Assessing slip resistance of wintery walkways with a novel portable slipmeter

Transcription

1 Paper presented to Walk21-V Cities for People, The Fifth International Conference on Walking in the 21 st Century, June , Copenhagen, Denmark Assessing slip resistance of wintery walkways with a novel portable slipmeter Carita Aschan, Mikko Hirvonen, Erkki Rajamäki and Tarmo Mannelin Finnish Institute of Occupational Health, Department of Physics Contact details: Carita Aschan Finnish Institute of Occupational Health, Department of Physics Topeliuksenkatu 41 a A FIN Helsinki Finland Phone: Fax: carita.aschan@ttl.fi Abstract In Finland about pedestrian and bicycle falling accidents occur yearly at the street, walkways and courtyards leading to serious consequences. The total costs of these accidents including the indirect costs are estimated to be as high as 420 million euros. Maintenance of the walkways in winter for example to the City of Helsinki causes costs of about 4 million euros per year but the costs of pedestrian and bicycle falling accidents are estimated to be tenfold. In general, slipping is due to fairly complex causal pathways, involving both environmental and human factors. The primary environmental factor behind slipping accidents is the slip resistance characteristics of the underfoot surface. Especially in winter, when the walking surfaces are often covered e.g. by ice, snow, slush or frost, slipping accidents are mainly due to the inadequate friction between footwear and underfoot surfaces. Portable devices measuring friction, i.e. slipmeters, may be used for assessing slipperiness of different walking surfaces in situ. However, the validity of these portable test devices should be improved by bringing them within the range of human slipping conditions observed in biomechanical studies. Therefore, a new portable device was designed and constructed at the Finnish Institute of Occupational Health (FIOH). The applicable measurement parameters of the device represent the actual kinematics during critical gait phases. During winter seasons and a pilot study was performed in order to evaluate the usability of the novel portable device for measuring slipperiness of walkways in varying pavement and weather conditions. According to the results of the two winter

2 seasons, the developed Portable Slip Simulator is applicable for measuring the slipperiness of the wintery walking surfaces in situ. Also, the results indicate that the method can be used to evaluate the quality of the winter maintenance as well as anti-slip properties of the footwear in field conditions.

3 Biographies The research on pedestrian safety has been performed at the Finnish Institute of Occupational Health from the late 1980s. The goal is to reduce accidents involving slipping, tripping, and falling. Research in this area focuses on assessment of risks, determination of slipperiness in different environmental conditions, and assessment of the functioning of mechanisms regulating balance. Researcher Mikko Hirvonen (B.Sc.), research engineer Erkki Rajamäki (M.Sc.Eng.) and assistant researcher Tarmo Mannelin (B.Sc.) have been working on the field of pedestrian safety from the beginning of 1990s. The main author, senior researcher Carita Aschan (Ph.D. in Physics), has been working in the research group since the year 2000.

4 Assessing slip resistance of wintery walkways with a novel portable slipmeter Carita Aschan, Mikko Hirvonen, Erkki Rajamäki and Tarmo Mannelin Finnish Institute of Occupational Health, Department of Physics Introduction In Finland, about slipping and falling accidents occur yearly leading to serious consequences (Penttinen et al., 1998; Grönqvist, 1995), about of which take place outdoors at the street, walkways and courtyards (Vuoriainen et al., 2000). Around 2/3 of these accidents occur when the walking surface is covered by ice or snow (Grönqvist, 1995). The total costs of slipping and falling accidents including the indirect costs are estimated to be as high as approximately 420 million euros per year (Vuoriainen et al., 2000). In the City of Helsinki, the yearly costs due to pedestrian and bicycle falling accidents are estimated to be about 45 million euros (Vuoriainen et al., 2000). At the same time, winter maintenance causes costs of about 4 million euros per year (Tervonen et al., 2001). The amount of slipping accidents as well as the costs due to them could be decreased e.g. by improving the maintenance, warning the pedestrians about most slippery conditions and informing the public about the importance of the footwear selection in winter as well as encouraging the use of anti-slip devices. In general, slipping is due to fairly complex causal pathways, involving both environmental and human factors (e.g. Courtney et al., 2001). The primary environmental factor behind slipping accidents is the slip resistance characteristics of the underfoot surface (e.g. Redfern and Rhoades, 1996; Grönqvist et al., 2001). Especially in winter, when the walking surfaces are often covered e.g. by ice, snow, slush or frost, slipping accidents are mainly due to the inadequate grip between footwear and underfoot surfaces. In general, slipperiness is defined quantitatively as a coefficient of friction between surfaces which is a measurable physical quantity. Measured coefficient of friction values are connected to subjective evaluations with the grading system presented in Table 1. Table 1. The grading system connecting the dynamic coefficient of friction to the subjective evaluations (Grönqvist, 1995). Class Explanation Coefficient of kinetic friction 1 Very slip-resistant Slip-resistant Unsure Slippery Very slippery < 0.05 In past years road friction has been assessed in several studies with different types of devices (Wallman and Åström, 2001). Unfortunately, most of the effort has been concentrated on the assessment of slip resistance between vehicle tyres and road. Most devices used in road friction measurements are not suitable for assessing slipperiness experienced by the pedestrian due to the fact that the parameters used differ too much from the human biomechanical parameters (Chang et al., 2001a, b). Very little attention has

