A methodology for assessment of road structures for the PBS pilot project in South Africa

Abstract

This paper describes the methodology that is being used for the performance assessment of PBS vehicles in terms of road structures as part of the PBS pilot project in South Africa. The assessment approach has evolved from the standard “bridge formula” contained in the National Road Traffic Regulations that is applicable to legal heavy vehicles, to the less conservative “Abnormal Load” bridge formula that is used to evaluate permit applications for the movement of indivisible loads with a total combination mass of up to 125 tonnes. Since 2012, a more performance-based approach has been adopted, which involved the comparison of maximum bending moment (BM) and shear forces (SF) generated by the proposed PBS vehicle with a reference bridge design load (NA and NB30 from the South African bridge design code). Span lengths assessed range from a 5m simply-supported span to 2- and 3-span continuous structures up to 120m. The requirement for the PBS pilot project is that the maximum BM or SF generated by the PBS vehicle may not exceed 85% of the corresponding effects generated by the reference design load. The paper shows the results of a number of the current PBS vehicles and compares the BM and SF effects with a range of legal heavy vehicles. This assessment methodology could form the basis of a performance standard for road structures should the PBS approach for heavy vehicles be adopted in South Africa.This paper describes the methodology that is being used for the performance assessment of PBS vehicles in terms of road structures as part of the PBS pilot project in South Africa. The assessment approach has evolved from the standard “bridge formula” contained in the National Road Traffic Regulations that is applicable to legal heavy vehicles, to the less conservative “Abnormal Load” bridge formula that is used to evaluate permit applications for the movement of indivisible loads with a total combination mass of up to 125 tonnes. Since 2012, a more performance-based approach has been adopted, which involved the comparison of maximum bending moment (BM) and shear forces (SF) generated by the proposed PBS vehicle with a reference bridge design load (NA and NB30 from the South African bridge design code). Span lengths assessed range from a 5m simply-supported span to 2- and 3-span continuous structures up to 120m. The requirement for the PBS pilot project is that the maximum BM or SF generated by the PBS vehicle may not exceed 85% of the corresponding effects generated by the reference design load. The paper shows the results of a number of the current PBS vehicles and compares the BM and SF effects with a range of legal heavy vehicles. This assessment methodology could form the basis of a performance standard for road structures should the PBS approach for heavy vehicles be adopted in South Africa.