BY GAVIN HORN, ERNEST TIMMONS, AND PETER KURATH
EMERGENCY ESCAPES FROM BURNING BUILDINGS necessitate that firefighters have complete confidence in their equipment, especially if it must be deployed from an involved room. In many scenarios, these escape efforts involve the use of rope systems as life-critical components. The need for a more detailed understanding of escape rope performance is justified in light of the January 2005 incident in New York City where two firefighters were fatally injured and four others critically injured.1
The most common benchmark for the design and performance standards for rope and rope equipment used by fire and rescue services is NFPA 1983, Standard on Fire Service Life Safety Rope and System Components.2 The minimum breaking strength (MBS) of virgin rope (tested at room temperature) is required to be 3,034 pounds force (lbf), and rope material must have a minimum melting temperature of 400°F to qualify as an “escape rope” under the NFPA standard. Escape rope is dedicated solely for the purpose of supporting a single human life in a one-time-only emergency escape. This rope is typically of a smaller diameter and lower strength than “life safety rope,” which is intended for a 600-lbf rescue load (two people and gear). The strength testing protocols for both types of rope are nearly identical. However, it is important to note that these standards do not attempt to quantify the effect that realistic service temperature might have on escape rope strength and stiffness if deployed from an involved room.
The reliability of escape rope systems is often directly related to the system’s strength and stiffness, and these systems are often deployed at temperatures known to degrade the properties of rope. The magnitude of this degradation has not been completely documented. For example, a common text3 indicates that nylon ropes will not begin to lose strength before 300°F; the amount of strength lost at higher temperatures is not stated. The Fire Department of New York (FDNY) has done a remarkable job researching and subsequently deploying a new rope system in response to the aforementioned incident, but the influence of temperature on this system has not been released to the public at this point. Grieff4 published an excellent study of escape rope systems in Fire Engineering in 2001. One portion of this study was designed to understand the time to failure for various escape ropes when deployed from an involved room. This report indicated that static loads caused by the weight of a firefighter would result in failure in one to four minutes. However, the author points out that the tests were difficult to control for all possible variabilities in the burn buildings.
The data presented here are from laboratory experiments that allow a greater control of the experimental variables that can cause strength or stiffness degradation. Where possible, existing NFPA standards were followed to provide a straightforward comparison with previous data and potentially an avenue toward expanding the scope of current standards. This study considers only temperature effects, but other service factors (e.g., rope bending/kinking, knot tying, abrasions, and impact) will be required to determine the most damaging condition or combination of conditions for escape rope failure. For this initial study, a single brand of NFPA-certified nylon escape rope was employed.
UNDERSTANDING FAILURE OF ROPE SYSTEMS
The Advanced Materials Testing and Evaluation Laboratory (AMTEL) and Illinois Fire Service Institute (IFSI) at the University of Illinois at Urbana-Champaign have begun a series of investigations to more fully understand the failure of rope and rope systems. A controlled laboratory environment is used to isolate specific factors that affect rope strength to quantify which are the most damaging. For the elevated temperature testing of escape rope, a 1929 Riehle screw-driven test frame (photo 1) with a 10,000-lbf load range was employed. Test fixtures were designed and built to the NFPA 1983 standards. A furnace was designed and fabricated such that a uniform controlled temperature could be applied over a nine-inch section of rope while the test was being conducted. The furnace blocks the ability to visually measure extension of the rope as outlined in NFPA 1983, so an extensometer that uses a linear variable displacement transducer (LVDT) to collect this measurement was devised. An extensometer is a device that normalizes extension data over a given original measurement length (called the “gage length”) that can be used without visual contact with the rope. This normalized measurement is called “engineering strain” [change in length over original length (ΔL/Lo)]. Load and extension data are collected simultaneously on a computer system for subsequent analysis.
(1) High temperature escape rope testing apparatus. (Photo courtesy of University of Illinois.)
For these initial experiments, a 9mm nylon kernmantle NFPA 1983-certified escape rope was tested at room temperature, 210°F, and 390°F. Five repeats of each test were performed in accordance with NFPA 1983. Only one test was conducted at 300°F to observe trends, but no statistical data analysis will be performed at this temperature.
