Installation manual

Community S-Series - Operation and Installation Manual - Page 42
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 a 10:1 ratio, 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, hardware components used for rigging 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
All loudspeakers, lighting instruments and any other object that is rigged or suspended in
any way (this includes the use of wall brackets) must be fitted with one or more safety
cables. Safety cables are a backup method of suspension intended to keep the rigged
object in the air, in the event that the primary suspension system fails.
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
support device, than that of the primary suspension system. They should also be attached
to a different point or points on the objects that are being suspended.
Shock Loading of Safety Cables
A safety cable must have as little slack in it as possible so that if the primary suspension
system fails, causing the load to switch from the primary suspension to the backup safety
cable, the shock will be minimized. An object that falls and is caught up short places
enormous strain on its safety cable. A 100 lb loudspeaker falling just several inches, can
create a shock load as great as 500 lbs. It’s difficult to precisely calculate the strain of a
shock load unless you know the elasticity of the safety cable as well as all other
components in the suspension system. Suffice it to say that this is rarely known with
accuracy. Therefore, in order to insure safety, the total potential stress subjected to the
safety cable and any components that are used with it, should always be a small fraction of
the ultimate breaking strength of the weakest component.