Civil Engineering Reference
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WT ratio, however, and additional
recovery periods would have to be provided as described earlier.
2. Moderate duration defines lifting tasks that have a duration of more than 1 h, but not more than
2 h, followed by a recovery period of at least 0.3 times the work time (i.e., at least a 0.3 recovery
time to work time ratio [RT
The 2-min recovery periods would not count toward the RT
/
WT]).
For example, if a worker continuously lifts for 2 h, then a recovery period of at least 36 min would
be required before initiating a subsequent lifting session. If the recovery time requirement is not
met, and a subsequent lifting session is required, then the total work time must be added together.
If the total work time exceeds 2 h, then the job must be classified as a long-duration lifting task.
3. Long duration defines lifting tasks that have a duration of between 2 and 8 h, with standard indus-
trial rest allowances (e.g., morning, lunch, and afternoon rest breaks).
Note: no weight limits are provided for more than 8 h of work.
/
WT ratio for the short-duration (less than 1 h) category, which is
1.0, and the moderate duration category (1 to 2 h), which is 0.3, is due to the difference in the magnitudes
of the frequency multiplier values associated with each of the duration categories. Since the moderate
category results in larger reductions in the RWL than the short category, there is less need for a recovery
period between sessions than for the short-duration category. In other words, the short-duration cat-
egory would result in higher weight limits than the moderate duration category, so larger recovery
periods would be needed.
The difference in the required RT
/
46.4.5.3 Frequency Restrictions
Lifting frequency (F) for repetitive lifting may range from 0.2 lifts
min to a maximum frequency that is
dependent on the vertical location of the object (V) and the duration of lifting (Table 46.5). Lifting above
the maximum frequency results in an RWL of 0.0. (Except for the special case of discontinuous lifting
discussed earlier, where the maximum frequency is 15 lifts
/
min.)
/
46.4.5.4 Frequency Multiplier
The FM value depends upon the average number of lifts per minute (F), the vertical location (V) of the
hands at the origin, and the duration of continuous lifting. For lifting tasks with a frequency less than
0.2 lifts
min, set the frequency equal to 0.2 lifts
min. Otherwise, the FM is determined from Table 46.5.
/
/
46.4.6 Coupling Component
46.4.6.1 Definition and Measurement
The nature of the hand-to-object coupling or gripping method can affect not only the maximum force a
worker can or must exert on the object, but also the vertical location of the hands during the lift. A
“good” coupling will reduce the maximum grasp forces required and increase the acceptable weight
for lifting, while a “poor” coupling will generally require higher maximum grasp forces and decrease
the acceptable weight for lifting.
The effectiveness of the coupling is not static, but may vary with the distance of the object from the
ground, so that a good coupling could become a poor coupling during a single lift. The entire range
of the lift should be considered when classifying hand-to-object couplings, with classification based
on overall effectiveness. The analyst must classify the coupling as good, fair, or poor. The three categories
are defined in Table 46.6. If there is any doubt about classifying a particular coupling design, the more
stressful classification should be selected.
The decision tree shown in Figure 46.3 may be helpful in classifying the hand-to-object coupling.
46.4.6.2 Coupling Multiplier
Based on the coupling classification and vertical location of the lift, the coupling multiplier (CM) is
determined from Table 46.7.
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