Civil Engineering Reference
In-Depth Information
13.2 A Description of Cumulative Loading
Much of the confusion between the terminology cumulative loading and CTD could have been avoided if
a more appropriate or descriptive term had been chosen in place of cumulative loading. Cumulative
implies a simple addition of the loads that the individual or body joint has experienced over a fixed
duration of time. In reality, the exposure would be more accurately described as integrative loading.
Cumulative loading is a summation, but not of the loading magnitudes, rather of the areas under a
sequence of force-time relationships representing a worker's daily exposure (Figure 13.2).
Since factors such as cycle time and magnitude of load will alter the magnitude of cumulative loading
for a task, a common exposure period is used to assess the relationship between cumulative loading
exposure and risk of injury. The most commonly used duration is shift exposure, typically an 8-h
period. The shift exposure is determined by incorporating each of the tasks (where n
total number
¼
of tasks and i represents each task) performed over the shift (Equation [13.1]).
X
n
Shift Cumulative Loading
¼
Cumulative Task Loading (i)
(13
:
1)
i
¼
1
Often this is simplified by taking the cumulative loading in one cycle and extrapolating the data to
represent what the equivalent exposure would be for an 8-h shift. The example presented in
Figure 13.2 has a cycle time of 5 sec. To extrapolate this value to a shift dosage would require a multi-
plication by 5760 (12 times per minute
8 hours per shift). This would yield
a shift cumulative compression value of 37.5 MN s. An understanding of how cumulative loading is
calculated is fundamental to appreciate the unique challenges associated with developing a method to
assess the risk of injury from cumulative loading exposure. This will be explored in the following sections.
60 minutes per hour
2500
2000
1500
1000
500
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time (s)
FIGURE 13.2 A biomechanical model was used to calculate the compression value for each frame of video for a
sagittal lifting task. The area under the curve (solid area) is integrated to determine the amount of compression
to which the L4
L5 joint is exposed over the course of the task. The cumulative compression for this lift is the
value of this area (6514 N s).
/
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