Whole-Body Vibrations When Riding on Rough Roads

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The overall aim of this study was to ascertain the seriousness of the problem of whole-body vibration when driving on roads; ”Is the road roughness such that it entails a health hazard and/or a road safety hazard through its impact on drivers?”. Other objectives were to estimate the scope of the problem during non-frozen ground conditions, to examine the problems and potential related to measurement techniques and to point out the necessity of further research in this field.

The measurement data was collected when driving on 37 kilometres of National Highway No. 90 (Hw 90) and 21 kilometres of County Road 950 (Lv 950) in Västernorrland County. The road condition on the test stretches covered the entire range from very smooth (IRI20 = 0.43 mm/m) to very rough (IRI20 = 22.78 mm/m). Whole-body vibration was measured in compliance with the ISO 2631-1 (1997) standard “Evaluation of human exposure to whole-body vibration”. This was done on stretchers with patients in different types of ambulance and at different speeds, and on the floor and driver and passenger seats for seated occupants in some different truck configurations.

There are three main sources of vibration: road roughness, vehicle properties and driver behaviour (including choice of speed). The interpretation of the results supports the opinion that within reasonable variations in these factors, road roughness plays a considerably greater part than the other two. High-energy, multi-directional vibrations at many natural body part frequencies were found at the seats in trucks. This is serious due to the risk of resonance, meaning a greater reproduction of vibration in the parts of the body afflicted than at the surface from which the vibrations are transferred. Further, the study substantiates findings from earlier studies; i.e., that the high frequency of occupational diseases among commercial drivers, especially in the locomotor systems, is related to rough roads. This relationship is probably strongest in geographic areas where the road roughness level is high on a large percentage of the roads. Where the roughness was greatest, peak values were registered on ambulance stretchers that considerably exceed the level that completely healthy people are assumed to experience as ”extremely uncomfortable” by international standards.
During a 15-minute ride on a stretch of National Highway 90, the vibration level in one type of ambulance was high enough to pose a potential health hazard had a healthy person been exposed to it for as little as 10 minutes a day. It was shown that the vibration on the ambulance stretchers was as great as at the drivers’ seat in wheel loaders loading blasted rock, bulldozers clearing way in forests for new road construction, etc. Vibration problems are even greater in the spring due to seasonal frost damage related additional roughness.

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Whole-Body Vibrations When Riding on Rough Roads

  1. 1. Publication 2000:31EWhole-body vibration when riding on rough roads A study 2000-05
  2. 2. Date Document designationHead Office 2000-05-15 (2002-03-08) Publ 2000:31EAuthorRoad Engineering Division.Contact person: Johan Granlund.TitleWhole-body vibration when riding on rough roads.Main contentAt rural highway speeds, road roughness is a source of undesirable dynamic forces and displacement inthe interaction between road, vehicle and human. These vibrations can cause a sense of discomfort, andit cannot be ruled out that they could impair the health and performance ability of both drivers andpassengers alike.A study has therefore been conducted on National Highway 90 and County Road 950, aimed at ascer-taining the seriousness of the problem of whole-body vibration during travel. The roughness index onthe test stretches varied from very good (IRI 20 = 0.43 mm/m) to extremely poor (IRI 20 = 22.78mm/m). Vibrations that affect vehicle occupants were measured in different configurations of movingtimber lorries and ambulances. A separate report published by Ingemansson Technology AB presents adetailed account of how the measurements were carried out and how the data was stored and analysed.Another separate report published by the National Institute for Working Life presents the findingsfrom an analysis of the effect on the human body of the vibrations recorded.This report is a summary of the study. It also contains an interpretation of the findings from collatingthe vibration measurement data with the data collected in connection with the routine annual road co n-dition surveys. There are three main causes of vibration: road roughness, vehicle properties and driverbehaviour (including the choice of speed). The results of this study support the opinion that, withinreasonable variations in these factors, road roughness has a far greater impact than the other two vari-ables. Further, the study substantiates that the higher frequency of injury, especially in commercial driv-ers’ locomotor systems (as been found in earlier studies), is related to rough roads. This correlation isprobably strongest in geographical areas where long stretches on a large percentage of the roads have ahigh IRI, i.e. in the so-called ”forest counties” of Norrland, Värmland and Dalarna in Sweden. Ridingthe roughest road stretches, peak values were registered on the ambulance stretchers with vibration levelsthat are considerably above levels that completely healthy people are considered to experience as ”ex-tremely uncomfortable”, as per an international standard on evaluation of human exposure to whole-body vibration.PublisherEnvironmental Department.ISSN 1401-9612Vägverket printers in Borlänge 2002.Picture of the ambulance on the cover is published with the permission of Anders Wiman AB, ambulancemanufacturer.PublisherNational Road Management Division.Key wordsRoads, pavement, roughness, texture, ride, vibrations, shock, dampening, natural frequency, resonance,dynamic forces, displacement, fracture mechanics, road grip, ride quality, stress, discomfort, performanceability, health, motion sickness, living environment, working environment, road maintenance, surfacingDistributor (name, postal address, telephone, telefax)Swedish National Road Administration, Butiken, Internal Services Division, SE 781 87 BORLÄNGE,Sweden+ 46 243-755 00, fax +46 243-755 50Head OfficePostal address Telephone TelefaxSE 781 87 BORLÄNGE + 46 243 - 750 00 +46 243 - 758 25
  3. 3. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E PrefaceThis is a report of a study that was co-financed by the Västernorrland County Council,SCA Forest and Timber AB, Själanders Åkeri AB (haulier) and the Environmental De-partment at the Swedish National Road Administration (SNRA). The project was initiatedby the SNRA subsequent to a survey of the problems associated with driving on roughroads. Further, the trend towards ”smoother roads” revealed in the SNRA’s annual IRI(International Roughness Index) measurements seemed questionable in view of the intensedissatisfaction revealed in road user opinion surveys. Particularly perplexing was the acutedissatisfaction with the ride quality amongst commercial drivers, primarily in the north ofSweden. Our interest was stimulated even more when interviewing hauliers and transportpurchasers in Västernorrland County. After having studied reams of literature containingthe key word ”vibration”, including reports on the impact of road roughness on driver per-formance, driver fatigue, reports on incubators in ambulances being badly shaken duringtransport, and the high frequency of health problems amongst commercial drivers, particu-larly in their locomotor systems, sufficient research material had been collected to warrantinvestment in this project.Kjell Ahlin, Licentiate in Engineering and employed at Ingemansson Technology AB wasresponsible for the surveys and analyses. Professor Ronnie Lundström of the National In-stitute for Working Life was in charge of examining the impact on the human body of ex-posure to those vibrations measured. The vibration data was collated with the SNRA’sexisting road surface condition data (collected through laser/inertial technology) by theundersigned. The ambulances were driven by Leif Leding, medical orderly, and the trucksby Hans Selin and Bengt Själander. Vibration measurements were conducted on non-frozen roads, to comply with the SNRA routine road surface condition surveys. It is im-portant to keep in mind that the vibration problem is considerably greater during the springthaw, when roads are still partially frozen and roughness even more pronounced.I would like to take this opportunity to express my sincere appreciation to those who pro-vided the financial backing for this project, as well as the persons mentioned above andtheir colleagues, as well as to my own fellow colleagues throughout the Swedish NationalRoad Administration.Finally, I would especially like to thank Kathleen Olsson at the SNRA International Secre-tariat, for making the English translation possible.Borlänge 15 May 20001Johan Granlund, MSc (Civil Engineering)Project Manager21Translation finished on 8 March 2002.2Translation comments: Johan Granlund is now leading road roughness profilometry operations within SNRA Consulting Services. KjellAhlin is now Professor at Blekinge Institute of Technology. Professor Ronnie Lundström is now head of the Biomedical Engineeringand Informatics Department at the University Hospital of Northern Sweden. 1(79)
