2023 International Technical Rescue Symposium—St. Louis. MO

AHD No-go: Exploring the Limits of Artificial High Directionals

Rich Siemer

Harken, Inc.

Introduction:

As artificial high directionals continue to gain acceptance in rope rescue and rope access, technicians continue to come up with configurations to solve rigging problems that the designers and engineers may never have imagined. Likewise, there are some commonly used configurations that technicians rig with confidence that may be unwarranted. This paper will discuss the results of testing artificial high directionals conducted at Harken’s SMC location in Ferndale, WA over several years and provide recommendations for rigging.

Background:

Testing was conducted using the TerrAdaptor which is produced under the SMC brand. The author is the Product Director for Harken Safety and Rescue and previously presented “What’s your Vector, Victor?” at ITRS in 2018. That presentation catalogued many unorthodox TerrAdaptor and Vortex configurations, analyzed the forces in those configurations and offered recommendations for things to avoid. However, the presentation lacked hard data about the load capacities of cantilevered configurations and was of limited practical use.

In the intervening years, others in the industry grew concerned about ubiquitous internet posts of exotic cantilevered configurations that popped up on social media. In February 2020, Richard Delaney of RopeLab published the test report “Cantilevered Tripod Tubes” which can be found at https://www.ropelab.com.au/members-cantilevered-tripod-tubes/ (paywalled). In the report, he calculated the predicted load where the Mid Tubes (TerrAdaptor nomenclature) or Inner Tubes (of an Arizona Vortex) would yield or permanently deform.

SMC conducted similar testing in 2012. The test setup consisted of a rigid fixture for the supported end of the tube and the load was applied to the unsupported cantilever with a hydraulic cylinder as shown below:

Fig. 1: 2012 SMC testing

For Richard Delaney’s testing, he had to source his own materials instead of sacrificing expensive TerrAdaptor or Vortex tubes. Conveniently, the dimensions of these tubes are similar enough between the two brands that his test results would be applicable to both TerrAdaptor and Vortex users. His test results more or less confirmed his calculations that “a 206 kg mass suspended from the tip of a horizontal 850mm long cantilevered tube will result in yield to the tube.” Richard’s conclusion was that “a 272kg/600lb. load suspended from a 1m horizontal cantilevered tube… will likely result in a permanently damaged tube. It may even fail completely.”

SMC 2012 & Richard Delaney 2020 Test Results:

Note the different cantilever distances between tests 1 and 4. The maximum cantilever you can create with a production TerrAdaptor is 27.25” (692mm) when measured to the center of the last hole 2 . It appears that Richard Delaney’s testing did not accurately replicate the supported portion of the cantilevered tube, at least in the TerrAdaptor. As you can see in Fig. 1, in order to create a cantilever, the Perf Tube (silver, but normally gray) must be pinned into the Mid Tube (orange) as well as the Leg Clamp. This leaves a maximum unsupported cantilever of 27.25” (692mm) with a supported portion of 15.375” (391mm) when the load is suspended from the last hole in the tube as in the SMC 2012 testing.

Fig. 2: TerrAdaptor maximum possible cantilever

The difference can be seen in Fig. 3 which shows Delaney’s test setup with a leg tube that has already yielded. Note that Richard Delaney did not account for passing the leg tube through a leg clamp in addition to connecting it to the Mid Tube. Delaney’s test also shows an added sleeve which loads the tube at the very tip.

¹ Richard Delaney’s test report includes three other tests not shown here since they replicate conditions that only apply to the Arizona Vortex

² The discrepancy between the SMC 2012 testing distance and the actual distance may be due to use of prototype parts or simply a miscalculation at the time of testing

Fig. 3: Delaney 2020 test

These dimensional differences mean that the lever arm is longer in Delaney’s testing than what is possible when the tubes are installed in a full cantilevered tripod unless you’re using a Space Station to extend the cantilever as shown in Fig. 2. This contributes to the significantly lower test result in the Delaney testing (674 lbf/3kN) than in the SMC 2012 testing (1649 lbf/7.36kN). Additionally, both these tests isolate a single leg tube mounted in a rigid test fixture which may not be representativ40%40%e of a full cantilevered tripod.

