Civil Engineering Reference
In-Depth Information
interviewing workers and supervisors. Jobs are then video filmed according to the job sampling strategy
discussed previously. Two synchronized cameras are used in order to capture both sides of the body while
the worker is performing tasks. The camera crews should be well coordinated so that when the worker
moves the cameras should be moved accordingly in order for at least one camera to capture both sides of
the body. This will help the off-site data processing in the laboratory.
During the observation period, forces applied in the task are noted and later measured. As it is not
feasible to measure all forces that the worker applies in the task, a subjective determination of “significant
force” is made. Operationally, when one of the ergonomists considers that the force is obvious and may
be of importance to the exposure, the force data will be collected. This is similar to most ergonomics
consultations where an ergonomist takes measurements he or she thinks necessary. Conceptually, a “sig-
nificant force” is defined as a lifting force of
0.9 kg, a pinch grip force of
0.9 kg, a power grip force of
4.4 kg. The force value is not known until measured. Therefore, in
practice, forces that are lower than the defined levels are sometimes measured. A lifting force is measured
by the object weight. This is typically measured by using a force gauge or a weight scale. Object weights
can also be obtained from the company. A push
4.4 kg, and a push
pull force of
/
pull force is also measured using a force gauge. For prac-
tical purposes, no distinction is made between push and pull forces, though they may have different phys-
iological impacts. Additionally, both lifting weight and push
/
pull force are also estimated by an
ergonomist using a 1 to 10 rating scale. A pinch or power grip hand force is measured using the force
matching method (Bao and Silverstein, 2005). This is done by asking a subject to recreate the amount
of force he or she uses in the task on a force dynanometer using similar hand
/
wrist postures. This
process is repeated three times, and the median of the three is used in the analysis. Borg ratings by sub-
jects and researchers are also collected for force applications (CR-10, Borg, 1982). Different measurement
methods are used for the same exposure parameters in order to study the differences and similarities
between the different methods.
Other observed parameters, such as the HAL, and parameters for computing the Strain Index (i.e.,
duration of exertion, efforts
/
wrist posture, speed of work, and duration per day) are also col-
lected during the on-site data collection period. This allows several event-based exposure estimations to
be made.
If the worker uses vibrating tools, the tool information is collected. This is used to crudely estimate
vibration exposure to the worker.
It is important to ensure that the data collection process of the on-site analysis does not interfere with
the normal performance of the task. Some workers may have the tendency to modify their performance
in front of video cameras. This must be discouraged. At the end of the data collection, it is also important
to check the completeness of the data and be sure all data are collected properly.
min, hand
/
/
44.3.3 Job Analysis (Significant Force Analysis)
Job analysis is done in the laboratory and is based on the video recording and data collected at the work-
site. The purpose of this analysis is to obtain the frequency and duration of significant forces. This can be
done by performing time studies on the recorded tasks. A software program called MVTA (multi-video
task analysis), developed by the University of Wisconsin (Yen and Radwin, 1995), is used in the SHARP
Study. A typical data processing screen is shown in Figure 44.4. The record shows a time line where a
certain event (activities of various significant forces) occurred. Significant forces are listed in the right
panel. The video window shows the recorded task performance. The analyst can use any video clip
from the two synchronized cameras to obtain the best view for the analysis. Time-line marks are inserted
at the time when significant forces occur. The analyst may often play the video at normal speed first in
order to understand the task activity contents and then play the video in slow motion mode to set the
event marks.
After the data processing, a time-study report can be generated. For instance in Table 44.1 it is shown
that at recorded cycle #3, the worker lifted an object of 56 lb for a duration of 142 frames (or 4.7 sec) and
spent 286 frame time (or 9.5 sec) performing other activities where no significant force was applied.
Search WWH ::
Custom Search