5 been paid to the evaluation of slip resistance of footwear in various winter conditions. In order to assess pedestrian safety on walkways, a more human-centred approach is needed. Therefore, a new portable device (Aschan et al., 2003) was designed and constructed at the Finnish Institute of Occupational Health (FIOH) based on the slip simulator developed for slip resistance measurements at FIOH in the late 1980s (Grönqvist, 1995). The applicable measurement parameters of both devices represent the actual kinematics during critical gait phases. The aim of this study was to evaluate the usability of a novel portable slip simulator for measuring slipperiness of walkways in various pavement and weather conditions. In addition, the effects of footwear selection, weather conditions and winter maintenance were evaluated. The results obtained are also utilized by the Finnish Meteorological Institute (Ruotsalainen et al., 2004) in developing a model for predicting pavement conditions in winter in order to give a warning of very slippery days. Materials and methods DCOF measurements Slip resistance, i.e. dynamic coefficient of friction (DCOF), of various walking surfaces was measured in situ by using the Portable Slip Simulator of FIOH (Aschan et al, 2003). The used measurement parameters of the device were as follows: normal force 170, 250 or 500 N, normal force build-up rate 4.5 kn/s leading to normal force build-up time of approximately 110 ms at F = 500 N, horizontal sliding velocity 0.2 m/s, contact angle 5 i.e. heel contact, and evaluation of DCOF between 100 and 300 ms after achieving the full normal force. The result of the measurement was the mean value during the measurement period of 200 ms. Means and standard deviations of at least nine repeated DCOF measurements were calculated. During the late winter 2003, the measurements were performed with four different footwear: i) winter shoe found to be slippery in use, ii) slip-resistant safety footwear (European prestandard ENV 13287:2000), iii) slippery footwear the outsole of which was made of hard (92 Shore A), unpatterned thermoplastic polyurethane (TPU) which is a commonly used material in women's shoes, and iv) footwear with soft polyurethane (PU) outsoles the tread pattern of which has been totally removed by using an orbital sander. Since the measured DCOF values were similar to each other for footwear i) and iii) and for ii) and iv), respectively, selection was restricted only to the footwear ii) and iii) during the measurements of the second winter period (October, March, 2004). Locations and weather conditions The DCOF measurements were started in February At the time, measurements were performed both at pavements with rather low pedestrian traffic as well as in busy locations, such as the surroundings of the Helsinki Central Railway Station, where many slipping and falling accidents have occurred (Vuoriainen et al., 2000). In addition, the slipperiness of walking surfaces was measured at the locations in which no proper winter maintenance existed. Because the maintenance was found to affect the measured results strongly, the DCOF measurements of the second winter period were performed at the locations with no regular winter maintenance in order to take better into account the needs of the Finnish

6 Meteorological Institute for their weather and pavement condition model development work (Ruotsalainen et al., 2004). The measurements were performed at different locations in different weather conditions. Air temperature and humidity as well as the surface temperature of a measurement point were recorded. All the locations were photographed, and the description of the surface (ice, snow, slush, etc.) and the winter maintenance used (graveling, ploughing, etc.) were written down. Results DCOF measurements were performed in different winter conditions, and the walking surfaces were mainly covered by ice, snow or slush. Since the results obtained were similar between the footwear samples i) and iii) as well as between ii) and iv), the results are reported only by using terms "slippery footwear" and "slip-resistant footwear" also indicating the meaning of footwear selection in different weather conditions. Slip-resistant footwear had better grip in all measured pavement conditions being occasionally even more than four times higher than that of the slippery one. On average, the DCOF values measured with slip-resistant footwear were two times higher than those measured with slippery footwear. Similar results with these different types of footwear were obtained when measuring very slip-resistant, well-graveled surfaces as well as wet, smooth icy surfaces in which the DCOF values measured even with the slip-resistant footwear were < Figure 1. The measured DCOF values for slippery wet ice were 0.09 and 0,12 when using slippery and slip-resistant footwear, respectively.

7 The average value of all measurements performed with the slippery footwear was 0.18 ± 0.06 the lowest value being 0.06 (melting ice of 0 C) and the highest 0.42 (trampled newly-fallen wet snow covering trampled snow). Similarly, the average for the slipresistant footwear was 0.38 ± 0.12 the lowest and the highest values being 0.09 (smooth ice covered by water) and 0.63 (trampled newly-fallen wet snow), respectively. The DCOF values measured with the slippery footwear correlated rather weakly (r = 0.59, p<0.0001) with those measured with the slip-resistant footwear.some examples of measurement points in different weather conditions are presented in Figures 1-5. Figure 2. The DCOF values of 0,12 for slippery and 0,15 for slip-resistant footwear were obtained when measuring ice (T=-6 C) covered by a thin layer of newly-fallen snow. Figure 3. Lightly graveled, trampled snow had DCOF of 0.50 when using slip-resistant footwear and 0.18 when using slippery footwear.