I was in contact with Richard prior to his testing and provided him with the dimensions of the TerrAdaptor tubes so he could make the test parts as accurate as was reasonable. I was also able to review his test set-up and results. After my review, I pointed out that his test set-up isolated a single tube in an effectively rigid fixture (similar to the 2012 SMC testing), but when used in a tripod, the forces imparted on a single cantilevered leg are distributed to the rest of the components in the tripod. This can be observed in testing where there is elastic deformation (temporary flexing) of all the leg tubes when a cantilevered leg tube is
loaded. In light of this, Richard added a caveat to the end of his report:

The other key point to note is that these theoretical values assume a cantilevered tube that has a rigid fixing. In reality, this will not be the case as the rest of the structure may lessen the effect of the applied torque.

Later, we will see how much a cantilever in a full tripod will lessen the effect of the applied load.

Testing:

We conducted the following testing at Harken’s SMC location in Ferndale, Washington in the summers of 2021, 2022 and 2023.

1) Horizontal Cantilevers (2023)
    a) Basic Horizontal Cantilever
    b) Supported Horizontal Cantilever
2) Basic Cantilevers (2021, 2022)
    a) Pinned vs. rigid
    b) Cantilever vs. supported cantilever
3) Monopods (2022)

The testing from 2021 and 2022 attempted to replicate lazy leg cantilevered tripod configurations as closely as possible. The test setup was complicated, time consuming and resulted in data with limited practical applications. There was some useful data that was collected which will be presented later.

Testing 2023:

The testing in 2023 was a simpler set-up, gave us more solid data and it is easier to compare to the SMC 2012 and Delaney 2020 testing so we will start there. A horizontal cantilever maximizes the bending moment, making it a worst-case scenario. The test sample consisted of a TerrAdaptor built from cosmetic second parts. The cantilevered leg was oriented horizontally with an articulating foot bolted to a vertical post and a Space Station pinned to the #9 hole on the Mid Tube. The load was applied with a 20,000lb. hydraulic cylinder connected via an Enforcer load cell to the horn of the Space Station in order to create the longest feasible cantilever (Figs. 2, 4 & 5). All tests had a sample size of one. Testing with a small sample size is not considered statistically relevant and further testing is required to confirm or disprove any of the following results.

Testing started with a single hole exposed (#9) and loaded to 3kN to see if the tripod could support a generous two-person static load. We then attempted to load it to 12kN to simulate a dynamic event. Then the lower two legs and head were moved back to the next hole so that two holes were exposed (#9 and #8) and the same 3kN and 12kN tests were conducted. We repeated this cycle until we had the maximum of 6 holes exposed. Twelve kilonewtons was chosen because it is the upper limit when using force limiting systems (Mauthner 2014 & 2016 ITRS papers). It also allowed us to conduct more testing since testing to yield for every test would quickly eat up our limited supply of TerrAdaptor leg tubes.

As expected, the results show that the longer the cantilever is, the weaker it gets. While a 3kN load can be supported by a horizontal cantilever, only the 10.75” cantilever distance will survive a 12kN load without permanent deformation. Note that any of the cantilevered configurations are significantly weaker than a standard tripod.

The results between the 2012 SMC, the 2020 Delaney and the 2023 SMC testing are compared below:

We did not do any tests in 2023 that directly compare to the 2012 SMC or 2020 Delaney tests. However, there are some interesting comparisons in the data: 1) Comparing test 4 (RD 2020) with test 6 (SMC 2023), the SMC sample has a cantilever that was 6.8% longer but was 68% stronger, a dramatically different result. This is likely due to test 4 being an isolated leg tube in a rigid fixture and test 6 using a full cantilevered tripod; 2) Comparing test 2 (SMC 2023) with test 2 (SMC 2012), the 2012 test, which was in a rigid fixture, has a cantilever that is 4.8% longer than the 2023 test sample but is 15.67% stronger. This is the opposite correlation to the previous comparison. Because of the small sample size (N=1 in all these tests) more testing would be required to explain this result. However, Delaney’s prediction that a 600lb. load suspended from a one meter long cantilevered tube would likely fail appears to be incorrect. In fact, test 6 shows that the tube’s load capacity is 1.6x what he predicted.

