Rigging Math
(Made
Simple)
A
Primer by
Delbert L.
Hall, Ph.D.
ETCP Certified
Rigger
ETCP
Recognized Trainer
Lesson 6: Deadhang
Tension
(on one end of a truss)
The most efficient way to hang anything is to be able to suspend from directly
above the object that you want to hang. When that is not possible, you
can use bridles to create a point somewhere between two established hanging
points. However, when you hang a truss, it is possible to hang it
so that neither end is directly below a hanging point, yet each end only has
one bridle leg. The diagram below illustrates how this can be
done. If the hanging point is directly
above the Load, then the tension on this leg would be equal to the Load.
But since the Load is not directly below the hanging point, the tension on the
Leg will be greater than the load. So what is the tension on the Leg?
The
equation to solve this problem is:
Tension on L1 = Load (L1/V1)
Example:
If H1 = 4Õ, V1 =10Õ, and the Load = 400 lbs., what is the tension on L1?
The
first thing we need to do is to compute the length of L1 (we did this in Lesson
3).
L1 = 10 [X2] [+] 4 [X2] [=] [
]
L1 = 10.77 feet
Now
we put this length into the equation above and getÉ
Tension on Leg (L1) = Load (10.77/10) or
Tension on Leg (L1) = Load
(1.077)
or
Tension on Leg (L1) = 400 x
1.077
or
Tension on Leg (L1) = 430.8 lbs.
If we have multiple loads on the truss, we would compute the load that would be
transferred to each of the two suspension points on the truss (this will be
covered in Lessons 8, 9 and 10), and then compute the load on each
leg. For now, just work with this single load that is directly
below the suspension point on the truss.
