LEARNING THROUGH A PRACTICAL EXAMPLE
Workplace Safety Accident Analysis
Are you a Safety Professional? Maybe an industry professional who was tapped on the shoulder to fill a Safety Practitioner role?
Was there an accident on your worksite? What did you do? Did you have the proper preparation to perform a workplace safety accident analysis?
As a Safety Professional/Practitioner in the field you may have asked yourself, Where do I start? How do I begin? What is the process?
Below I provide an example to help answer these questions and more.
Incident Description
During an onsite corporate leadership tour an accidental fall occurred. A Systems Engineering Staff Member ascended a Combat All-Terrain Vehicle (C-ATV) with the intent of entering its cabin. As the individual began to enter the interior of the vehicle they released their hold of the exterior grip and extended their arm to access the interior handle. They were standing with both feet securely placed upon the lower rung of the vehicle. The individual missed the interior handle and lost their footing. They then fell directly backwards from the vehicle landing on their upper back and posterior skull. The posterior impact to their skull caused a concussion and linear temporo-parietal skull fracture.
Documenting the Nature of the Injuries Sustained
The injuries sustained from the impact after the fall can be grouped into two categories. The first is the fracture injury. The fracture was a linear temporo-parietal skull fracture. It was determined to be a closed fracture because the skin covering the fracture area had not been broken or cut. After the fall had ended and the impact occurred the individual was complaining of severe pain in the form of a headache. Additionally, the incident report indicated that the posterior portion of the skull exhibited bruising and swelling at the impact site. Figure 1 is an x-ray of a skull that has experienced the same type of fracture. The small red arrow indicates the location of the linear fracture line.
Figure 1 Linear Temporo-Parietal Skull Fracture X-Ray (Khan, 2015)
The second is the concussion. A concussion is a type of traumatic brain injury that is caused by a blow to the head or body, a fall, or another injury that jars or shakes the brain inside the skull. Although there may be cuts or bruises on the head or face, there may be no other visible signs of a brain injury (WebMD, n.d.). After the fall had ended and the impact occurred the individual was complaining of headache, nausea, blurred vision, and confusion. These were all signs that they were experiencing a concussion.
Identifying the Mechanism of Injury
The method by which the injury occurred was a compressive impact force to the posterior area of the skull. This impact contributed to the linear temporo-parietal fracture. The concussion occurred when the victims falling head struck the ground. The head decelerated abruptly while the intracranial contents continued moving forward to the point of impact. The result was a contusion in a region opposite the point of impact. The backward fall caused contrecoup contusions at the frontal and temporal poles of the brain. A contrecoup contusion is a contusion resulting from a blow to the head with damage to the opposite side by transmitted force (Dictionary, 2011).
Understanding the Human Kinematics
Figure 2 shows the body position of the individual prior to impacting the asphalt. As shown in the figure the body’s upper back and posterior skull were nearly parallel with the ground. With the buttocks and lower back raised, along with the legs and arms extended out and upwards. This caused nearly all the impact force to be distributed through the areas highlighted by the red lines in the image. Prior to impact the body was in a state of freefall. There were no additional forces applied to or created by the body to generate increased impact force.
Figure 2 Body Position Prior to Impact
Determining the Forces Involved
The individual weighed 86.18 kg and was 1.82 m tall. They ascended the C-ATV to enter its cabin. They were standing with both feet evenly placed upon the lower rung (0.58 m above the ground) of the vehicle. Figure 3 depicts the C-ATV and shows the fall that occurred.
Figure 3 C-ATV Fall Impact Diagram
To determine the Impact Force applied to the body of the individual the Kinetic Energy of the fall and the Deceleration Distance that the head deflects prior to the fracture must be determined. Figure 4 shows the calculations of the impact force.
Figure 4 Impact Force Calculation
Based on the bruising that was observed on the individual it is believed that they landed such that their upper back (shoulder blades region), and posterior skull struck the asphalt first and at the same time which resulted in this their absorption of nearly all the impact force. The bruising to the back was elliptical so to determine the surface area of the region where the force was applied the equation for calculating the area of an ellipse is used. The bruising to the skull was much smaller. An estimate of 1 cm2 was used to account for the surface area of that impact site. For the calculations, it is also assumed that the Impact Force was equally distributed over the before mentioned surface areas for both the upper back and posterior skull impact site. Figure 5 shows the calculations necessary to determine the Impact Force Average which is equal to the impact force applied to the posterior portion of the skull when it struck the ground.
Figure 5 Average Impact Force Calculations
The calculation of the Impact Force Average is 6.23 kN/cm2. Keep in mind that this is an approximation but it corresponds very closely with the Force (N) for 50% Risk of Temporo-Parietal Skull Fracture. Table 1 shows the forces for four different impact cases. Per the analysis performed by Sahoo, Deck, Yoganandan, and Willinger a 6.37 kN Temporo-Parietal impact will result in a fracture 50% of the time. This data supports the resulting skull fracture that the individual experienced.