Continuing with the 2023 SMC testing, we also conducted tests using a tie-back for the cantilever to see if it was possible to increase the performance of cantilevered tripods. One of the vertical legs was extended past the head and equipped with a Lash Ring. Two strands of Aramid-core “tech cord” were run between the Space Station and the Lash Ring, tensioned and tied off with a round turn and a mule knot. The tie-back was approximately 45 degrees from the horizontal and vertical leg tubes for each test. The test set-up can be seen in Fig. 6.

Fig. 6: Tie-back detail, test #7

*For tests 9, 11 & 12, the head was reversed with the Half Plate was pointing forward as shown in fig. 7.

2021-2022 Basic Cantilever Testing

The 2023 horizontal cantilever testing was intended to simulate a worst-case scenario and is not necessarily representative of the way that most people use tripods. The 2021-2022 testing is more representative of how cantilevered tripods are typically used in the field. The test set-up consisted of a TerrAdaptor built from cosmetic second parts. The load was applied with a 20,000lb. hydraulic cylinder connected to a cable, chain and turnbuckle that were used in place of a rope to eliminate rope stretch and accommodate the limited throw of the hydraulic cylinder. The TerrAdaptor was in a lazy leg configuration as it would be in an over-the-edge rescue scenario. The length of the lazy leg was constant for this entire series of tests. The front feet were tied off to prevent movement away from the load. The legs were hobbled using cam straps or rope hobbles. No testing was conducted with any artificial high directionals from other manufacturers so these results may not be directly applicable to any tripod but the TerrAdaptor.

Rigid vs. Pinned Cantilever Testing

In statics, connections between structural elements such as columns and beams can be characterized as rigid or pinned. A rigid connection constrains the rotation between a column and beam and therefore can resist lateral loads without any additional bracing. A pinned connection allows a beam to rotate at the column/beam joint. A structural frame using pinned connections would need cross bracing to resist any lateral forces. This test was intended to determine if the tripod reacted differently when the cantilevered leg was a rigid or a pinned connection at the head which allows the rear leg to pivot. The rigid tests were conducted with all three load locking head pins engaged in the half plate of the TerrAdaptor head with the rear leg clamp in the “C” position in the TerrAdaptor head. The pinned tests were conducted with only one load locking head pin engaged in the pivot hole of the half plate so that the leg clamp and leg were free to rotate around the half plate. Two holes were exposed in the cantilevered leg for a cantilevered distance of 9”. All legs were hobbled. Each test set-up was first loaded to 3kN to see if it could support a generous two-person static load. We then attempted to load it to 12kN to simulate a dynamic event.

Looking at tests 1, 2 and 3, the rigid cantilever outperformed the pinned cantilever in 12kN loading. This indicates that the rigid cantilever distributes more of the force of the load to the front legs. The effect can be seen in figs. 10 and 11 where the pinned cantilever experiences considerably more deformation than the rigid cantilever at a much smaller load.

Tests 4,5 and 6 were removed from this test report for intellectual property reasons. For tests 7 through 18, we only conducted the 3kN tests since we did not have a lot of spare parts if we damaged the head or any of the leg tubes. We varied the amount of overlap between the Perf Tube and the Mid Tube where feasible to see if that had any effect on the strength of the cantilever. There were no conclusive results. We also measured forces at the hobbles to the rear legs to see if the rigid or pinned rear leg affected the load on the hobbles and found that for any given cantilever length, the forces on the hobbles are greater on the pinned configuration than on the rigid configuration. Again, this indicates that the rigid configuration more effectively distributes force throughout the entire system.

Supported Cantilever

We only conducted one supported cantilever test with the rear leg in a pinned configuration. It failed at 3.07kN (test 20) so all further testing was done with a rigid rear leg with the leg clamp in the “C” position. Tests 21-23 were conducted with variations in rear anchor position, which changed the resultant minimally and appeared to have no effect on the results. Since the end of the testing sequence was near, we decided to load the supported cantilever configuration to 12kN. On our first attempt at test 25, our rear anchor failed. After re-setting the anchor and re-starting the test, it reached 12.6kN and the test was stopped. Test 26 reached the 12kN threshold as well. Unfortunately, we did not test an unsupported cantilever with a 29” cantilever distance at 12kN, so we do not have direct points of comparison for tests 25 and 26. Again, it must be noted that even though a configuration with a 29” supported cantilever was able to withstand a 12kN load, it is still significantly less than a standard tripod.

