Civil Engineering Reference
In-Depth Information
factor to account for asymmetric lifting conditions and a coupling factor that accounts for whether or not
the load lifted has handles. The RWL is represented algebraically in Equation (11.4) (metric units) and
Equation (11.5) (US units).
RWL (kg)
¼
23(25
=
H)
½
1
(0
:
003
j
V
75
j
)
(0
:
82
þ
4
:
5
=
D)(FM)
½
1
(0
:
0032A)
(CM),
(11
:
4)
RWL (lb)
¼
51(10
=
H)
½
1
(0
:
0075
j
V
30
j
)
(0
:
82
þ
1
:
8
=
D)(FM)
½
1
(0
:
0032A)
(CM),
(11
:
5)
where H is the horizontal location forward of the midpoint between the ankles at the origin of the lift. If
significant control is required at the destination then H should be measured both at the origin and des-
tination of the lift; V is the vertical location at the origin of the lift; D is the vertical travel distance
between origin and destination of the lift; FM is the frequency multiplier shown in Table 11.4; A is
the angle between the midpoint of the ankles and the midpoint between the hands at the origin of
the lift; CM is the coupling multiplier ranked as either food, fair, or poor as described in Table 11.5.
In this revised equation the load constant has been significantly reduced compared to the 1981
equation. The adjustments for load moment, muscle length-strength relationships, and cumulative
loading are still integral parts of this equation. However, these adjustments or discounting factors
have been changed (compared to the 1981 Guide) to reflect the most conservative value of the biome-
chanical, physiological, psychophysical, or strength data upon which they are based. Recent studies
report that the 1993 revised equation yields a more conservative (protective) prediction of work-
related LBD risk (Marras et al., 1999).
11.4.4 Static Models
Biomechanically based spine models have been developed to help assess occupationally related manual
materials handling tasks. These models assess the task based upon both spine loading criteria as well as
through an evaluation of the strength required at the various major body joints in order to perform the
task. One of the early static assessment models was developed by Chaffin at the University of Michigan
(Chaffin, 1969). This original two-dimensional (2D) model has been expanded to a three-dimensional
(3D) static model (Chaffin and Muzaffer, 1991; Chaffin et al., 1999) and has been developed to help
1.0
0.9
Occasional,
Bench Height and Above
0.8
0.7
0.6
Continuous,
Bench Height and Above
0.5
Occasional,
Low Lifting
0.4
0.3
Continuous,
Low Lifting
0.2
0.1
123456789 0 1 2 3 4 5 6 7 8
Average lifts per minute
FIGURE 11.29
minute and the F
max
curve. The F
max
depends upon lifting
posture and lifting time. (Adapted from National Institute for Occupational Safety and Health (NIOSH), Work
practices guide for manual lifting. Department of Health and Human Services (DHHS), National Institute for
Occupational Safety and Health (NIOSH), Cincinnati, OH, 81-122, 1981. With permission.)
Frequency factor (FF) varies with lifts
/
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