Live load distribution factors for glued-laminated timber bridges

Over the past years the United States Department of Agriculture - Forest Products Laboratory and the Federal Highway Administration have supported several research programs. This thesis is a result of a study sponsored by the Forest Products Laboratory, with the objective of determining how truckloads are distributed to the structural members of glued-laminated timber bridges. Glued-laminated timber girder bridges with glued-laminated timber deck panels and longitudinal glued-laminated timber deck bridges were the focus of this paper. Currently, the American Association of State Highway and Transportation Officials LRFD Bridge Design Specification provides live load distribution provisions for glued-laminated timber bridges. This paper investigates the existing live load distribution provisions for glued-laminated timber bridges utilizing field test data collected by Iowa State University researchers, laboratory test data, and analytical finite element modeling. From this data, simplified live load distribution equations were developed following methods established for other bridge types where needed to improve the accuracy of determining how truckloads are distributed to structural members of glued-laminated timber bridges

Simplified live-load moment distribution factors for simple span slab on I-girder bridges

The primary goal of this effort is to identify and assess various methods of computing live load distribution factors and to use the results of laboratory and field tests to compare these methods. It is further a goal of this work to use these methods to perform a parametric study over a wide range of typical slab on steel I-girder bridges to assess the accuracy of both the AASHTO Standard and AASHTO LRFD specifications and to propose an empirical model that correlates better with the analytical results within the range of parameters that are to be studied.;These studies include: (1) a verification study into the FEA techniques used in modeling bridge geometry, (2) selection of procedure of calculating load distribution factors from FEA data, (3) a verification study of the selected procedure, (4) a parametric study to assess the influence of bridge parameters on the contribution to load distribution factors, (5) the development, using regression techniques, of a new equation for live load distribution factors, and (6) a comparison of proposed distribution factors against FEA, AASHTO LRFD, and AASHTO Standard Specifications. (Abstract shortened by UMI.)

Load Distribution Factors for Skewed Composite Steel I-Girder Bridges

The concept of load distribution factors have been used in bridge design for many decades as a simplified method to estimate load effects on bridge members. It enables bridge engineers to consider the transverse and longitudinal effects of truck wheel loads as two separate phenomena and thus simplifying the analysis and design of new bridges as well as for the evaluation of the load carrying capacity of existing bridges. Existing bridge design codes do not provide sufficient guidance to bridge engineers regarding the accurate assessment of load distribution factors for skew composite bridges. Thus leads to an extremely conservative design in some cases and to unsafe design in others, since these factors do not represent the actual behavior of the bridge structure. The presence of skew angle makes the analysis and design of composite slab-on-girder bridges much more complex in comparison to straight bridges. Over the past decade, several authors have drawn attention toward the steel I-girder twisting placed over highly skewed supports. These rotations are larger at the obtuse corners and difficult to predict due to the uneven load distribution across the bridge superstructure. In addition to girder twisting, skewed bridges can also lead to increased lateral flange bending stresses as well as increased shear and end reactions at girder obtuse corners that subsequently results in the reduction of girder shear and end reactions, and even possibly undesirable uplift in girders at the acute corners of the bridge

LIVE LOAD DISTRIBUTION FACTORS FOR HORIZONTALLY CURVED CONCRETE BOX GIRDER BRIDGES

Live load distribution factors are used to determine the live-load moment for bridge girder design when a two dimensional analysis is conducted. A simple, analysis of bridge superstructures are considered to determine live-load factors that can be used to analyze different types of bridges. The distribution of the live load factors distributes the effect of loads transversely across the width of the bridge superstructure by proportioning the design lanes to individual girders through the distribution factors. This research study consists of the determination of live load distribution factors (LLDFs) in both interior and exterior girders for horizontally curved concrete box girder bridges that have central angles, with one span exceeding 34 degrees. This study has been done based on real geometry of bridges designed by a company for different locations. The goal of using real geometry is to achieve more realistic, accurate, and practical results. Also, in this study, 3-D modeling analyses for different span lengths (80, 90, 100, 115, 120, and 140 ft) have been first conducted for straight bridges, and then the results compared with AASHTO LRFD, 2012 equations. The point of starting with straight bridges analyses is to get an indication and conception about the LLDF obtained from AASHTO LRFD formulas, 2012 to those obtained from finite element analyses for this type of bridge (Concrete Box Girder). After that, the analyses have been done for curved bridges having central angles with one span exceeding 34 degrees. Theses analyses conducted for various span lengths that had already been used for straight bridges (80, 90, 100, 115, 120, and 140 ft) with different central angles (5º, 38º, 45º, 50º, 55º, and 60º). The results of modeling and analyses for straight bridges indicate that the current AASHTO LRFD formulas for box-girder bridges provide a conservative estimate of the design bending moment. For curved bridges, it was observed from a refined analysis that the distribution factor increases as the central angle increases and the current AASHTO LRFD formula is applicable until a central angle of 38º which is a little out of the LRFD`s limits