  4. 4. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E Contents1 SUMMARY ........................................................................................................................................................ 42 READER INSTRUCTIONS ........................................................................................................................ 63 DEFINITION OF TERMS .......................................................................................................................... 74 BACKGROUND..............................................................................................................................................18 4.1 FROM PAST TO PRESENT.............................................................................................................................. 18 4.2 ROAD ROUGHNESS IN BRIEF ....................................................................................................................... 21 4.3 MONITORING OF THE ROAD CONDITION AT THE SNRA....................................................................... 23 4.3.1 Road roughness measurements........................................................................................................ 23 4.3.2 Road user opinion polls.................................................................................................................... 25 4.4 ANALYSIS OF ROAD ROUGHNESS ............................................................................................................... 28 4.5 TRANSMISSION OF VIBRATIONS THROUGH THE VEHICLE....................................................................... 29 4.6 W HOLE-BODY VIBRATION.......................................................................................................................... 31 4.6.1 Natural frequencies and resonance in the human body............................................................... 32 4.6.2 Examples of the effect of whole-body vibration in the 0.5-80 Hz range..................................... 33 4.6.3 Examples of the effect of extremely low frequency whole-body vibrations ................................ 33 4.6.4 Origin of whole-body vibration...................................................................................................... 34 4.6.5 Measurement of whole-body vibration........................................................................................... 365 METHOD ........................................................................................................................................................ 38 5.1 TEST STRETCHES .......................................................................................................................................... 38 5.1.1 National Highway No. 90............................................................................................................... 38 5.1.2 County Road 950.............................................................................................................................. 39 5.2 VEHICLES ...................................................................................................................................................... 40 5.2.1 Ambulances ....................................................................................................................................... 40 5.2.2 Heavy trucks...................................................................................................................................... 41 5.3 MEASUREMENT AND ANALYSIS OF WHOLE-BODY VIBRATIONS............................................................. 44 5.3.1 Variables ............................................................................................................................................ 44 5.4 E XPERT ANALYSIS OF THE EFFECT OF VIBRATION ON THE HUMAN BODY........................................... 46 5.5 COLLATION BETWEEN THE VIBRATION DATA AND THE DATA FROM THE ROAD CONDITION SURVEYS ................................................................................................................................................................... 46 5.5.1 Effect of emergency action, ”the devil’s choice”, on National Highway 90............................... 466 RESULTS ......................................................................................................................................................... 49 6.1 ROAD SURFACE CONDITION AS PER THE SNRA’S ”PMS” DATABASE ................................................... 50 6.1.1 Roughness expressed as International Roughness Index............................................................... 50 6.1.2 Crossfall.............................................................................................................................................. 52 6.1.3 Lane cross-sections............................................................................................................................. 53 6.1.4 Seasonal variation in road roughness, County Road 950 ........................................................... 54 6.2 CAB ACCELERATION MODEL AS A FUNCTION OF ROAD ROUGHNESS (IRI).......................................... 557 DISCUSSION.................................................................................................................................................. 58 7.1 ROAD STRETCHES WHERE THE ROUGHNESS PRESENTS A HEALTH HAZARD........................................ 59 7.2 VARIATIONS IN THE ROAD CROSSFALL ARE PARTICULARLY HAZARDOUS ............................................ 64 7.3 METHODS TO REDUCE WHOLE-BODY VIBRATION IN CONNECTION WITH ROAD TRANSPORT .......... 67 7.3.1 Changed travel speeds....................................................................................................................... 67 7.3.2 Changes in vehicles ........................................................................................................................... 69 7.3.3 Road maintenance............................................................................................................................ 70 7.3.4 Does the choice of road maintenance strategy matter? ................................................................ 71 7.4 CONCLUSIONS .............................................................................................................................................. 72 7.4.1 Evaluation of impact on humans of vibrations related to road roughness ............................... 72 7.4.2 Assessment of the need to take action on the road network, etc.................................................. 72 7.4.3 Need for further research and development................................................................................... 748 REFERENCE LIST...................................................................................................................................... 75 2(79)
  5. 5. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EIntroductionApart from the direct impact road roughness and texture has on vehicles and the environ-ment, these road characteristics are also an indirect source of the noise, infrasonic soundand whole-body vibration that cause stress on road users. The effect of this stress/load canbe divided into the following three categories, for which criteria can be stipulated and well-motivated limits set: 1. discomfort 2. performance ability 3. health impactThe effect on the human body depends on the type of load. It varies from individual toindividual, depending on the person’s own particular situation. Reactions can be acute (likespeech impairment), gradually increase during travel (like motion sickness) or steadily de-velop over time (like spinal injury). The effects can be transient, as in temporary visual im-pairment, or chronic as in kidney damage. Temporary exposure can cause stress reactions,like a faster pulse or higher blood pressure, which in turn entails a greater stress on theheart. Sustained exposure can tire the brain a nd produce drowsiness. Daily exposure can, inthe long run, impair health and result in long periods of sick leave or even early retirement.Sometimes these ailments can require medical treatment, which in turn can have side ef-fects that can substantially impair quality of life. Musculo-skeletal injury is by far the great-est working environment problem in the Western world today.In the mid 1970’s, the exposure of truck drivers to vibration was an issue raised at the fed-eral government level in the USA, formulated as ”Do vibrations (as well as noise, toxicfumes and other factors that contribute to truck “ride quality”) have a negative effect ondriver health and on highway safety?” A research programme that extended over severalyears, ”Ride Quality of Commercial Motor Vehicles and the Impact on Truck Driver Per-formance” was initiated in 1977 to answer this question. The findings were summarised ina report published in 1982 entitled ”Truck Cab Vibrations and Highway Safety” [66]. Thisreport was jointly produced by leading researchers, road authorities, vehicle manufacturers,hauliers and commercial drivers. It shows that the answer to the key question as to whetherthere is any correlation between cab vibrations and road safety is YES, that there is goodreason to believe that vibrations affect drivers’ health, and that vibrations must be elimi-nated at source through effective road maintenance rather than merely dampened. Thereport concludes that if the deterioration of the road network is allowed to continue, theresult will be serious health and road safety problems.Today, further on down the road, we can see how the American road network has beenupgraded. According to the FHWA report Life-Cycle Cost Analysis in Pavement Design,action is nowadays initiated on federal roads before the condition reaches a level corre-sponding to IRI 2.7 mm/m [67 ]. In the study conducted during summer on Swedish Na-tional Highway 90, IRI1 values close to 100 mm/m have been measured3, 37 times abovethe American limit. Hw 90 is known to be much rougher during the spring thaw.At the time of writing, an EC directive stipulating limits for exposure to whole-body vibra-tion based on health and safety criteria is in the process of being drawn up.3 Laser/inertial Profilometers have limitated laser measuring range (MR). On the Profilometers used in Sweden MR for vertical distanceis +/ - 100 mm. Since the distance from the laser beam to the front axle of the Profilometer vehicle is close to 1 m, profile slopes (usedwhen calculating IRI) will begin to be underestimated when they exceed about 100 mm / 1 m = 100 mm/m in static theory case. Inpractise, Profilometer pitch and roll dynamic motion reduces this range of use further. 3(79)
