Understanding Aerial Rigging Ratings: Minimum Breaking Strength and Working Load Limits

When it comes to aerial dance rigging, the equipment we use comes from two primary industries: recreational (like rock climbing) and industrial (like construction). Each industry uses its own ratings, which mean we need to understand multiple ratings and what those mean.

Recreational vs. Industrial Equipment

• Recreational Equipment: Includes carabiners, swivels, rescue 8s, and rig plates, commonly used in rock climbing. These are rated by their Minimum Breaking Strength (MBS), typically denoted in kilonewtons (kN), representing the maximum force the equipment can endure before breaking.
• Industrial Equipment: Includes round slings, shackles, and box truss. These are rated by their Working Load Limit (WLL), measured in pounds, indicating the safe load limit.

In other words, a carabiner is stamped with the number (in kN) of force it takes to break it, whereas a shackle is stamped with the number (in pounds) that can be safely applied. In aerial, we do not want to break our equipment! So, we are most interested in understanding and using the working load limit. 

Key Rigging Terms: Defining MBS, WLL, and Design Factor for Aerial Dance

• Minimum Breaking Strength (MBS): The force required to break a piece of equipment, often stamped on recreational gear (e.g., 25kN = 5,620 pounds). While important to know, we never want to approach this limit in use.
• Working Load Limit (WLL): The maximum force safely applied to equipment. It includes a built-in Design Factor (or safety margin), which ensures usage well below the breaking point. This is the number we are most interested in.
• Design Factor (DF): A ratio set by manufacturers to convert a MBS into WLL. Let’s take our fabric as an example. In destructive tests we have found our aerial fabric with shine (sign strand) breaks around 2,500 pounds. This is the minimum breaking strength. When using the silks, we don’t want to get anywhere close to that amount of force. As a manufacturer, we implement a design factor of 6. This is also called a safety factor and essentially cuts the minimum breaking strength by a factor of 6 to get a more conservative number.

To calculate the working load limit, the formula is:
WLL = MBS ÷ Design Factor

To continue with our fabric example. MBS (2,500 pounds) divided by 6 (design factor) means the fabric has a working load limit of 416 pounds. This working load limit ensures safe use far below the breaking point.

Calculating Working Load Limit for Recreational Equipment

Unlike industrial equipment, recreational gear (a carabiner) often lacks a stamped WLL. In these cases, you can:

1. Contact the manufacturer for a recommended design factor.
2. Use a standard design factor: Experts often recommend 5:1. For example:
• A carabiner with an MBS of 25 kN (5,620 pounds) would have a WLL of: 5,620 ÷ 5 = 1,124 pounds.

This calculated WLL provides a safe load limit for the equipment in use.

Why Understanding Aerial Equipment Ratings Matters

Every piece of equipment in an aerial rigging system is subject to the forces applied to it. Knowing how to calculate and adhere to the working load limit ensures you're operating within a safe margin. By understanding these principles and using equipment appropriately, you can rig safely and confidently for your aerial practice. Remember, if you aren’t sure it’s always best to contact a professional rigger or structural engineer!