A “Toolbox” for Forensic Engineers

Accidents in the Workplace 273
8.4 Ladder Accidents
Ladder accidents are one of the most common kinds of domestic and industrial
accidents because ladders are such widely used pieces of equipment, and
the user is exposed to a high level of risk unless safety rules are obeyed strictly.
Both of the accidents described here occurred because one or more of the
rules was broken, with serious personal injuries as a result. There are several
standards dealing with ladders, of which the most important in the U.K. is
the British Standard. 1
One of the accidents occurred on a dry, summer morning, just after the
homeowner had erected the two-stage extension ladder (which had just been
purchased) against the back wall of his house to clean the upper windows (on
the first floor of a two-story detached property). The ladder was supported on
the level concrete paving slabs of the patio adjacent to the back garden of the
house. According to the witness statement, the owner was at or near the top
where he was washing a fanlight above an upper window. As he moved down
the ladder to clean the pane below, the ladder suddenly slipped from the sill,
and “walked down” the wall, and he fell to the ground where he sustained very
serious injuries. The ladder ended up at right angles to the wall. One of the
plastic tips to the ladder had broken (Figure 8.13A, B and C), but the feet were
intact. There was no damage or visible defects to the metal ladder itself. It was
thought that the broken tip could have caused the fall, by allowing the ladder
to slip down the wall, so the owner, who was also the injured party, initiated
litigation against the ladder manufacturer. Following funding from legal aid,
the lawyer approached an expert for a report.
8.4.1 The Material Evidence
The key evidence was thus the broken tip (Figure 8.13). It showed a brittle
fracture in an unidentified, relatively rigid plastic. The fracture surface was
very fresh, and there was no sign of old or subcritical cracks. FTIR analysis
showed that the material was in fact a copolymer of polypropylene, which
should normally be tough and resilient. But what did the fracture reveal, and
were there any obvious features that could represent defects exposed in the
surface?
The central void is clearly a stress concentration with a stress concentration
value of at least two. However, analysis of the crack surface itself showed
the origin to lie elsewhere, at an external corner produced as a feature of the
design of the tool used to injection mold the component, known as a mold
parting line. It is simply where two mating parts of the steel tool meet, and
often show a mismatch owing to mold tool wear (Figure 8.13B). The