Installation manual

Community WET II - Operation and Installation Manual - Page 23
2. Safety Factor
No component in a rigging system should be stressed to its maximum load rating. A suitable Safety Factor
or Design Factor must be determined and applied, so that every component in the rigging system has
reserve load capacity. The manufacturer’s stated SWL or WLL is the quotient of the component’s ultimate
strength, or breaking point, divided by the manufacturer’s stated safety factor. If the safety factor is not
stated on the component or in the manufacturer’s technical specification sheet, the component should not
be used, as its ultimate strength is unknown.
In addition to the manufacturer’s rated Safety Factor, the prudent installer will determine and apply an
additional Safety Factor to insure that the suspension system is installed in a manner that cannot fail under
the prevailing conditions.
Safety factors may be regulated by local, state, federal or international agencies and therefore may vary
based on your geographic location. Safety factors typically assume normal environmental conditions;
additional consideration must be given when unusual conditions are encountered, such as in corrosive
marine environments (e.g. loudspeakers mounted on a cruise ship or in a venue located near sea water), or
in conditions involving high-vibration, high wind-loads, or other unusual conditions. A common safety factor
is a 6:1 ratio; however many theatrical rigging companies voluntarily work at a 7:1 ratio or higher. A 10:1 ratio
is required in most countries that are members of the European Union. At 10:1, this means that a component
rated at 10,000 lbs of ultimate strength should never be subjected to a load greater than 1,000 lbs. The de-
rated value of 1,000 lbs should take into account the additional force of dynamic loading, such as when a
motorized hoist starts and stops. Such dynamic loading can easily exceed 200% of the static load and is
dependent on the hoist design, how close to capacity it is loaded, the length of the cable or chain, and the
elasticity of the supporting structure.
Seismic Considerations
Seismic events (earthquakes) can cause great dynamic loading of a rigging system. In the case of strong
quakes, which occur regularly in some regions, a rigged system may merely sway back and forth with no
damage, or it may be subjected to many times its static load rating, particularly in the case of unequal
loading of a multi-part suspension system due to uneven roof or beam movement. It’s also possible that
one loudspeaker rig may contact another rig or a lighting truss, during the quake. The results cannot be
predicted because seismic events occur with differing forms of wave motion at varying axes to the
suspended system. An earthquake might produce long, rolling waves in one axis then later, an aftershock on
the same day could produce short, violent waves in another axis.
Aging of Components
Over time, rigging components will degrade causing them to lose some, or even much, of their load capacity.
Factors include strain, oxidation, exposure to corrosive or acidic atmospheric conditions, exposure to water
and chemicals, exposure to UV light, and metal fatigue. In consideration of long-term durability, a high
safety factor should always be chosen, particularly when future conditions cannot be clearly foreseen
(which is almost always the case).
3. Safety Cable
Safety cables must be sized appropriately for the loads they will carry, and should be attached to a different
point or points on the wall, ceiling, truss, frame, bumper, or other, than that of the primary suspension
system, as well as to a different point or points on the objects that are suspended.