Impact of Posted Load Limits on Highway Bridge Reliability

The effectiveness of posted load limits in reducing annual maximum live load effects, thus enhancing bridge reliability, is investigated for 12 and 40 m simple span highway bridges. Novel analytical expressions are derived for event gross vehicle weight (GVW) distributions that account for violation of posted load restrictions, and the corresponding annual maximum GVW distributions are presented. Annual reliability indices associated with load restrictions computed using typical bridge posting criteria and different compliance levels are compared to the target reliability index. For the case of perfect compliance, a posted load restriction can significantly reduce maximum annual live load effects and so enhance the reliability. Under imperfect compliance, however, a violation rate as low as 2.5% (i.e., one illegal truck in 40 ignores the posting) causes the mean value and variability of the annual maximum live load effect distribution to increase significantly, resulting in a significant loss in reliability. Thus, unless posted loads are strictly enforced, the effectiveness of enhancing existing bridge reliability with a posted load restriction is questionable.     

Effects of State Legal Loads on Bridge Rating Results Using the LRFR Procedure

The main objective of this research was to study the effects of different specified trucks on bridge rating with the load and resistance and factor rating (LRFR) procedure. Twelve specified trucks were selected for this study, which include one AASHTO design truck, three AASHTO legal trucks, and eight state legal trucks. These rating trucks were applied on 16 selected Tennessee Dept. of Transportation bridges to obtain the LRFR ratings. The selected bridges covered four commonly used bridge types, including prestressed I-beam bridges; prestressed box beam bridges; cast-in-place T-beam bridges; and steel I-beam bridges. The research results revealed that (1) LRFR AASHTO legal load ratings factors were enveloped by the LRFR HL-93 truck ratings factors, thereby confirming the validity of the LRFR tiered approach with regard to AASHTO legal loads; (2) the lighter state legal trucks were enveloped by the HL-93 loads, whereas the heavier state trucks with closer axle spacing typically resulted in load ratings that governed over the HL-93 loads; and (3) the bridges with both high average daily truck traffic and short spans were more likely to be governed by state legal load ratings instead of HL-93 load ratings.     

State-Specific LRFR Live Load Factors Using Weigh-in-Motion Data

The LRFR Manual, within commentary Article C6.4.4.2.3, contains provisions for development of site-specific live load factors. In Oregon, truck weigh-in-motion (WIM) data were used to develop live load factors for use on state-owned bridges. The factors were calibrated using the same statistical methods that were used in the original development of LRFR. This procedure maintains the nationally accepted structural reliability index for evaluation, even though the resulting state-specific live load factors were smaller than the national standard. This paper describes the jurisdictional and enforcement characteristics in the state, the modifications used to described the alongside truck population based on the unique truck permitting conditions in the state, the WIM data filtering, sorting, and quality control, as well as the calibration process, and the computed live load factors. Large WIM data sets from four sites were used in the calibration and included different truck volumes, seasonal and directional variations, and WIM data collection windows. Finally policy implementation for actual use of the factors and future provisions for maintenance of the factors are described.     

Development of Truck Weight Regulations Using Bridge Reliability Model

Historically, truck regulations have maintained controls on axle and gross weights with legal load formulas based on limiting allowable stresses in certain types of bridges. These stress limitations do not usually lead to consistent or defensible safety levels and also ignore the cost impact of the weight regulation on the national bridge network. This paper illustrates how new truck weight regulations can be developed to provide acceptable safety levels. Target safety levels are derived from existing AASHTO bridge evaluation and rating procedures applied to structures showing adequate performance levels. Reliability indices are used to relate the statistics of bridge load effects, based on either existing or proposed truck weight regulations, to the dynamic behavior and resistance variables of existing bridges. The sensitivity of the results to various assumptions and errors in the database is also analyzed. An accompanying paper reviews the consequences of adapting such a formula on the safety of existing bridges. The deterministic analysis as well as a reliability assessment are performed in the accompanying paper to review the consequences of adapting such regulations on the U.S. bridge network using the National Bridge Inventory files.     

