Failure of firefighter escape rope under dynamic loading and elevated temperatures

Transcription

1 Proceedings of te SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA 2010 Societ for Experimental Mecanics Inc. Failure of firefigter escape rope under dnamic loading and elevated temperatures G.P. Horn, P. Kurat Department of Mecanical Science and Engineering, Universit of Illinois at Urbana- Campaign 1206 W. Green St. Urbana, IL ABSTRACT Fire Service escape rope sstems are a firefigter s last resort for a controlled exit from elevated floors of a burning structure wen conditions rapidl deteriorate. Te escape rope is liel to be deploed from a room tat is at least partiall involved wit fire and must support a full loaded firefigter maing a ast exit, resulting in dnamic loading. Tese dnamic loads are accompanied b elevated temperatures, wic will affect te strengt and stiffness of te rope. Current standards onl require tat escape rope sstems be tested at room temperature and under quasi-static loading in order to be certified as a personal escape rope. A pilot stud will be presented tat provides te first experimental quantification of te decline in rope strengt and stiffness at elevated temperatures. Tese data are interpreted wit a simple inetics model for te dnamic loading of rope for various escape scenarios, suggesting tat te factor of safet tat is assumed in current escape rope standards is quicl eliminated b te dnamic forces during escape and temperature dependent strengt deterioration. Suggestions and guidelines for an improved certification protocol are forwarded. I. INTRODUCTION Emergenc escapes from burning buildings require firefigters to ave complete confidence in teir equipment, especiall in te liel scenario wen it must be deploed from a room full involved wit fire. In tese scenarios, escape efforts require te use of rope sstems as life-critical components. Te Fire Service as been faced wit several ig profile incidents in recent ears were line of dut fatalities ave occurred in scenarios were escape ropes ave not been available or could not be deploed safel. One of te most ig profile incidents occurred on Januar 2005 in New Yor Cit were two firefigters were fatall injured and four oters were criticall injured [1]. Escape rope sstems consist of a lengt of rope, usuall 50 to 75 feet long, and tpicall a device to control te rate of descent and a metod of attacing te rope to te bod and to an ancor. Te onl part tat is required of an sstem is te escape rope itself. Tese ropes are designed to allow te firefigter to mae an emergenc egress and descend 3-4 stories to reac te ground or a floor below te fire. Te total time to ancor te rope, mae a controlled descent, and possible enter a lower floor can require several minutes even for te most seasoned firefigter. In recent ears, tere ave been significant advances in ancoring and descent control device design and rope materials. Ancors and descent control devices are tpicall made from metals tat can witstand te temperatures in an involved room. On te oter and, escape rope is made from polmeric materials wit a muc lower termal capacit; te rope still must ave enoug strengt and stiffness to old te firefigter, but not be too stiff or it will cause te escaping firefigter severe injur. Wen a firefigter must escape a structure fire from an elevated floor during an emergenc scenario, dnamic loading of te rope sstem will invariabl occur. Te firefigter will be facing no oter escape options and rapidl deteriorating conditions witin te structure. He or se will be loaded wit eav and restrictive firefigting equipment and ma liel be fatigue from suppression or searc activities and possibl burned. Ancoring te rope in tese conditions will commonl result in an excess amount of rope being paid out resulting in slac line. Te rapid exit scenario ma ten result in a free fall condition causing dnamic loading on te rope. Te most common bencmar for te design and performance standards for rope and rope equipment tat is used b fire and rescue services is NFPA 1983: Standard on Fire Service Life Safet Rope and Sstem Components [2]. Te minimum breaing strengt (MBS) of virgin rope (tested at room temperature) is required to be 13.5 N and rope material must ave a minimum melting temperature of 200 C for te rope to qualif b te T. Proulx (ed.), Dnamic Beavior of Materials, Volume 1, Conference Proceedings of te Societ for Experimental Mecanics Series, 353 DOI / _53, Te Societ for Experimental Mecanics, Inc. 2011

