Page 147 - The Complete Rigger’s Apprentice

You could say that angles are to rigs as genes are at the other, and is long enough that if you pick it up
to bodies: They determine the shape, size, and pro- at the middle, exerting 150 pounds of force, it will
portions of the finished structure. Rig design involves deflect about 12 inches (305 mm) over its 30-foot
working out realistic relationships among angles (9.1 m) run (see Figure 5-7). By clipping the shackle
and the loads they produce. The angle a shroud of a safety harness around this wire, a crewmember
makes relative to its mast, for instance, determines can walk forward and aft, secure from the danger
the ratios of compressive and lateral forces that of going overboard. If the lifeline is strong enough.
shroud exerts and therefore how strong both mast That chilling “if” might make you receptive to
and shroud need to be. Very steep shrouds can exert a funny little picture. That is, we can draw what is
compressive loads that could crumple any mast, so called a “stress diagram” that will show us just how
in practical terms angles limit rig design options. much load would come on the wire if a hypothetical
With that notion in mind, let’s take a break from 150-pound crewmember were to fetch up against it.
mast building and look at the effect of angles more In Figure 5-8, the load (150 pounds) is represented
closely on a simpler structure. by a vertical line of arbitrary length. Lines parallel
to the two sides of the lifeline are drawn from the
A Lifeline top and bottom of this vertical line. Their lengths
1
Consider a deck lifeline—say, a piece of ⁄8-inch where they intersect show the relative load on each
(3 mm) 1 x 19 stainless steel wire with a break- leg. So if they are five times longer than the vertical
ing strength rated at 2,100 pounds (955 kg). It is line, each leg of the lifeline will experience a load
secured at the cockpit at one end and at the foredeck five times greater than the weight of the crewmem-
ber, or about 1,125 pounds.
You can get to the same answer by means of a
Figure 5-7. A load of 150 pounds applied at the mid- formula. If formulas make your eyes glaze over, skip
dle of a lifeline run of 30 feet causes a deflection of 12 this part: Load 5 Length of one leg ÷ Deflection =
inches. The tension on the lifeline can be calculated Tension on both legs combined.
from this observation using the formula in the text or In this case that’s 150 pounds 5 15.0333 feet ÷
the diagrammatic representation in Figure 5-8. 1 foot = 2,255 pounds. This is the combined tension,

12 inches
30 feet

Figure 5-8. To calculate tension on a line or wire graphically, draw a vertical line of any convenient length to represent the
deflecting load at the midpoint (in our case, 150 pounds). Now draw in the deflected legs, starting from the top and bottom
of the vertical; in our example we get the angles of deflection from Figure 5-7. (We could walk them over to Figure 5-8 with
parallel rules if desired.) Line ac or bc divided by line ab and multiplied by 150 pounds yields an approximation of the
tension on each end of the lifeline.

a

150 lbs
c
b

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