On the evolution of highway live load models

Author: Adrian Furrer
Language: English

Abstract

Over the past century, transportation has undergone a significant transformation due to technological advancements. Consequently, traffic live load models in various countries have evolved substantially over a relatively short period of time. Despite having the same objective of reproducing realistic traffic loads with practical live load models, the resulting effects can vary considerably depending on the model used for structural assessment.

The initial objective of the study was to compare the current traffic live load models defined in the EN1991-2, the SIA 261, the AASHTO LRFD Bridge Design Specifications (BDS), and the Chinese JTG D60. The loads from the traffic models were applied on a simply supported beam, varying the span and number of traffic lanes. The main conclusion of this comparison was that the loads and resulting force effects due to the Eurocode and SIA load model 1 (LM1) were significantly higher than the AASHTO LRFD BDS HL-93 load model.

This difference motivated the investigation into the evolution of the HL-93 and LM1 load models to find the root causes for the significant variations in load magnitudes and patterns. Historical live load models were reviewed, following a more detailed investigation of the development processes of the HL-93 and LM1 models. Key findings from this investigation were the different traffic data used to determine target traffic effects, on which these models were calibrated. The Eurocode LM1 used real traffic data from the heavily trafficked A6 motorway near Auxerre, collected from 1977 to 1982 using weight-in-motion systems. Compared to older code provisions in Europe, the new load model increased the load applied by an average of over 30%. The American HL-93 model was developed using data from the Ontario Truck Survey conducted in 1975 under the assumption that future truck weight limits and the truck population would not change.

Finally, four existing bridges, including short and medium span girder bridges, a network tied arch and a cable-stayed bridge were evaluated by applying the loads from the four aforementioned traffic models to see how the corresponding force effects affect the dimensioning of specific components. The general conclusion of this comparative study was that bridges designed according to the AASHTO or JTG D60 provisions would not meet some of the corresponding dimensioning requirements had they been subjected to the Eurocode/SIA load models. Consequently, when considering general design recommendations on certain bridge types, it is important to frame those in the context of the corresponding local design provisions that the given recommendations are based on.

Considering the current trend of designing more efficient and hence lighter bridges, this conclusion may become even more pronounced, as the applied live load gains more importance in the total load considered in the dimensioning of bridges.