Girder Distribution Factors Research Articles

Live Load Distribution in Girder Bridges Subject to Oversized Trucks

A significant challenge facing motor carriers and engineers in this nation is the limitation of vehicle size and weight based on pavement and bridge capacity. However, the current demands of society and industry occasionally require a truck to carry a load that exceeds the size and weight of the legal limit. In these cases, engineering analysis is required before a permit is issued to ensure the safety of the structures and roadways on the vehicle's route. A truck with a wheel gauge larger than the standard 1.83 m (6 ft) gauge requires additional engineering effort because the wheel load girder distribution factors (GDFs) established by AASHTO cannot be used to accurately estimate the live load in the girders. In this study, the finite-element method is used to develop modification factors for the AASHTO flexure and shear GDFs to account for oversized trucks. The results of the analysis showed that the use of the proposed modification factors with the specification-based GDFs can help increase the allowable loads on slab-on-girder bridges.

Response of Prestressed Concrete I-Girder Bridges to Live Load

Design and evaluation of prestressed concrete I-girder bridges is in large part dependent on the transverse load distribution characteristics and the dynamic load amplification, as well as service level, live load, and tensile stresses induced in the girders. This study presents the results of field tests conducted on three prestressed concrete I-girder bridges to obtain dynamic load allowance statistics, girder distribution factors (GDF), and service level stress statistics. The field-based data are also compared to approximate and numerical model results. Bridge response was measured at each girder for the passage of test trucks and normal truck traffic. The dynamic amplification is observed to be a strong function of peak static stress and a weak function of vehicle speed and is independent of span length, number of axles, and configuration. GDFs for one- and two-lanes are less than code specified GDFs. Results from the numerical grillage models agree closely with experimentally derived results for tra...

Performance of Five-Span Steel Bridge with Fiber-Reinforced Polymer Composite Deck Panels

To better understand the performance of bridges with fiber-reinforced polymer (FRP) composite decks, the short-term and long-term responses of a 207-m, five-span bridge retrofitted with four different FRP panel systems were monitored through controlled truck load and modal tests at various stages and through long-term monitoring of key load-transfer mechanisms and panel responses. The overall aspects of the panel systems, connection details, construction techniques, and experimental program are presented followed by presentation of the measured responses. Key design issues (impact factors, girder distribution factors, level of composite action, and overall structural stiffness) for FRP and reinforced concrete decks are evaluated and compared against published design provisions. Performance of panel-to-panel connections and panel-to-girder connections is also discussed.

Live-Load Girder Distribution Factors for Bridges Subjected to Wide Trucks

An important problem facing engineers and officials in the United States is the constraint imposed on transportation due to limitations of bridges. These limitations typically constrain vehicles to minimum heights and widths, to minimum and maximum lengths, and to a maximum allowable weight. However, with current demands of society and industry, there are times when a truck must carry a load that exceeds the size and weight of the legal limit. In this situation, the trucking company requests from the state departments of transportation an overload permit. For a truck with a wheel gauge larger than 1.8 m (6 ft), the process of issuing a permit for an overload truck requires a tremendous amount of engineering efforts. This is because the wheel load girder distribution factors (GDFs) in the design specifications cannot be used to estimate the live-load effect in the girders. In some cases, an expensive and time-consuming finite element analysis may be needed to check the safety of the structure. In this study, the finite element method is used to develop a modification factor for the GDF in AASHTO’s LRFD Bridge Design Specifications to account for oversized trucks with a wheel gauge larger than 1.8 m. To develop this factor, nine bridges were considered with various numbers of girders, span lengths, girder spacings, and deck slab thicknesses. The results indicated that use of the proposed modification factor with the GDF in the design specifications can help increase the allowable load on slab-on-girder bridges.

Control of Live Load on Bridges

Application of field testing for an efficient evaluation and control of live-load effects on bridges is described. A system is considered that involves monitoring of various parameters, including vehicle weight, dynamic load component, and load effects (moment, shear force, stress, strain) in bridge components, and verification of the minimum load-carrying capacity of the bridge. Therefore, an important part of the study is development of a procedure for measuring live-load spectra on bridges. Truck weight, including gross vehicle weight, axle loads, and spacing, is measured to determine the statistical parameters of the actual live load. Strain and stress are measured in various components of girder bridges to determine component-specific load. Minimum load-carrying capacity is verified by proof load tests. It has been confirmed that live-load effects are strongly site specific and component specific. The measured strains were relatively low and considerably lower than predicted by analysis. Dynamic load factor decreases with increasing static load effect. For fully loaded trucks, it is lower than the code-specified value. Girder distribution factors observed in the tests are also lower than the values specified by the design code. The proof load test results indicated that the structural response is linear with the absolute value of measured strain considerably lower than expected. Field tests confirmed that the tested bridges are adequate to carry normal truck traffic.

