WIM and dynamic loading on pavements

Transcription

1 IM and dynami loading on pavements M. HUHTALA, LiTeh, Head of Road Researh, J. PIHLAJAMAKI, MS(ng), Researh Sientist, and P. HALONN, BS(ng), Researh ngineer, Tehnial Researh Centre of Finland (VTT), Road and Traffi Laboratory, spoo, Finland. The axle load of a heavy freight vehile is not onstant but varies at eah point beause of the dynami effs of a vehile and its suspension and the roughness of the road. eight-in-motion (IN) measures the axle load in one point. The eff of dynami loadings on IN is disussed in this paper. 1. INTRODUCTION The auray of INs depends on the IN itself and on the dynami axle loads. The axle loads are not onstant but beause of the roughness of the road the real axle load is osillating around the stati axle load. The dynami axle loads depend on the roughness of the road and on the vehile, espeially on its suspensions. Usually only the auray of the IN itself has been studied and the effs of dynami axle loads have been negled. Its eff has been tried to minimise in ertain few ases by using multiple INs in a row. Axle loads may vary +/- 15 perent on good roads (it means that eg if the stati load is 1 ton the real dynami axle load may be any value bween 8.5 and 11.5 tons). It is known that the same vehile at the same speed auses highest dynami axle loads at the same point of the road (spatial onentration) but little is known about the effs of the whole vehile fle. These dynami effs an be avoided to ertain degree with multiple sensors but very little researh work has been done in this area. The dynami axle load effs for IN an be studied with omputer simulation and with dynami axle load measurements in a vehile. 2. COMPUTR SIMULATION The omputer simulation was made with ADAMS (Automati Dynami Analysis of Mehanial Systems) whih is a ommerially available multibody system analysis software. Today it is the most widely used software pakage of its kind. ADAMS is used in a large variy of industrial appliations throughout the world, automotive, aerospae, general mahinery. ADAMS offers an extendable library of modelling elements. The elements are very general, so they do not restrit the type of mehanism being modeled. In fat they are kind of basi building bloks of dynamis i.e. fores, parts, joints. By seling and ombining these elements, the user an build up a dynami model in the required level of dail. Beause most of the ADAMS elements math very losely to their ounterparts in the real world, the model is, indeed, defined by imputing the real physial properties of the system. In this study the dynami loads on pavement were studied using omputer simulations. The original truk model was reated in ooperation with the manufaturer, Oy Sisu-Auto Ab, whih is speialized on heavy transport equipment under extreme driving onditions. The model was first used in ride omfort analysis. This presentation onentrates on dynami axle loads, espeially by varying the number of axles, spring types, damping and speed of the truk, the repeatability of road-vehile interation fores on different road profiles were examined. Originally a four-axle truk was hosen to be studied, beause of its omplex dynami behaviour. There are also three-axle truks available quite similar to the one being studied. The four-axle versions are used in heavy duty and they are ommon espeially in lumber transportation in Finland. The equipment of a truk varies very muh depending on the intended usage of it. For the truk model the equipments needed to arry logs was hosen in. order to represent typial use of suh a truk in Finland. In this truk the first two axles steer and there are single tyres on them. On the both axles of the bogie there are twin tyres, so altogher the truk has 12 tyres. The first axle is suspended by paraboli leaf springs, the seond by air springs and on the bogie there are two multileaf springs. The two rear axles are kinematially oupled by swing arms, so that one multileaf spring on eah side of the bogie an suspend both of the axles. The seond and the fourth axle an be lifted off the ground in order to save fuel and tyres, when the truk is driven without load. The original four-axle truk model was reated in ooperation with the manufaturer to be used in studying the ride omfort. In Fig. 1 a side view of the model is shown. It onsists of 35 inertial masses, whih are onned to eah other with kinematial joints and nonlinear fore elements. ah joint redues the amount of ative degrees of freedom in the 272 HeavyvehiJes and roads: tehnology, safy and poliy. Thomas Telford, London, 1992.

