Holding forces of two-stranded Figure-8 knotted slings in sandstone cracks: Experiments and analytical modelling

In the Elbe Sandstone Mountains two-stranded Figure-8 knotted slings are commonly used as climbing protection.These protections are experimentally examined on the basis of 8 mm technical accessory cord in an artificial crack test rig.The resulting load-bearing capacity by means of the maximum force is characterized for different friction linings while varying the crack angle and crack width.Based on the experimental results and an analytical model, it is demonstrated that for crack angles below 20° the required crack widths are less than the 1.5-fold rope diameter to effectively withstand typical forces encountered during a lead climber fall.