2 354 NFPA standard as escape rope. However, it is important to note tat tese standards do not attempt to quantif te effect tat realistic service temperature migt ave on escape rope performance if deploed from an involved room. Te reliabilit of escape rope sstems is often directl related to its strengt and stiffness, and tese sstems are often deploed at temperatures tat are nown to degrade te properties of rope. Te magnitude of tis degradation as not been completel documented. Tis stud presents te design of and initial results from a series of experiments to stud te cause of strengt or stiffness degradation of firefigting escape ropes in service conditions to allow estimation of dnamic loads tat ma be encountered wen deploing te escape sstems in emergenc scenarios. Were possible, existing NFPA standards were followed to provide a straigtforward comparison to previous data and potentiall an avenue toward expanding te scope of current standards. Tis stud considers onl temperature effects but oter service factors (e.g. rope bending/ining, not ting, and abrasions) will be studied to determine te most damaging combination of conditions for escape rope failure. For tis initial stud, a single brand of NFPA certified nlon escape rope was emploed. II. MODEL OF FIREFIGHTER DYNAMIC LOADING To model te impact forces applied to a firefigter during an emergenc escape, a simplified inetics description of te rope deploment and dnamic loading scenario will be emploed, Figure 1. It is assumed tat te rope acts lie a linear spring. Even toug te load displacement plots are not entirel linear, te initial stiffness reported in Table 1 will be emploed as an approximation. Te firefigter will be modeled as a solid mass tat does not deform and te ancor is assumed to be completel rigid. In most situations, te ancor will deflect sligtl to elp absorb a small amount of energ, toug not as muc as for sport climbers wo can use te friction of carabiners and a belaer to significantl reduce te loads. Tese assumptions provide a worst case scenario, but one tat is realistic. A inematics description of te scenario is provided below: m m Figure 1. Simplified model of dnamic loading of rope. were m is te mass of te firefigter and is te rope stiffness. Te solution to tis set of equations is: cos m t (1). (2) For tis stud, we are most interested in maximum forces generated b te rope, wic occurs at te point of te maximum deflection: m m 2 m max 1 (3) F max 2 max m m m 1 (4) Important to te accurate utilization of tis simple model is te experimental determination of te rope stiffness,, for te conditions in wic te escape rope sstem will be deploed. For te axial loading scenario of interest ere, AE M M, (5) L L L p L s

3 355 were M is te rope modulus, L p is te pa out lengt of te rope (between te ancoring point and exit from te building), and L s is te lengt of slac in te rope tat ma lead to a fall outside of te exit point of te same distance. Te units of rope modulus require experimental collection of load-strain data from te rope test, wic presents a particular callenge in rope testing experiments due to te extensive slippage wit standard rope fixtures and particularl wen onl a portion of te rope is maintained at elevated temperatures. Tus a new experimental protocol was developed as outlined in te next section. Te rope pa out lengt will depend on te ancoring scenarios Tere are tree ancoring scenarios tat we will consider wit tis model. Te first metod requires te firefigter to ancor te rope at te window sill. From anecdotal evidence, once a firefigter as made teir wa to te window, te are not liel to go bac into te room to find an ancor, tus tis ma be te most common location to ancor te rope. It also results in te least amount of rope being paid out (and tus less rope available to absorb energ). Te second scenario is one were an ancor is available at te bottom of te wall under te window, suc as a radiator. In tis case, approximatel 2 feet of rope will be paid out prior to te exit location at te window sill. In te final scenario, te firefigter as ancored te rope to some remote location in te room and tus can pa out between 5 and 10 feet of rope before bailing out of te window. Tis scenario provides te most rope lengt for energ absorption, but also increases te ris of rope burn troug, reduces te lengt available to mae a safe descent to te ground, and ma require a significant amount of time to access te window due to te large frictional forces required to pa out te rope over suc long distances. Equation (4) can also be derived using an energ balance approac using appropriate assumptions and as been utilized to stud te effect of falls during sport climbing [4,5]. Due to te incredibl stiff ropes utilized in te Fire Service, including te direct ancoring scenarios required and te lac of a belaer (individual wo plas out rope to minimize impact forces in sport climbing) in tis application, man of te assumptions tat bring tese models into question for te climbing application are not as significant in tis case. III. EXPERIMENTAL PROCEDURES For te elevated temperature testing of escape rope, a Riele screw driven test frame (Figure 2) wit a 44.5 N load range was emploed. Rope loading fixtures were designed and built to te NFPA 1983 standards. A furnace was designed and fabricated suc tat a uniform controlled temperature could be applied over Grips a 9 inc section of rope wile te test was being Furnace conducted. An extensometer was devised tat utilizes a linear variable displacement transducer (LVDT) to collect deformation data in te eated section. For tese initial experiments, a 9mm Nlon ernmantle NFPA 1983 certified escape rope was tested at room temperature, 100 C and 200 C. Five repeats of eac test were performed in accordance wit NFPA 1983.To set up te tests, te rope was wrapped tree times around te top fixture, and ten te extensometer was attaced to te gage section. Tis assembl was ten passed troug te rope furnace. Next, te rope was pretensioned around te lower fixture and ten wrapped tree times prior to final attacment to a cleat to minimize rope slippage. Te rope was ten ccled from 0.4 to 1.3 N (10% of required MBS) multiple times until te rope ad settled into te grips. Te rope samples were ten loaded until failure. For elevated temperature tests, te oven was ramped up to te maximum temperature in about 30 minutes and was ten eld at maximum temperature for approximatel 30 minutes before testing to ensure a uniform temperature in te Data Collection Test Frame Figure 2. Experimental ig temperature escape rope testing apparatus.

