Rigging
Math
(Made
Simple)
A
Primer by
Delbert
L.
Hall, Ph.D.
ETCP
Certified
Rigger
ETCP
Recognized Trainer
Lesson
3: Bridle Lengths
Bridles are typically hung in pairs and converge to create a new
hanging point
somewhere between two existing hanging points.
While bridles can have
more than two ÒLegs,Ó this lesson will only deal with the two-legged
variety. Each leg of a bridle can hang from different
heights, be
different lengths, and be at different angles. Since we will
be computing
the lengths of both bridle legs, we will call one Leg 1 (L1) and the
other one
Leg 2 (L2).
In other to compute the length of each leg, we will need to know a) how
low the
bridle point is below the hanging point for that leg of the bridle
(vertical
distance), and b) how far the bridle point is away from the hanging
point for
that leg of the bridle in a horizontal distance. See drawing
below.
By
knowing the V and H lengths, we can compute the
length of L (the hypotenuse of the right triangle) by using the
Pythagorean
Theorem (A2 + B2 = C2),
only we will use V2
+ H2 = L2,
converted into the equation L = 
So,
letÕs do it.
Example:
Calculate the lengths of L1 and L2 where, V1 = 10Õ, H1 = 4Õ, V2 = 6,
and H2 =
3Õ.
L1 =
10 [X2] [+] 4 [X2] [=]
[
]
L1 =
10.77
feet
L2 =
6 [X2] [+] 3 [X2] [=] [
]
L2 =
6.7
feet
When
working with any type of bridle problem, I
like to draw a diagram, similar to the one above, and label the known
distances. I find it much easier to solve most rigging
problems when I
can ÒseeÓ what it looks like visually. Try it and see if it
helps you.
But
what is the angle of the bridle?
For those of you who are really interested in this question, here is
the equation:
Angle
= (TAN-1 (H1 / V1) )+ (TAN-1 (H2 / V2))
Note:
TAN-1 is another way of denoting the arc tangent (aka ATAN).
To find the
angle of the bridles above, we do the following: [2nd]
[TAN] is
[TAN-1]. So...
Angle
= [ON/C] 4 [Ö] 10 [=] [2nd] [TAN]
Note: Write the
result down (21.8)
[ON/C]
3 [Ö] 6 [=] [2nd]
[TAN] Note: Write the result
down (26.56)
Angle
= 21.8 [+] 26.56
Angle
= 48.36 degrees
Why is it important to know the bridle angle? There can be several reasons: 1) the angle of pulley on a shackle should not exceed 90 degrees - if so, you should de-rate the shackle's WLL by 50%; and 2) bridle angles greater than 120 degrees can put a greater force on the support point/structure than the weight load being supported.