  6. 6. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E1 SummaryThe overall aim of this study was to ascertain the seriousness of the problem of whole-body vibration when driving on roads; ”Is the road roughness such that it entails a healthhazard and/or a road safety hazard through its impact on drivers?”. Other objectives wereto estimate the scope of the problem during non-frozen ground conditions, to examine theproblems and potential related to measurement techniques and to point out the necessityof further research in this field.The measurement data was collected when driving on 37 kilometres of National HighwayNo. 90 (Hw 90) and 21 kilometres of County Road 950 (Lv 950) in Västernorrland County.The road condition on the test stretches covered the entire range from very smooth (IRI20= 0.43 mm/m) to very rough (IRI20 = 22.78 mm/m). Whole-body vibration was measuredin compliance with the ISO 2631-1 (1997) standard “Evaluation of human exposure towhole-body vibration”. This was done on stretchers with patients in different types ofambulance and at different speeds, and on the floor and driver and passenger seats forseated occupants in some different truck configurations.There are three main sources of vibration: road roughness, vehicle properties and driverbehaviour (including choice of speed). The interpretation of the results supports the opin-ion that within reasonable variations in these factors, road roughness plays a considerablygreater part than the other two. High-energy, multi-directional vibrations at many naturalbody part frequencies were found at the seats in trucks. This is serious due to the risk ofresonance, meaning a greater reproduction of vibration in the parts of the body afflictedthan at the surface4 from which the vibrations are transferred. Further, the study substanti-ates findings from earlier studies; i.e., that the high frequency of occupational diseasesamong commercial drivers, especially in the locomotor systems, is related to rough roads.This relationship is probably strongest in geographic areas where the road roughness levelis high on a large percentage of the roads. Where the roughness was greatest, peak valueswere registered on ambulance stretchers that considerably exceed the level that completelyhealthy people are assumed to experience as ”extremely uncomfortable” by internationalstandards.4 seat, seat back, floor, stretcher 4(79)
  7. 7. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E Vibration 2(weighted acceleration [m/s ])on stretchers, and in the case of the bulldozer, at the operator’s seat. Some studies indicate that exposure to vibrations of 1.6 2 1.30-1.35 m/s for 10 1.4 minutes a day can be harmful even for healthy 1.2 people. The journey by 1 ambulance on the rough stretch of highway took a 0.8 little more than 15 minutes. 0.6 Clearing forest for new road construction, bulldozer 0.4 CASE 1150 C 0.2 Mobile Intensive Care Unit Ambulance 0 Emergency Ambulance Rough road, IRI average = 4.0 mm/m Smooth road, IRI average = 1.2 mm/mDuring a 15-minute ride on a stretch of National Highway 90, the vibration level in onetype of ambulance was high enough to pose a potential health ha zard had a healthy personbeen exposed to it for as little as 10 minutes a day. It was shown that the vibration on theambulance stretchers was as great as at the drivers’ seat in wheel loaders loading blastedrock, bulldozers clearing way in forests for new road construction, etc. See the figureabove. Vibration problems are even greater in the spring due to seasonal frost damage re-lated a dditional roughness. 5(79)
  8. 8. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E2 Reader instructionsThe project is presented in three separate reports:1. The first is a technical analysis of the whole-body vibrations measured in trucks and ambulances. This report was compiled by Kjell Ahlin, Licentiate in Engineering at In- gemansson Technology AB [64], and may be of interest for researchers etc.2. The second analyses the impact on the human body of the vibrations measured. This report was compiled by Professor Ronnie Lundström at the National Institute for Working Life [65]. A summary of the conclusions is presented in Chapter 7. The report is available (in Swedish) at SNRA as well as NIWL websites, using the following links: http://www.vv.se/aktuellt/pressmed/2000/VVRapport.pdf or http://umetech.niwl.se/Published/.Publ.html3. The third is the report at hand, compiled by Johan Granlund, MSc (Civil Engineering), of the Swedish National Road Administration. This report presents the results from collating the data collected in the annual road condition surveys with the whole-body vibrations measured on the test stretches. It also compares the results with the ISO limits for whole-body vibrations, and assesses the magnitude of the problem on the state network. This report is available on the SNRA website, using the following link for the Swedish version http://www.vv.se/publ_blank/bokhylla/miljo/2000_31/intro.htm and this link for the English version http://www.vv.se/for_lang/english/publications/index.htm 6(79)
  9. 9. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E3 Definition of termsThe definitions of the following terms are based on those in the Swedish National Enc yclopaedia [1] with itsappurtenant dictionaries [34], the Swedish Centre for Technical Terminology glossaries [9], EngineeringMechanics [40], Handbook of Human Vibrations [11], Vägtrafikteknisk nomenklatur (Highway Engineer-ing Terminology) [16] published by the Transport Research Institute and ASTM´s Terminology Relating toVehicle-Pavement Systems [20]Accident frequencyNumber of accidents at a certain intersection, stretch or unit of distance. Differences in the accident ratio between two road networks show that one is ”more dangerous” for an individual than the other. Differences in acci- dent frequency between two road networks depends partially on the differ- ence in the accident ratio, and partially on the difference in the number of vehicles using the road networks. A simple way to reduce the accident fre- quency on a road with heavy traffic volume is to divert certain parts of traf- fic to other smaller roads. However, as the accident ratio is generally higher on smaller roads, this would increase the total number of accidents. From this perspective, the accident ratio is better than the accident frequency for assessing how dangerous roads are. The road network in Jämtland County (known to have low traffic volumes but poor roads) has the highest accident ratio in Sweden.Accident ratioNumber of accidents related to units of measure in traffic; i.e., the term vehicle kilometresis the unit commonly used on road stretches. At junctions the unit of measure is the num-ber of vehicles entering the intersection.AccuracyThe ability of the measurement instrument to give results close to the true value for theparameter measured. The greater the accuracy, the less the error.AlignmentThe design of the road profile in space.AmplitudeAmplitude is the maximum deviation from the mean of a signal (e.g., road roughness, orvibration), see Figure 10.ComfortA subjective state of well-being or absence of mechanical disturbance in relation to theinduced environment (mechanical vibration or repetitive shock). 7(79)
  10. 10. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EAgreeable and practical convenience {relaxed conditions under which to live and work}. Comfort connotes the absence of significantly disturbing or intrusive physi- cal factors. It is a complex subjective entity depending upon the effective summation all the physical factors present in the induced environment, as well as upon individual sensitivity to those factors and their summation, and such psychological factors as expectation. (For these reasons, for example, the same values of vibration that might be judged by most riders to be un- comfortable in a limousine may be judged acceptably comfortable in a bus.)The main factors behind comfort reduction (discomfort) are shown in Figure 9. 8(79)
  11. 11. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31ECrest factorThe ratio between the frequency-weighted peak value and the frequency-weighted rootmean square for the parameter studied. See Figure 3.CriteriaA criterion is a verbal description of the effect, e.g. discomfort, reduced performance abilityor physical injury that is of interest. Limits (threshold values, target values, etc.) are set toensure an acceptably low probability of the effect that the criterion defines. In other words,the criteria explain the reasons for the different limits.CrossfallThe angle between the horizontal plane and the surface of the roadway, carriageway orshoulder, measured at a right angle to the longitudinal direction of the road.ErgonomicsStudy of the relationship between people and their work environment, especially theequipment they use. See also [52].Estimated vibration dose value, eVDV)An estimation of a cumulative measure of the vibrations and shocks that a person is ex-posed to during the period under study, based on the frequency-weighted root mean squarefor the vibration. See Formula 1. If the vibration level varies or contains shock elements, the vibration dose value must be determined directly from the complete measurement series. This is usually the case when the crest factor exceeds 6 – 9, which makes eVDV less useful for ride quality assessment on the rougher roads.eVDV = 1.4 * a rms * T1/4Formula 1 Estimated vibration dose value during exposure time TFracture mechanicsThe science of how solid material breaks. This is often characterised by one or more cracksspreading throughout the mass of a structure, ultimately resulting in its splitting into two ormore parts. Cracks can increase through different mechanisms, like fatigue. An increase infatigue occurs in structures exposed to repeated load. The increase can be very little at anyindividual load. However, major cracks can form in a very short time through exposure tovibration. The research that laid the foundation for fracture mechanics was carried out dur-ing the Second World War. Since the 1950’s, fracture mechanics has developed into animportant element in the mechanics of materials. Most research has been conducted in theUSA and has been motivated by safety demands, primarily within the nuclear power andaviation industries. Fracture mechanics can be used to answer the question ”how quicklydoes a small crack grow through fatigue at the load spectrum to which the structure is ex-posed?”.HealthA condition of complete physical, psychological and social well-being, and not only theabsence of illness or disability [World Health Organisation (WHO), 1946]. 9(79)
