A “Toolbox” for Forensic Engineers

164 Forensic Materials Engineering: Case Studies
done but was instead simply resting on top. The inability to lock the unbroken
claw was readily demonstrated by placing it over a cross tube; there was no
snap action as it was pushed down — the crescent just hung vertically —
and it could be lifted off again without any resistance. In contrast the spring
at the broken end was rusted but was still operative, so the two steps were
able to butt against each other and exert their normal locking action. Hence
at the time of the accident the unbroken claw would have been resting on
the frame tube while the broken claw was clamped.
The cause of the guard rail coming off while the engineer was leaning
against it and reaching upward thus had nothing to do with the weakness in
the broken claw. What must have happened is that his movement lifted the
unclamped claw off the frame and caused the rail to swing outward. This
movement applied bending force to the claw at the other end, which was still
clamped, and this caused it to break as the engineer was already falling. When
the rescuers reached the scene the guard rail was lying on the floor and the
piece broken off the claw was not recovered. But the freshly produced fracture
had exposed the porosity in the casting and the initial investigators assumed
this must have been responsible for the accident. They made no mention of
the state of the claw springs or that the legs of the one at the unbroken end
had rusted away and rendered the clamping action inoperative.
One other possibility was considered. The broken claw might have
already been cracked, making it much more likely to twist off as the guard
rail moved outward. Such partial fractures are often found when tubes are
thrown to the ground as scaffolding is dismantled, instead of handing down
the tubes to someone on the ground. If the claw lands on a hard surface endon
the impact may bend it inward with sufficient force to crack it or break
it off. However, in this instance there was no staining or oxidation of the
fracture surface to suggest that the claw had been partially cracked for some
time and, unfortunately, the allegations in the Further and Better Particulars
that the “hook” had snapped due to having insufficient strength were not
substantiated. Because the broken end was not recovered it was not possible
to discover whether this exhibited any witness marks from an impact.
The case did not go to court. The allegations that the hook had snapped
due to a manufacturing fault and that was the direct cause of the accident
could not be substantiated. Instead, the firm that had hired out the scaffold
(who would have been the second defendant) settled the claim. They failed
in their duty to inspect the scaffolding regularly to ensure all parts were in
a safe condition. There was no case against the manufacturers of the scaffold.
Despite the fact that the casting exhibited a degree of gas porosity, it was not
a weakness that caused the accident. If it had been seriously defective, then
failure would be expected almost the first time it came under load in service