Effect of Changing Truck Weight Regulations on U.S. Bridge Network

Historically, truck weight regulations have maintained controls on axle and gross weights with legal load formulas based on limiting allowable stresses in certain types of bridges. These stress limitations do not usually lead to consistent or defensible reliability levels and also ignore the impact of the weight regulation on the existing highway bridge network. This paper is the second part of a two-paper series. The companion paper by the first writer illustrated how new truck weight regulations can be developed to provide an acceptable reliability level. The target reliability level was derived from bridge structures designed to satisfy AASHTO standard design specifications that showed safe and adequate performance levels under current truck loading conditions. In this part of the two-paper series, a deterministic load capacity evaluation as well as a reliability assessment are performed to review the consequences of adapting such regulations on the existing U.S. bridge network. A sensitivity analysis shows how changes in the safety criteria used to develop the truck weight regulations would affect the existing bridge network. Detailed load capacity evaluations and reliability analyses also are performed on a representative sample of bridges to provide specific examples of expected changes in rating and safety levels if the proposed truck weight regulation is to be adopted.     

Predicting Truck Load Spectra under Weight Limit Changes and Its Application to Steel Bridge Fatigue Assessment

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.     

Impact of Commercial Vehicle Weight Change on Highway Bridge Infrastructure

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.     

Assessment of Load Transfer and Load Distribution in Bridges Utilizing FRP Panels

A primary means of demonstrating the feasibility and effectiveness of fiber-reinforced polymer (FRP) composite bridge materials is via in situ bridge load testing. For this study, the prescribed or assumed design factors for each of the study bridges were compared to those exhibited by the performance of the bridge. Specifically, the wheel load distribution factors and impact factors as defined by AASHTO were considered in order to assess the load transfer and distribution in structures utilizing FRP panels. The in situ testing configurations for the study bridges are outlined, including the truck and instrumentation placement to obtain the desired information. Furthermore, comparisons were drawn between the design values for deflection and those experienced by the structures during testing. It was found that although the deflections exhibited by the bridges were well within the design limits, further research is needed to be able to prescribe bridge design factors for FRP panels.     

Lateral-Torsional Buckling of Stepped Beams with Continuous Bracing

Continuous span multibeam steel bridges are common along the state and interstate highways. The top flange of the beams is typically braced against lateral movement by the deck slab, and in many bridges the cross section is stepped at discrete points along the span. Design equations for lateral–torsional buckling (LTB) resistance in the American Association of State Highway and Transportation Officials “Load and resistance factor design bridge design specifications” are for prismatic beams and ignore the lateral restraint provided by the bridge deck. A new design equation is proposed that can be applied to I-shaped stepped beams with continuous top flange lateral bracing. By including the effects of the change in cross section size and the continuous top flange bracing, the calculated LTB resistance is significantly increased. Critical bending moment values from the proposed equation are compared to values from finite element method buckling analyses. The new equation is sufficiently accurate for use in design and in the evaluation of existing bridges.     

Load and Resistance Factor Calibration For Wood Bridges

The paper presents the calibration procedure and background data for the development of design code provisions for wood bridges. The structural types considered include sawn lumber stringers, glued-laminated girders, and various wood deck types. Load and resistance parameters are treated as random variables, and therefore, the structural performance is measured in terms of the reliability index. The statistical parameters of dead load and live (traffic) load, are based on the results of previous studies. Material resistance is taken from the available test data, which includes consideration of the post-elastic response. The resistance of components and structural systems is based on the available experimental data and finite element analysis results. Statistical parameters of resistance are computed for deck and girder subsystems as well as individual components. The reliability analysis was performed for wood bridges designed according to the AASHTO Standard Specifications and a significant variation in reliability indices was observed. The recommended load and resistance factors are provided that result in consistent levels of reliability at the target levels.     

Design and Field Evaluation of Hybrid FRP/Reinforced Concrete Superstructure System

Fiber-reinforced polymers offer several advantages over conventional construction materials but are also faced with several challenges. These include increased first cost, relatively low stiffness, and a lack of field experience. To address these challenges and to advance the state of the art, a hybrid fiber reinforced polymer/reinforced concrete bridge was designed and constructed in Texas. The bridge design and field evaluation are unique in several respects. Design considerations, the bid process, and the results of intermittent live load evaluations that have been conducted over a period of approximately 2 years are presented. Recommendations for the design of future similar bridges are provided.     