  12. 12. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EHorizontal curveThe curve indicating the direction of the road alignment in the horizontal plane.International Roughness IndexThe IRI value is a substitute measure for the vertical vibration that occurs in the suspen-sion of a model (”the Golden Car”) of a quarter of a standard passenger car during a hypo-thetical journey at the speed of 80 km/h on the road stretch studied. The value describesthe accumulated vertical displacement between the car body and the non-suspended massof the wheel, divided by the distance travelled. The unit of measure for the IRI is [mm/m],which is low when the road is smooth along the wheel track5 in which the roughness pro-file is measured. The IRI is currently the preferred unit of roughness measure used in Swe-den and many other countries around the world that conduct objective surveys of the roadcondition. Index notation such as IRI 400, IRI 20, IRI 1 etc is used when explaining the length of report/averaging interval, such as 400 metre, 20 metre and 1 me- tre. Up until now, the basic report storage interval in the SNRA PMS has been 20 metre. (As a comparison; the sampling spatial frequency used by ve- hicle manufacturers fatigue researchers typically must be no longer than about 1 decimetre, not to lose information about shock that causes damage).JerkThe first time-derivate of acceleration. Jerk is thus a measure of how fast the magnitude ofthe acceleration changes. When assessing damage potential, the relation between load and bearing ca- pacity is studied. The “bearing capacity” of the human body depends strongly of the state of muscular brace, comparable to the case where a small child is learning to stand and walk. When exposed to unexpected occasional shock, an intensive jerk may reduce the chance for the body to suddenly increase its “bearing capacity” through instinctive brace. This implies that among differ- ent motions with a similar peak acceleration, motions having a more inten- sive jerk may be more serious than those with a less intensive jerk.5 In Sweden, the IRI value is measured in the outer wheel track as seen from the centre of the road. In some countries, it is measuredfrom a mean profile of the outer and inner wheel tracks instead. 10(79)
  13. 13. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31ELimitsThe value stating the maximum permitted limit for a source of discomfort or injury. A limit is generally only set for activities that are planned and governed by directives issued by public authorities. The general trend in most countries is towards reducing limits. It is usually the authority responsible for a specific field of expertise that sets these limits. The health and hygienics limits are particularly important in the work environment. A health and hygienics limit is not a sharp line between harmful and non- harmful exposure. In Sweden health and hygienics limits have a legal status. See also the Swedish Environmental Code (SFS 1998:808) and the Health and Safety at Work Act (SFS 1977:1160). 11(79)
  14. 14. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EMacrotextureTerm for those aberrations in the road surface (compared to an ideal plane) that have cha r-acteristic wavelength and amplitude dimensions from 0.5 mm and upwards to those thathave no effect on the interaction between tyre and roadway.Measurement errorDifference between the measurement value a nd the true value.Measurement resultsThe product of the measurement value and the unit of measure. The measurement valuecan have been corrected in connection with this through calibration in order to take knownsystematic errors into consideration.Measurement valueThe value for the parameter compared to the unit of measure. Can be identical with themeasurement result.Motion sicknessA physiological reaction in people induced by vibration, where the central nervous systemis incapable of co-ordinating information obtained visually, from the balance organ in theear and from joints and muscles. The reaction can cause drowsiness and affect perform-ance ability. Symptoms include greater salivation, perspiration, depression, apathy, pallor,nausea, dizziness and vomiting. Motion sickness seldom occurs in connection with vibra-tions with a higher frequency than 0.5 Hz. When the reaction occurs in a moving vehicle, itis usually called ”travel sickness”. The most renowned hypothesis for a qualitative explanation for the origin of motion sickness is called ”the sensory conflict hypothesis”[36]. A schematic dia- gram of this hypothesis is shown in Figure 1. Several other conflict hypotheses are discussed in Griffins ”Handbook of Human Vibrations” [11]. 12(79)
  15. 15. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E Stimuli Receptors Central Nervous System Responses Active Motor controlmovement Volitional system and reflex move- ment Eyes Updates internal Internal model model (adaption) Semicir- neural store of Motion cular expected signals stimuli canals Otoliths Leaky Neural centre Motion and other Compa- integ- mediating signs sickness gravi- rator & symptoms of symptom ration receptors motion sickness Passivemovement Mismatch signal Threshold Figure 1 Schematic diagram of the sensory conflict hypothesis. This figure has been modified by Förstberg [36], originally developed by Benson (1988). Natural frequency The most fundamental property of an oscillating system. Natural frequency constitutes the free oscillation frequency of a system after having been disturbed. Every real system has several natural frequencies, and each of these has a given pattern of movement. When a system is subjected to an external disruptive (driving) force whose frequency is equal to a natural frequency in the system, resonance occurs and the magnitude of the vibration in- creases. See Figure 2. 13(79)
  16. 16. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EFigure 2 Mechanical model of the human body specifying natural frequencies for a few parts of the body [51]. Observe that the body lacks the female bosom. The natural frequencies refer to vibrations in the axial direction of the body parts (e.g. the spinal column) 14(79)
  17. 17. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EParameterA characteristic that is the object of measurement.Pavement Management Systems, PMSA system that in an organised, co-ordinated way manages the road administration process.Peak valueThe maximum deviation from the mean of a parameter during a given interval. See Figure3. The peak value is used especially when assessing the risk of mechanical damage frommotion/force sequences of short duration - shock.PrecisionThe degree of agreement between a number of values measured, determined through re-peated measurements. Precision has nothing to do with the deviation of the values ob-tained from the true values for the parameter. Precision is sub-divided into repeatabilityand reproducibility.RepeatabilityThe precision of the values measured for a given parameter, determined in a uniform wayand under similar conditions.ReproducibilityThe precision of the values measured for a given parameter, determined in a uniform waybut under different conditions, such as another measurement method, another operator,another instrument or another point in time.ResonanceGeneral phenomenon in oscillating systems implying that even a weak intermittent externaldisruption (driving force) within a narrow frequency range can result in a large increase inthe oscillation amplitudes, accelerations and energy content of the system. This increasedepends on the frequency and becomes maximal when the frequency is largely equal to thefree natural frequency of the system. Through resonance, large amounts of energy can betransferred by the driving force to the oscillating system, in connection with which damageor disruptions in operation often occur. This phenomenon is of key importance from asafety perspective, etc.Road alignmentThe (imagined) large scale vertical and horizontal curvature of a road.Road roughnessTerm used for deviations in a road surface compared to a real plane, which affect vehiclemovement, ride quality, dynamic loads, drainage and winter maintenance.RoadwayCarriageway including the shoulders.RollMovement of rotation around the x-axis. See Figure 24. 15(79)
  18. 18. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31ERoot mean square, rmsThe root mean square of a variable during the period studied. See Formula 2 and Figure 3. When assessing ride quality, effects of occasional shock are often of great in- terest. The running root mean square of the weighted acceleration (using in- tegration time 1 s) is then often the preferred definition of vibration, since this definition has been proven to correlate very closely with perceived an- noyance [75]. To assess the risk of mechanical damage to the spine, the weighted positive (compression phase) peak acceleration is the preferred defi- nition. t2 ∫ a (t ) 2 dta rms = t1 t 2 − t1Formula 2 Root mean square for accelerationRoot sum of square, r.s.s.A summation procedure for vectors in different directions. For the root meansquare, the square root is taken from the sum of the vectors squared root meansquares.Running rmsA filtering procedure that smoothens a very transient measurement series (as where occa-sional shocks have occurred) that have a high crest factor.Second Law of NewtonThe acceleration of a particle is proportional to the force acting upon the particle and oc-curs in the direction of that force. Normally expressed in dynamic analysis as F = m*a.StressThe physiological/hormonal reactions in the organs of the body that are triggered byphysical and mental ”stress factors”. Threatening or strenuous situations stimulate in-creased secretions of adrenaline and noradrenaline. These hormones function such thatthey increase the heart rate, blood pressure and circulation of blood to the skeletal muscles,while decreasing the circulation of blood to other organs. Further, breathing is stimulated,the trachea expand and the level of sugar and fatty acids in the blood increases. When peo-ple are unable to control their own situation, the cortisol level also increases substantially.Cortisol increases the amount of glucose in the blood, as well as the turnover of fats andproteins. These and some 1400 other reactions to stress mean that the body, through all itsendeavours to adapt to the situation, is prepared to destroy itself after being subjected to anall too extended or strenuous load.SurveyA series of measures to determine the value of a parameter. 16(79)
