Major advances have been recently achieved in developing methodologies for the structural analysis of cascading failures and in understanding the behaviour of different types of systems under suddenly applied extreme loads. Yet, a main issue related to defining objective measures of redundancy and quantifying the levels of redundancy that exist in structural systems remains vastly unresolved. This paper reviews the work done by the authors and their colleagues on the quantification of system redundancy of typical highway bridges and reassesses previously made proposals for including system redundancy and robustness during the structural design and safety evaluation of bridge superstructure and substructure systems. These proposals, which are based on system reliability principles, consider structural system safety, system redundancy and system robustness in comparison to member safety, and account for the uncertainties associated with determining member and system strengths as well as future loads in a consistent and rational manner.

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.

This paper presents a procedure that describes how site-specific truck weight and traffic data collected using Weigh-In-Motion (WIM) systems can be used to obtain estimates of the maximum live load for the 75-year design life of new bridges or the two-year return period to be used for the load capacity evaluation of an existing bridge. The application of the results for the reliability-based calibration of live load factors is also presented.

Heavy trucks represent a major load to highway bridges in the transportation infrastructure system. These loads are directly related to the truck weight limits of the jurisdiction, and largely determine the standard loads for bridge design and evaluation. Thus, truck weight limit is one of the major factors affecting bridge deterioration and expenditure for maintenance, repair, and/or replacement. Truck weight in this paper not only refers to the truck gross weight but also to the axle weights and spacings that affect load effects. This paper presents the concepts of a new methodology for estimating cost effects of truck weight limit changes on bridges in a transportation infrastructure network. The methodology can serve as a tool for studying impacts of such changes. The resulting knowledge is needed when examining new truck weight limits, several of which have been and are still being debated at both the state and federal levels in the United States. The development of this estimation method has considered maximizing the use of available data (such as the bridge inventory) at the state infrastructure system level. In application examples completed (but not reported herein), the costs for relatively inadequate strength of existing bridges and for increased design requirement for new bridges were found dominant in the total impact cost.

This report contains the findings of research performed to develop recommended revisions to the legal loads for posting as depicted in the for Condition Evaluation of Bridges and the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges. The report details the development of the new loads and includes recommended revisions to the manuals to incorporate these loads. The material in this report will be of immediate interest to bridge managers and load raters. The research identified and quantified the types of short multi-axle legal vehicles operating on the public thoroughfares and the subset of these vehicles that cause overstressing.

The use of load-factor procedures for the design of bridge superstructures is expanding rapidly. However, substructure design is still based exclusively on allowable stress methods. This paper presents an approach to load-factor design for pile foundations. The load factors suggested follow the current American Association of State Highway and Transportation Officials recommendations, while the resistance factors recommended are based on the capacity-determination methods and the construction control procedures used. Actual values are selected to be consistent with currently used procedures where they are available. The proposed specification can provide a framework for the use of more-appropriate resistance factors as they become available from ongoing research.

This paper introduces the American Association of State Highway Officials' (AASHTO) new Guide Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges that was completed in March 2000 under a National Cooperative Highway Research Program research project and adopted as a Guide Manual by the AASHTO Subcommittee on Bridges and Structures at the 2002 AASHTO Bridge Conference. The new Manual is a companion document to the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications in the same manner that the current Manual for Condition Evaluation of Bridges is to the AASHTO Standard Specifications. The new Manual is consistent with the LRFD Specifications in using a reliability based limit states philosophy and extends the provisions of the LRFD Specifications to the areas of inspection, load rating, posting and permit rules, fatigue evaluation, and load testing of existing bridges. This paper presents an overview of the manual; specifically, the new Load and Resistance Factor rating procedures are explained and the basis for their calibration is discussed.

The use of load-factor procedures for the design of bridge superstructures is expanding rapidly. However, substructure design is still based exclusively on allowable stress methods. This paper presents an approach to load-factor design for pile foundations. The load factors suggested follow the current American Association of State Highway and Transportation Officials recommendations, while the resistance factors recommended are based on the capacity-determination methods and the construction control procedures used. Actual values are selected to be consistent with currently used procedures where they are available. The proposed specification can provide a framework for the use of more-appropriate resistance factors as they become available from ongoing research.

An automated prediction scheme is presented which utilizes both force and acceleration records measured at the pile top during driving to compute the soil resistance forces acting along the pile. The distribution of these forces is determined, and the dynamic and static resistance forces are distinguished such that a prediction of a theoretical static load versus penetration curve is possible. As a theoretical basis stress wave theory is used, derived from the general solution of the linear one-dimensional wave equation. As a means of calculating the dynamic pile response, a lumped mass pile model is devised and solved by the Newmark B-method. Wave theory is also employed to develop a simple method for computing static bearing capacity from acceleration and force measurements. Twenty-four pile tests are reported, 14 of them with special instrumentation, i.e., strain gages along the pile below grade. The piles tested were of 12-in. (30-cm) diameter steel pile with lengths ranging from 33 ft. to 83 ft. (10 m to 25 m).

