Characteristic traffic load effects from a mixture of loading events on short to medium span bridges

A mixture peaks over threshold approach for predicting extreme bridge traffic load effects

Traffic load model in standard or specification for bridge design should guarantee all newly designed bridges to have sufficient security margin for future traffic. Many different methods have been used to model extreme traffic load effects on bridges for predicting characteristic value for short or long return period. In order to provide some guidance on selecting parameter estimation when applying POT to bridge traffic loading, we focus on the effect that method used to estimate the parameters of the GPD has on the accuracy of the estimated characteristic values. Through this qualitative discussion on the methods, several available methods for traffic loading are selected. Numerical simulation data, Monte Carlo simulation traffic load effects and in-field traffic load effect measurements are analyzed and presented. Literature points out that the traffic load effect is induced by loading event that involves different number of vehicles, and the distribution of the load effects from different loading events are not identically distributed, which violates the assumption of classic extreme value theory that the underlying distribution should be identically independent distributed. Methods using mixture distribution (exponential or generalized extreme value) has been proposed in the literature to model the extreme traffic load effect by loading event. However, the traffic loading may be also importance if the bridge encounter traffic induced fatigue problem, components like orthotropic steel deck is governed by traffic induced fatigue load effects. We intend to explore the influence of traffic load on the fatigue behaviour of orthotropic steel deck, especially the influence of the loading position in terms of transverse location of vehicle. Measurements of transverse location of vehicle collected from by weigh-in-motion (WIM) systems in 2010 and 2011 four French highways showed a completely different distribution model of transverse location of vehicle to that recommended in EC1. Stress spectrum analysis and fatigue damage calculation was performed on the stresses induced traffic on orthotropic steel deck of Millau cable-stayed bridge. By comparing the stresses and damages induced by different traffic patterns (through distributions of transverse location of vehicle), it was found that the histogram of stress spectrum and cumulative fatigue damage were significantly affected by the distribution of transverse location of vehicle. Therefore, numerical analysis that integrates finite element modelling and traffic data with distributions of transverse location of vehicles can help to make an accurate predetermination of which welded connections should be sampled to represent the health of the deck

Statistical analysis of traffic loads and their effects on bridges

An objective of the European Commission 5th Framework Research project, Samaris ,i s to determine the bridge repair needs of new and possible future member states of the European Union (EU). This paper reports the findings of a study into the differences in bridge traffic loading between such countries and long-standing EU member states. A comparison is made of bridge traffic loading for two-lane short- to medium-length bridges in two countries deemed to be representative: Slovenia and the Netherlands. Truck classification and weight data from weigh-in-motion sites in each country are used in conjunction with other European data in a wide range of simulations to assess the implications of the differences for bridge assessments. Significant differences are identified between countries, both in the truck volumes and in the statistical distributions of truck weight. The implications for a range of bridge load effects are calculated and compared to the characteristic load effects implied by the Eurocode. Country- or network-specific bridge assessment load models are recommended to reflect the significant differences in traffic between European regions.

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Bridge assessment loading : a comparison of West and Central/East Europe

The Load and Resistant Factor Design (LRFD) approach is based on the concept of structural reliability. The approach is more rational than the former design approaches such as Load Factor Design or Allowable Stress Design. The LRFD Specification for Bridge Design has been developed through the 1990s and 2000s. In the development process, many factors were carefully calibrated such that a structure designed with LRFD can achieve a reliability index of 3.5 for a single bridge girder (probability of failure of about 2 in 10,000). As the initial development of the factors in the LRFD Specification was intended to be applied to the entire nation, state-specific traffic conditions or bridge configuration were not considered in the development process. In addition, due to lack of reliable truck weigh data in the early 1990s in the U.S., the truck weights from Ontario, Canada measured in the 1970s were used for the calibration. Hence, the reliability of bridges designed with the current LRFD specification needs to be evaluated based on the Missouri-specific data and the load factor needs to be re-calibrated for optimal design of bridges. The objective of the study presented in this report is to calibrate the live load factor in the Strength I Limit State in the AASHTO LRFD Bridge Design Specification. The calibration is based on the Missouri-specific data such as typical bridge configurations, traffic volume, and truck weights. The typical bridge configurations and the average daily truck traffic of the bridges in Missouri are identified from statistical analyses of 2007 National Bridge Inventory. The Weigh-In-Motion (WIM) data from 24 WIM stations in Missouri are used to simulate realistic truck loads. Updated material and geometric parameters are also used to update the resistance distributions. From this study, it was found that most representative bridges in Missouri have reliability indices higher than 3.5. For many bridges in rural areas with Average Daily Truck Traffic (ADTT) of 1,000 or less, the average reliability indices are higher than 5.0. This study proposes a table of calibration factors which can be applied to the current live load factor of 1.75. The calibration factor is developed as a function of ADTT such that bridge design practitioners can select a calibration factor considering the expected ADTTs of a bridge throughout its life span. Impact of the calibration factor on the up-front bridge construction cost is also presented.

https://rosap.ntl.bts.gov/view/dot/18380/dot_18380_DS1.pdf

Calibration of the Live Load Factor in LRFD Design Guidelines

The paper describes a field trial of the use of a simple statistical extrapolation technique for the determination of design load effects in existing bridges. Deflections were measured directly using lasers in the Foyle Bridge, and data were recorded for 155 daily 48-min samples. As only traffic load effects were of interest, wind-induced deflections were removed by Fast Fourier transform analysis and temperature-induced deflections were removed through identification of traffic-free periods. A simple linear regression analysis using probability paper has been employed to determine the parameters which characterise the statistical distribution. The cumulative distribution function was then used to estimate the level of deflection with a 1000-year return period. Empirically derived formulas have been utilised to determine the variability in the 1000- year estimates and to calculate a design deflection which allows for this. (A)

Traffic load 'fingerprinting' of bridges for assessment purposes

/pdf/traffic-loading-fingerprinting-of-bridges-of-assessment-o9bdnx0nme.pdf

K. M. Butler

Papers

Traffic load 'fingerprinting' of bridges for assessment purposes

Traffic Loading 'Fingerprinting' of Bridges of Assessment Purposes