Evaluation, Comparison, and Design of Two Experimental Bridges in Tennessee

This thesis describes the design and evaluates the adequacy of the moment connection of an experimental two-span highway bridge designed by the Tennessee Department of Transportation. The Massman Drive bridge is an experimental design that unifies the construction economy of simple span bridges and the structural economy of continuous span bridges. The experimental connection, consisting of cover plates and kicker wedge plates, is used to connect the two adjoining girders over the center pier. As a result, the bridge is designed to function as a continuous bridge during the deck pour and behave compositely with the reinforced concrete deck under the live load. After completing a moment comparison analysis, it is concluded that the Massman Drive bridge indeed acts as continuous over the pier as it was designed. This thesis also compares the measured lateral wheel load distribution factors for two experimental two-span highway bridges designed by the Tennessee Department of Transportation. The measured load distribution factors were then compared to distribution factors from several methods commonly in use such as AASHTO 1996, AASHTO 2001 LRFD, and Henry’s Method. Results from American Association of State Highway and Transportation Officials (AASHTO) 1996 produced load distribution factors that were deemed to be conservative. Interior girder load distribution factors from both the DuPont Access and Massman Drive bridges compared well to the AASHTO 2001 Load and Resistance Factor Design (LRFD) specifications. Exterior girder distribution factors compared well with Henry’s Method, while the values from AASHTO were consistently high. Also, the factors were consistent between the Massman Drive and DuPont Access bridges

Live load distribution factors for exterior girders in steel I-girder bridges

In lieu of a complex three-dimensional analysis, live load distribution factors (also referred to as girder distribution factors or wheel load distribution factors) are commonly employed by bridge engineers to simplify the analysis of a bridge system. Specifically, instead of looking at the bridge system as a whole, these factors allow for a designer or analyst to consider bridge girders individually by determining the maximum number of wheels (or lanes) that may act on a given girder.;The development of the relatively new distribution factors for beam-and-slab bridges incorporated in the current AASHTO LRFD Specifications are primarily the result of NCHRP Report 12-26. This report, however, does not take into account the different live load responses of interior and exterior girders. Numerous research studies have shown that the distribution of live load in a bridge system differs between interior girders and exterior girders.;The current AASHTO specifications employ three methods to determine the distribution to exterior girders: a statical based procedure called the lever rule, a rigid body rotation procedure called special analysis, and an empirical equation that calculates an adjustment factor that is applied to the interior girder distribution factor. While several studies have shown that for many cases these methods do not accurately predict the load in the exterior girder little work is available to actually evaluate the distribution of live load to exterior girders.;Therefore, the goal of this research is to develop new expressions for the distribution of live load to the exterior girders of steel slab-on-beam bridges. To accomplish this, a commercial finite element software package (Abaqus) is employed. The finite element modeling technique used in this project is first compared with physical data from the August 2002 field test of the Missouri Bridge A6101. Once validated, this modeling technique is then used in a sensitivity study to determine the effect of key parameters on exterior girder live load distribution. Subsequently, a parametric matrix employing these key parameters is developed and analyzed. Data correlation techniques are then used to relate the parameters which were varied throughout the course of this study to develop empirical equations for live load distribution factors

A New Proposal for Live Load Distribution Factors of Bridges with Transverse Beams

Many bridge superstructures use transverse beams as load carrying components. In these systems, usually the transverse beams are connected to the main longitudinal girders or trusses on the two sides of the bridge. Such systems are commonly used in plate girder, box girder, cable-stayed and truss bridges.  The live load distribution factor (LLDF) for bridge superstructures with transverse beams in AASHTO-LRFD bridge design specification has remained unchanged for decades and is prescribed as a function of the distance between the transverse beams. However, for slab-beam superstructures in which longitudinal beams at close spacing carry the loads to the substructure, the LLDFs have gone through many changes throughout the years and in their current forms depend on many parameters such as concrete slab thickness, beam span, longitudinal beam stiffness as well as the distance between the longitudinal beams. This study investigates the factors affecting the LLDF for transverse beams and intends to obtain new equations similar to AASHTO’s longitudinal beam equations. For this purpose, 3D finite element models of different sample bridges were developed and critical parameters affecting the LLDF were identified and varied. Accordingly, the LLDFs for moment and shear forces of transverse beams were obtained through regression analyses. The proposed equations have less than 3.1% of average error for the cases considered

Assessment of live load distribution characteristics of press-brake-formed tub girder superstructures

The scope of this thesis project was to refine the development of live load distribution factors for tub girders. This was done in three stages. First, experimental data was gathered to assess live load distribution on the Amish Sawmill Bridge located in Fairbank, Iowa. Then, finite element analysis models were developed to benchmark against experimental data. Finally, a series of parametric studies were performed to explore the distribution factors of steel tub girders under various design conditions and to generate more accurate live load distribution factors. Results drawn from this research project demonstrate that press-brake-formed steel tub girders exhibit consistent performance and are a practical option in short span bridge construction. In addition, it was found that the current AASHTO LRFD Bridge Design Specifications can overestimate distribution factors for interior girders and fails to estimate distribution factors for exterior girders depending on girder spacing and length of bridge