Civil Engineering Reference
In-Depth Information
46.4.4 Asymmetry Component
46.4.4.1 Definition and Measurement
Asymmetry refers to a lift that begins or ends outside the midsagittal plane (see Figure 46.2).
In general, asymmetric lifting should be avoided. If asymmetric lifting cannot be avoided,
however, the recommended weight limits are significantly less than those limits used for symmetrical
lifting.
3
An asymmetric lift may be required under the following task or workplace conditions:
1. The origin and destination of the lift are oriented at an angle to each another
2. The lifting motion is across the body, such as occurs in swinging bags or boxes from one location
to another
3. The lifting is done to maintain body balance in obstructed workplaces, on rough terrain, or on
littered floors
4. Productivity standards require reduced time per lift
The asymmetric angle (A), which is depicted graphically in Figure 46.2, is operationally defined as the
angle between the asymmetry line and the midsagittal line. The asymmetry line is defined as the line that
joins the midpoint between the inner ankle bones and the point projected on the floor directly below the
midpoint of the hand grasps, as defined by the large middle knuckle. The sagittal line is defined as the line
passing through the midpoint between the inner ankle bones and lying in the midsagittal plane, as
defined by the neutral body position (i.e., hands directly in front of the body, with no twisting at the
legs, torso, or shoulders). Note: The asymmetry angle is not defined by foot position or the angle of
torso twist, but by the location of the load relative to the worker's midsagittal plane.
In many cases of asymmetric lifting, the worker will pivot or use a step turn to complete the lift. Since
this may vary significantly between workers and between lifts, we have assumed that no pivoting or step-
ping occurs. Although this assumption may overestimate the reduction in acceptable load weight, it will
provide the greatest protection for the worker.
The asymmetry angle (A) must always be measured at the origin of the lift. If significant control is
required at the destination, however, then angle A should be measured at both the origin and the desti-
nation of the lift. Remember that A should be measured at the liftoff point, when the load actually begins
to move upward, rather than at the point when the object begins to move horizontally. This is often
easiest to see when the job is videotaped and the videotape is played back at a slow speed or frame by
frame.
46.4.4.2 Asymmetry Restrictions
The angle A is limited to the range from 0 to 135
8
.IfA
135
8
, then the asymmetric multiplier (AM) is
.
set equal to zero, which results in a RWL of zero, or no load.
46.4.4.3 Asymmetric Multiplier
The AM is 1
(0.0032A). The AM has a maximum value of 1.0 when the load is lifted directly in front of
the body. The AM decreases linearly as the angle of asymmetry (A) increases. The range is from a value of
0.57 at 135
2
8
of asymmetry to a value of 1.0 at 0
8
of asymmetry (i.e., symmetric lift). If A is greater than
135
8
, then AM
0, and the load is zero. The AM value can be computed directly or determined from
ΒΌ
Table 46.4.
3
It may not always be clear if asymmetry is an intrinsic element of the task or just a personal characteristic of the
worker's lifting style. Regardless of the reason for the asymmetry, any observed asymmetric lifting should be con-
sidered an intrinsic element of the job design and should be considered in the assessment and subsequent redesign.
Moreover, the design of the task should not rely on worker compliance, but rather the design should discourage or
eliminate the need for asymmetric lifting.
Search WWH ::
Custom Search