2 TSTING TCHNOLOGIS Fig. 1. The side wiew of the truk model. model. After adding the kinemati ouplings there are still 32 of them left. Altogher the model forms a s of 1166 differentialalgebrai equations, whih are solved iteratively by ADAMS. The other models used in this study were derived from the original one by varying hassis harateristis suh as number of axles, spring types and shok absorbers. The geomry and kinematis of different parts were derived from drawings. The mass and inertial properties needed were measured as far as possible. For omplex strutures, as the frame, the inertial data were alulated by dividing them in smaller piees. The properties of eah piee were found by using a separate omputer program, after whih they were put togher to represent the whole struture. The harateristis of shok absorbers, rubber mountings and bump stops, tyres and springs are based on the data derived from the manufaturer in question. Most of those dynami properties, whih were not dirly available, were measured during the proj. Some data, whih was diffiult to ahieve through measurements, had to be approximated. very approximated value was first arefully onsidered to have a minor eff to the dynami behaviour of the whole truk. The three road profiles studied were measured by the Road Surfae Monitoring Vehile fabriated by Tehnial Researh Centre (VTT). It measures the profiles by aeleromers and lasers and thus the wavelength is limited approximately to 3 mres. The first, alled Road 6, represents a rather good highway (IRI value 1.52). Its profile onsist of long waves and there are not very severe loal unevenness. The Road 7 (IRI value 2.4) inludes muh more short radial hanges in its profile, espeially in the end part of it. The third profile used, Road 8 (IRI value 1.89), the first 1 mres is the worst part (IRI value 2.73) used in this study. Unfortunately even the Road 8 is not an extreme example of existing roads, beause most of the measurements have taken plae on national highways. The lengths of the Roads 6 and 7 were about 1 mres and the Road 8 about 2 mres. The dynami properties of only one truk was available. It was realisti, however, to take off one axle and the seond type was available. The paraboli springs ould be replaed by realisti multileaf springs. Beause the number of truks was very limited three different speeds (7, 8 and 9 km/h) were used espeially for the studies onerning the spatial onentration of dynami axle loads. The legal speed limit for truks, 8 km/h, was hosen to be the most frequently used speed. In this study the other speeds were used only with one model version on two different road profiles. The ombinations are shown in Table 1. Table 1. Simulated ombinations. 4L = four axle leaf spring, 4P = four axle paraboli spring, 3P = three axle parapoli spring. ROAD Suspension type 4 L 4 P 3 P Speed km/h Speed km/h Speed km/h * * X * X 7 * * * 8 * * * X = with three shok absorbers 273

3 HAVY VHICLS AND ROADS The simulations were run on Sun 4/39 - workstation. CPU-time used for a 1 mre simulation with 2 output steps per seond was hours depending on the speed and the model version. The total number of time steps during the simulation varied bween 1-15 and the number of umulative orror iterations needed were roughly 4. Fig. 2 shows dynami axle loads of the front axle as a funtion of the road station at three speeds and the longitudinal profile of the road (note, wavelengths over 3 mres are negled). The small frequeny due to the pith of the truk body dominates and the higher frequeny from the axle boune is muh smaller. All the frequenies are in the same phase in the beginning beause the road before the beginning of the simulation is perfly smooth and it takes some time before the truk body begins to osillate. From about 4 mres the dynami axle loads begin to separate as a funtion of the spae but not in time (the frequeny is the same). A stronger unevenness at 75 mres is in pith phase and gs the dynami loads in about the same phase. This happens even the IRI-value is only The dominating frequeny in Fig. 3 (the driving bogie axle on the same Road 6) is higher beause it is due to the boune of the axle and not to the pith of the truk body. However, the phenomenon is the same, the dynami axle loads separate in the beginning but a single, a slightly greater unevenness gs them bak into the same phase. Fig. 4 presents the front axle on the Road 8 whih is rougher (IRI 2.73) and onsequently greater dynami axle loads exist. It is possible to distinguish both lower frequenies due to truk body pith and higher frequenies from axle boune. Higher frequenies are more important than those in Fig. 2. The dynami axle loads at different speeds begins to separate but a sharp and short unevenness gs them in the same phase. The spatial repeatability is perf at the 4-6 mres but then begins to separate. At 9 mres the fores g one again in the same phase. The dynami axle loads of the driving axle in the bogie (Fig. 5) shows similar behaviour but the repeatability is learly stronger. The axles 2 (the seond axle) and 4 (the arrying axle in the bogie) behaved like the axle 3 (driving axle in the bogie). The eff of the road roughness and the speed an be seen in Figs. 6 (Road 6) and 7 (Road 8). The Dynami Load Coeffiients (DLC = standard deviations of dynami axle loads divided by the mean dynami axle load) vary bween.3 and.11. The DLC of the front axle on the Road 6 is relatively high beause of the pith but beause there are only few smaller unevennesses the DLCs of the tridem axle are small. The small, sharp unevenness inrease all DLC values but beause the pith is smaller the DLC of the front axle is even smaller than that on the Road 6. The DLC of the air suspension is relatively bter on the Road 8 whih is more uneven. As the speed inreases, the DLC inreases linearly. The three-axle version of the vehile behaved exatly like the four-axle vehile and thus it is not handled here. The basi idea of the use of simulation was to find the eff of dynami loading and its repeatability in order to estimate the errors due to dynami axle loads and to find riteria for optimum plae for IM. The relative dynami axle loads of the driving axle on Road 6 are presented in Fig. 6 (total axle loads in Fig. 3). Limits for +/- 3 perents has been drawn in the Fig. 8. The longitudinal profile bween stations 3 and 8 has been drawn in Fig. 9 for further analysis. The dynami axle loads are presented as earlier, all three speeds separately and added the differene bween the dynami loads of 7 km/h and 9 km/h (bold line). Those parts of the Road where the dynami load differs less than three perent are marked in the lower part of the figure, separate marks for eah speed and the ombination of them. IM should be installed in plae where all three speeds are marked whih is marked as the "ombination". The road is relatively smooth bween 3 and 4 mres and even the axle loads begin to separate most of the part is aeptable for IM. Seond good point is after 4 mres whih is still aeptable beause the top of the "hill" is smooth and its radius great. The rough area bween 5 and 7 mres is not aeptable even it is not very rough. The smooth part of the road ould be surprisingly short if the road is relatively smooth everywhere. Only very few profiles have been analyzed. General rules have not been found y. 3. MASURMNTS Tehnial Researh Centre (VTT) has measured dynami axle loads with strain gauges glued to the axle. Inertial effs an be orred by aeleromers. The alibration has been only stati but it is estimated that the errors in dynami axle loads are less than +/- 1 perent if orrions from aeleromers are used. Thus it is not possible to make any exat omparisons bween measured values in the vehile and those from the IM. The phases are, however, orr and if various speeds and several vehiles are used the eff of spatial distribution an be estimated and the results from the simulation validated to ertain degree. The following researh was planned in 1991 but the finaning was delayed. It will be arried out during summer season 1992 and more dailed plans will be made in near future. One instrumented vehile will be taken in the first phase, the dynami axle loads will be measured and the exat moment as the vehile The signal from the elri eye is exat and the plae of the dynami axle load an be exatly dermined (Fig. 1). passes IM will be monitored with elri eye. Various speeds as well as loads will be used on several IMs. The profiles will be measured. In the seond phase more instrumented vehiles will be used. 274

