the load being applied to your structure but should not be used for calculations or deciding if any other, structure or installation is safe to be used. Always consult with a competent person (engineer or rigger) prior to rigging your equipment to a structure.’
Presently in the circus industry, there are two ways of calculating working load limit.
BASIC LOAD CALCULATION METHOD
This is the 10:1 ‘basic method’, which is often most widely used.
Pros: Extremely easy and quick to calculate. Can be performed by anyone.
Cons: This method is only good for use in beginner to intermediate acts and skill level. It can lead to use of equipment at loads they are not designed for.
How to Calculate it:
It is quite simple. It’s 10 for 1 on the Minimum Breaking Load. That means you take the MBS, divide it by ten, and you have the maximum User+prop+rigging allowed weight.
Example: So, let us say you have a structure with 3750 lb of MBL, you would assume a working load limit of 375 Lb. So, you could have a Working Load totalling 375 lb on it.
But now, as mentioned earlier I have witnessed and even measured artists generating loads of up to 10 times their body weight due to acceleration vector – that means that using that technique, even in a perfect setting, they would reach or surpass the MBL. This is not acceptable and can easily cause injuries to the performer. And, of course, no setup is perfect...Meaning - The structure is tested in ideal conditions/perfect condition. But your setup, position, age of structure might differ. Meaning it probably would break at lower load than the MLB. This is why Safety Factor is important.
How to improve it:
If you wish to adjust/improve it your safety margin, you could increase the ratio according to your use/level. This is the IMPROVED BASIC LOAD CALCULATION METHOD
- You can read information below about the other techniques which could guide you, but in short:
- If you have a highly dynamic act, use at least 15:1 ratio.
- If you do high drops on Spanish Web, use 20:1 ratio.
- If you do a trick that seems to create extremely high load, get someone to measure the loads, or use 20:1 ratio.
- Considering what we know about the loads often resulting from these acts, this is already a better indicator.
ADVANCED LOAD CALCULATION METHOD
Then, there is the Advanced Method. This technique, which is the one approved and used by riggers and engineers, is what we like to use at CircusConcepts. It is a more complicated method, but we use it as we consider it safer.
Pros: Allows to know precisely what the loads are that can be used safely on the equipment and limits the risk of damage or failure.
If you are an advanced user, you will highly benefit from it.
Cons: You need to know exactly the load your tricks generate to your structure and rigging. The reality is that most people do not have access to load cells to measure the loads. But you can use reference table to give you a fair idea, or ideally get or borrow a load cell for tests.
This method will give you the maximum load you can apply on an equipment’s, which includes dynamic loading/g-force . Do not load any item more than the working load approved by a manufacturer.
The best practice to calculate the working load in America and Europe, for lifting a human, is 5 times less than the MBL (Minimum Breaking Load). So that is a Design Factor of 5.
That maximum load includes dynamic loading and peak loads. It is not the maximum user+prop+rigging weight. The factor of 5 is to account for all “side factors” and fatigue.
Further on, I will explain why this “side factors” safety’ is important to consider.
Breaking loads are always tested in near perfect loading conditions. The Design Factor accounts for Side factors like aging, material impurities, misuse, and any other factor we might not even think about when designing and building an aerial equipment.
How to calculate it:
The Design Factor varies on the construction material.
Generally, we can say:
On metal structure/items: 5 for 1(Example: Carabiner, 20 Kn MBL, 5 Kn WLL)
You divide by 5 the MBL of an item, you get the WLL.
- Steel and Aluminum Freestanding structures
- Carabiners, Swivels etc. etc.
- Steel Cable cored Trapeze or Hoop Ropes
On fibre and textile equipment: Minimum 8:1, ideally 10:1 (Example, Spanish Web, 6000 lb MBL, 600 lb WLL)
You divide by 10 the MBL of an item, you get the WLL.
- Slings
- Aerial Straps
- Spanish Webs
- Aerial Silk
- Rope
This will result in the first Calculation = Maximum WLL of the equipment.
Nb: In most industries, G-Force are very low so they are neglectable, resulting in a Working Load Limit being the maximum static weight you can safely put on an item. But in the Aviation and Circus Industry, we must work differently. Thus, WLL is NOT the total of artists weight + props.
Second Calculation - Calculating G-Force / Dynamic Factor.
G-Force can vary a lot, but often the limitation is the human body. Most human will pass out with a 5G force of a few seconds.
Most acts will create 2 to 3G of force, and some professionals up to 5. In rare cases, mostly in recent Spanish Web development, up to 10.
G-Force is linked to Acceleration and Deceleration. The variation of speed versus time give the result. Which means that for the same Deceleration, the longer the time for this to happen, the lower the G-Force.