  19. 19. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EUnit of measureReference value for a parameter; e.g., in the case of distance, a metre could be used as aunit of measure.VibrationOscillation in mechanical systems, where parts of the human body and human organs canbe included. (See Figure 2). This is governed by different kinds of force: mass, restoration,calming and disruptive (driving, instigating) forces. Vibration can be measured in terms of displacement, speed or acceleration. The unit of measure used for acceleration is [m/s2]. Results are usually pre- sented as peak values (mechanical spinal damage etc) or as a root-mean-square or running root-mean-square (perceived ride quality etc). See Figure 3. Peak value Root mean square Mean value Mean valueFigure 3 Peak value, root mean square and mean value for a signalVibration dose value, VDVA cumulative measure of the vibrations and shock elements to which a person was exposedduring the period under study. See Formula 3. t2VDV = 4 ∫ a w (t ) 4 dt t1Formula 3 Vibration dose value for accelerationWavelengthThe shortest distance between two of the signal’s points with an equal phase. See Figure10. 17(79)
  20. 20. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E4 BackgroundTo most people, a roadway is merely a charcoal grey surface of infinite length. They expectit to provide safe driving conditions, a smooth and quiet ride, minimal splash and spraywhen it is raining, good visibility during poor conditions and that it will last a long time (toavoid disturbance from road works).A closer examination of the road reveals that it has several important characteristics, suchas surface texture. Texture is needed to provide road grip, minimise spray and mist when itis raining, and reduce the glare from high beams at night. But the texture can cause morenoise as well as reduce the life span of both roads and tyres. As the road surface ages and isworn down by studded tyres, heavy vehicle loads and climate, road damage begins to a p-pear. Deformation (or road roughness), which is one type of damage, can limit bothshorten the life span of a road as well as reduce the quality of the ride. Roughness, primar-ily longitudinal, can also be built into the road from the outset due to poor geometric de-sign/construction.Roughness is the source of many kinds of irritation that road users encounter; flickeringheadlight reflections, deep pools of water, the dynamic forces that increase pavement stressand damage to vehicles and cargo, and poor ride quality. Most road users are very sensitiveto ride quality, making this a prime criterion when setting road maintenance priorities [68].Road roughness can mean reduced travel speeds. This has led many to believe that roughroads are safer than smooth ones, since speed is generally acknowledged as dangerous.However, after collating databases with information on accidents, road surface condition,climate, road geometry, speed, etc at VTI (Swedish Road & Transport Research Institute),it was concluded that ”the accident ratio increases with an increase in the roughness” and”roughness has a major impact” [18]. A strong correlation between road roughness and theaccident ratio on the paved part of the state road network in Sweden has thus been ascer-tained, implying that the idea of rough roads being safer is probably a serious misconcep-tion.However, such a statistical correlation is not clearly tantamount to the accident risk actuallybeing caused by roughness. For instance, it is likely that the vehicles on the roads in ruralareas, where roughness is worst, are older and less roadworthy than those found on thesmoother roads in and between the larger cities. It is therefore necessary to verify a statisti-cal correlation through experiments that provide information about possible mechanismsfor an actual cause and effect relationship. This could include the effect of roughnessthrough mechanical interference on the steering and braking properties of road vehicles,the effect on winter road maintenance and on drivers’ performance ability.4.1 From past to presentThe mechanisation of human transport on roads has taken place in a very short time. Peo-ple were still basically travelling on the backs of animals or on foot in the 18th century,despite the fact that the invention of the wheel 3 500 years before Christ had made thedevelopment of animal-drawn carts possible. The reason was that roads were often almostentirely impassable, which explains why the carts were primarily used to transport goods,and even then only at average speeds up to about 10 km/h. This meant that travellers usu- 18(79)
  21. 21. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31Eally experienced more discomfort from dirt than from vibrations. On the narrow streets ofStockholm, the upper class avoided the dirt by being carried on palanquins. A tariff for thiswas laid down in 1726 [58].Through Carl Snoilsky’s classical poem on Stenbock’s courier, generations of Swedish stu-dents have learned about how Captain Henrik Hammarberg, who was sent off by GeneralMagnus Stenbock from Helsingborg with a message to the king and government in Stock-holm on February 28th 1710, rode so hard that ”a horse collapsed behind him at everystation”. Modern historians maintain, however, that the courier, who covered the journeyof 900 kilometres at an average speed of 18 km/h, probably did not ride on horseback atall. Verification ”Folio 2282” in the 1710 treasury records (preserved in the National Ar-chives) for Hammarberg’s travelling-expenses account clearly shows that he travelled by acarriage drawn by a team of horses from station to station. It is believed that he sufferedfrom travel sickness during the journey; ”this coach is swaying so frightfully on these terri-ble roads” [59].The bicycle could enter the scene at the turn of this century, as a result of the soft non-bituminous roads, which could be evened out by simple means as needed. Bicycles werecrowded further and further out to the periphery as motoring became more widespread. Inthe past 130 years, mobility for people in Sweden has grown a thousand fold. See Figur 4. Mobility trend in Sweden, 1850 - 1990 50000 45000 40000Decade Travelled distance [m] 35000 1850 40 30000 1870 200 25000 1890 350 20000 1910 900 15000 1930 3000 10000 1960 20000 5000 1980 40000 0 1850 1870 1890 1910 1930 1950 1970 1990 DecadeFigur 4 The average daily distance travelled by vehicle by adults in Sweden from the 1850’s to the 1990’s. Data in the tables extracted from [61], supple- mented in the figure with data from [62].Roads became steadily harder throughout the years, necessitating more sophisticated careand maintenance routines. The long distances covered at the high speeds that characterisemodern road traffic, mean an exposure to vibration and shock that, in the presence of sig-nificant road roughness, can mean people being exposed to mechanical energy that is sub-stantially higher than at any other time in history. According to the second law of Newton,the magnitude of this mechanical load can be estimated through measuring the accelerationof whole-body vibration. 19(79)
  22. 22. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EThe mental stress is more difficult to measure. At the macro level, it has been shown thatthe risk for certain types of cardio-vascular disease in Sweden is more than three timeshigher for commercial drivers than for the average worker. Mental stress under certaindriving conditions is considered to explain the raised level of stress hormones found incommercial drivers, and is believed to cause the problem [69, 70].Amongst older commercial drivers, musculo-skeletal problems and cardiovascular diseasesare the primary reasons for changing their occupation [71]. 20(79)
  23. 23. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E 4.2 Road roughness in brief Road roughness can result from faulty basecourse adjustment (usually through insufficient repair of crossfall variations or longwave deformations); incorrect initial construction; post- compaction of added layers; subgrade settlement; material abrasion -- primarily by studded tyres -- or through uneven frost heave during the spring thaw. A general rule, based on laboratory tests, is that drained road structures can stand being driven on by trucks six to seven times more than those without drainage before unaccept- able deformation occurs in the unbound layers. Ditching is thus a very effective mainte- nance measure for preventing roughness, if it is executed so that the gradient of the inner embankment is not steeper than 1:3 (otherwise there is the risk of edge deformations due to insufficient lateral counterstay). A drained road structure is also a prerequisite for avoid- ing frost-related roughness in winter. The binder stiffness affects the ability of the asphalt to distribute the load and thus the risk of deformation in underlying layers. Temperature is a key factor for this stiffness, which means that dark asphalt roads become rough faster than those with a light surface. The mechanical properties of vehicles can also increase roughness. As early as in the 1930’s, a large-scale experiment showed a substantial increase in roughness on gravel roads when the test vehicles had high-pressure tyres, while there was not even enough roughness to measure when low-pressure tyres were used (the roadway had actually been smoothed out). Speed and suspension were also shown to be major factors affecting roughness [56]. These conclusions could even be valid today for roads with a thin surface, like single sur- face treatment (Y1G), which is very similar to the dust abatement measures undertaken on the old gravel road. 8,00 7,00 6,00 5,00IRI [mm/m] 4,00 3,00 2,00 1,00 0,00 01 01 -01 -01 -01 -01 -01 -01 -01 -01 -01 -01 1- 0- -12 -03 -06 -09 -04 -07 -02 -05 -08 -11 -0 -1 00 00 00 00 00 00 00 00 00 00 00 00 Tid på året Figure 5 Conceivable variation in roughness per month, on a specific road sec- tion 21(79)
