Roof Hatch Accident
In this feature article, we will look at an accident that occurred to the friend of an employee at a fast food restaurant. It was after hours at the restaurant and the young employee and his friend were headed to the roof of the small one story building. Roof access was gained from inside the building via a fixed ladder leading up to a hinged roof hatch. The hatch is shown in Figure 1. As the friend was climbing up through the hatch, the hatch suddenly dropped injuring her head and neck.
Inspection and testing revealed that hatch was equipped with a built in bar to prop it in the fully open position, however the handle for the prop bar extended well into the hatch opening; the prop bar was designed to release when pushed upward. It was likely that the friend was climbing through the opening and inadvertently dislodged the prop bar allowing the hatch to fall. Normally, a counterweight spring on the hatch would prevent its free fall, but the device was damaged and not operable.
Measurements found that the hatch weighed 70 pounds; this imparts a downward static force of 35 pounds at the lip. An external spring had been added, apparently to replace the broken counterweight spring, but was not stiff enough to fully arrest the hatch – a 15 pound static force still remained. A falling hatch can impart significantly higher forces than its weight alone depending on its speed, and the contact stiffness.