Force Measurement in Rock Climbing Ropes

 

 

Introduction

The objective of this project was to quantify the loads that occur in an 11 mm diameter dynamic rock climbing rope during a simulated lead fall with a fall factor of 2. The fall factor is defined as the length of the fall divided by the length of the rope between the climber and the belayer (or anchor). Thus, a fall factor of 2 is the largest possible.

Methods and Equipment

The drop tests were conducted indoors in the Bridge Structures Laboratory on the University of Nevada, Reno campus. As illustrated in Figure 1, a 25-ton overhead crane served as the anchor and a tandem 5-ton overhead crane served as the drop point. Due to height limitations, the maximum length rope tested was approximately 3.5 meters (12 feet).

  

Figure 1. Tandem cranes used as release and anchor points for the drop tests. A 1.5 m (4 ft) test rope is shown just prior to release of the dead weight.

 

The total weight used was 84 kg (185 lbs). This included the 80 kg (176 lbs) dead weight and 4 kg (9 lbs) of chain, carabiners, and bolts. The test set-up consisted of the following items (see Figures 2 and 3):

1.      3 loops of chain to secure the load cell to the anchor crane. Each loop was approximately 2 feet long. The chain was an ISO certified 3/8 inch G80 alloy chain rated to 5000 lbs. (Figure 2).

2.      The chain was connected to the load cell using three ISO certified 4000 lb capacity inch eye-bolts. (Figure 2).

3.      An Instron 50 kN (10,000 lb) load cell, model 2518-802 (Figure 2).

4.      3 parallel slings constructed of 450 N (2000 lb) nylon webbing (Figure 2).

5.      2 gate-opposed locking 27 kN carabiners.

6.      The 11mm test rope (double-8 knots at both ends). Rope lengths from 0.75-3.5 m (2-12 feet) were tested.

7.      2 gate-opposed locking 27 kN carabiners (Figure 3)

8.      3 nylon web slings (Figure 3).

9.      3 loops of 3/8 inch G80 alloy chain - each loop was about 1 foot (Figure 3).

10.  Three inch eye-bolts mounted in the dead weight (Figure 3).

11.  Dead weight - 80 kg steel mass (Figure 3).

  

Figure 2. The load cell was suspended from the anchor crane using chains and connected to the test rope using slings and carabiners.

 

Figure 3. The 80 kg dead weight was attached to the test rope using 3 loops of chain and 3 nylon webbing slings.

 

All load data was collected using an IBM compatible laptop computer equipped with LabVIEW data acquisition software, a DAQCard 1200 data acquisition card, and an SC-2043-SG strain gauge amplifier (National Instruments, Austin, TX).  The system was configured to sample at 200 Hz and had a force resolution of 11.38 N (2.56 lbs).  Data collection began just prior to release and ended once the weight stopped bouncing (approximately five to ten seconds, depending on rope length).

Due to the tandem crane configuration, some horizontal offset between the release crane and the anchor crane was unavoidable (see Figure 1).  The two cranes were separated by approximately 2 feet, which allowed the weight to miss the anchor crane as it fell.

A custom-built release mechanism was used to manually release the dead weight from the upper crane. The device consisted of a notched wheel which was loaded slightly over-center by the dead weight. A pin inserted into the wheel prevented rotation/release. The pin was manually removed to release the weight.

A total of five safety ropes were also used. One rope was used in case primary (test) rope broke. This backup rope was approximately twice the length of the primary rope. A second rope was used as a guide rope to minimize swinging.  The guide rope was stretched taught between the upper crane and the floor. A 4 foot runner with carabiners on both ends connected the primary and guide ropes.  Finally, 3 belay ropes attached to the dead weight were used to control the amount of swinging after the fall.

 

Results and Discussion

A total of 14 drop tests were completed, of which 2 resulted in inaccurate data due to systematic problems.  The test data for the 2.5 m (8 ft) rope lengths is shown in Figure 4. The data shown has been truncated to start at the onset of load (bottom of the fall) and end at 2.5 seconds. For these tests, the maximum rope load during the initial impulse was approximately 9 kN (2000 lbs). Subsequent impulses were much smaller in magnitude (typically a factor of 3 less).

The initial impulse lasted approximately 0.35 seconds for all ropes tested. For rope lengths longer than 1.5 m, a “bounce” was evidenced by a short ( second) period where a zero rope force was measured directly after the initial impulse.

  

Figure 4. Sample data of rope load as a function of time for the 2.5 m rope length.

 

 

Figure 5. Maximum Rope loads as a function of rope length for a fall factor 2. Fall distance is twice the rope length.

 

Figure 5 illustrates the maximum rope load as a function of rope length, which is half of the total fall distance. The peak rope load was found to increase with rope length over the range of  rope lengths tested. The maximum rope load recorded was 9.7 kN for a 3.5 m rope (7 m fall).   These values are fairly consistent with the often quoted “shock force” of 9 kN for a 4.0 m rope and 7 kN for a 0.6 m rope. Both Figures 4 and 5 indicate that the rope load data collected is very repeatable between tests.

Rope lengths were measured both just prior to and just after the drop tests. Table 1 summarizes the amount of rope stretch that occurred during the testing. The general trend is for increasing percent rope stretch with decreasing rope length. This is most likely due to the fact that the knots constitute a larger portion of the total rope length and significant elongation occurs due to tightening of the knots.

 

Table 1. Summary of drop test data.

 

Length Before

Length After

Rope

Max Force

Drop (m)

Drop (m)

Stretch

(kN)

3.91

4.52

116%

8.25

3.48

4.09

118%

9.67

3.28

3.76

115%

9.58

3.38

3.91

116%

8.27

3.25

3.68

113%

9.56

2.57

2.95

115%

9.08

2.62

2.97

114%

9.15

1.98

2.34

118%

7.31

2.06

2.59

126%

7.40

1.42

1.70

120%

6.83

1.35

1.63

121%

6.97

1.32

1.60

121%

6.78

0.74

0.91

124%

6.16

0.79

0.97

123%

5.77

Summary

1.      An experimental method of measuring the loading history of a rock climbing rope was developed. The equipment is portable and can be used in real rock climbing environments.

2.      The system developed was used to measure the loads in an 11 mm dynamic rope that occurred during a fall factor 2 with a total weight of 84 kg. Rope lengths tested varied from 0.75 to 3.5 m (2-12 feet).

3.      Repeated measurements for a given rope length indicated that the rope load as a function of time is very repeatable between tests.

4.      The maximum load measured was 9.7 kN for a 3.5 m rope length (7 m fall).

5.      Peak rope loads increased with fall distance over the range tested.

6.      The peak force is substantially less than the typical load rating for rock climbing equipment.