Bala Sivakumar

Protocols for Collecting and Using Traffic Data in Bridge Design

This report provides a set of protocols and methodologies for using available recent truck traffic data to develop and calibrate vehicular loads for load and resistance factor design (LRFD) superstructure design, fatigue design, deck design, and design for overload permits. The protocols are geared to address the collection, processing, and use of national weigh-in-motion (WIM) data. The report also gives practical examples of implementing these protocols with recent national WIM data drawn from states/sites around the country with different traffic exposures, load spectra, and truck configurations. The material in this report will be of immediate interest to bridge engineers. This report replaces NCHRP Web Document 135.

Enhancement of bridge live loads using weigh-in-motion data

Over the past century, the American Association of State Highway and Transportation Officials (AASHTO)-specified live loads have been evolving to catch up with the ever-changing knowledge and trends of vehicular traffic. The latest loads are based on past Canadian traffic data, and may not represent modern or future traffic conditions in some US jurisdictions. Today, weigh-in-motion (WIM) systems can provide a solution to this dilemma by providing valuable site-specific traffic data for bridge design and evaluation, weight-limit enforcement, and the likelihood of illegally overloaded trucks causing premature bridge deterioration. Bridge owners have long recognized the importance of incorporating site- or state-specific truck loads in their bridge evaluation and preservation programs. The AASHTO LRFD and LRFR Bridge Specifications have evolved to advance the trend toward information-sensitive specification formulation. When warranted, the specified live loads can be enhanced utilizing more relevant site or...

Bridge System Safety and Redundancy

This report develops a method to calibrate system factors that can be applied during the design and load capacity evaluation of highway bridges to account for bridge redundancy and system safety. The proposed system factors can be used during the design and safety assessment of bridges subjected to distributed lateral load being evaluated using the displacement-based approach specified in the AASHTO Guide Specifications for LRFD Seismic Bridge Design or the traditional force-based approach. Also, the report presents system factors calibrated for application with bridge systems subjected to vertical vehicular loads.

Development of State-Specific Load and Resistance Factor Rating Method

Recognizing the limitations of the generic truck weight data and conservative assumptions made during the calibration of live load factors for bridge rating, the AASHTO load and resistance factor rating (LRFR) manual for bridge evaluation provides sufficient flexibility and allows state agencies to adjust the live load factors based on their individual conditions and site-specific or state-specific information. This paper describes a reliability-based process that can be followed to perform such adjustments and illustrates its application using an example in which the procedure was followed during the calibration of a LRFR methodology for New York State bridges. This methodology is applied to the rating of existing bridges, posting of understrength bridges, and checking of permit trucks. The live load models used during the calibration are based on weigh-in-motion data collected from several representative sites. The LRFR live load factors developed using the proposed calibration process would provide uniform and consistent levels of bridge safety and reliability for the bridge classes and configurations targeted. The target reliability levels used during the calibration should reflect the experience gained by state bridge engineers from evaluation of existing bridges under current loading conditions.

Calibration of Load and Resistance Factor Rating Methodology in New York State

In 2003, AASHTO adopted the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges, which was calibrated to provide uniform reliability levels represented by a target reliability index β = 3.5 for inventory ratings and β = 2.5 for operating ratings. These target values were selected to remain consistent with the AASHTO load and resistance factor bridge design specifications and provide average reliability levels similar to those of the allowable stress operating ratings and load factor ratings (LFR). The calibration process was based on a generic live load database considered to be representative of truck weight and traffic spectra at typical U.S. bridge sites. Recognizing the limitations of this generic data set, the LRFR provides sufficient flexibility and allows state agencies to adjust the LRFR load factors on the basis of their individual conditions and site-specific or state-specific information. A reliability-based procedure was developed for calibrating a state-specific NYS-LRFR that would reflect current bridge loading conditions in the state of New York, including the intensity and shape of the truck weight spectra as well as measured truck traffic data. The proposed calibration would lead to bridge ratings consistent with the current practice of the New York State Department of Transportation and provide uniform and consistent levels of bridge safety and reliability. The methodology can be easily adapted by other states on the basis of their unique truck loads and traffic conditions.