The use of load-factor procedures for the design of bridge superstructures is expanding rapidly. However, substructure design is still based exclusively on allowable stress methods. This paper presents an approach to load-factor design for pile foundations. The load factors suggested follow the current American Association of State Highway and Transportation Officials recommendations, while the resistance factors recommended are based on the capacity-determination methods and the construction control procedures used. Actual values are selected to be consistent with currently used procedures where they are available. The proposed specification can provide a framework for the use of more-appropriate resistance factors as they become available form ongoing research.

Truck weight-limit regulations have significant influence on truck operating weights. These regulations directly influence loads applied to highway facilities, such as bridges and pavements. "Truck weight" herein collectively refers to a vehicle's gross weight, axle weights, and axle configuration. Truck load spectra as a result of truck weight limits are important to bridge engineering in many respects, such as that of determining requirements for evaluation and design of bridges for both strength and fatigue. This paper's objective is to present a new method for predicting truck weight spectra resulting from a change in truck weight limits. This method is needed to estimate impacts of the change on highway bridges such as accelerated fatigue accumulation. Historical and recent truck weight data are used to test and illustrate the proposed method, and the results show its good prediction capability. This method is also applied here to an example of estimating the impact on steel bridge fatigue due to a possible increase in the gross-vehicle-weight limit from 356 kN (80 kips) on five axles to 431 kN (97 kips) on six axles. Also included is an investigation of the AASHTO fatigue truck model for steel bridge evaluation. Results show that the current fatigue truck model may become invalid under the studied scenario of truck weight-limit increase.

This report contains the findings of a study to develop a methodology for estimating the impact of changes in truck weight limits on bridge network costs. The report describes the research effort and the recommended methodology and illustrates application of the methodology. A software module for automation of the recommended methodology also is included. The material in this report will be of immediate interest to bridge engineers and planners.

The US National Cooperative Highway Research Program is developing, under project 12-51, a new methodology and decision support tool to be used by highway agencies to estimate the network-level bridge costs due to changes in truck weight limits. Combined with other tools for pavements and other affected assets, the system will help agencies to assess the full costs and benefits of allowing heavier trucks to use the highway network, or even restricting the network to lighter trucks. Using bridge inventory data, the methodology addresses four cost impact categories: steel fatigue consumption, deck fatigue consumption, overstress deficiencies, and higher new bridge design loads. For the first two categories, it quantifies the change in the number and magnitude range of loading cycles and assesses the probability of fatigue damage for cost estimation. For the latter two categories, it estimates the change in load rating requirement and design load requirement, respectively. Then the impact costs are estimated based on these requirements. To enable practical application of the methodology, a decision support tool is being developed in Microsoft Excel and Visual Basic. The system consists of a connected set of workbooks for preparing inventory data, describing scenarios of weight limit changes, and analyzing the impacts. Paul D. Thompson, et al.

Conference paper written by Gongkang Fu, Fred Moses, Dyab A. Khazem and Waseem Dekelbab presented at IABSE Conference: Cable-Supported Bridges - Challenging Technical Limits, Seoul, South Korea, 12-14 June 2001.

This report contains the findings of a study to determine load factors for use in evaluating the load capacity of existing bridges. The report includes recommended values for load factors and presents the methodology and data used to calibrate the factors to provide appropriate safety margins. The material in this report will be of immediate interest to bridge engineers involved in bridge load rating and to engineers interested in the development of load and resistance factor rating procedures.

The design of buildings, bridges, offshore platforms and other civil infrastructure systems is controlled by specifications whose purpose is to provide the engineering principles and procedures required for evaluating the safety of structural systems. The calibration of these codes and specifications is a continuous process necessary to maintain a safe national and global infrastructure system while keeping abreast of new developments in engineering principles, and data on new materials, and applied loads. The common approach to specification calibration is to use probabilistic tools to deal with the random behavior of materials and to account for the uncertainties associated with determining environmental and other load effects. This paper presents a procedure to calibrate load factors for a structural design specification based on cost and safety optimization. The procedure is illustrated by determining load factors that may be applicable for incorporation in a bridge design specification. Traditional code calibration procedures require a set of pre-determined safety levels that should be used as target values that each load combination case should satisfy. The procedure in this paper deduces the failure cost implied in present designs, and provides consistent safety levels for all load combination cases. For greater accuracy, load effects showing variance in time have been modeled by separating them into two random variables; time dependent r.v. (wind speed, vehicular loads, etc.) and time independent r.v. (modeling uncertainties). The total expected lifetime cost is used in the optimization to account for both initial construction cost and future equivalent failure costs.