To set up the tests, the rope was wrapped three times around the top fixture, and then the extensometer was attached to the gage section. This assembly was then passed through the rope furnace. Next, the rope was pretensioned around the lower fixture and then wrapped three times prior to final attachment to a cleat, to minimize rope slippage. The rope was then cycled from 100 to 300 lbf (10 percent of required MBS) multiple times until the rope had settled into the grips. The rope samples were then loaded until failure. For elevated temperature tests, the oven is ramped up to the maximum temperature in about 30 minutes and is then held at maximum temperature for approximately 30 more minutes to ensure a uniform temperature in the gage length of the rope. This procedure was established using multiple thermocouple temperature measurements along the gage length of the rope. No significant elongation or drop in load was observed during the heating or hold time, indicating some basic difference with Greiff’s experiments.
ELEVATED TEMPERATURE TESTING
Rope strength and extension data are summarized in Figure 1 and Table 1. In Figure 1, the applied load is plotted on the vertical axis, and engineering strain is displayed on the horizontal axis. A representative curve from each of the temperatures tested is provided. The maximum value attained on the load axis is the failure load of a specific sample (MBS will be calculated from this value later). Figure 1 shows that as the temperature is increased, the maximum height of the curves decreases, signifying that the rope is getting weaker. As Table 1 indicates, the average failure strength of the rope decreases from over 4,800 lbs/ft at room temperature to just over 2,900 lbs/ft at 390°F. The standard deviation of each of the repeated tests is less than two percent, demonstrating excellent repeatability of the experiments. The failure load of each of the individual rope samples is summarized in Figure 2, showing the downward trend in strength as well as the excellent repeatability of the test procedure.
Figure 1. Rope Load-Strain Behavior
Figure 2. Failure Strength
Following NFPA 1983, MBS was calculated as the average of the individual failure loads minus three times the standard deviation. MBS ranges from almost 4,500 lbf at room temperature, which satisfies the NFPA standard requirement, to 2,640 lbf at 390°F, which is significantly below the required value. This tested rope is significantly stronger than required (certified at 4,350 lbf at room temperature) and has a larger diameter than other available NFPA-certified escape ropes. For lower-rated, smaller-diameter ropes manufactured with similar materials, the temperature at which the strength drops below the NFPA standard may be even lower. The temperatures tested here are well below the windowsill level temperatures recorded during Grieff’s escape rope testing (4) and thus may underestimate strength loss that could be encountered in an actual structural fire.
These results are somewhat encouraging in that this rope maintained some strength at the NFPA mandated minimum melting temperature (~400°F), because nylon has a much higher melting point than required. However, they are discouraging from the perspective that conservative estimates of service temperature cause a nearly 40 percent drop in the breaking strength. Furthermore, a significant drop in strength is measured at temperatures as low as 210°F for nylon ropes, contradicting the notion that these ropes retain room temperature strength up to 300°F. Combining this strength loss with expected load-carrying reductions presented by deployment in realistic firefighting scenarios, coupled with likely dynamic service loading, suggests that testing for rope strength at elevated temperatures may be critical to determining reliability in actual service environments.
Another quantity that is important to understand from laboratory testing is the rope stiffness. The initial stiffness of the rope is determined by the initial slope of the data shown in Figure 2. A stiffer rope in essence has a steeper slope on this figure. It is apparent that as the temperature is raised from 75°F to 390°F, the initial slope of the data is much shallower, and thus the rope is not nearly as stiff. Figure 3 summarizes the approximately 60 percent drop in initial stiffness from room temperature testing to 390°F. As discussed in the next section, a reduction in stiffness can be beneficial when the rope is subjected to dynamic loading as long as the dynamic forces do not exceed the reduced strength of the rope.
Figure 3. Rope Stiffness
A picture of a typical rope that failed after being tested at 390°F is shown in photo 2. It is apparent from photo 2 that there is significant discoloration of the rope in the region exposed to elevated temperatures. Along with this visible change, the rope is significantly less pliable (i.e., is harder to bend). The rope has experienced obvious physical changes just by raising it to a higher temperature environment once. To fully understand the normal service life expectation for rope, it would be necessary to determine the strength of the rope after repeated heating and cooling (which has been done for some material systems5). The effect of temperature cycling on knotability, knot strength, abrasion resistance, and descent device performance should also be evaluated, as each of these properties depends, in part, on the surface properties of the rope.
(2) Failed escape rope tested at 390°F. Significant discoloration is evident as well as changes in the material’s stiffness and surface. (Photo by author.)
An obvious question is, Why would you care about this reduction in rope strength since the rope still has an MBS of more than 2,600 pounds at 390°F? This result may be taken to imply that this strength level ensures a “safety factor” of 10 times based on the static weight for a typical fully equipped firefighter (this is the weight you would read on a bathroom scale). However, it is important to consider the often overlooked fact that dynamic loading greatly increases the load that the firefighter applies to the rope. A simple example of the effect of dynamic loading is the difference between a hammer resting on your thumb and your thumb’s being struck by an errant blow to a nail.
