The South African heavy vehicle load limit study

Transcription

1 The Suth African heavy vehicle lad limit study H. P. VAN TONDER, DepartmentfTransprt,J. P. HASLUCK, Lexetran(Pty) Limited, and D. J. W. WIUM, Van Wyk and Luw Inc., Pretria, Suth Africa The presentatin relating t the Suth African Heavy Vehicle Lad Limit study was made, unprepared, at the Third Internatinal Sympsium, and this paper was prduced after the Sympsium t be included in the prceedings. The study was f interest as it represented the latest applicatin f the principles being researched by ther members f the sympsium. A brief utline f the study was given, cvering the effect f changing axle and vehicle lads in Suth Afric This invlve analysing the ecnmic issues as well as briefly examining ther issues related t the subject. BACKGROUND The Department f Transprt, assisted by a team f cnsultants (Lexetran Pty Limited, Van Wyk and Luw Inc., Transprtek (CSIR), Van Niekerk Kleyn and Edwards Inc., Jrdaan and Jubert Inc. and Transprt Research Assciates) has fr the past eighteen mnths been reassessing the axle and vehicle lading limits as cntained in the Rad Traffic Act, 1989 (Act N. 29 f 1989) (RTA) and the Cnslidated Rad Traffic Regulatins (the Regulatins) prmulgated thereunder. The RTA and Regulatins cntain prvisins, that, as well as giving legal frce t vehicle manufacturers lading ratings fr their heavy vehicles, limiting lading n heavy vehicles t the lad capacities f the tyres f the vehicles cncerned and setting a lwer limit t pwer!mass rati, als set definite maximum limits fr axles, grups f axles and permissible cmbinatin lads, t prtect rad infrastructure. The axle and axle unit lads are cntrlled by direct limits, while the lads n grups f axles and n vehicles (and vehicle cmbinatins) are cntrlled by a bridge frmul These limits are as shwn in Table 1. Table 1 AXLE, AXLE UNIT AND AXLE GROUP LOAD LIMITS Axle/Unit! Tw wheel Fur wheel Grup axle (tnnes) axle (tnnes) Single 7,7 8,2 Tandem 15,4 16,4 Tridem 21,0 21,0 Grup! vehicle! cmbinatin 1.8L+ 16* 1.8L+ 16* *L = distance in metres between the extreme axles f the grup r f the vehicle r cmbinatin * Maximum cmbinatin length = 22,0 m * Maximum articulated vehicle length = 18,5 m These limits mean that a 7 r mre axle vehicle cmbinatin can carry 52 tnnes while a 5 r 6 axle articulated vehicle may carry 41 tnnes. The limits have been in the law fr many years, sme since befre 1966, sme since These limits were therefre due fr review and pssible revisin. This situatin was aggravated by a severe crisis that had develped in the rad freight transprt industry. This crisis was due in part t the fact that the ecnmy generally was in recessin, and in part due t the effect f a plicy f deregulatin in the transprt sectr. This has meant increased cmpetitin and aggressive pricing tactics by peratrs. The entire situatin had been exacerbated by a pr lad limit enfrcement strategy which has recently been revised and was demanding better cmpliance with the law. The fact that all vehicles in Suth Africa are surced frm Eurpe, UK, USA r Japan and as such are all capable f exceeding bth the axle lad and vehicle lad limits in the RT A and Regulatins, als cntributed t dissatisfactin. The transprt peratrs were requesting larger legal axle and vehicle lad limits t assist them t smth ut and recup the deficits due t lw pricing f transprt services. Against this the pavements in Suth Africa were generally designed fr relatively light traffic with thin, granular pavement layers, while bridges have been generally designed t British design cdes. The assessment f the elfect f a change in lad limits n these was essential. OBJECTIVES OF THE STUDY The Department f Transprt therefre wished t review the lad limits as a part f a strategy t assist the transprt industry, while als demanding better cmpliance. The lad limits shuld be changed t the extent this culd be dne efficiently in ecnmic terms. It is generally accepted that higher lads n vehicles means lwer average transprt perating and fixed csts (vehicle csts) per tnne f paylad carried. It is als accepted that higher lads als mean greater rad rehabilitatin and maintenance csts (rad csts). The Department set as its bjective, the determinatin f the ecnmic ptimum level Heavy vehicles and rads: technlgy, safety and plicy. Thmas Telfrd, Lndn,