Let’s see it as jumping from 1m high onto Concrete or Water. The water will ‘slow you down’ gently, creating less G-Force, thus less load on your legs. The Concrete obviously will create quick deceleration resulting in high load on your legs.
Applied to Circus:
As a rule of thumb, you must understand that:
The more ‘stretch’ your system has, the lower the dynamic load in the system you will generate (because the load will be dampened by the elasticity)
Thus, the less stretch your system has, the higher the possible dynamic force.
Examples:
- Using a non-stretch fabric
- Using a Spanish Web, you will create higher G-Force than Silks.
- Using a Trapeze with Wire Rope Core Ropes, you will create a higher G-Force.
Then, you add the type of trick. Everyone knows Newton and his apple…
- The higher you fall, the higher the possible load.
- The faster you stop the fall, the higher the load
- The slower you stop the fall, the lower the load
Then, you need to assess all of this data…
Using gathered data over the years and in experiments, we now know that:
Duos and Trios will usually create low dynamic loads, because of the counterbalance involved, and/or the partnering, where one dampens the other’s trick (otherwise the ‘base’ would get injured)
Solos are usually the ones able to create the higher loads.
Common examples of configurations and resulting loads are:
- A 2m drop on a Trapeze or Aerial Hoop/Lyra
- No stretch
- High Fall
- G-Force ranging usually from 2 to 5, in exceedingly rare cases, up to 10
Result: You will create forces so high that you will probably be over the WLL it of most standard equipment and components of your rigging system.
- A 2m drop using a Spanish Web, Aerial Straps or Low Stretch Aerial Silk
- Low stretch
- High Fall
- G-Forceranging from 2 to 5, and in some case, we have seen up to 9 on Spanish webs.
Result: You will create forces so high that you might surpass the WLL of most standard items used in rigging, and higher than the WLL of the apparatus itself.
Note that most Spanish web acts are now recommended to use bungee packs.
- A 2m drop using a Spanish Web or Low Stretch Aerial Silk rigged onto a Bungee Pack
- Medium stretch
- High Fall
- G-Forceranging from 2 to 4.
Result: You will create high forces that are probably within the higher range of acceptable WLL of most items.
- A 2m drop using a Medium to High Stretch Aerial Silk
- Medium to high stretch
- High Fall
- G-Forceranging from 1.1 to 3 usually.
Result: You will create forces that are within the medium range of acceptable WLL of most items.
- A Duo Hoop or Duo Trapeze act
- No stretch
- No high falls, or if any, done in partnering.
- G-Forceusually ranging from 1.1 to 2.
Result: You will create forces that are within the medium range of acceptable WLL of most items.
Of course, these are only general and reference measurements. For any high-level acts, it is advised you should use a Load Cell to measure the exact loads your act creates and consult with a qualified person to help establish the loads generated by your act and select the appropriate components for your system, or adjust your act according to the available rigging.
N.B. your body is one of the best instruments around. How hard you ‘feel’ some tricks are on your body will hint you also on how much dynamic force your using.
Overloading Structures and Equipment
Advanced and Pro-users, if you are performing high drops (2m or more) on Straps, Spanish Web, Non/Low Stretch Silk, or flip-over drop on Aerial Ring, you might be creating higher loads than those expressed above. Most probably the apparatus and rigging components you are using are not approved for such loads nor the anchor point. In this case, it is recommended to use a Bungee Pack or any dampening device. It will save your body from possible injuries and will avoid possibly overloading the rigging you are using.
It is also known that such loads, If not dampened, will eventually create irreparable damage to your body’s joints and tendons.
Fixed rigging points higher than 15 feet (5 m) should have ideally 750-1000 lb WLL, or 3750 lb to 5000 lb+ Breaking Load.
Now, it is important to note that you could overload a freestanding structure, even as an intermediate, by using common rigging techniques. The way your rig your equipment can act as a multiplicator on the loads you create.
Freestanding aerial structures and the use of pulleys
Most users of freestanding aerial structures often forget that threading a rope through a pulley on top of your freestanding rig will double the load on the rig.
Example: A Dynamic move on a straps act can easily create 600lb of load. Through a pulley, you are doubling that load at 1200 lb, which is more than any rig on the market is designed for.
Note that a rig designed with the correct Design Factor will be able to withstand such overload, hence why this does not generally cause an incident. But, as time has proven over centuries, we should always do the necessary not to load structure or any items more than its working load limit. You do not want to be that exceptional incident that ‘proves the rule’
Since the higher the rig is, the weaker it tends to be, and that usually a higher rig will often require using a rope and pulley (as you can’t easily reach the top), it is very common that people overload freestanding structures.