To consider dynamic loading, we present a very simple physics description from an engineering text6 (a process called “modeling” by engineers). There is no need to have a detailed knowledge of physics courses to understand this section. For the purpose of this model, a few assumptions must be made. It is assumed that the rope acts much like a linear spring. The farther the spring stretches, the more force it exerts, and the magnitude of this force is directly related to the stiffness of the rope. (For example, the farther you stretch a rubber band, the harder you have to pull.) Even though the load displacement plots (Figure 1) are not entirely linear, the initial stiffness reported in Table 1 will be employed as an approximation. The firefighter will be modeled as a solid nondeforming mass, and the anchor is assumed to be completely rigid. In most situations, the anchor will deform slightly to help absorb a small amount of energy, though not as much as for sport climbers, who can use the friction of carabiners and a belayer to significantly reduce the loads. These assumptions provide a worst-case scenario, but one that is realistic.
For the purposes of modeling, a realistic escape scenario, a length (L*) of two feet is assumed to be the necessary minimum length between a firefighter and the rope’s anchoring point. Any additional line that has been played out while anchoring or preparing to make an emergency exit will be considered as “slack.” This slack in the line will result in a fall distance (L) and dynamic loading when the firefighter exits the building. For example, if five feet of line are played out prior to exiting the building, a “fall” of L = 3 feet may be encountered.
Without details of the equations, even an L = 0 feet distance fall results in dynamic loads that are twice the static weight of the firefighter. A good analogy for this effect comes from those who garden or have dug trenches. It is remarkably easier to make the shovel penetrate the ground by suddenly applying your weight to the shovel as opposed to gently putting your weight on it. The load applied by the test machines used in this project as well as those specified by NFPA 1983 apply the force gently. A firefighter bailing out of a burning building in an emergency scenario will apply load in a much more sudden manner. If there is some amount of free fall prior to the rope’s starting to carry some weight, these dynamic loads will increase dramatically.
Figure 4 displays the results from this model for various rope lengths using 250 lb/ft as the static firefighter weight. The horizontal lines on this figure represent the MBS of the tested rope at 75°F and 390°F. As can be seen by the four dynamic loading curves, forces on the rope increase as the fall distance increases. The different curves result from the fact that the rope stiffness decreases as the temperature increases (Table 1). Because of its reduced stiffness, rope at elevated temperatures generates less force than one at room temperature (just as “softer” springs on a truck will transmit less road roughness to the driver). From the perspective of dynamic loading, a decrease in stiffness is advantageous, yet one can see that even for relatively small free falls that may reasonably be expected in service, the forces in the rope may approach or exceed the MBS.
Figure 4. Dynamic Loads During Fall
Some additional important insights can be gained from this simple exercise without too much additional detail. It is important to remember the lesson that every action has an equal and opposite reaction. In this context, that means that the loads calculated and shown in Figure 4 not only apply to the potential failure of the rope but also are felt by the firefighter and the anchor point. While the strength of the rope may be sufficient to withstand 2,000-lbf dynamic loads, this same force will be transmitted to the firefighter through a harness (in an ideal scenario!); a ladder belt; or, in a worst case scenario, directly from the rope. Loads of this magnitude could cause localized bone and skin injuries as well as potential internal organ trauma. Furthermore, if an anchor is not properly set into a solid material or structure, it can potentially pull out or fail under this level of loading.
(3) Definitions for model of rope dynamics. (Photo by author.)
Dynamic loading becomes increasingly important when one considers using a Kevlar® (or similar aramid) rope as escape rope. Kevlar® rope has been introduced as a high-strength, low-weight option that does an excellent job retaining strength properties at elevated temperatures. Unfortunately, these systems also have a structural stiffness that has been (most likely conservatively) estimated to be five to 10 times stiffer than similar strength nylon ropes. Assuming a stiffness that is 7.5 times that measured for nylon at room temperature in this study, if a 250-pound firefighter takes a fall (L = 2 ft, L* = 2 ft) on Kevlar® rope, the dynamic load would be 7,831 lbf (compared to 3,091 lbf for the tested rope at room temperature). As a result of these calculations, it is highly recommended that escape ropes, and especially Kevlar® ropes, be employed with a device that assists in absorbing dynamic loading to minimize impact force on the rope and body.