2 HEAVY VEHICLES AND ROADS f lad limits, where the sum f the rad and vehicle csts is a minimum. The Department ais wished t examine the impacts n safety, traffic, envirnment and scial cnsideratins, and n ther mdes f freight transprt. In rder t examine the effects f a pssible change in the axle and vehicle lad limits n bridges, these were examined separately, as decisins relating t these wuld be based n the extent f a nce ff investment, which culd be discunted against any nett benefit received frm the changes fr the rad transprt system as a whle. T achieve the maximum c-peratin and disseminatin f infrmatin, a Wrking Grup f transprt peratrs, vehicle manufacturers, rad authrities, enfrcement agencies and ther interested parties was cnvened t prvide a cnsultative frum thrugh which t btain and discuss varius view pints, and t assist in frmulating final recmmendatins. PffiWSOPHY OF THE STUDY As a basis t carry ut the ecnmic analysis f the study a cnstant paylad was initially assumed t be travelling n the radway system. The effect n ther mdes and the resulting mde transfer r change in market share was examined after the ecnmic ptimum was determined fr the rad transprt sectr. The basis fr the study was the determinatin f the effect f the change in axle and vehicle lads upn the vehicle csts and the rad csts, each in respect f transprting ne tnne f paylad. Figures 1 and 2 shw the schematic relatinships derived in the study. Figure 1 shws that fr each type f vehicle, the vehicle cst per paylad tnne kilmetre reduces with an increase in axle and vehicle lads. The vehicle csts are higher fr the smaller vehicles, and the rate f reductin in cst with increase in lad is lwer fr the larger vehicles, due t the lwer percentage change in ttal lad n each vehicle. Figure 2 shws that the rad cst per paylad tnne kilmetre increases with increasing axle lad. This figure shws that fr the lightly cnstructed pavements (thse designed t carry less than 1,0 millin equivalent single axle lads ESAL) the rate f increase in cst with increase in axle lad is very rapid, upn the expnential curve, while fr strnger pavements, csts increase mre slwly ver the range f interest. In analysing the csts and benefits the effect f each type f vehicle n each type f vehicle must be determined. Figure 3 shws typical results fr a three axle vehicle. In determining the ttal cst and benefit the distance travelled by each vehicle n each pavement must be determined t prvided a weighted average f the ptimum axle lad - as shwn in Figure 4. The verall benefit f changing t the ptimum limit is then the sum f all the benefits (r disbenefits) f the ttal vehicle kilmetres travelled by each type f vehicle n each type f pavement. With regard t bridge structures the principle was slightly different in that these create a cnstraint upn the grss vehicle lads and the cncentratin f the lad n vehicles. The apprach was t determine the theretical lad carrying capacity f the bridges n the netwrk, with the cst implicatins, if any, f a change t the existing legal limits. These cst implicatins culd then be discunted as an investment cst against the benefits derived frm the first part f the study. EXECUTION OF THE STUDY With regard t the analysis f the vehicle and rad csts and benefits, the fllwing steps were fllwed. Traffic flw and vehicle lading data was required. General statistics n traffic n the rad netwrk were reasnably available frm the Departments Cmprehensive Traffic Observatins (CTO) system, but the lading data was nt generally available. A number f specific vehicle lad surveys were cnducted n a number f different rad categries. These surveys were cnducted with weigh-inmtin devices, and allwed the categrisatin f the general traffic infrmatin btained frm the CTO. It als prvided a database f axle and vehicle ladings n each categry f vehicle, t be used in the later stages f the study. Six categries f heavy vehicle r vehicle cmbinatin were used. Netwrk data was als required. The length f each categry f pavement in the netwrk was btained, partly frm pavement data and partly frm traffic dat The latter was nt an accurate representatin f the actual pavement, but was an indicatin f what the pavement fr a particular sectin f rad shuld be. The heavy vehicle traffic n each sectin f each categry f pavement was then estimated frm traffic data and the generalised categrisatin f the traffic frm the detail lading surveys was applied t this infrmatin t prvide an estimatin matrix f the number f kilmetres travelled by each categry f vehicle n each categry f pavement. A predictin mdel was then develped t attempt t determine the number f vehicles that wuld utilise the new lading limits if they were changed. A number f cmplex mdels were cnsidered and tried but a simplistic mdel using eighty percent f vehicles within 0,6 