  24. 24. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31ERoad roughness varies throughout the year, which often is more noticeable than the annualdeterioration in the road condition. An estimation of what the variation might be like overthe year on a road that is not treated with salt de-icer and that is slightly frost-damaged isshown in Figure 5. Variable weather conditions in addition to winter maintenance measuresaccounts for the greater variation shown in the graph for the winter months, while the ex-tremes in the spring are explained by the thaw. While there is a high rate of steadily increas-ing deterioration on roads with deficient bearing capacity and/or problems related to fro-zen ground conditions, the deterioration on well-constructed roads is minor and disappearsin time (except during the late stage when surface abrasion occurs if no preventive meas-ures are undertaken). Roughness can be eliminated through appropriate periodic mainte-nance. Road strengthening serves to reduce roughness immediately, while also retarding itsfuture speed of increase. Needless to say, this applies regardless of whether the improvedbearing capacity has meant a change in the administrative bearing capacity class of the road.For microtexture, as well as that part of the macrotexture with wavelengths shorter than ca25 mm, it is important that the road roughness amplitudes are neither too large nor toosmall. To a certain extent, this kind of roughness produces desirable effects; like friction,noise reduction, a certain amount of drainage, etc. Some effects are undesirable, like greaterwear and tear on tyres [2].All roughness with wavelengths above ca 25 mm increases transport costs [2]. It is possibleto correct roughness with amplitudes under ca 15 - 30 mm and with wavelengths up toabout 10 metres simply through a new wearing course. Roughness with larger amplitudes,or of a more longwave nature, is remedied through milling or more fill works. Frost-relatedroughness normally demands highly extensive and expensive measures, such as deep drain-age and extensive material replacement. The maintenance and repair budget (per squaremetre road surface) must therefore be several times higher for roads damaged by frost thanfor roads damaged by traffic. 22(79)
  25. 25. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E4.3 Monitoring of the road condition at the SNRA Roads with an IRI > 5 mm/m 14% 12% 10% Northern Region Central Region 8% Stockholm Region Western Region 6% Mälardalen Region South-Eastern Region Skåne Region 4% 2% 0% 95 96 97 98 99 YearFigure 6 Development of severe roughness on the paved state road network, expressed as time series of IRI 20 > 5 mm/m per road management re- gion. A lower percentage indicates fewer very rough stretches on the road network.4.3.1 Road roughness measurementsThe SNRA regularly measures roughness on paved state roads using high technology sur-vey vehicles. European and National Highways are surveyed annually, and other roads atleast every third year. Up until now, the parameters that have been of greatest interest areruts, crossfall and roughness. The IRI value [mm/m] is the most important measure ofroughness, and is calculated from the road roughness profile measured. The IRI value canbe said to describe the vertical vibrations in the suspension of a mathematically simulatedpassenger car driving at a speed of 80 km/h, and is affected primarily by roughness withwavelengths between about 1 and 30 metres. IRI is very similar to the measures of rough-ness used in the USA as early as the 1920’s when roughness began to be measured usingsimple vehicles. These roughness measures were successfully used to stimulate competitionamong civil engineers and contractors to achieve better ride quality through their beingofficially published as objective comparisons of different road projects [54].Today’s survey results are analysed and interpreted as the basis for budget discussions, set-ting priorities, research projects, evaluating performance contracts, etc [8]. Figure 6 showsthe percentage of roads with excessive roughness (very high IRI values) in all road man-agement regions. Signs of improvement can be seen, particularly up to 1999. 23(79)
  26. 26. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EHowever, an entirely different, negative trend was found when reviewing road users’ opin-ions on ride quality. See Section 4.3.2. The discrepancy between the results from the roadcondition and road user surveys can perhaps be attributed to the fact that people are travel-ling more (which can increase the exposure to vibration even if the road roughness is un-changed) and that the annual road condition surveys are only performed when there is noground frost, for reasons of measurement precision. Roughness on frozen roads can bemuch worse than on non-frozen roads, and ground frost conditions vary substantially fromyear to year. Much higher local IRI values have been measured on frost-damaged roads,than what has been registered in the routine surveys in the summer months.During the quality a ssurance of road condition surveys, it was observed how the vehicleoperators found it much more difficult on rough roads to follow the driving instructionrequirements. In other words, roughness has a strong adverse effect on driver performance[private comment made by Kerstin Svartling, administrator for the SNRA’s road conditionsurveys].The significance of different types of roughness and different speeds can be studied inFigure 7. 24(79)
  27. 27. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EFigure 7 Typical vertical motions at the rear axle and in the car body when driv- ing at different speeds on different road roughness [12]4.3.2 Road user opinion pollsThe road user opinion polls conducted by the SNRA between 1995 and 1998 included 30000 people. The questions cover new roads, care and maintenance. On the whole, the find-ings were not too negative. The majority of the interviewees were satisfied in most respects,with one major exception being road roughness.The smoothest roads in Sweden are found in Skåne Region (southern Sweden). Despitethis, the percentage of commercial drivers in Skåne who are satisfied with ride quality onthe national road network is as low as 30 – 35 percent. The percentage of satisfied roadusers is much lower in other parts of the country and with respect to other types of road. 25(79)
  28. 28. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EDissatisfaction is greatest and growing most rapidly in the northern half of the country. Forexample, it is expected that 0% (none) of the commercial truck drivers in the counties ofVästernorrland, Jämtland, Gävleborg and Dalarna (the Central Road Management Region,being part of “northern Sweden”) will be satisfied with the ride quality on regional thor-oughfares in the winter of 1999. See Figure 8. SNRA Central Region (Västernorrlands, Jämtlands, Gävleborgs och Dalarnas counties) Marks given by commercial drivers for the ride quality on regional roads Results 1996 Results 1997 8% 18% 82% 92% Percentage dissatisfied Results 1998 Percentage satisfied Prognosis 1999 0% 5% 95% 100%Figure 8 Commercial drivers’ marks for ride quality on regional roads. Väs- ternorrland, Jämtland, Gävleborg and Dalarna Counties [6].At the national level, the percentage of commercial truck drivers who are satisfied with thequality of the ride is about half that of passenger car drivers. However, even the percentageof passenger car drivers who are satisfied in this respect is low [6][37].Factors that are known to influence people’s sense of discomfort are shown in Figure 9. 26(79)
  29. 29. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E Texture Vibrations Vibration- Roughness related discomfort Alignment Jerky ride Vision disor- ders Megatexture Shock Difficulties in Roughness handling Noise and Sleeping disor- Roughness Infrasound ders Texture Visually Variance between Noisiness Roughness individuals Road signs Information Speech diffi-Road markings culties The individual Ride quality Rest areas Food / beverages Sweating / (sum of discomfort) freezing ? Odours Air quality Variance for the individual ? Temperature Glaring lights ? Body posture Uncomfortable posture ? Privacy Social discom- fort Disruptive Other Other road works Figure 9 Factors associated with road management that produce discomfort in connection with road transport. The figure has been modified on the basis of [11]. The figure also helps us understand for example that faulty, irregular crossfall, unsuitable texture and major road roughness cause many different kinds of discom- fort. Road damage, along with recurrent disruptive road works, thus results in poor ride quality. 27(79)