Simplified AASHTO Load and Resistance Factor Design Girder Live Load Distribution in Illinois

Illinois began full transition to the American Association of State Highway and Transportation Officials load and resistance factor design (LRFD) bridge design specifications from the traditional load factor design code or standard specifications in 2002. To facilitate implementation of the new specification, engineers from the Illinois Department of Transportation undertook a series of investigations. The studies focused on interpretation of LRFD for the design of typical bridges in Illinois and the simplification of its procedures for determination of live load lane distributions to primary superstructure girders. Some important presented results from the conducted investigations are believed not only relevant to bridge design in Illinois, but to other states and jurisdictions which employ or will employ LRFD in the near future. The initial simplifications and interpretations focused on concrete deck-on-steel girder bridges and were subsequently expanded to include concrete deck-on-prestressed concrete girder structures. These types of structures comprise a large portion of Illinois’ inventory. Illinois Department of Transportation engineers continue to build on the studies described in the paper such that policies and procedures for other types of typical bridges can be formulated.     

Routing and Permitting Techniques of Overweight Vehicles

Overweight vehicles require permits to cross the highway bridges, which are designed for “design load vehicles” (prescribed in the national standards). A new, fast, and robust method is presented for the verification of bridges, which requires minimal input only: the axle loads, axle spacing, the bridge span(s), and the superstructure type. The bridge can be a single or a multispan girder, an arch bridge, a frame structure, or a box girder. The overweight vehicle may operate within regular traffic or it may cross the bridge at a given lane position while other traffic is prohibited on the bridge. The method is illustrated by numerical examples for deck-girder bridges and for a box girder.     

Performance Evaluation of Jointless Bridges

A recent trend in bridge design has been toward the elimination of joints and bearings in the bridge superstructure. Joints and bearings are expensive in both initial and maintenance costs and can get filled with debris, freeze up, and fail in their task to allow expansion and contraction of the superstructure. They are also a “weak link” that can allow deicing chemicals to seep down and corrode bearings and support components. Because the behavior is unknown and designs are cumbersome, jointless bridges are not widely used despite their enormous benefits. There are no standardized design procedures for these bridges, only a list of specifications and design recommendations are available. The objective of this research on jointless bridges conducted at the Constructed Facilities Center-West Virginia University is to study the behavior of jointless bridges supported on piles and spread footings and subjected to varying load conditions. In addition, time-dependent material properties have also been incorporated in this study. In this paper, the following items are presented: (1) synthesized analytical data that aids in understanding the performance under varying load combinations; (2) effects of primary versus secondary loads, boundary conditions, and system flexibility on induced stresses at various bridge locations; and (3) field testing and monitoring results of a jointless bridge in the state of West Virginia. Based on analytical and experimental results, conclusions are drawn in terms of design alternations.     

Transverse Posttensioning Design of Adjacent Precast Solid Multibeam Bridges

Adjacent precast, prestressed multibeam bridges have often been used for medium- and short-span bridges. However, there have been longitudinal cracking problems in shear keys and overlays commonly seen on some adjacent precast multibeam bridges during their service years. The fundamental reason for the problem is the poor transverse connection. Transverse posttensioning is important to the transverse connection design, although the posttensioning varies largely from state to state. Especially for adjacent precast solid multibeam bridges without diaphragms, there are no theoretical justifications for designing the transverse posttensioning. In this study, an approach based on the concept of shear friction, which is used for designing the transverse posttensioning in adjacent precast solid multibeam bridges, is presented. Furthermore, a newly rehabilitated bridge was load tested with the primary purpose of evaluating the effect of transverse posttensioning under truck load. Also, the calibration of a numerical model was conducted. At last, suggestions about design and construction of shear keys, with reference to the experience in other states, are presented for the practice in the state of Maryland.     

Equivalent Wheel Load Approach for Slender Cable-Stayed Bridge Fatigue Assessment under Traffic and Wind: Feasibility Study

In the current AASHTO LRFD specifications, the fatigue design considers only one design truck per bridge with 15% dynamic allowance. While this empirical approach may be practical for regular short and medium span bridges, it may not be rational for long-span bridges (e.g., span length >152.4?m or 500?ft) that may carry many heavy trucks simultaneously. Some existent studies suggested that fatigue may not control the design for many small and medium bridges. However, little research on the fatigue performance of long-span bridges subjected to both wind and traffic has been reported and if fatigue could become a dominant issue for such a long-span bridge design is still not clear. Regardless if the current fatigue design specifications are sufficient or not, a real understanding of the traffic effects on bridge performance including fatigue is desirable since the one truck per bridge for fatigue design does not represent the actual traffic condition. As the first step toward the study of fatigue performance of long-span cable-stayed bridges under both busy traffic and wind, the equivalent dynamic wheel load approach is proposed in the current study to simplify the analysis procedure. Based on full interaction analyses of a single-vehicle–bridge–wind system, the dynamic wheel load of the vehicle acting on the bridge can be obtained for a given vehicle type, wind, and driving condition. As a result, the dimension of the coupled equations is independent of the number of vehicles, through which the analyses can be significantly simplified. Such simplification is the key step toward the future fatigue analysis of long-span bridges under a combined action of wind and actual traffic conditions.     

