ABSTRACT
The accuracy of bridge system safety evaluations and reliability assessments obtained through refined structural analysis procedures depends on the proper modeling of traffic load effects. In fact, traffic load intensities need to be defined based on available real traffic data supplemented by a statistical model able to project the data in time to estimate the maximum load effect expected over a specific bridge design or service period. This paper proposes a procedure to calibrate appropriate live load models that can be used for advanced analyses of bridges. The calibration procedure is demonstrated using actual truck data collected at a representative weigh-in-motion (WIM) station in NewYork State. Extreme value theory is used to project traffic load effect to different service periods using the upper tail of the load effect probability distribution. The live load models are developed for a 5-year typical rating interval and for a 75-year design life.
The load model proposed in this paper is obtained by calibrating reasonable weights for the reference trucks that should be used during the analysis process to determine the maximum expected load effects. The calibration models the effects of real truck data on several representative bridge configurations. Specifically, the methodology is applied to a set of multi-beam bridges having various span lengths, number of beams, and beam spacing.
It has been found that two side-by-side trucks having the configurations of the AASHTO 3-S2 Legal Truck can provide an acceptable live load pattern for simulating the maximum traffic load effect. The weight of the axles of the interior truck are those of the AASHTO 3-S2 Legal Truck which add up to 320 kN (Figure 1), while the axle weights of the external truck are scaled by a bias factor a that varies as a function of span length, number of beams and beam spacing. AASHTO 3-S2 legal truck configuration. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig26_1.tif"/>
Figure 2 shows the side-by-side trucks configurations and introduces the parameter α as a multiplier of the weight of the most eccentric truck such that the maximum moment produced by the two side-by-side trucks of weights P and αP on the most critical member is equal to the maximum traffic moment effect obtained from real traffic. Section view of side-by-side truck loading pattern. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig26_2.tif"/>
The parameter α can be well represented by a quadratic equation of the form: () α = c o n s t + a 1 S L 15.2 + b 1 B S 1.2 + c 1 N B 4 + a 1 ( S L 15.2 ) 2 + b 2 ( B S 1.2 ) 2 + c 2 ( N B 4 ) 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq140.tif"/>
where SL is the span length in meters, BS is the beam spacing in meters, NB is the number of beams, const is a coefficient accounting for the reference time under study and a 1, b 1, c 1, a 2, b 2, c 2 are coefficients calibrated to minimize the error in estimating a value of the parameter α.
Live load models obtained using the proposed calibration procedure are readily implementable for deterministic refined analyses of typical highway bridges (simple span bridges less than 70 meters in length) and for evaluating the reliability of bridges at serviceability and ultimate limit states considering the system’s behavior.