  30. 30. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E4.4 Analysis of road roughnessThe deviation of the road surface from a horizontal plane can be described by the wave-lengths and amplitudes of the roughness, see Figure 10. The very shortest roughness wave-lengths are classed as microtexture, which is determined by the properties of the aggregateand binder in the surface. Somewhat longer wavelengths are classed as macrotexture,which is determined by such things as the shape of the aggregate and the particle size dis-tribution. Longwave deviations are quite simply designated as roughness [2], often causedby more or less extensive settlement, frost heave or ice lenses in or under the road struc-ture in the winter. λ A λ/2 AFigure 10 Wavelength (λ) and amplitude (A). Above at corrugation, below at a pothole.The basic relationship between travel speed (velocity) v [m/s], wavelength λ [m] and verti-cal vibration frequency f [s-1] is shown in Formula 5. Depending on the travel speed andtype of vehicle, vehicle properties are a key factor where the wavelengths are up to 25 - 50m. Where the wavelengths are longer (or more to the point, at lower frequencies) thedampening property of the vehicle is insignificant [11]. The equation should therefore pro-vide a reasonable estimation of the vibration frequencies where the roughness is of longerwavelength. Vibrations with a frequency of 0.1 Hz are caused by roughness (unevenness)with wavelengths of about 85 m at a travel speed of 30 km/h (8.3 m/s) and wavelengths ofabout 360 m at 130 km/h (36.1 m/s). A vibration frequency of 0.5 Hz is caused by rough-ness with wavelengths of some 15 - 20 m at a speed of 30 km/h and 70 m at 130 km/h. λ = v/fFormula 5 The basic relationship between roughness wavelength, travel speed and vertical vibration frequency (one wheel, no suspension). 28(79)
  31. 31. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E4.5 Transmission of vibrations through the vehicleThe magnitude of the vibration transmitted to vehicle occupants through the vehicle de-pends on the road roughness and the speed of travel, as shown clearly in Figure 7. This isaccentuated by the mechanical properties of the vehicle. For instance, those riding inhigher vehicles are exposed to a greater amount of pitch and roll than those in low vehicles[13].Two-axle cars, are said to have three natural frequencies for vertical vibrations: one that isrelated to the car body bouncing on its suspension, one that is connected to the wheel axlehop between the body suspension and the tyre suspension, and one that originates fromthe rocking of the car seat. The car body has a natural frequency of about 1 Hz, and vibra-tions close to this frequency are amplified by a factor of 1.5 – 3.0. The wheel axles of a carhave a natural frequency of 10 – 15 Hz, which means that at this frequency they tend tovibrate more than what the car body and tyres together with the road surface would di-rectly cause [13][14].Formula 5 indicates that a vibration frequency of 1 Hz when travelling at 90 km/h iscaused by roughness with wavelengths of about 25 metres. Vibration frequencies of 10 –15 Hz at 90 km/h seem to be caused by roughness with wavelengths of about 2 - 3 metres.Multi-axle vehicles that are both heavy and long may have considerably different mechani-cal behaviour than normal cars, particularly if they are towing heavy trailers. Moreover, theproperties of heavy vehicles are changed substantially by the actual weight of the payload.Some types of heavy-duty vehicles lack suspension altogether.The natural frequency of the roll of heavy vehicles is less than 3 Hz. Since roll motions atfrequencies under 5 Hz are not common when driving on roads with ”normal”(?) rough-ness and at normal speeds, it is not usually considered to be of any greater significance.[55]. This item will be under further discussion later in the report.The current European trend towards fewer and more specialised hospitals is resulting in agreater percentage6 of ambulance transports having to cover longer distances while simul-taneously administering intensive care. To manage this, more -- and heavier -- medicalequipment is required on board. Ambulances must then have a greater load capacity thanbefore, which means that large vehicles (”container ambulances”) designed similarly totrucks are needed. See Figure 11. An effective load capacity of more than a tonne is notunusual.In many cases it has been shown how even slight road roughness can, through vibrationand dynamic weight transfer, cause the wheel load to temporarily exceed twice the staticload and then revert just as suddenly to 0 (zero!) during the ride. See Figure 12. A feelingfor how dynamic loads can originate can be created by bouncing a little on the bathroomscales. That the road grip varies between the wheels – and moreover is occasionally non-existent – involves a major risk of skidding when hitting the brakes in an emergency. [3].Dynamic loads have been proven to be a large problem when weighing vehicles in motion,even on very smooth road stretches [48].6Today, there already are some 850 000 ambulance transports per year in Sweden. Of these, about 200 000are emergencies [29, 17]. 29(79)
  32. 32. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E Figure 11 Well-equipped “Mobile Intensive Care Unit” type of ambulance. The effective load capacity of the vehicle in the picture is 1.44 tonnes. Notice the heavy- duty wheels, which are even mounted in pairs at the rear. Wheel load Wheel axle hopRoad roughness profile Figure 12 Dynamic change in the wheel load when driving on a rough road [57]. The static wheel load is designated as ”p” in the figure. As seen here, the actual wheel load -- which determines the road grip and thereby the risk of skidding when braking -- varies between 0 (zero!) and twice the static load as roughness in the road profile causes vibrations and weight transfer in the vehicle. 30(79)
  33. 33. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E4.6 Whole-body vibrationDescribing the consequences of shaking the whole human body -- a complex, active, intel-ligent structure -- is not a completely simple matter. The National Board of OccupationalSafety and Health has compiled the known effects of humans being over-exposed towhole-body vibration. A few of the conclusions were: ”It can be assumed without a doubtthat the human being is negatively affected by whole-body vibration, from both a subjec-tive and objective perspective” and ”It would obviously be desirable from everyone’s pointof view if vibrations could be totally eliminated” [51].Needless to say, there are also vibrations that are positive. For instance, vibrations thatinform drivers about the movement of their vehicle, [11, 66], that they are driving over azebra crossing or that the right front wheel ha s a puncture. For safety reasons, such vibra-tions -- ”a sense of the road” -- should not be dampened.Like auditory stimuli, sensory impulses impart strong impressions. These should thereforebe used sparingly, since they partially block or suppress other sources of information. [57].For instance, unlike operators of forestry machinery working out in the woods, those driv-ing on public roads find it completely reasonable to expect the underlying surface to besmooth enough that any vibration generated would be insignificant. In an upcoming ECdirective for limiting exposure to whole-body vibration, it is stipulated that ”the risks aris-ing from exposure to mechanical vibration shall be eliminated at source or reduced to aminimum [with the aim of reducing exposure to below the threshold level].”The survey conducted by the National Board of Occupational Safety and Health on theeffects of overexposure to whole-body vibration showed that although this primarily causesfatigue, it also gives rise to visual acuity disorders, motion sickness, dizziness,back/abdomen/face pain, headaches and a frequent need to urinate [51].That very extreme acceleration causes bodily injury is a factor that has set limits on themanoeuvrability of manned fighter aircraft. Based on fracture mechanics, it is not unlikelythat even the substantially less intensive forces (but at higher frequencies) that cause more”normal” whole-body vibrations can cause physical injury in connection with long-termexposure.When being subjected to vibration, human body reflexes try to protect organs that are sen-sitive to resonance through a tightening of the muscles (this is only successful for veryshort periods - seconds) [38]. Lengthy exposure to vibration therefore often results in highmuscular tonicity [15], which is dangerous to health on many accounts.A governmental working committee on public health has estimated that the cost to societyfor back problems, which is the primary reason for people reporting in sick and for earlydisability retirement in Sweden, exceeds SEK 20 billion per year (1991). In its report, thecommittee also ascertained that whole-body vibrations are of ”key importance” as a sourceof back problems [53]. However, in the general health statistics, the concept of whole-bodyvibration is lacking. ”Vibration injuries” primarily refers to hand/arm vibrations [33]. InEngland a direct relationship has been found between the frequency of back problems andthe distance travelled per year [60]. 31(79)
  34. 34. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EThe findings of a recent review of epidemiologic studies conducted between 1986 and 1997on the relationship between exposure to vibration and problems in the lumbar part of theback provided ”clear evidence for an increased risk for LBP disorders in occupations withexposure to WBV. Biodynamic and physiological experiments have shown that seatedWBV exposure can affect the spine by mechanical overloading and excessive muscularfatigue, supporting the epidemiologic findings of a possible causal role of WBV in the de-velopment of (low) back troubles”. The review also mentions that it is estimated that 4-7%of the working population in the EU is exposed to potentially harmful whole-body vibra-tion [74].Surveys have shown that truck drivers are exposed to considerably greater vibrations thanmost other categories of road user. The exposure often exceeds the recommendations andlimits in the International Vibration Standards [10, 7], as well as the limits proposed by theEU.Sensitivity to vibration differs substantially between men and women. Women (and thefoetus) are particularly sensitive during pregnancy [30].Those who are most sensitive to vibration are injured, sick or disabled people who oftenrequire ambulance transportation. The National Swedish Institute for Working Life hascompared the noise and vibration properties in traditional ambulances and the increasinglymore common larger MICU container ambulances. A major difference was found. The rmsfor vertical vibration (0,5 – 80 Hz) at the