Performance of Bridge Materials by Structural Deficiency Analysis

The diagnostic concept of the structural deficiency of bridges is an essential engineering and management consideration with implications of performance. The structural deficiency analysis reflects the constructed system performance at the serviceability limit states. This paper analyzes trends in the structural deficiency of bridge inventory on the basis of material kind. A multiple-criteria diagnostic approach defines measures for condition, durability, and longevity performances and determines the overall equivalent performance. Thus, the structural performance levels reflect the structural reliability and vulnerability indices for bridge serviceability. The application of the approach analyzes the raw database of the entire bridge inventory in the United States. This comprehensive operational experience provides a national network-level comparative basis. The comparison suggests a relative need for improvements in one or more areas, such as design details or maintenance level, to increase the desirability of bridge construction materials. The results support more objective bridge management and decision making on distribution of funds, updating of policies, perfection of practices, and trade-off analyses for design, construction, maintenance, and replacement.     

Behavior of Reinforced Concrete Bridge Decks on Skewed Steel Superstructure under Truck Wheel Loads

This paper focuses on the behavior of skewed concrete bridge decks on steel superstructure subjected to truck wheel loads. It was initiated to meet the need for investigating the role of truck loads in observed skewed deck cracking, which may interest bridge owners and engineers. Finite-element analysis was performed for typical skewed concrete decks, verified using in?situ deck strain measurement during load testing of a bridge skewed at 49.1°. The analysis results show that service truck loads induce low strains/stresses in the decks, unlikely to initiate concrete cracking alone. Nevertheless, repeated truck wheel load application may cause cracks to become wider, longer, and more visible. The local effect of wheel load significantly contributes to the total strain/stress response, and the global effect may be negligible or significant, depending on the location. The current design approach estimates the local effect but ignores the global effect. It therefore does not model the situation satisfactorily. In addition, total strain/stress effects due to truck load increase slightly because of skew angle.     

Model Validation for Bridge-Road-Vehicle Dynamic Interaction System

The dynamic response of highway bridges subjected to moving truckloads has been observed to be dependent on (1) dynamic characteristics of the bridge; (2) truck configuration, speed, and lane position on the bridge; and (3) road surface roughness profile of the bridge and its approach. Historically, truckloads were measured to determine the load spectra for girder bridges. However, truckload measurements are either made for a short period of time [for example, weigh-in-motion (WIM) data] or are statistically biased (for example, weigh stations) and cost prohibitive. The objective of this paper is to present results of a 3D computer-based model for the simulation of multiple trucks on girder bridges. The model is based on the grillage approach and is applied to four steel girder bridges tested under normal truck traffic. Actual truckload data collected using a discrete bridge WIM system are used in the model. The data include axle loads, truck gross weight, axle configuration, and statistical data on multiple presence (side by side or following). The results are presented as a function of the static and dynamic stresses in each girder and compared with code provisions for dynamic load factor. The study provides an alternate method for the development of live-load models for bridge design and evaluation.     

Vehicular Overloads: Load Model, Bridge Safety, and Permit Checking

All states in the United States issue special permits for nondivisible and∕or divisible truck overloads exceeding the weight limit of the highway jurisdiction. This causes stress levels higher than those induced by normal truck traffic. The rationality of such overstress levels has not been documented. This paper addresses several aspects of this issue. It presents (1) a method to develop live load models including overload trucks; (2) associated reliability models for assessing structural safety of highway bridges; and (3) proposed permit-load factors for overload checking in the load and resistance factor format. It shows that the proposed overload checking procedure leads to relatively uniform reliability of bridge structures. A sensitivity analysis is also presented here to assure that possible variations of the input data used to prescribe the proposed load factors will not adversely affect bridge safety. The proposed procedure is intended to be used by engineers responsible for checking overload permits. It may be included in evaluation specifications for highway bridges.