Live Sound University Article Thu, July 03, 2008

Design Factor:
Design factor is a term used by the rigging industry to denote theoretical reserve capability. The rated capacity / of all lifting and hanging equipment b based upon the nominal strength of the equipment reduced by the design factor.

Design factor is a number representing the fraction of equipment nominal strength chosen to be appropriate for the particular application.

RATED CAPACITY = NOMINAL STRENGTH
DESIGN FACTOR

Example:
Design factor = 5
Rated capacity of equipment is only l/5 of its nominal strength.

Minimum design factors vary according to the application, and may be regulated from location-to-location. No design factor discussed herein should be assumed to represent a recommendation on the part of JBL. Users must assume all responsibility for the determination of design factors suitable for local conditions.

Shock Loading:
When a load is suddenly moved or stopped, its weight may be magnified many times the original value. This is known as shock loading. Shock loading of lifting equipment should be avoided at all times.

Shock loads will usually be instantaneous and may go undetected unless equipment is visibly damaged. No equipment is designed to compensate for poor rigging practices or foolish planning, however. Every tool and piece of equipment has limitations. Safe working practices demand that these limitations he known and fully understood, and that they never be intentionally exceeded.

A 900 pound loudspeaker cluster dropped four inches cod cause a shock load of 4500 pounds if the rigging were attached to rigid structures and of a material that would not stretch. However, because all rigging will stretch under shock loading, the exact shock load on a piece of equipment isn’t easily predicted. To protect people and property, all tools and equipment should be limited to stresses that are several times smaller than their minimum breaking strengths.

Although shock loading of equipment and structure is usually confined to lifting and installation, it should also be recognized that other forces (such as earthquakes) can impose shock loads upon structures many times that of the static load. It is therefore imperative that hardware and structures be capable of supporting several times the weight of the equipment being hung.

Center of Gravity:
The center of gravity of an object is the point at which the weight of the object acts as though it were concentrated. It is the point at which the object may be completely supported or balanced by a single force.

The center of gravity of a regularly shaped object may be estimated fairly accurately by determining its approximate center. Finding the center of gravity of irregularly-shaped objects can be more difficult, but it is necessary, nevertheless. A load will always hang from its attachment point through the center of gravity. It is important to visualize this before making a lift.

All loads to be lifted should be rigged above the center of gravity in order to prevent tipping and possible hazards to equipment and workers. The lifting force should always be located above the center of gravity and exert a straight vertical pull to prevent swinging of the load.

Ropes:
Before discussing actual rigging hardware and systems, it is appropriate to examine ropes and their proper use. Ropes are used for many rigging functions. Although synthetic ropes of great strength are available, most codes prohibit their permanent use in rigging for a variety of good reasons. Nevertheless, ropes are necessary to lift approved cables, fixtures, tools and equipment into position.

In the interest of safety it is important that ground workers be familiar with the proper use of rope and a few basic knots used in rigging.

Rope Terminology (Figure 1 below):
1. The Standing Part is the end of the rope which is inactive.
2. The End is the part of the rope that is free—typically the part in which knots are tied.
3. A Bight is the central part of the rope between the standing part and the working end.
4. An Overhand Loop is formed by crossing the end over the standing part.
5. An Underhand Loop is made by crossing the end under the standing part.
6. Tightening. Once formed, a knot must be tightened slowly and with care. Failure to do so could result in a tangle, or an untrustworthy knot.

Figure 1. Rope Terminology

Knot Efficiency:
Knot efficiency is the approximate strength of a rope with a knot as compared to the full strength of the rope. It is expressed at a percentage of the ropes rated capacity, and refers to the stresses that the knot imposes upon the rope. When a knot is tied in a good rope, failure under stress is certain to occur at the knot. This is because bends result in uneven stresses upon the fibers, with the outsides of the bends taking a greater share of the load. lt follows that the tighter the knot, the greater the percentage of the total load that is carried on fewer fibers.

Bends:
Bends are used to join two pieces of rope, usually temporarily. Typical knot efficiency is 56%. Bends offer some advantage over binding knots, as they resist untying when slackened or jerked. The Sheet Bend is a simple knot to tie, consisting of an overhand loop on one piece, with the second rope end fed up through the loop from behind, around the standing part of the first rope and back down through the loop from the front.

