Loading tests on aged safety nets

Status:

completed 01/1996

Aims:

Safety nets, when they are used on construction sites, are exposed to climatic factors. Consequently, safety nets are most likely to suffer from ageing due to UV-radiation, heat and humidity. Two aspects should be investigated: the quantitative loss in safety functions of different safety nets due to ageing after a two-years-period of weathering and the suitability of test methods to evaluate the ageing behaviour of safety nets.

Activities/Methods:

Quasi-static loading tests were carried out on safety nets made from twisted polyamide yarns (white colour) and plait polypropylene yarns (white or green colour). The net samples (3.3 m x 3.1 m) were fixed on the edges and subjected to loading by means of a 500-mm-sphere until the net bursted. The force/distance diagrams were used to calculate the net's energy absorption before destruction. Characteristic values were the maximum tensile force, the breaking distance and the breaking energy. These characteristics could also be determined in simple yarn and knot tensile tests. During the two-years-weathering phase, UV- and global radiation of the sun light were measured. Samples were taken every three months. Safety loss due to ageing was recorded as a function of time. It was thus possible to compare different safety nets, and data obtained for different net samples, yarns and knots.

Results:

The breaking energy which was determined on net samples in quasi-static loading tests turned out to be the best criterion to evaluate the ageing behaviour of safety nets. Breaking energy values determined in knot tensile tests are an equally reliant basis for evaluation. Yarn characteristics, however, were less suitable to evaluate the ageing behaviour of safety nets. In the beginning polyamide nets achieved higher breaking energy values that polypropylene nets. As the latter material shows a better ageing resistance, there was but an insignificant difference between the two materials at the end of the weathering phase. Green polypropylene nets turned out to resist more easily to ageing than white ones, although there was no big difference. After a two-years-period of weathering in the open air, the safety loss came to 42% of the breaking energy for polyamide nets and to 29% of the breaking energy for white polypropylene nets. The results are used to formulate recommendations for the user as to the selection of materials with an increased resistance to ageing and concerning the replacement state of wear of safety nets.

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