Monopod Testing

The intention of this testing was twofold: to analyze the strength of monopods built with only two tubes and to determine the forces imparted to guylines.

The setup was a monopod built from a Mid Tube (bottom) and a Perf Tube (top) with the minimum overlap at the joint. Initially, a Lash Ring was used as a head but after it broke we replaced it with the Free Module of a Space Station. The guylines were heavy chain to make the system as rigid as possible. The front ties were two strands of Aramid core tech cord. The load was applied with a 20,000lbf hydraulic cylinder connected to a combination of cable, a turnbuckle and chain in place of rope to eliminate rope stretch and accommodate the limited throw of the cylinder. Enforcer load cells were used to record the peak forces. We attempted to align the resultant with the monopod.

Each test set-up was first loaded to 3kN to determine if it could support a generous two-person static load. We then attempted to load it to 12kN to simulate a dynamic event.

Test 27 reached a target load of over 12 kN but failed the Lash Ring. We replaced it with the Space Station module for the remainder of the tests. We stopped test 28 due to instability because the front guyline anchors were too close together. All subsequent tests used the wider guyline anchors. Tests 29 and 30both reached the 12kN target. Note the rear guyline force values of 958lbf (4.26kN) and 772lbf (3.43kN) respectively. This indicates that the resultant was forward of the foot of the monopod. Even though we tried to align the resultant with the monopod, the front guylines would go slack and the pulley was observed to swing slightly forward when the load was applied. This was due to a natural bias in the pulley resulting from an imbalance in the forces between the input and output legs of the cable. The difference in those forces is created by the friction in the bearings which can easily be seen in the difference of the resultant between lowering and hauling operations even at two-person loads. Test 31 had the highest recorded load at the rear guyline probably because the Mid Tube was yielding before it finally broke at 11.95kN.

Conclusions

The 2023 Horizontal Cantilever testing suggests that cantilevers as part of a full tripod outperform single, rigidly fixtured tubes since the forces imparted on the cantilever are distributed to the rest of the tripod. We also saw that supporting a horizontal cantilever with a tie-back to an extended front leg tube increased the strength of the cantilever. However, the quality of the tie-back appeared to affect the strength significantly.

The 2021-2022 Basic Cantilever Testing showed that a cantilevered tripod with a rigid lazy leg outperformed one with a pinned leg by a substantial margin. We recommend that if users choose to use cantilevered tripod configurations, they should not use a tripod with a pinned (pivoting) rear leg. The other result of note is that a supported, maximum possible cantilever (using a Lash Ring) can withstand a 12kN load in that specific configuration. The load capacity of any cantilever will vary depending on the angle between the cantilevered leg and the resultant.

The 2022 Monopod testing demonstrated that a monopod built from only two tubes with the minimum possible overlap can withstand a 12kN load, but the monopod is likely to become unstable at loads that high. Monopod strength is heavily dependent on how it is rigged, which is why the TerrAdaptor manual does not show that the monopod configuration meets any applicable standards. We recommend maximizing the overlap of tubes in monopods, which is best accomplished by using three tubes.

Although some of the configurations we tested were able to withstand 12kN loads, that force is significantly lower than any of the configurations that are shown in the TerrAdaptor manual. Using the TerrAdaptor in cantilevered configurations is outside of the manufacturer’s recommendations. If site conditions require additional reach, configurations such as a forward-leaning tripod or a luffing A-frame are preferred alternatives.

Credits

Numerous people deserve my thanks since this project was a team effort. Design engineer, Erik Warmenhoven did all the hard work of fitting out the test area, conducted the testing with me and gave me feedback on this paper. SMC General Manager Chris Starr also provided feedback. Harken allowed us to devote the many, many hours that the testing required.