tnnes f the present legal limit increasing their lads was used. The lad increase was then determined in a distributin between the present.limit and the new legal limit. This mdel was then used fr a number f different ptins f bridge frmula and axle lad. The mdel was cmplicated by the fact that the data shwed cnsiderable ccurrence f verladed vehicles and axles, sme f which were already abve the prpsed new legal limits. In the final analysis it was assumed that all these verladed vehicles were at the present legal limits and wuld mve accrding t the predictin mdel t maximum f the new legal limits. The next step was t calculate the rad csts per paylad tnne kilmetre. Typical crss sectins fr the varius categries f rad pavement were assumed and the rehabilitatin cst per kilmetre fr each was determined. This was cnverted t a cst per ESAL kilmetre fr each crss sectin and pavement. The number f ESAL' s were then calculated fr each categry f vehicle under firstly the present lad limits and secndly the new lad limits. The AASHTO relatinship using an expnent f 4,0-4,5 was used in this calculatin. These were then multiplied by the cst per ESAL and divided by the paylad in each case. The result shwed the rad cst fr each type f vehicle per paylad tnne kilmetre and the 414

3 VEHICLE COST AGAINST ~ :~~~-=--~~~= ~1 z ~ ROAD COST AGAINST 3 AXLE VEHICLE 120Ti ~ ; 100J r...-. Cl... <t: 2:;~...J ~ 20~'- 0:: 1t:i~ l:r=~~ "* -"''El-- 1 E-< 80..._.. ""...,"/..L.:."" S;; L/ ' ~ 60-~ &l <t: 2~ b:; ;::.:::= ~ 8 0 ~.-;;:;==-=---. O~!-,! ~r--r--r--r--r--T"-T"-.--,--.-~ I' ~2 AXLE ~ 3 AXLE -- 5 AXLE -e-- 7 AXLE - LIGHT PVMNTS ~ MEDIUM PVMNTS -- STRONG PVMNTS FIGURE 1 FIGURE 2 ~ TOTAL COST AGAINST 3 AXLE VEHICLE 100Ti ~ , ::; 90+- :.: i ::--=~= ~ ::::r" -_._/....;/ /=~=~_: ~ 501-~ :::::::::c~-: 13 u 40i ~~---,,< 30 i!!!!!!!!!!!!!! :< TONNES FIGURE 3 TOTAL COST AGAINST ALL VEHICLES ::; :.: 100+;/-.. 1 z Z E-< Cl <t:...j > <t: 0:: :;:' [~ ALL VEHICLES FIGURE 4 trj i U'l ~

4 HEA VYVEHICLES AND ROADS difference was the increase in rad cst as a result f the lad limit change fr the vehicle cncerned. This prcess was repeated fr each vehicle n each pavement and the additinal cst was summed fr all vehicles n all pavements. The fllwing step was t calculate the vehicle csts per paylad tnne kilmetre. Vehicle perating and fixed csts were btained frm the Rad Freight Assciatin Vehicle Csts Schedules and the cst per paylad tnne kilmetre were determined using firstly the present lad limits and secndly the prpsed limits. The change in paylad was assumed t induce an perating cst change in the vehicles given by the fllwing frmula, but the result was tested and fund t be largely insensitive t the relatinship assumed. c=0,23iw W where C = vehicle perating cst and W = ttal weight f the vehicle. The difference in vehicle cst per paylad tnne kilmetre was then the benefit fr the particular vehicle categry, and these were aggregated fr the amunt f travel f the varius vehicle categries n the netwrk. A number f axle lad changes were tested t prvide a trend t indicate the apprximate rder f magnitude f the ptimum axle lad. Then a number f specific ptins f axle and axle unit lad limits and bridge frmulae were tested. As an additinal assessment the change in transprt ~perating cst as a result f the necessary rad rehabilitatin nt being dne was calculated. In the analysis f the effect f a change in axle and vehicle lading n bridges, a theretical cmparisn f the traffic lads and the design lads fr the bridges was carried ut. Frm the traffic survey data, traffic streams were simulated using statistical techniques, and these were passed ver a range f representative bridges. The effects in terms f bending mment and shear frces were then cmpared with the effects induced under the design ladings fr which the bridge had been designed. In calculating the effects under traffic lading, allwance was made fr impact and dynamic effects. It was further realised that if better cntrl f verlading culd be btained, then the traffic lad simulatin wuld change. A simulatin using "ideal" verlading cntrl,cnditins was then als used. Simulatins under the present lad limits as well as a number f ptins f increased axle and vehicle lad limits were then tested. It was fund that certain shrt span (5-15 m) bridges were experiencing stresses exceeding thse prvided fr in the design lading cases, under the present simulatin f verlading. If the axle l~ds were increased then the number f bridges experiencing this verstress als increased. Using this number f bridges the cst t strengthen all