Binding Knots:
Binding knots are also used to join two pieces of rope. In general, binding knots have a knot efficiency of 50%, but can untie easily when a free end is jerked.

In the square knot, the end and the standing part of each line tie together through the bight of the other. In the untrustworthy granny knot, the end and the standing part are separated by the bight. The granny knot is particularly treacherous in that it will appear to be secure--only to slip under load. The thief knot is deceptively similar to the square knot, but has the two loose ends coming out of the opposite sides, instead of from the same side as in the square knot. This knot is almost certain to fail under load.

Loop Knots:
Loop knots are used to hold objects where security is of paramount importance. The bowline, widely used in rigging, won’t slip, yet is easily tied and untied. It may be tied in the hand or used as a hitch and tied around an object, usually for lifting purposes (Figure 2 below).

Figure 2. Tying Bowline

To tie: Make an overhand loop with the end toward you (Step 1). Pass the end up through the loop from behind (Step 2) then up behind and around the standing part-then down through the loop again (Step 3). Draw up tight. The bowline has a knot efficiency of approximately 60%.

Hitches:
Hitches are used for temporary fastenings that untie readily. They are generally tied directly around the object instead of first being tied in the hand and then placed over the object. Hitches must be drawn up tight, as they have a tendency to slip if loose.

The clove hitch (Figure-3 below) consists of two underhand loops, which may be tied in the hand and slid over an object at any point along the length of a rope. Knot efficiency is 60%.

Figure 3. Tying Clove Hitch

Wire Rope:
Vast wire ropes are constructed from plow steel, improved plow steel, or extra improved plow steel wire. The wires are woven into strands, which are woven to form the wire rope. Typical wire rope may consist of six strands wound around a central core. The central core supports the outer strands and helps to prevent the rope from crushing under stress. Wire rope core materials may be fiber (abbreviated FC), independent wire rope (abbreviated IWRC), or wire strand (abbreviated WSC).

Wire rope is classified by diameter, number of strands, number of wires making up each strand and core material construction. Rope diameter is measured at its widest dimension. Wire rope is also classified according to the direction the strands and wires are twisted. The distance along the rope required for a strand to make one full revolution is one Lay.

In Right Regular Lay construction, strands twist to the right, wires twist to the left.

Right Lang Lay construction finds both strands and wires twisting to the right.

Left Regular Lay ropes are constructed with strands twisted left and wires twisted right.

The Left Lang Lay configuration twists both strands and wires left.

Regular lay ropes are less susceptable to crushing and deformation because the wires lie nearly parallel to the rope. Lang lay ropes twist the wires across the direction of the rope, and are therefore more flexible and resistant to abrasion damage. If both ends of a lang lay rope are not fixed, however, it will rotate severely when under load.

Most sound and stage rigging requirements are easily handled by two wire ropes: 3/V and l/2” 6 X 19 IWRC classification. These ropes in improved plow steel have a nominal strength of 13120 pounds and 23000 pounds, respectively. If we assume a design factor of 5, rated capacities become 2600 and 4600 pounds.

Just as knotting a fiber rope reduces the nominal strength of the rope, bending of a wire rope also results in a reduction in its nominal strength. The tighter the radius of the bend in the rope, the greater percentage of the load is concentrated on fewer wires and strands. This results in a reduction in the rope’s nominal strength and rated capacity.

Figure 4 (below) shows the relationship between wire rope efficiency and the ratio of bend radius to rope diameter. The chart is for 6 X 19 class wire ropes. Note that the chart is nearly asymptotic as the bend radius approaches the rope diameter--such as might occur in wrapping a beam with a basket sling. Overloading of a cable under these conditions could result in irreparable damage to the wire rope, or a possible failure.

Experienced riggers always pad beam edges with softeners before wrapping the beam with a sling, and avoid sharp or jagged edges that could possibly injure the wire rope or sling. Heavy burlap or thick polyester is usually used for this purpose.

Figure 4. Wire Rope Bend Efficiency

Excerpts from JBL Professional Technical Note Volume 1, Number 14: “Basic Principles for Suspending Loudspeaker Systems.” Copyright and courtesy of JBL Professional. The entire document is available at: http://www.jblpro.com/pub/technote/tn_v1n14.pdf