FIELD TESTS AND IN-SERVICE MONITORING OF NEWBURGH-BEACON BRIDGE, NEW YORK

Field tests previously conducted on the Newburgh–Beacon Bridge in New York captured anomalous stress spikes in the critical hangers of the truss bridge. The very high magnitude spikes occurred sporadically, without warning and under various conditions. The cause and nature of the spikes were of great concern to the bridge owner, the New York State Bridge Authority. Originally, the spikes were thought to be due to some combination of wind, thermal effects, and an unlocking of stresses due to fabrication errors in the pin-and-hangers systems. A series of short- and long-term field tests were recently conducted on the bridge to help identify the cause of the spikes. Sections of the bridge were instrumented with strain gauges around two different pin-and-hanger members. One site was continuously monitored for over 3 months to obtain statistical data on the spikes. Through a series of different test setups, designed to debug the problem, the anomalous spikes were found to be caused by radio interference in the environment, which was being picked up by the strain gauge wires. The test setup, test program, and test results are described. The tests resolved a long-standing concern of the bridge authority and avoided costly and unnecessary retrofits.

TRIAL RATING OF BRIDGES USING LOAD AND RESISTANCE FACTOR PROCEDURES

A draft version of the Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges has been compiled through National Cooperative Highway Research Program 12-46. The manual extends the Load and Resistance Factor approach to the evaluation of highway bridges in the United States. The objectives of bridge rating are different from the objectives of new bridge design and these differences result in different limit states and reliability requirements. The draft manual proposes a system for bridge load rating and permit evaluation based on a uniform reliability index (where possible) that uses site specific information to narrow the uncertainty in some of the evaluation variables. The results of this system are compared to the current load factor ratings based on trial evaluations.

Adjustment of Load and Resistance Factor Design Live Load Factors Using Recent Weigh-in-Motion Data

Traffic loads on bridges exhibit significant variations regionally, from state to state, and from site to site. Accounting for actual live loads in the bridge design process is important to improving the overall reliability and safety of bridges. In some cases, the code-specified live loads may underestimate traffic loading on a bridge. The current load and resistance factor design (LRFD) live load calibration is based on a biased sample of truck weights collected as part of an Ontario, Canada, truck weight survey conducted in 1975. In the past 35 years, truck traffic has significantly increased in volume and weight, which may necessitate adjusting the LRFD live load factors in certain cases on the basis of current truck traffic conditions. Although the quality and quantity of traffic data being collected by highway agencies has improved since 1975, it has not been used to update the bridge design loads. NCHRP Project 12–76 was initiated in 2006 to develop a set of protocols and methodologies using recent truck traffic data to update live loads for LRFD bridge design. Various levels of complexity are available using the site-specific truck weight and traffic data to calibrate live load models. One simplified calibration approach focuses on the lifetime maximum live load for updating the live load model or the load factor for current traffic conditions. Another, more robust, reliability-based approach for calibration is proposed in the protocols. The models are applicable for the design of bridge members, for both ultimate capacity and cyclic fatigue, and are implementable for both main structural members and the design of bridge decks.

Collecting and using Weigh-in-Motion data in LRFD bridge design

The HL-93, a combination of the HS20 truck and lane loads along with the AASHTO LRFD live load factors were calibrated using 1975 truck data from the Ontario Ministry of Transportation to project a 75-year live load occurrence. Because truck traffic volume and weights have increased and truck configurations have become more complex, the 1975 Ontario data does not represent present U.S. traffic loadings. Updating bridge live load models needs representative samples of unbiased truck weight data that meets accepted quality standards. A method that has been developed over the last 3 decades to capture truck loads in an undetected manner and obtain a true unbiased representation of actual highway loads is known as the Weigh-in-Motion, or WIM technology. The implementation of WIM systems in recent years has led to improving the quality and quantity of traffic data, which can be used to update the bridge design loads. The goal of NCHRP Project 12-76 was to develop a set of protocols and methodologies for using available recent WIM data collected at different U.S. sites and recommend a step-by-step procedure that can be followed to obtain live load models for LRFD bridge design. The protocols are geared to address the collection, processing, and use of national WIM data to develop and calibrate vehicular loads for LRFD superstructure design, fatigue design, deck design, and design for overload permits. The recommended protocols were implemented using recent traffic data from 26 WIM sites in 5 states (California, Texas, Florida, Indiana, and Mississippi) across the country. The states and WIM sites were chosen to capture a variety of geographic locations and functional classes, ranging from urban interstates, rural interstates, and state routes.