Distribution of forces in full body harnesses

Status:

completed 01/1996

Aims:

For reasons of mechanical strength, straps used in body harnesses are generally broad and hard, thus causing discomfort to the user. To make sure workers are more easily prepared to use personal protective equipment against falls from a height, the question was risen whether or not it would be possible to have harness straps designed thinner, softer and more elastic. Two aspects were to be investigated: the distribution of forces and the load acting on an articulated dummy in the moment of fall arrest.

Activities/Methods:

Drop tests were conducted with an articulated dummy under constant framework conditions. Measurements concerned the acceleration on the dummy, tensile forces on the straps, distribution of pressure between the straps and the dummy, distances before and after the drop test, tensile forces at the point of anchorage during the drop test and the suspension angle of the dummy after the test. The readings were transformed into characteristic values to compare different harness constructions and allow for a comparison with biomechanical load limits and other prescribed limit values.

Results:

The distribution of forces in a full body harness depends on the harness construction and on the type of fall. If falling happens in a vertical, upright body position, the force passed into the fall arrest attachment element at the back is mainly led to the sub-pelvic straps. The force measured in the shoulder straps comes to approximately 1/7 to 2/3 of the force occurring in the back straps. If the harness includes a sit strap, the latter transfers the main force to the body. The distribution of forces in the thigh straps depends on the harness construction. Corresponding to the distribution of forces in the harness there are defined points of force transmission into the body, where the pressure is particularly high. High pressure values are mostly measured at points where buckles, slings or fittings are sewed (e.g. shoulder straps). Points of high pressure are also found at the thighs and buttocks. Before modifying the straps, it would therefore be useful to improve the harness design at the before mentioned critical points. Biomechanical limit values were not found to be exceeded.

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