4 356 gage lengt of te rope. Tis procedure was establised using multiple termocouple temperature measurements along te gage lengt of te rope. No significant elongation or drop in load was observed during te eating or old time. IV. HIGH TEMPERATURE EXPERIMENTAL RESULTS AND DISCUSSION Experimental rope strengt and stiffness data is summarized in Figure 3 and Table 1. In Figure 3, a representative curve from eac of te temperatures tested is provided. As Table 1 indicates te average failure strengt of te rope decreases from over 21 N at room temperature to just under 13 N at 200 C. Te standard deviation of eac of te repeated tests is less tan 2%, demonstrating excellent repeatabilit of te experiments. Figure 3. Escape rope load-strain beavior at various temperatures. Following NFPA 1983 standard, te minimum breaing strengt (MBS) was calculated as te average of te mean failure load minus tree times te standard deviation. MBS decreases from 20.0 N at room temperature - wic satisfies te NFPA standard requirement - to 11.7 N at 200 C - wic is significantl below te required value. Tis tested rope is significantl stronger tan required (certified at 19.3 N at room temperature) and as a larger diameter tan oter available NFPA certified escape ropes. For lower rated, smaller diameter ropes tat are manufactured wit similar materials, te temperature at wic te strengt drops below te NFPA standard is liel to be even lower. Te temperatures tested ere are well below te windowsill level temperatures recorded during Grieff s escape rope testing [ 6] and tus ma underestimate strengt loss tat could be encountered in an actual structural fire. Tese results are somewat encouraging in tat tis rope maintained some strengt at te NFPA mandated minimum melting temperature (~200 C) because nlon as a muc iger melting point tan required. However, te are discouraging from te perspective tat conservative estimates of service temperature cause a nearl 40% drop in te breaing strengt.

5 357 Table 1. Failure load and stiffness data for 9 mm nlon ernmantle escape rope. Temp ( C) Failure Load (N) Structural Stiffness (N/m/m) Average Standard Deviation MBS Average It is apparent tat as te temperature is raised from 75 to 200 C, te rope becomes significantl more compliant. Table 1 summarizes te approximatel 60% drop in initial stiffness from room temperature testing to 200 C. Tis reduction in stiffness can be beneficial wen te rope is subjected to dnamic loading as long as tese dnamic forces do not exceed te reduced strengt of te rope. V. DYNAMIC LOADING DISCUSSION Reviewing te functional form of equation (4), it is apparent tat even an L = 0 m distance fall load is applied suddenl to te rope, but no fall taes place - results in dnamic loads tat are twice te static weigt of te firefigter. A firefigter bailing out of a burning building in an emergenc scenario will liel experience some amount of freefall due to te slac in te rope tat must be taen up before te rope starts to carr some weigt and tese dnamic loads increase dramaticall. Figure 4a displas te results from tis model for various different rope lengts assuming te static firefigter weigt is 1.3 N. It is important to note tat te individual maing tis emergenc exit will be in full firefigting buner gear (approximatel N) and wearing a self contained breating apparatus (minimum 110 N) in addition to oter equipment and tools tat te will be carring. Tus, te static weigt of te individual is significantl more tan just teir bod weigt. Te orizontal lines on tese figures represent te MBS of te tested rope at 20 and 200 C. As can be seen b te four dnamic loading curves, forces on te rope increase as te fall distance increases. At te same time, te forces increase as less rope is paid out prior to te emergenc exit. As Figure 4b indicates, rope at elevated temperatures generates less force tan one at room temperature due to its reduced stiffness. From te perspective of dnamic loading, a decrease in stiffness is advantageous, et one can see tat even for relativel small free falls tat ma reasonabl be expected in service, te forces in te rope ma approac or exceed te MBS at tese elevated temperatures. Wile te strengt of te rope ma be sufficient to witstand 9+ N dnamic loads, tis same force will be transmitted to te firefigter troug a arness (in an ideal scenario), ladder belt, or, in a worst case scenario, directl from te rope. Loads of tis magnitude could cause severe bone and sin injuries as well as potential internal organ trauma. Furtermore, if an ancor is not properl set into a solid material or structure, it can pull out or fail under tis level of loading. Dnamic loading becomes increasingl important wen one considers using a stiff aramid escape rope. For instance, Tecnora ropes ave been introduced as a ig strengt, low weigt option tat does an excellent job retaining strengt properties at elevated temperatures. Unfortunatel, tese sstems also ave a structural stiffness tat is almost an order of magnitude stiffer tan similar strengt nlon ropes. If a 1.3 N firefigter taes a 0.6 m fall on Tecnora rope wit a 0.6 m paout(l p = 0.6 m, L s = 0.6 m), te dnamic load would be 37.6 N (compared to 15.2 N for te tested nlon rope at room temperature). As a result of tese forces, it is igl recommended tat escape ropes, and especiall Tecnora ropes, be emploed wit a device tat assists in absorbing dnamic loading to minimize impact force on te rope and bod.

