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Combined fall of both materials and worker into safety nets is ‘problematic’

HSE has published the results of research evaluating the effectiveness of safety nets to identify the risk (if any) of premature failure. There is little historical information on the effectiveness of nets and an incident at Old Trafford, raised concerns about their performance in non-ideal loading situations (ie pre-existing damage, sharp or heavy material entering the net or variations in installation practices).

Given the increased emphasis on safety nets the research aimed to evaluate the effectiveness of safety nets, as used in the UK, and to identify the risk (if any) of premature failure in the less than ideal situations that could occur in the workplace.

The project included a range of variables including:

• net material and manufacture (knotted or knotless nets);
• effect of falling onto different positions in the net such as edges and corners;
• spacing of the attachment points;
• effect of repeated dynamic loading at one position (to simulate multiple falls);
• effect of differently shaped items falling into the net;
• effect of sag in the net;
• use of alternative techniques to control sag in oversized nets;
• presence of defects; and
• effects of ageing and degradation due to ongoing service damage and the effectiveness of test meshes in monitoring degradation

To evaluate these variables a range of tests were carried out using a purpose built drop facility at HSL, Buxton. The resulting loads were measured at various positions around the periphery of the net and damage to the net monitored.

Conclusions

The research report draws the following conclusions (bold text NOT in report) :

• The safety nets used, generally withstood multiple tests, with penetration occurring only in unusual circumstances. No serious safety concerns were identified.
• The larger the attachment spacing, the higher the load at the attachment point and the greater the displacement of the net under load.
• The highest peak loads are generally recorded adjacent to the drop point.
• Peak loads are generally higher for drops at centres of edges and lower for drops at corners.
• Load is transmitted along linear axes from the point of drop, which is likely to be an effect of the square mesh configuration.
• Where damage to the net occurred, it was more likely to occur at attachment points than at drop points.
• There were no significant trends in performance differences between any of net types.
• The highest load on a first drop was 4.84 kN, well below the 6 kN characteristic limit.
• There was a cumulative reduction in elasticity on repeated drops into a safety net. FASET guidance is that nets which have experienced a dynamic load should be replaced.
• The calculated sag was not necessarily evident in the suspended safety net. Inducing extra sag into the net, increased the displacement of the net under load but didn’t have a significant effect on loading.
• The FASET recommended technique of under-rolling excess netting did not produce significantly different results to the alternative technique of gathering. However, under-rolling is a more controllable and should remain the preferred option.
• Small defects at the drop point represented a potential source of failure on repeat dropping. However, small defects at the attachment point increased displacement under load but did not represent a source of failure.
• All retained items tended to move towards the centre of the net. The 95%-ile test dummy moved more erratically than the sphere.
• The test dummy induced higher loading on the net than the BS EN 1263-1 sphere. While this may be a more realistic test, the dummy’s erratic movement makes it less controllable.
• The safety nets were incapable of retaining the compact mass of the 100 kg cylinder. The possibility of aggressive material (such as heavy and compact or sharp edged objects) entering the net with a falling worker remains a concern.
• Despite the poor condition of the nets tested in phase 3, intact mesh samples retained between 67% and 97% of their estimated original strength.
• The mesh retained its properties even in close proximity to areas of damage. Significant deterioration in performance, only occurred when there was obvious damage to the net sample.
• There was consistent agreement between test meshes results and net samples from within the net indicating that test meshes provide a realistic predictor of net performance.

Recommendations for further work

1. This programme was specified to “sample” a wide range of variables and has identified several areas where further consideration and more detailed investigation may be required.
2. Investigation of the loads experienced by a falling body on entering a safety net, using an instrumented test dummy. The erratic movement of the test dummy and resulting damage may indicate that workers entering a net may be at risk of injury.
3. Comparison with the loads on a net resulting from a body falling into the net from greater heights. Personnel working above the recommended UK working limit of 2 m may be a foreseeable misuse.
4. Investigation of the effects of damage (such as damage from hot work or abrasion) on small scale material samples. In the absence of a reliable source of samples from industry (such as was encountered during phase 3 of this programme) damage could be artificially introduced.
5. Further investigation into the behaviour of “real” construction materials entering nets. The 100 kg cylinder penetrated the net on every test, retention of other construction material, which could easily enter the net with falling personnel, could prove similarly problematic.

This article was posted on 10th May 2011 in ALL LATEST NEWS, WORK AT HEIGHT.
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