Finally, a longer attachment length relative to the fall distance will tend to reduce the forces in the rope (as an analogy, one would require less force to get the same extension from a longer rubber band). Though not always practical, it may be possible to anchor the escape rope farther from the exit point, potentially across the room from an escape window. If, for example, the attachment length could be increased to 10 feet, the maximum force encountered by the rope for the 250-pound firefighter taking the same fall distance (L = 2 ft, L* = 10 ft) is reduced from 3,091 to 1,903 lbf at room temperature for the rope tested in this program.
From some simple laboratory tests, we have been able to quantify the effects that elevated temperatures have on some typical properties for nylon escape rope. The small increase from 75°F to 390°F results in a nearly 40 percent reduction in strength and 60 percent reduction in stiffness. The rope tested in this initial study was nearly 50 percent stronger than the NFPA standard requirement and yet when tested at 390°F, its strength was below the NFPA 1983 minimum. Significant qualitative changes in the rope materials were noted after cooling to room temperature after testing. Rope that has been repeatedly raised to high temperatures is potentially significantly different from the original material; even if this rope is deployed at room temperature, it may not behave as expected.
A basic study of the effect of dynamic loading was presented in conjunction with these test results, pointing out that the actual loads a rope may encounter are significantly higher than the static weight of a firefighter. Even for relatively small free falls that may reasonably be expected in service, the forces in the rope may exceed the minimum breaking strength of the rope. This simple model also provides some helpful hints for deploying escape rope systems. If operationally feasible, minimize dynamic loads to reduce the risk of escape rope failure. Even if the rope does not break, the large forces generated by dynamic loading highlight the need for training in exit strategies and rope-anchoring procedures. Techniques for exiting windows that significantly reduce or eliminate dynamic loads (such as “spidering”) should be taught and employed whenever possible.
Dynamic forces can be significantly reduced by employing a descent device or tie-off technique that allows the rope to gradually slip so that friction absorbs a significant portion of the energy from these falls. It is also possible that an additional energy absorption device between the firefighter and the rope can be employed to reduce these loads. The issue of high-impact loads is magnified when dealing with higher stiffness escape ropes such as Kevlar® systems. Appropriate placement and fastening of the rope anchor are critical for surviving even small falls without injury. Finally, although not always practical, fall loads can be significantly reduced if it is possible to anchor a greater distance from the point of exit.
Testing rope properties at elevated temperatures is an important process that NFPA may want to consider for future editions of the NFPA 1983 standard for escape ropes. Collection of both strength and stiffness data is recommended, as it allows a more detailed analysis of the damage caused by elevated temperatures. For the nylon rope tested here, the significant reduction in stiffness resulted in lower forces from dynamic loads, which somewhat mitigated the effect of reduced strength at 390°F. However, if other escape rope systems display a reduction in strength without the significant stiffness reduction, dynamic loading effects are even more critical. The only way to quantify changes in these rope properties at elevated temperatures is to develop a standardized test procedure.
1. O’Donnell, M., “City Firefighters Build Their Own Escape System,” The New York Times, June 6, 2005. http://www.nytimes.com/2005/06/06/nyregion/06ropes.html?ex=1153022400&en=1a9c55bc589e5d0b&ei=5070.
2. NFPA 1983, Standard for Life Safety Rope and System Components, 2001 Edition. National Fire Protection Association, Quincy, MA.
3. Firefighter’s Handbook: Essentials of Firefighting and Emergency Response, 2nd Edition, Delmar Thomson Learning, Clifton Park, NY, 2004.
4. Greiff, J.S., “Performance Tests of Personal Escape Rope Systems,” Fire Engineering, May 2001, 45-60.
5. McKently, J., Parker, B., and Smith, C., Escape line bake off , presented at ITRS, 2003.
6. Juvinall, R.C. and K.M. Marshek, Fundamentals of Machine Component DesignI, 4th Ed. (John Wiley & Sons, 2006).
GAVIN HORN is research program coordinator at the Illinois Fire Service Institute and a research scientist at the Advanced Materials Testing & Evaluation Laboratory, The University of Illinois at Urbana-Champaign. He is also a firefighter with the Savoy (IL) Fire Department.
ERNEST TIMMONS is student at the Advanced Materials Testing & Evaluation Laboratory, Department of Mechanical Science and Engineering-University of Illinois at Urbana-Champaign.
PETER KURATH is director of the Advanced Materials Testing & Evaluation Laboratory, Department of Mechanical Science and Engineering-University of Illinois at Urbana-Champaign.