6 358 Figure 4. Dnamic loads on a rope for various fall lengts (L) and pa outlengts ( L p ), for a 1.3 N firefigter at room temperature and 200 o C. Finall, a longer ancor lengt (L p ) relative to te fall distance will significantl reduce te forces in te rope. Toug not alwas practical, it ma be possible to ancor te escape rope furter from te exit point; potentiall across te room from an escape window. If, for example, te attacment lengt could be increased, te maximum force encountered b te rope for te 1.3 N firefigter taing te same fall distance (L = 0.6 m) is reduced from 15.2 N (L p = 0.6 m) to 9.4 N (L p = 3.0 m) at room temperature for te rope tested in tis program.

7 359 V. CONCLUSIONS A basic stud of te effect of dnamic loading was presented in conjunction wit te development of a ig temperature rope testing apparatus. Te experiments ave allowed te first quantification of strengt and stiffness of rope sstems at elevated temperatures. Te relativel small increase from 20 ºC to 200 ºC results in a nearl 40% reduction in strengt and 60% reduction in stiffness. Te rope tested in tis initial stud was nearl 50% stronger tan te NFPA standard requirement at room temperature and et wen tested at 200 ºC, its strengt was below NFPA 1983 minimum. Using a inetics description of te fall event, te dnamic loads tat ma be experience due to a fall on tis tpe of rope were estimated. Even for relativel small free falls tat ma reasonabl be expected in service, te forces in te rope ma exceed te minimum breaing strengt of te rope. Tis simple model also provides some elpful ints for deploing escape rope sstems. If operationall feasible, minimize rope slac and free falls to reduce te ris of escape rope failure. Even if te rope does not brea, te large forces generated b dnamic loading igligt te need for training in exit strategies and rope ancoring procedures to reduce injuries. Tecniques for exiting windows tat significantl reduce or eliminate dnamic loads sould be taugt and emploed wenever possible. Dnamic forces can be significantl reduced b emploing a descent device or tecnique tat allows friction to absorbs a significant portion of te impact from tese falls. It is also possible tat an additional energ absorption device between te firefigter and te rope can be emploed to reduce tese loads. Te issue of ig impact loads is magnified wen dealing wit iger stiffness escape ropes suc as Tecnora sstems. Appropriate placement and fastening of te rope ancor is critical for surviving even small falls witout injur. Finall, wile not alwas practical, fall loads can be significantl reduced if it is possible to ancor a greater distance from te point of exit. Testing rope properties at elevated temperatures is an important process tat NFPA ma want to consider for future editions of NFPA 1983 standard for escape ropes. Collection of bot strengt and stiffness data is recommended as it allows a more detailed analsis of te damage caused b elevated temperatures. For te nlon rope tested ere, te significant reduction in stiffness resulted in lower forces from dnamic loads, wic somewat mitigated te effect of reduced strengt at 200 C. Te onl wa to measure canges in tese rope properties at elevated temperatures is to develop a standardized test procedure. ACKNOWLEDGEMENTS Te autors tan Ernie Timmons for is assistance in building te furnace for elevated temperature rope testing. REFERENCES 1. O Donnell, M., Cit Firefigters Build Teir Own Escape Sstem, Te New Yor Times, June 6, ttp:// NFPA 1983: Standard for Life Safet Rope and Sstem Components, 2001 Edition. National Fire Protection Association, Quinc, MA. 3. Firefigter s Handboo: Essentials of Firefigting and Emergenc Response, 2 nd Edition, Delmar Tomson Learning, Clifton Par, NY, Vogwell, J., and Minguez, J.M., Te safet of roc climbing protection devices under falling loads, Engineering Failure Analsis, 14, , Pavier, M., Experimental and teoretical simulations of climbing falls, Sport Engineering, 1, 79-91, Greiff, J.S., Performance Tests of personal escape rope sstems, Fire Engineering, 45-60, McKentl, J., Parer, B., and Smit, C., Escape line bae off, presented at ITRS, 2003