Verification of Girder-Distribution Factors for Short-Span Steel Girder Bridges by Field Testing

The procedure and results of field tests performed on five simplysupported, short-span steel girder bridges to verify girder-distribution factors (GDFs) are presented. Prior analytical studies have shown that, in most cases, currently used GDFs are too conservative. However, these studies also showed that, for short spans and short girder spacing, the GDFs can be too permissive. Therefore, this paper focuses on the experimental evaluation of girder-distribution factors for short-span steel girder bridges. The research work involved formulating the testing procedure, selecting the structures, installing the equipment, determining the measurements, and processing the results. Strains and deflections were measured and filtered to calculate GDFs. The results were compared with the distribution factors specified by the AASHTO Standard Specifications (1996) and the AASHTO LRFD Code (1998). The measured GDFs were found to be lower than the AASHTO values in all cases.

Live Load Distribution in Integral Composite Steel Bridges

In recent years, there has been an increased interest in integral bridges for highway applications due to their inherent economy, serviceability, and strength. This study deals with the live load distribution of integral composite steel bridges near the abutments. Girder distribution factors for shear and bending moment in interior as well as in exterior beams are determined. A comparison between the response of integral bridges and their jointed counterpart is also included. Single and continuous spans are considered in the investigation. The applied loading is composed of several side-by-side HS20 trucks. Linearly elastic finite element analysis is used to analyze the three-dimensional bridge systems. The results of the study indicated that shear in the interior beams and the corresponding girder distribution factor are lower in integral bridges than in simply supported bridges. As the pile length to fixity increases, the shear and moment in the beams near the integral abutment decrease. Further, as the length of the wingwalls in integral bridges increases, the shear and moment in the interior beams increase.

Load Distribution and Impact Factors for I-Girder Bridges

This paper presents the procedure and results of field tests that were performed on two simply supported steel I-girder bridges to assess girder distribution and impact factors. The measurements were performed under normal truck traffic. Strain data were taken from bottom flanges of girders in the middle of a span. Additional strain data were obtained under passes of a control truck with known weight and configuration. A computerized data acquisition technique enabled selective recording of the significant blocks of the strain data under normal traffic. Strains were measured for two consecutive days on each bridge. Measured data consist of strain blocks from approximately 900 trucks. The strain records were filtered with a lowpass digital filter to remove the dynamic components and to obtain an equivalent static strain. The data were further processed to obtain statistical parameters (mean and standard deviation) of the girder distribution and impact factors. The results were compared with the values calculated according to American Association of State Highway and Transportation (AASHTO) methods. Measured girder distribution factors are lower than AASHTO values. Measured impact factors are well below AASHTO values.

Approximate Analysis of Irregular Slab-on-Girder Bridges

A simple, approximate method for determining the live-load girder distribution factors of irregular slab on I-beam bridge layouts is presented. The method can also be used for the live-load analysis of bridges subjected to one truck, as in the case of fatigue, or two (or more) different truck configurations, as in the case of permit loading. The method takes into account the longitudinal as well as transverse effects of the truck loads. It is based on isolating strips of the deck slab in the transverse direction directly under the wheel loads and treating them as beams on elastic supports. The stiffness of the supports at the strip locations is computed based on the geometric properties of the beams and location of the trucks in the longitudinal direction. The accuracy of the method for shear and flexure of I-beams in simple and continuous span bridges is verified using a detailed three-dimensional finite-element analysis.

Live load model for highway bridges

Load models are developed for highway bridges. The models are based on the available statistical data on dead load, truck loads and dynamic loads. The paper deals mostly with the static live load. The model is derived from truck surveys, weight-in-motion measurements and other observations. Simple span moments, shears and negative moments are calculated for various spans. Extreme 75 year loads are determined by extrapolation. The important parameters also include girder distribution factors and multiple presence (more than one truck on the bridge). Multiple presence is considered in lane and side-by-side with various degrees of correlation between truck weights. The maximum load is calculated by simulations. The developed live model served as a basis for the development of new design provisions in the United States (LRFD AASHTO) and Canada (Ontario Highway Bridge Design Code).