# Load Models Representative of Brazilian Actual Traffic in Girder-Type Short-Span Highway Bridges
## 1 Introduction  
Codes for highway bridge design provide live load models for both ultimate limit states and fatigue evaluation. In the first case, the model intends to represent extreme traffic load effects, and in the second one, the magnitude of fatigue damage generated by the traffic or to ensure an unlimited fatigue life. Distinct techniques for traffic simulation as well as for the analysis and treatment of traffic data [1, 2, 3, 4, 5, 6, 7, 8, 9] have been developed, with the aim to assess bridge load effects by means of probabilistic methods and to improve the (re)calibration of live load models for design codes. Eurocode 1 and AASHTO are reference design codes in relation to live load model development. Eurocode 1 [10] provides four models for vertical loads on highway bridges calibrated to represent the effects of the actual traffic, including dynamic effects [11]. Bridges spanning 5-200 m were chosen as representative structures. The basic model LM1 consists of three design lanes, each one loaded with a 2-axle vehicle and a uniformly distributed load. The original calibration of the Eurocode 1 load models was carried out using data from weigh-in-motion (WIM) traffic recorded between 1986 and 1987 [11, 12]. A recalibration of the models was performed with new traffic data obtained from 1997 to 2001, which confirmed the values of the effects obtained in the original calibration [13]. On the other hand, the United States' design code AASHTO LRFD [14] provides two load models for actual traffic: the former AASHTO HL-93 design truck, for long spans, and a tandem axle for shorter spans. Both are accompanied by a 9.3 kN/m uniformly distributed design load. An impact factor is considered. Traffic simulation included the presence of multiple trucks on the slab, both in a single lane (convoys) and in adjacentlanes (side-by-side and staggered), to determine the probability density functions (PDF) of the traffic effects on bridges [15]. Calibration of the models involved weighing 9250 heavy trucks in Ontario, Canada, in 1970s [12; 16]. Approximately 200 bridges in the U.S. spanning 9-60 m were chosen as representative structures [17]. As a large amount of new WIM data has been collected within the U.S. over the past two decades [16], a recalibration has recently been performed. It was found that the current AASHTO load model is no longer suitable for short spans; therefore, [PERSON] and [PERSON] [18] recommended increasing the tandem's load per axle from 110 kN to 130 kN. In Hong Kong, WIM data from over 800,000 trucks were collected from five different locations; statistical analysis of traffic parameters was performed, such as GVW, axle weights, and wheelbases. Extreme daily bending moments and shear forces were calculated on simply supported bridges spanning 5-40 m. A 47-t 6-axle load model was obtained to represent the real traffic [19]. In Australia, load models have been developed considering the effects of traffic, future trends, and the presence of very long heavy trucks. The main ones are a \"moving traffic load\" and a \"stationary traffic load\"; both consist of four tridem groups plus a non-interrupted uniformly distributed load. The reference internal forces were shear and positive moments in simply supported bridges and negative moments in two-span continuous bridges spanning up to 100 m [20]. With the widespread use of WIM systems worldwide, the mentioned calibrations have already been carried out in several countries, e.g., South Africa [21] and Mexico [22]. To account for extreme load effects, the design code NBR 7188 [23] for highway bridges in Brazil presents a live load model, as shown in Figure 1, named TB-450, which is composed of a 450 kN weight vehicle and a distributed load \(p\) of 5.0 kN/m\({}^{2}\), both multiplied by an impact factor. This configuration originated from an older version of the design code.