4 TSTING TCHNOLOGIS Z a. >< 6 SOkmlh 5 9kmIh 4 PROFIL Fig. 2. The dynami axle loads of the driving axle on road 6. u:: Y2 Z >< 1 9-7kmlh 8 ---SOkmlh kmIh 15., u:: 7 -+ ==PR5OFIYL_5 o Fig. 3. The dynami axle loads of the driving axle on road 6. 5 a. o r2 o 9 Z 8 7 >< a. o _ O Fig. 4. The dynami axle loads of the front axle on road

5 HAVY VHICLS AND ROADS 13o oo 11 z >< 5 15 g. Q _ _ _4 o Fig. 5. The dynami axle loads of the driving axle on Road Bll 7 km/h 8 km/h I 9 km/h.12.1 l 7km/h 8 km/h 9 km/h.8.8 Y ::::; Fig. 6. The Dynami Loading Coeffiients (DLC) on Road 6.. Fig. 7. The Dynami Loading Coeffiients (DLC) on Road ";!! w. : 5 z > 1 15 ROAD PROFIL 2 15 g ii: Q Fig. 8. Dynami axle loads at various speeds and the differene at speeds 9 km/h and 7 km/h. 276

6 -1 o TSTING TCHNOLOGIS w. i! -15 :: D.. Cl: :: km/h 4IIIID-=miIIIIIIIVGI[D..:> 7 kmlh 4IIIIIIIIIm) 4II> 4IIIIa> filllililll!l!llilllijg! mo I!IIIIIJ DIJJIJID 4 5 CJ :> aiiiid ad GIIIaID 4DIIIIIIC GIIIIIIIIC C> 4IIIIIIIII> GIIIIIt> """IC I]ID rn'ii" NIII I QU!IIIIY ". IIl11lDlJ Fig. 9. The longitudinal road profile and plae where IM ould be installed. 1y Z ::. Cl:.. X Cl: 5-5 POINTING SIGNAL Fig. 1. Dynami axle load measured in a vehile and a signal from an elri eye. 4. CONCLUSIONS It has been found that if a truk passes the same road at the same speed the dynami loadings our at the exatly same points, so the repeatability or the spatial onentration of dynami pavement loads is perf. Very little is known if the speeds and vehiles are different, that is to say what is the spatial onntration of the dynami axle loads of the whole vehile fle. The spatial onentration of dynami axle loads was simulated driving the same vehile at different speeds. ah axle has its own speifi frequeny. If the speeds of a vehile are different the maximum axle loads should our at different points. The results were shown as a funtion of the station of the road; not as a funtion of time as usually. The results showed that if the road is very smooth there is no spatial onentration of dynami loadings but only a very small unevenness in the road put the dynami loadings in phase and the spatial onentration of dynami loadings is very lear. The same phenomenon was found on all the axles and on both roads. Simulation was used in order to find riteria for best possible position for IM and to estimate errors due to dynami axle loads in IM measurements. The smooth part of the road ould be surprisingly short if the road is relatively smooth everywhere. ven long radius "hills" are aeptable if they have no short wave irregularities. Only very few profiles have been analyzed. General rules have not been found y. Dynami axle load measurements an be used for the same purpose and a researh program will be realized during this summer season. 277