EDCON 2018: RIGGING
THE BIG PICTURE LOADS AND FORCES
Rigging for aerial performance is
extremely situational, and there are
always choices to be made. These
choices should be dictated both by
following accepted minimum standards
and practices (including but not
limited to ACE Guidelines and ANSI
Standards), and through case-by-case
risk assessment by people who know
what they are doing. Every situation is
different, and we need to understand
WHY a particular choice may be
appropriate in one situation but not
another. The problems, solutions,
and—sometimes unintended—
consequences, may not be obvious. This starts with the weight of the performers and equipment, but is
never just that weight. It is always more, and sometimes much more.
In order to determine how much more, we need to take into account
at least the following factors:
Perhaps the most common pitfall we
run into is assuming that because
someone else does it in a certain way,
or with certain equipment, or that
we saw it on the internet, that it is ok
to do it that way. Without doing our
own risk assessment, and without an
understanding of the dynamics of what
we are doing, we are operating in the
dark.
Beyond this, there are two fundamental
questions to be addressed in any
rigging situation:
1/
What are the loads and forces that
our activity will impose on the
rigging system and the structure that
supports it?
2/
Are all components of the rigging
system, and the structure that
supports it, strong enough to support
those loads, as we are using them?
• The “shock loading” and other dynamic forces that acrobatic
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aerial performance typically imposes
The “resultant forces” that using pulleys, for example, impose on
the structures holding them
The extreme effect that the angle of multi-point (“bridle”)
connections can have in multiplying the force imposed on our
systems and equipment.
Each of these factors increases the force applied to the rigging
system, and each is frequently overlooked, misunderstood, or
misapplied. Each is also a complex subject in its own right.
MATERIALS AND EQUIPMENT
Once we know how strong the system needs to
be, we need to figure out how strong it actually
is. In order to do this, we need to be sure we
are looking at the entire system, focusing in
particular on the weakest link.
To start, we should be using hardware and
equipment with known strength and properties.
In most cases this means a “rating” provided by
the manufacturer. We need to know what those
rating numbers mean, and this is a constant
source of confusion for many. (See the following
section for a simplified introduction to ratings
and equipment strength).
The challenges do not stop there. A rating means
little on its own. It has to be looked at in the
context of the complete rigging system and how
it is being used. Common pitfalls include:
At a very basic level, Newton’s three laws of motion tell us that
starting and stopping (climbing and dropping, for example) add
force to the system. What is commonly called “shock loading”
is the rapid application of force due to acceleration (and, of
course, deceleration). In a typical aerial performance scenario,
it is reasonable to expect that the forces may end up in the
neighborhood of five or more times the static weight. Some
performances can generate as much as ten times that weight.
• Assuming that a beam, building, tree, or
other structural element used as an anchor
is strong enough simply because it “looks”
strong and “those things don’t usually fall
down.” This includes taking into account
the direction of the forces being applied,
since components may be much stronger
in one direction than in others. This is often
seen in buildings with roofs supported by
open web steel joists or eyebolts installed
in ceiling members. Assessing the strength
of structures is beyond the scope of most
riggers’ expertise, unless they also happen
to be qualified structural engineers.
This reality is further magnified
when we add a pulley to the
system to redirect and control
the load. The amount of force,
and its direction, are changed
as a result of the angle of the
line through the pulley. A typical
90% pulley angle multiplies the
load (including the dynamic
force) by around 1.5 times
(actually 1.41) and changes the
direction of the force. Make
sure that the structure you are
hanging the pulley from can
support that load in the (now 45
degrees off vertical) direction
of the pull. Different angles
produce different resultant
forces.
Finally, and frequently
overlooked, is the effect
of “bridle” rigging on the
magnification of forces. A
bridle is a connection of two
points to one point, and is most
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commonly seen when we wrap a sling around a beam or
truss to make a point. Since we usually want to preserve
as much working height at possible, there is a tendency
to wrap the sling as tightly as possible around the beam
to make the connection. This common practice can,
depending on the angle, create tremendous increases in
the force being applied to the structure and equipment.
This is the reason that “tri-loading” of carabiners is a
problem. It also explains why tight-wire rigs require such
heavy-duty structure.
• Not taking into account the dramatic
reductions in strength of materials
and equipment that results from the
concentration of forces, including knots and
other terminations. A knot in a fiber rope
can reduces the strength of that rope by
up to 50%. Using a pulley with a sheave that
is too small will both reduce strength and
cause the components to wear out faster.
The unpadded edge of a beam is a point of
concentrated force like a knife edge that can
cut through ropes and slings very quickly.
There are other factors involved, but the bottom line is
that we have to know the loads in order to know if our
rigging system can support them. Too often, we don’t.
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