Table 1 Force for 50% Risk of Skull Fracture (Sahoo, Deck, Yoganandan, & Willinger, 2016)
Identification of Preventative Measures
The Hierarchy of Controls typically characterize preventive measures. When a hazard is identified, it is important to apply controls that reduce or eliminate its likelihood of occurrence. The purpose of preventive measures is to control the risk of hazards that can contribute to or result in a mishap. For the C-ATV entry system there are many preventive measures that could be exercised to avoid/eliminate the risk associated with a fall hazard. Sections 6.1 through 6.6 discuss possible approaches to each of the controls as they pertain to the system.
Elimination
The fall related hazard could be eliminated if a system was created that allowed for vehicle entry at ground level. One way of achieving this would be to design a trench system that the vehicle drives into. While this provides the greatest level of safety it would prove to be the least practical. Elimination of the hazard could only be realized if the trench system was used and strict controls were applied when vehicle entry and exit are permitted. It would stand to reason that once the occupants of the vehicle had entered they would be required to remain in their seated positions for the duration of the mission. This is not practical and therefore elimination of this hazard would have a negative effect on the value of the overall mission.
Substitution
The fall related hazard could be mitigated if an alternative system was created that allowed for vehicle entry through the substitution of the existing means of entry. One way of achieving this would be the use of deployable stairs. This would be identical to the Airstair system that is used to enter aircraft. Incorporating such a design would reduce the likelihood of trips and falls because the Airstair also includes hand railings to improve the safety of vehicle ascent and descent. The practicality of this substitution is debatable depending on its ease of use. Success hinges upon the implementation. A poor implementation could negatively affect the mission.
Engineering Controls
The fall related hazard could be mitigated if the existing system design was altered such that it allowed for deployable hand railings. As mentioned in the proposed substitution approach such a design would reduce the likelihood of trips and falls because the hand railings improve the safety of vehicle ascent and descent. The practicality of this engineering control is considerable. The practicality can be attributed to its simplicity and potentially low cost implementation. Such an engineering control could be implemented with a foldable design that would make the hand railings less bulky and thus more easily storable when not being used. Incorporating foldable hand railings would have little if any impact on the mission.
Warnings
The fall related hazard could be mitigated if placarding was added to both the interior and exterior of the vehicle to create a heightened awareness of the associated risk of falling during vehicle ascent and descent. Placarding is a cheap preventive measure that can contribute to the reduction of mishap occurrence. The effectiveness of placarding is very dependent upon workplace safety culture. It is especially important for the personnel ascending and descending the vehicle to be trained properly in recognizing safety placarding when it is present along with acting upon it correctly. Otherwise, the usefulness of such a preventive measure is reduced to zero. The incorporation of warnings has no effect on the success of the mission.
Administrative Controls
The fall related hazard could be mitigated if administrative controls are added such that only those who have taken the necessary training are permitted to ascend and descend the vehicle. This administrative control coupled with other preventive measures such as the Airstair and retractable hand railings could reduce the likelihood of mishap occurrence even further. Taking this approach is an example of layering safety mechanisms which typically contributes to improved safety. Although, it should be noted that sometimes this can have negative impacts on the systems usability and thus degrade the mission. Regardless, training on its own is a useful preventive measure that can also positively influence an organizations safety culture.
Personal Protective Equipment (PPE)
The fall related hazard could be mitigated if PPE were added such that the use of a harness system and/or protective head and body gear was required for vehicle entry. This preventive measure is a low-cost solution that brings significant benefit through the reduction of mishap occurrence. Like the usage of warnings, for PPE to be effective it is very important that an adequate level of training complements it. In previous examples of preventive measures, it was mentioned that an organizations safety culture will have an impact on their effectiveness. This is especially true for PPE. It can be easy for workers to brush PPE off as a hindrance or burdensome. This is the attitude of workers who exist in an organization whose safety culture is poor. Incorporating a training curriculum can change that and increase the effectiveness of PPE. It is highly unlikely that the incorporation of PPE would have a negative impact on the systems mission.
References
Khan, A. N. (2015, October 03). Imaging In Skull Fractures. Retrieved from Medscape: http://emedicine.medscape.com/article/343764-overview
Sahoo, D., Deck, C., Yoganandan, N., & Willinger, R. (2016). Development of Skull Fracture Criterion Based on Real-World Head Trauma Simulations Using Finite Element Head Model. Journal of the Mechanical Behavior of Biomedical Materials, 24–41.
WebMD. (n.d.). Concussion — Topic Overview. Retrieved from WebMD: http://www.webmd.com/brain/tc/traumatic-brain-injury-concussion-overview#1
