The Collapsing Chair - A Case of Workplace Injury
What Happened?
An office worker sustained personal injuries when her office chair collapsed as she attempted to sit down. The chair, which had been in regular use for approximately nine (9) years, failed suddenly and catastrophically beneath her, causing her to fall to the ground, injuring her wrist in the process. This case highlights how hidden structural defects in everyday objects can lead to accidents, and how forensic engineering can be key to identifying the root cause if and when failure occurs.
How did we analyse it?
The entire chair was sent to me for forensic examination. Upon initial inspection, I noticed an obvious crack running through one of the plastic spokes in the base of the chair, which supports the five caster wheels. The crack was located near one of the wheels. The position and path of the crack, and the final position of the wheel relative to the chair and spoke, indicated a clock-wise torsional or twisting force acting on the spoke.
Further examination revealed that the central column of the chair was in direct contact with the ground, a clear sign that the chair's base had become displaced relative to the seat. This displacement and imbalance caused some of the wheels to not be in contact with the ground, effectively redirecting the weight-induced stresses to the wheel that was in contact with the ground, and spoke connecting that wheel to the base, in a manner similar to a 3-point bend test.
I conducted a series of detailed examinations, including Fourier Transform Infrared Spectroscopy to identify the type of plastic used in the chair, and light microscopy of the fracture surface. The material was identified as nylon, which is commonly used in office furniture. Microscopy examination showed the fracture was brittle in nature, which is common in plastic materials subjected to sudden high-stress loads. The lack of plastic deformation along the fracture edges confirmed that the failure occurred suddenly and without significant prior warping of the material.
Through analysis, I determined that the failure originated in the plastic spoke. In this case, the weight of the office worker acted as a force load on the chair's base and spoke, with additional torsional forces (rotational stress) contributing to the failure due to the dynamic motion of sitting down. The location of the crack, adjacent to the wheel rather than halfway along the length of the spoke, suggests that a crack had already formed at that location.
There was likely a progression of crack growth over time, exacerbated by regular use of the chair, which put the plastic spoke under cyclic loading conditions. When the worker sat down on the chair at the time of the accident, the stresses on the compromised spoke reached a critical point, resulting in the catastrophic failure.
What was the resolution?
The failure of the chair’s base was caused by the brittle fracture of one of the plastic spokes, and the formation and growth of a crack over a period of time. This failure was facilitated by the displacement of the central chair column, such that the column was in contact with the ground. This placed the spoke of the chair base in a scenario similar to a three-point bend test, but with additional torsional forces.
Regular maintenance of the chair might have identified the crack growing within the spoke, or prevented the displacement of the central column. The failure may have thereby been prevented.
To prevent similar accidents in the future, it is recommended that manufacturers use more durable materials in high-stress components, such as the spokes of a chair's base, and regularly test for cyclic fatigue. For users, regular inspection of office equipment for signs of wear or damage is also advised. Scheduled replacement of office equipment after a given service life may be an alternative method to prevent such failures.
I prepared an Expert Technical Report on my findings, which was used to reach an agreement between the parties. A policy of regular maintenance and replacement of dated office equipment has also been implemented by the employer.
Forensic examination is a crucial aspect of any investigation, and can be pivotal in the progression of a case. If you are involved in a case which may benefit from a forensic examination, Contact Us today!
