Civil Engineering Reference
In-Depth Information
90
80
70
60
50
40
30
20
10
0
30
40
50
60
70
80
90
100
Loading Magnitude (% of maximum compressive strength)
FIGURE 13.13 Scatter plot of cumulative load tolerated at failure (MN
s) vs. loading magnitude (% of maximum
compressive strength), with the power curve fit to the data (r
2
0.9024). Symbols indicate the cumulative magnitude
sustained until failure by specimens in the different compressive exposure groups.
¼
relationship (r
2
0.9024) with the cumulative loading a specimen could withstand prior to failure
(Figure 13.13). Based on this relationship a weighting factor was developed that created an equal cumu-
lative exposure magnitude for a given risk of injury. The equation effectively increases the cumulative
exposure for higher acute exposures to represent the same risk of injury that would result from a
longer exposure at a lower force magnitude. This nonlinear weighting equation (Equation 13.2) incor-
porates the magnitude of exposure as a percentage of maximum strength. This can be implemented in
biomechanical models by using regression approaches to predict an individual's lumbar spine compres-
sive strength. An example of such an equation is developed by Genaidy et al. (1993) which includes age,
gender, body weight and spinal level as input variables and is based on the work of Hansson et al. (1987).
This work provides an initial estimate and a usable equation of the force weighting relationship when
calculating cumulative exposure. However, this theoretical foundation should be assessed on a popu-
lation data set of worker exposure and corresponding injury reporting to determine if it strengthens
this association.
¼
10
8
5
10
5
Weighting Factor ¼
5
:
4617470
ðloading magnitudeÞ
1
:
3802063803
4
3
10
3
ð
loading magnitude
Þ
þ
1
:
460100508173
ð
loading magnitude
Þ
10
2
2
7
:
8813626392557
ð
loading magnitude
Þ
þ
2
:
141251917806310
ðloding magnitudeÞ
22
:
2341862486371
(13
:
2)
A final issue related to setting a dose exposure limit is the length of standardized exposure period that
should be adopted. Should an hourly dose be set, a shift, a day, a week or an even longer period? The most
common reported exposure dose is a shift exposure. This will allow for a point exposure to be assessed
and intervention decisions to be made without requiring information on long periods of exposure.
However, biologically, injury pathways can progress over long periods of time and an injury is in response
to the loading history prior to the injury or pain event. This extended cumulative exposure was the
rationale behind the two studies linking medically documented injuries and cumulative loading exposure
(Seidler et al., 2001, 2003). An additional factor is that loading exposure does not end when a worker
leaves the work place. Cumulative and peak loading documented in activities external to the industrial
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