This is the reason why we have designed, for example, the tripods that can go up and down in 30 seconds total, so you can change the apparatus easily without compromising its strength.
Conclusion:
Maximum recommended loads
Considering most of the data available today, and the current WLL on most of commonly sold apparatuses and rigging components, it would not be recommended to put more than 600-700 lb (3 Kn) on any given apparatus.
Beginners and intermediate users/artists will not usually create extremely high loads and can easily use the BASIC LOAD CALCULATION METHOD
Intermediate to advanced users should consider the IMPROVED BASIC LOAD CALCULATION METHOD, and if possible, use the ADVANCED LOAD CALCUALTION METHOD.
Finally, should you have any doubts about your calculations you should contact CircusConcepts. We will be glad to help you stay safe.
Glossary:
As with any art form, it is important to use the same vocabulary when describing the intent of our work, especially when discussing with a rigger, engineer. These terms and their definitions are some of the most used considered industry standards.
Aerial Arts: Art performed above the ground. In most cases Circus Wise, using an apparatus suspensed to a higher point.
Competent person: A person who has the necessary academic and work experience related to the field of expertise needed.
Design factor (previously known as safety factor) : A ratio of the design load limit to the ultimately load carrying capacity ( I.E., breaking strength) of a material or component. Example
dampened: make less strong or intense
design load limit: The maximim amount of force an item is designed to handle frequently
dynamic forces: Multiplication of a weight by acceleration. Example, any dynamic movement of a mass will increase it's force( Newton's law )
engineer: As practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the limitations imposed by practicality, regulation, safety and cost.[1][2] The word engineer (Latiningeniator[3]) is derived from the Latin words ingeniare ("to create, generate, contrive, devise") and ingenium ("cleverness").[4][5] The foundational qualifications of an engineer typically include a four-year bachelor's degree in an engineering discipline, or in some jurisdictions, a master's degree in an engineering discipline plus four to six years of peer-reviewed professional practice (culminating in a project report or thesis) and passage of engineering board examinations.
equipment( material,Circus ) : It reprensents the contraptions, devices and links used by the artist and/or rigger.
G-Force : The amount of times your bodyweight is the resuting force created by a acceleration/deceleration.
Load: The amount of force applied at a certain moment in time. Loads can be static( constant ) or Dynamic( vary with accelleration/decelaration )
Load-bearing hardware: The material that will be put into tension or compression due to the loads.
Load cell: A device that measures the amount of load in one point of a system.
Minimum Breaking Load (MBL): The lowest amount of force at which an item can catastrophically fail( break ). In tests, often items will break at different loads due to many factors. The lowest load at which an item breaks is considered the highest load this item can bear before breaking.
Peak Load: The maximum load that was applied during a certain time lapse. Often used for dynamic foces.
Performer: A person, usually human, who is on stage and presents a piece of art and/or performance.
Rigger: A competent person in the knowledge of rigging. This term is used in the Building Industry, but also in the Circus Industry. In our articles, we refer to a Rigger as an acrobatic rigger, a person who has knowledge of safe practies to attach to a higher point acrobatic apparatus.
Qualified person: A person who has the necessary academic and work experience related to the field of expertise needed.
Risk: noun - a situation involving exposure to danger. as a Verb expose (someone or something valued) to danger, harm, or loss.
Side factors: At large, Other factors that are are inherent to the situation.
Working Load Limit (WLL): is the maximum working load designed by the manufacturer. This load represents a force that is much less than that required to make the lifting equipment fail or yield. The WLL is calculated by dividing MBL by a safety factor (SF). An example of this would be a chain that has a MBL of 2000 lbf (8.89 kN) would have a SWL or WLL of 400 lbf (1.78 kN) if a safety factor of 5 (5:1, 5 to 1, or 1/5) is used.
Resources:
Albeit in French, the French circus association Hors les Murs has made extensive research and papers on the manufacturing of circus equipment. This memento is full of extremely knowledgeable information on the subject.
This Master Thesis from engineer Marion Cossin at the University of Montreal, titled “Measures of Dynamic Forces through the use of circus apparatuses”, where they measure the different strength, loads, ratings and effects on the human body during circus arts is also extremely interesting. All the data was taken at the ENC in Montreal and Tohu (Show theater) See link here.
It is very interesting because it even dresses a list of the maximum dynamic loads measured, per item, per trick, albeit again, it is in French. If you have relevant documentation in English on the subject please contact me I will add it here.
Entertainment Rigging For the 21st century very detailed chapters on forces loads and aerial equipment
Automated-Performer-Flying-The-State-of-the-Art while pretty advanced goes into great details about load on performers.