8 Figure 4. By using the slippery and slip-resistant footwear, respectively, the DCOF values of 0.12 and 0.18 were measured for wet ice with no gravel. The DCOF values were doubled when measuring at the same time in an adjacent measurement point with gravel. Figure 5. Well-graveled trampled snow was found to have a good grip when measured both with slippery (DCOF = 0.39) and slip-resistant (DCOF = 0.42) footwear. Conclusions According to the results obtained, the new portable slipmeter is applicable for measuring the slipperiness of the wintery walking surfaces in situ. Significant differences between the slip resistance characteristics of different footwear were found indicating the importance of the right footwear selection in prevention of slipping accidents. In addition, the measurement method was found capable for evaluating the quality of winter maintenance in field conditions.

9 Acknowledgements The authors wish to express their appreciation to Finnish Meteorological Institute and "Jaloin"-project for their good cooperation. The work has been supported by the Ministry of Transport and Communications and the Ministry of Social Affairs and Health who are gratefully acknowledged. References Aschan, C., Hirvonen, M., Mannelin T. and Rajamäki, E. 2003, Prevention of slipping accidents - development of the portable slip meter II. IN: Proceedings of the XVth Triennial Congress of the International Ergonomics Association, August 24-29, 2003, Seoul, Korea Chang, W.-R., Grönqvist, R., Leclercq, S., Myung, R., Makkonen, L., Strandberg, L., Brungraber, R.J., Mattke, U., Thorpe, S.C., 2001a. The role of friction in the measurement of slipperiness, Part 1: Friction mechanisms and definition of test conditions. Ergonomics 44 (13), Chang, W.-R., Grönqvist, R., Leclercq, S., Brungraber, R.J., Mattke, U., Strandberg, L., Thorpe, S.C., Myung, R., Makkonen, L., Courtney, T.K., 2001b. The role of friction in the measurement of slipperiness, Part 2: Survey of friction measurement devices. Ergonomics 44 (13), Courtney, T.K., Sorock, G.S., Manning, D.P., Collins, J.W. and Holbein-Jenny, M.A., Occupational slip, trip, and fall-related injuries - can the contribution of slipperiness be isolated? Ergonomics 44 (13), European prestandard ENV 13287:2000. Safety, protective and occupational footwear for professional use - Test method and specifications for the determination of slip resistance. March Brussels, CEN. Grönqvist, R., A dynamic method for assessing pedestrian slip resistance. Ph.D. Thesis. People and work, research reports 2. Helsinki, Finnish Institute of Occupational Health. Grönqvist, R., Abeysekera, J., Gard, G., Hsiang, S.M., Leamon, T.B., Newman, D.J., Gielo-Perczak, K., Lockhart, T.E. and Pai, C.Y.-C., Human-centred approaches in slipperiness measurement. Ergonomics 44 (13), Penttinen, M., Nygård, M., Harjula, V. and Eskelinen, M., Jalankulkijoiden liukastumiset, vaikeimmat kelit ja niiden ennustaminen sekä tiedottamiskokeilu pääkaupunkiseudulla. VTT Research Notes. Espoo, Valtion teknillinen tutkimuskeskus (VTT). (In Finnish, English abstract: Pedestrian slipping accidents, detecting and forecasting of hazardous road conditions. Information campaign on the Helsinki region.

10 Redfern, M.S. and Rhoades, T.P., Fall prevention in industry using slip resistance testing. IN: A. Bhattacharya and J. McGlothlin (Eds.), Occupational Ergonomics: Theory & Applications. New York, Marcel Dekker, Inc. Ruotsalainen, J., Ruuhela, R. and Kangas, M., Preventing pedestrian slipping accidents with help of a weather and pavement condition model. In these proceedings. Tervonen, J., Hämekoski, K., Myllynen, M., Pihlaja, M. and Penttinen, P., Liukkaus, hiekoitus ja katupöly - Yhteiskuntataloudelliset vaikutukset. Helsingin kaupungin Rakennusviraston julkaisut 2001:30. Helsinki, Helsingin kaupunki, Rakennusvirasto. (In Finnish). Vuoriainen, T., Helenius, M., Heikkilä, J. and Olkkonen, S., Jalankulkijoiden ja pyöräilijöiden kaatumistapaturmat, Espoo, Helsinki, Jyväskylä ja Oulu. Finnra Reports 48/2000. Helsinki, Finnra, road and traffic technology. (In Finnish, English abstract: Incidence of falls by pedestrians and cyclists in Espoo, Helsinki, Jyväskylä and Oulu. ). Wallman, C.-G. and Åström, H., Friction measurement methods and the correlation between road friction and traffic safety. A literature review. VTI meddelande 911A. Linköping, Swedish National Road and Transport Research Institute.