Civil Engineering Reference
In-Depth Information
TABLE 46.4 Asymmetric Multiplier
A (
8
)
AM
0
1.00
15
0.95
30
0.90
45
0.86
60
0.81
75
0.76
90
0.71
105
0.66
120
0.62
135
0.57
.
135
0.00
46.4.5 Frequency Component
46.4.5.1 Definition and Measurement
The frequency multiplier is defined by (a) the number of lifts per minute (frequency), (b) the amount of
time engaged in the lifting activity (duration), and (c) the vertical height of the lift from the floor. Lifting
frequency (F) refers to the average number of lifts made per minute, as measured over a 15-min period.
Because of the potential variation in work patterns, analysts may have difficulty obtaining an accurate or
representative 15-min work sample for computing the lifting frequency (F). If significant variation exists
in the frequency of lifting over the course of the day, analysts should employ standard work sampling
techniques to obtain a representative work sample for determining the number of lifts per minute.
For those jobs where the frequency varies from session to session, each session should be analyzed sep-
arately, but the overall work pattern must still be considered. For more information, most standard
industrial engineering or ergonomics texts provide guidance for establishing a representative job
sampling strategy (e.g., Eastman Kodak Company, 2004).
46.4.5.2 Lifting Duration
Lifting duration is classified into three categories based on the pattern of continuous work-time and
recovery-time (i.e., light work) periods. A continuous work-time (WT) period is defined as a period
of uninterrupted work. Recovery time (RT) is defined as the duration of light work activity following
a period of continuous lifting. Examples of light work include activities such as sitting at a desk or
table, monitoring operations, light assembly work, etc. The three categories are short duration, moderate
duration and long duration:
1. Short-duration defines lifting tasks that have a work duration of 1 h or less, followed by a recovery
time equal to 1.0 times the work time (i.e., at least a 1.0 recovery time to work time ratio [RT
/
WT]). (Note: the RT
/
WT ratio has been changed from 1.2 to 1.0 since the equation was originally
published.)
For example, to be classified as short duration, a 45-min lifting job must be followed by at least a
45-min recovery period prior to initiating a subsequent lifting session. If the required recovery
time is not met for a job of 1 h or less, and a subsequent lifting session is required, then the
total lifting time must be combined to correctly determine the duration category. Moreover, if
the recovery period does not meet the time requirement, it is disregarded for purposes of deter-
mining the appropriate duration category.
As another example, assume a worker lifts continuously for 30 min, then performs a light work
task for 10 min, and then lifts for an additional 45-min period. In this case, the recovery time
between lifting sessions (10 min) is less than 1.0 times the initial 30-min work time (36 min).
Thus, the two work times (30 and 45 min) must be added together to determine the duration.
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