Rigging Fundamentals

Loads are not only static. A moving hoist, a load that snatches onto a slack chain or a flying performer produces dynamic forces well above the static weight, and the system is rated for that peak. Geometry also increases forces: in a bridle, the tension in each leg rises as the included angle widens, and at 120 degrees each leg carries the full load. Truss has its own load tables for span, distributed loads and point loads (ANSI E1.2 in North America, BS 7905-2 in the United Kingdom, EN 17115 for truss design and EN 17206 for temporary truss installations in Europe), and a heavy point load at mid-span uses far more of the capacity than the same weight close to a support.

Chain hoists are classified by use in a German scheme that is also widely applied outside Germany. A D8 hoist lifts loads but needs a secondary suspension (a safety) when the load is held above people; a D8+ hoist has a second brake and is derated, typically to half its capacity, so that it can hold a load above people without one; a C1 hoist is designed for loads moved above people. The British National Rigging Advisory Group notes that a system of one or two hoists remains exposed to the failure of a single hoist, even with double brakes. Every shackle, sling, hoist and clamp is rated, marked, within its inspection interval and inspected before use. In the United Kingdom, LOLER 1998 requires a thorough examination at least every 6 months for lifting accessories and equipment that lifts people, and every 12 months for other lifting equipment, unless a competent person has drawn up an examination scheme; in Germany, IGVW SQ P2 calls for D8+ and C1 hoists to be inspected by a competent person every 12 months and by an expert every 48 months. Installation is carried out by competent people.