Calibration of Alberta Fatigue Truck

The Province of Alberta uses one of the heaviest design trucks in Canada for the design of its highway bridges. Despite the use of a CL‐800 design truck, most of the fatigue damage is caused by the more frequent trucks rather than the heaviest trucks used for design at the ultimate limit states level. The Canadian Highway Bridge Design Code, CSA‐S6‐14, uses a fatigue design truck with a GVW of 52% of the design truck and a further reduction of 27% is applied when the volume of heavy trucks not more than the greater of 200 trucks per day or 5% of the ADTT. In order to verify whether these fatigue truck factors are still valid for traffic on Alberta highways, a re‐calibration of the fatigue truck was conducted using an extensive database of weigh‐in‐motion data collected from six sites from September 2004 to July 2013. The data collection sites include Highway 2 at Leduc and at Red Deer, Highway 2A at Leduc, Highway 3 at Fort MacLeod, Highway 16 at Edson, and Highway 44 at Villeneuve. 

The data processing consisted of filtering the data to eliminate data that were found to be unreliable either because of excessive vehicle speed, unrealistic axle spacing or weights, and light trucks that would not have any impact on the fatigue damage of bridges. Filtering of the raw data resulted in the elimination of about 90% of the collected data at each site. However, approximately 30 million trucks were retained for the calibration of a fatigue truck for Alberta highways.

The calibration of a CL‐800 truck was conducted for the double slope fatigue curves defined in CSA‐S6‐ 14 using four different influence lines, namely, the midspan moment of a simply supported span and the moments at midspan of the end span, at the midspan of an interior span and at an interior support of a four span continuous beam. Span lengths from 2 m to 70 m were investigated. The calibration for all six WIM sites indicated that the calibration factor varies with span length, but is essentially constant for span lengths longer than 12 m and decreases significantly for shorter span lengths. Although the results for most WIM sites were similar, the Edson site showed slightly heavier trucks than at the other sites. The trucks at Highway 2A at Leduc (having the lowest traffic volume of all sites) were found to be significantly lighter than at the other sites, resulting in smaller calibration factors for all span lengths.

The calibration of the fatigue truck was conducted for the number of equivalent stress cycles specified in CSA‐S6‐14. The calibration process supported a fatigue truck factor of 0.52 for bridges with span lengths greater than or equal to 12 m. A linearly variable fatigue truck factor is proposed for spans shorter than 12 m. It was found that the factor CL as presented in CSA‐S6‐14 is adequate for low traffic roads, although this could be verified at only one location in Alberta.