driver’s seat in a large ambulance amounted to asmuch as 1.44 m/s2. Interpretations of the findings indicate that levels above 0.5 m/s2 entailan excessive risk for any normally healthy person sitting behind the wheel 6 hours a day.The surveys also showed that the vibrations in an infant incubator on board are often evengreater than at the driver’s seat. On one occasion, the rms for the vertical vibration in theincubator was ranked as a 5 (very uncomfortable) on a six-grade scale of discomfort in theISO 2631-1 “Evaluation of human exposure to whole-body vibration” standard. [17, 29].According to ambulance orderlies, acute motion sickness is a common problem for staffand patients alike.More can be learned about the effects of whole-body vibration in the report by Prof.Ronnie Lundström [65]. Additional information – including the effect of such loads as lowfrequency noise and infrasound – can be found in other reference literature compiled inChapter 8 as well as from such sources as the National Board of Occupational Safety andHealth, the National Swedish Institute for Working Life’s Vibration Committee [31], Upp-sala Academic Hospital [15] and the Swedish Road and Transport Research Institute.4.6.1 Natural frequencies and resonance in the human bodyAll material bodies have a natural frequency, which to some extent can be compared to thenatural frequency of a swinging pendulum. When a body is exposed to a frequency vibra-tion that coincides with its own natural frequency, it will vibrate strongly.The various parts and organs of the human body have different natural frequencies. Thismeans that the body does not vibrate uniformly, but rather that the different parts behavelike individual, albeit interlinked, material bodies in this respect. (See Figure 2). Externalvibrations with frequencies of about 6 Hz are amplified through resonance in the abdomenby up to 200%. Certain vibrations are amplified in the spine by up to 240%. The head has a 32(79)
  35. 35. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31Enatural frequency of about 25 Hz, which means that vibrations with frequencies aroundthis are amplified by up to 350% [41][42]. The resonance phenomenon leads to a greaterload on the body and thereby a greater risk of injury.4.6.2 Examples of the effect of whole-body vibration in the 0.5-80 Hz rangeShortwave road roughness produces vibrations with a frequency content that among otherthings includes the 0.5 - 80 Hz band.According to the ISO 2631-1 standard, vibrations with frequencies between 0.5 - 80 Hzcould probably cause a greater risk of injury to the vertebrae in the lumbar region and thenerves connected to these segments. Exaggerated mechanical strain can be a factor in thedeterioration of the lumbar segments. Vibrations are reported as affecting the bodythrough causing deformation of the spine (spondylosis deformans), damaging the cartilagebetween the vertebrae (osteochondrosis intervertebralis), and by producing chronic pro-gressive change in the cartilage and bone tissue (arthrosis deformans). Exposure to whole-body vibration can also exacerbate certain endogenous pathological disorders of the spine.It is not considered unlikely that the digestive system, the urinary and sexual organs and thefemale reproductive organs are affected. Health impairment caused by whole-body vibra-tion normally only occurs after several years of exposure [10]. Spontaneous abortion is anexception.4.6.3 Examples of the effect of extremely low frequency whole-body vibrationsLongwave road roughness produces low frequency vibrations. Vibrations with greater am-plitudes within the 0.1 – 0.63 Hz frequency band have a particularly strong effect on peo-ple.According to the ISO 2631-3 standard, these vibrations cause various degrees of motionsickness, ”travel sickness”, even after only short exposure. Motion sickness can affect peo-ple for hours, and even up to days after an arduous trip. It has been observed that motionsickness lowers performance ability and reduces alertness.A survey conducted amongst 300 students revealed that about 58% had felt nauseous dur-ing car rides. Some 33% could remember actually having vomited during car trips beforethe age of 12 [11].A nationwide questionnaire revealed that motion sickness is a frequent working environ-ment problem amongst ambulance orderlies. 23% replied that they easily felt nauseousduring the ride. [27]. Orderlies in Sollefteå Municipality reported having observed palpabletravel sickness symptoms in patients (in the worst case vomiting, uncontrollable bowelmovements, etc) in 20-25% of the most acute (high speed) transport situations. In the careunit of the vehicle, it is impossible to watch the horizon.Vehicle manufacturers are aware that the suspension properties affect the risk of passen-gers developing motion sickness. A sports car type of suspension is recommended forpeople who easily get car sick. It cannot be ruled out that ”comfort suspension” -- byAmerican standards -- can mean that the high frequency vibrations caused by road surfacedamage are converted to an exceptionally high degree into that very type of low frequencyvertical vibration that is known to cause motion sickness. It is also known that rotationvibrations are a factor in motion sickness. Perhaps even the differences in roll-stability be- 33(79)
  36. 36. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31Etween different vehicles has a major impact? Added to the ”motion sickness vibrations”created by the ride are the low frequency vibrations that ensue from the billowing align-ment that roads originally built for horse drawn carriages often have.4.6.4 Origin of whole-body vibrationAccording to the Academy of Engineering Sciences (IVA) road roughness is a muchgreater cause of vibration in road vehicles than in-vehicle factors (wheel imbalance, drive-line, etc.). IVA has also ascertained that vibrations have a major impact on the steering andbraking properties of the vehicle, and on the working environment, ride quality, health andpossibly even performance ability of vehicle occupants [19].Whole-body vibrations originate from two different types of force. A random and suddenforce designated as shock. When the wheel hits a bump or sinks into a pothole, shock oc-curs. If this shock is strong enough, passengers without a safety belt can be thrown fromtheir seat. They could also be hit by a loose-flying object. Shock can also cause severe spi-nal injury [32], such as in several Scandinavian cases due to riding in buses over trafficcalming road humps. Less sudden displacements and forces occur during a normal ride onmore or less rough roads. See Figure 13 - Figure 15. These are the most common motioninduced forces that we experience during a normal day [42]. The second law of Newtoncan be used to calculate the dynamic forces that vibrations transfer to human organs.Certain types of vibration are known to cause car sickness. These include extremely lowfrequency vertical vibrations (0.1 – 0.63 Hz) and roll (often in combination with lateraldisplacement). The low frequency vertical vibrations are caused by exceedingly longwaveroughness (up to 350 m), but can also occur when a vehicle with worn or poorly designedwheel suspension transforms high frequency vibrations to low frequencies. Roll occurswhen there is an unfavourable variation in the gradient between the wheel tracks (crossfall);this often is caused by roadway deformations and all too sharp curves in the alignment.The limits for whole-body vibration in the ISO 2631-3 Standard can be converted intostandard specifications for the road roughness profile. See Figure 16. On roads where thereis substantial roll (caused for example by sharp curves or deformed edges) the acceptablelongwave road roughness must be reduced by 25%. Major vertical motionFigure 13 Origin of vertical vibrations on roads where the roughness wavelength coincides with the distance between the vehicle axles. Adapted from [66] 34(79)
  37. 37. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EMajor pitch and thus major longitudinal vibration Little vertical movement Figure 14 Vibrations in the direction of travel occur when the wavelength of the road roughness does not coincide with the distance between the vehicle axles. Adapted from [66]. Major roll and thus substantial lateral vibration Figure 15 Origin of lateral vibrations on a road with deformed edges 35(79)
  38. 38. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31E 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 153 122 95 76 61 49 38 31 24 Length of the hollow or ridge (i.e., half the roughness wavelength), [m]Figure 16 Limits for longwave roughness at a speed of 110 km/h, set with respect to the criterion for decreased performance ability. The values are derived from the vibration limits in the ISO 2631-3 standard. The corresponding limits with respect to the discomfort criterion are considerably stricter. The limits assume no surface defects at all (aggregate stripping, potholes, etc) or damage that cause rota- tion vibration (unevenness at culverts, edge deformation, etc).4.6.5 Measurement of whole-body vibrationThe measurement of whole-body vibration must comply with ISO 2631 “Evaluation ofhuman exposure to whole-body vibration” (1997). The equipment consists of accelerationsensors, arranged as shown in Figure 17 and Figure 18.The reaction time for the sense of motion has been found to be 0.24 – 0.80 s, with a meanvalue of 0.72 seconds [57]. This is one of the reasons why comfort-related measurementsare normally done through integration over 1-second intervals. A vehicle travels 20 m in asecond, at the speed of 72 km/h. This means that vibration data measured in compliancewith ISO 2631 at rural highway speeds on sub-stretches are fairly comparable in length toroad roughness data in the SNRA road surface condition database, which after sampling atthe mm-level was ultimately averaged over 20 m intervals. 36(79)
  39. 39. Whole-body vibration when riding on rough roads SNRA Publ. no. 2000:31EFigure 17 Vibration measurement gauge on the seatFigure 18 Vibration measurement gauge on the floor 37(79)

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