Stage Rigging Handbook, Third Edition

Part 1. Loads and Reactions

 Loads and Reactions The 4 Ks Rigging is a tool used in the theatre. It supports and provides movement of overhead objects that are part of a production. If it works as it should, it rarely calls attention to itself. If something goes wrong, it may not only be noticeable but life threatening as well. The functions of the rigging equipment and the rigger are to do the job as the designer designed it, and the director directed it, and do it safely. In order to do that, there are four principles that a rigger needs to follow, called the 4 Ks of rigging. 1. Know the rigging system you are working with. 2. Keep the equipment in safe working order. 3. Know how to use it. 4. Keep your concentration. Everything that follows in this book is an elaboration of these four principles. Part 1 1.01  Knowing the Rigging System A typical rigging system is made up of individual line sets. The line sets may be hemp, counterweight, or motorized (or a combination ). Each line set is made up of individual components, such as rope, a head block, loft blocks, and so on. Each line set is a separate subsystem, that is, a group of components interrelated and working together. Knowing the rigging system means knowing three things. 1. The capacity of the equipment. You must know how much weight each line set is designed to hold, the maximum speed that motorized line sets are designed to travel, the maximum weight that all of the line sets together can hold, and the maximum additional weight that the grid steel can support. 2. The capacity of the components. You should know the maximum load that each component is designed to carry. A sixline counterweight set may be designed with a total capacity of 1,500 lb. The individual loft blocks may be designed to support only a load of 250 lb. each. The head-block support steel may or may not be designed to support all of the line sets at full load. 3. The operating characteristics of the system. Each line set is unique. It has its own sound and feel; it even has its own smell. In order to know the system, the rigger should know the individual peculiarities of each line set in the system. By knowing how a line set operates normally, the rigger can detect abnormal operation and possible problems. A. Load and Force A part of knowing the rigging system is understanding the forces that an object places on each component of a rigging system and the structural members supporting the components. When an object—whether it is a piece of scenery, a lighting instrument , or an actor—is flown, it exerts a force on all of the elements that support it. Within the confines of this book, the object will be referred to as the load. The elements include all of the components of the rigging system and the structural members of the building supporting those rigging components. The term force can be thought of as a strength or energy trying to cause motion or change. B. Static Equilibrium Newton’s third law of physics states that for every action there is an equal and opposite reaction. Applied to rigging, this means 1.02  that the rigging components and supporting members resist the applied force of the load with a reactive force exactly equal and opposite to the applied force. For example, imagine yourself holding a cup of coffee. The muscles in your arm and hand apply only enough force to support the weight of the arm and the cup of coffee. If less force is applied, your arm bends down, and the coffee spills. If more force is applied, the coffee is thrown up in the air. A lighting instrument is hung on a batten. If the light is steady and does not move, all of the supporting components— the C-clamp, batten, lift lines, loft blocks, loft block support steel, head block, head-block beams, arbor, rope lock, and lock rail— are exerting exactly enough force to hold it in place. Two distinct branches of mechanics, statics and dynamics, apply to stage rigging. Statics is the study of forces and the effect of forces acting on rigid bodies at rest. Dynamics deals with motion and the effect of forces acting on rigid bodies in motion. A body is...