the relevant bridges was estimated. If the verlading can be prperly cntrlled then fewer bridges will be affected, but a number f these will still experience stresses in excess f the design limits. The mst imprtant results f the analysis are shwn in the fllwing sectin. RESULTS OF THE STUDY The first assessment shwed the tendency f csts and benefits with changes in axle lad. Cst Tendency n Theretical Axle Mass Changes (Apprximate Values) Axle Lad Tns TPI' Cst Saving Rad Cst Increase r , , , , Nett Benefit US$ $1.00 = R2,85 These results indicated that 9 tns was the apprximate ptimum axle lad As indicated a number f ptins f axle lad and bridge frmula were assessed, the mst imprtant f these were the fllwing: Mst Imprtant Optins Assessed Optin Axle Lads Tns Bridge Frmula N. Single Dual Tridem ,2 16,4 21,0 1,8L ,2 16,4 21,0 2,IL ,0 18,0 21,0 2,lL ,0 18,0 21,0 2,lL ,0 18,0 24,0 2,lL ,0 18,0 21,0 1,8L ,0 18,0 27,0 2,5L Benefits and Csts f Specific Optins Optin TPI' Cst Rad Cst Benefit Bridge Cst Saving Increase N. R mill R R R 135* R R R R 240* * Obtained by interplatin. These results high-ligbted a number f pints. Firstly if nthing was changed (ptin zer), R100 mill wuld have t be spent n bridges anyway. Secndly the highest benefit wuld be generated by increasing the axle lad limits withut increasing the bridge frmula (ptin 5). 416

5 ECONOMIC ISSUES This culd nt be recmmended hwever, because it wuld cause lads that culd be carried by heavier vehicles, t mve nt smaller" vehicles, which wuld be less efficient. This effect was nt taken int accunt in the calculatins fr the study. The study therefre recmmended ptin 2 as being the mst beneficial and balanced ptin. It was further recmmended that an immediate change in bridge frmula t accmmdate higher vehicle lads was bth affrdable and ecnmically advantageus, and als assisted in allwing better law enfrcement by bringing the bridge frmula limit in line with the axle lad limits n mst vehicles. It was als determined that if the necessary funds were nt made available t repair the additinal rad damage caused, the increase in vehicle perating cst wuld rapidly grw t a level exceeding any pssible benefit. Superficial assessment f safety, envirnmental factrs and traffic indicated that althugh heavier vehicles affected these aspects negatively, the reductin in the number f heavy vehicles required t carry the same paylad was reduced, indicating the nett change t be f a very lw rder. Scial effects were nt investigated in detail. The effect f pssible changes n the ther mdes f transprt was being assessed, but the results were nt available at the time f ging t press. CONCLUSIONS It was acknwledged that a number f parameters used in the calculatin f the results f the study culd nt be determined precisely. Hwever given the assumptins made the results prvided a gd indicatin f the ecnmic benefit that may be btained as a result f changing the legal axle and vehicle lad limits, and f the csts that will be impsed n the rad system as a result f such change. The results were reasnably rbust when tested fr sensitivity against changes in the assumptins made. It was cncluded that a change in the legal axle and vehicle lads culd be justified in terms f the verall benefit generated. The need t analyse certain bridges in detail and pssibly t strengthen such bridges, irrespective f whether the axle and vehicle lad limits were changed, was identified. If the lad limits were changed this analysis and strengthening (if necessary) wuld have t be expanded t mre bridges. The results indicate that there is n real ecnmic justificatin in increasing the maximum axle lads abve 9 tnnes, given the present rad pavement inventry in Suth Afric Changing the axle lads t ptin 2 generated an verall benefit f R19 millin per year. T release this benefit an annual additinal amunt f R107 millin wuld have t be invested in rad maintenance and rehabilitatin. This als induced an estimated extra R35 millin cst fr bridge strengthening. The need t carry ut the additinal rad maintenance was essential as the benefit generated wuld rapidly be negated by an increase in vehicle perating csts, if the rad netwrk was allwed t deterirate mre rapidly as a result f the change in legal limits. It was therefre prpsed that the change in limits be implemented nly when a better lading cntrl prgramme was in place, and when assurance f the necessary financial prvisins has been btained frm Gvernment. T assist the transprt industry the bridge frmula cntrlling the axle grup n vehicle cmbinatin lads was recmmended fr immediate change. 417