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This study aims to develop Load and Resistance Factor Design (LRFD) provisions for movable bridges’ mechanical components. It requires the formulation of the limit state functions, determi...

Twenty-five years ago, a bridge live-load model was developed along with the calibration of the AASHTO LRFD Specifications. The statistical parameters of truck loads were based on the Onta...

The objective of this study is to summarize the load and resistance criteria for highway bridge plank decks, and to estimate the reliability of plank decks designed by the AASHTO Code. Both transverse and longitudinal planks for a variety of typical stringer spacings and plank sizes are considered. Truck traffic load data are based on the model used to calibrate the 1994 AASHTO LRFD Code. However, for plank decks, wheel load rather than whole vehicle weight is most important, and these statistics are developed for this study. For wood planks, dead load and dynamic load are not significant. The limit state considered is flexural strength, and resistance statistics are presented for wood planks in terms of modulus of rupture. Special flat-wise use data are presented to account for section aspect ratio as well as edge of load application. The reliability analysis is carried out using the procedure developed for calibration of AASHTO LRFD. Reliability indices for both the AASHTO Standard and AASHTO LRFD Code are presented for plank decks. The results indicate that there are considerable differences in plank reliability indices. Causes of inconsistencies in safety are identified.

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.

Secondary elements such as barriers, sidewalks, and diaphragms may affect the distribution of live load to bridge girders. The objective of this study is to evaluate their effect on girder reliability if these elements are designed to be sufficiently attached to the bridge so as not to detach under traffic live loads. Simple-span, two-lane structures are considered, with composite steel girders supporting a reinforced concrete deck. Several representative structures are selected, with various configurations of barriers, sidewalks, and diaphragms. Bridge analysis is performed using a finite-element procedure. Load and resistance parameters are treated as random variables. Random variables considered are composite girder flexural strength, secondary element stiffness, load magnitude (dead load and truck traffic live load), and live load position. It was found that typical combinations of secondary elements have a varying influence on girder reliability, depending on secondary element stiffness and bridge geometry. Suggestions are presented that can account for secondary elements and that provide a uniform level of reliability to bridge girders.

This research investigates the effects of barriers, sidewalks, and diaphragms (secondary elements) on bridge structure ultimate capacity and load distribution. Simple-span, two-lane highway girder bridges with composite steel and prestressed concrete girders are considered. The finite-element method is used for structural analysis. For the elastic range, typical secondary elements can reduce girder distribution factors (GDF) between 10 and 40%, depending on stiffness and bridge geometry. For the inelastic response, steel is modeled using von Mises yield criterion and isotropic (work) hardening. Concrete is modeled with a softening curve in compression with the ability to crack in tension. At ultimate capacity, typical secondary elements can reduce GDF an additional 5 to 20%, and bridge system ultimate capacity can be increased from 1.1 to 2.2 times that of the base bridge without secondary elements, depending on bridge geometry and secondary-element dimensions.

This paper deals with distribution of truck load on girder bridges. Previous analytical studies based on finite-element method indicated that AASHTO code-specified girder distribution factors (GDFs) are inaccurate. In particular, GDFs appear to be conservative for longer spans and larger girder spacing, but too permissive for short spans and girder spacings. Therefore, a field testing program was carried out including about 20 steel girder bridges with spans up to 45 m. For each tested structure, GDFs were determined by measuring strains in the girders under heavy trucks. Test trucks were 11-axle vehicles, loaded to the legal limit in Michigan (over 650 kN). The strains were recorded for a single truck and for two trucks side-by-side. The tests were repeated for crawling speed and normal traffic speed for the location. In all tested bridges, the GDFs determined from the field measurements are lower than code-specified values. In addition, the considered bridges were analyzed using a commercial finite-elem...

Proof load tests were carried out on three deteriorated steel girder bridges with simple supports. Selected bridges are more than 60 years old, and their span lengths range from 11.0 to 15.5 m. Two military tanks were used to load the bridges up to a predetermined proof load level, which depended on the maximum allowable legal load. The midspan moment was increased in several steps by gradually moving the tanks closer to the midspan. Each tank weighed over 530 kN. Stresses and deflections in steel girders were measured at selected locations. Small stresses and a linear response were considered as the indication of a safety reserve. All bridges were able to sustain the applied proof load. Observed stresses and displacements were considerably smaller compared with those from analytical models. The composite action between concrete slab and steel girders was observed even at the maximum load level, although no shear connectors were provided. Nonstructural members such as parapets, railings, and concrete faca...

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.