Journal of Bridge Engineering

Conventional precast I-girder bridge systems are widely used in North America for short and medium spans, up to 45 m. Spliced standard precast I-girder segments made continuous by longitudinal posttensioning have been used for spans of up to 75 m, making them far more competitive with the steel plate girder and concrete box girder alternatives. The span and/or girder spacing capabilities of the standard I-sections of Nebraska University, Florida, American Association of State Highway and Transportation Officials-Precast/Prestressed Concrete Institute (AASHTO-PCI), and Canadian Prestressed Concrete Institute (CPCI) are determined for both spliced posttensioned and conventional pretensioned girder systems. This investigation shows that the Florida and Nebraska University I-sections are the most efficient girders for spliced posttensioned and conventional pretensioned bridges, respectively. Using a nonlinear optimization program, the optimum girder shape is found to be a bulb-tee for spliced posttensioned girders and a quasi-symmetrical I-section for conventional pretensioned girders. A new set of five I-sections that achieve a balanced efficiency for both spliced posttensioned and conventional pretensioned bridge girder systems are proposed. Three examples of alternative preliminary bridge designs using both the existing standard and the newly proposed I-sections illustrate the practicality of the presented results.

This paper compares the performance of four finite-element modeling techniques reported in the literature used in evaluating the wheel load distribution factors of steel girder bridges. A typical one-span, simply supported, two-lane, composite bridge superstructure was selected for this study. American Association of State Highway and Transportation Officials (AASHTO) HS20-44 design truck loads were positioned to produce maximum moments in the girder. Two finite-element programs, SAP90 and ICES-STRUDL, were used to perform the analysis along with their preand postprocessing capabilities. The results of these modeling techniques were compared with AASHTO wheel load distribution factors (in 1996 and in 1994) and published experimental results. The four finite-element modeling techniques yielded similar load distribution factors. Further parametric study, varying the span length and girder spacing, was conducted and the distribution factors obtained using two of the four finite-element modeling techniques correlated well with AASHTO's 1994 load and resistant factor design (LRFD) based formula, but not with AASHTO's 1996 simple formula (S/5.5). The AASHTO LRFD formula also correlated well with prototype experimental results reported in the literature by various researchers. This paper will assist bridge engineers in gaining confidence using the new AASHTO wheel load distribution factors and adopting a finite-element modeling technique when analyzing new bridges or evaluating the load carrying capacity of existing bridges.

The conventional analysis and design of highway bridges ignore the contribution of sidewalks and/or railings in a bridge deck when calculating the flexural strength of superstructures. The presence of sidewalks and railings or parapets acting integrally with the bridge deck have the effect of stiffening the outside girders and attracting more load while reducing the load effects in the interior girders. This paper presents the results of a parametric study showing the influence of typical sidewalks and railings on wheel load distribution as well as on the load-carrying capacity of highway bridges. A typical one-span, two-lane, simply supported, composite steel girder bridge was selected in order to investigate the influence of various parameters such as: span length, girder spacing, sidewalks, and railings. A total of 120 bridges were analyzed using three-dimensional finite-element analysis. American Association of State Highway and Transportation Officials (AASHTO) HS20 design trucks were positioned in both lanes to produce the maximum moments. The finite-element analysis results were also compared with AASHTO wheel load distribution factors. The AASHTO load and resistance factor design (LRFD) wheel load distribution formula correlated conservatively with the finite-element results and all were less than the typical empirical formula (S/5.5). The presence of sidewalks and railings were shown to increase the load-carrying capacity by as much as 30% if they were included in the strength evaluation of highway 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.

In this paper an analysis of the natural periods of long-span cable-stayed bridges is developed for both H-shaped and A-shaped towers. The analysis is carried out by using both an analytical and a numerical model of the bridge. The analytical developments refer to a continuous model of the bridge, which is founded on the assumption of diffused distribution of the stiffness of stays along the girder and of the prevailing truss behavior of the bridge. The dynamical equilibrium equations governing free vibrations are obtained considering the oscillations from the starting equilibrium configuration corresponding to the dead load action. Then, the tangent stiffness of the bridge corresponding to this configuration is considered. The analytical treatment of the dynamical equilibrium equations is based on the assumption that for long spans the bending stiffness of the girder is negligible compared to the global flexural stiffness of the bridge. Simple analytical results that are capable of capturing the main behavior characteristics of the bridge are obtained. Moreover, to give useful and suitable numerical comparisons, a discrete model of the bridge that accounts for the actual spacing of stays and the actual flexural stiffness contribution of the girder has also been developed. The numerical results obtained validate the simple analytical continuous model.

A rational approach for evaluating structural integrity of aged and deteriorated steel bridges—incorporating analytical, mechanical, and microstructural characterizations—is presented. Critical regions of built-up members, salvaged from decommissioned truss bridges, were identified using structural finite element analysis and the results guided the sampling process for microstructural analysis. A set of mechanical and metallurgical characterization procedures were designed to understand the mechanical history of critical sections of the structure. Observations made on the microstructure of a naturally stressed region revealed segregation of pearlite and MnS inclusions and flow of grains. The mechanical flow and microcracks observed in the stress-concentrated tensile region of the member indicate a high probability of eventual brittle failure of the material. Since stress-assisted aging seems to affect the microstructure, this may be used as a means of unobtrusive testing for susceptibility to failure. It follows that the microstructural analysis procedure, to detect stress-assisted aging and failure, may serve as a novel field-evaluation technique for aged steel structures.

The 29-span highway bridge across the Sava River at Brcko, Bosnia, dates to 1894 and suffered the destruction of four deck trusses during the recent war. Following the deployment of U.S. peace keepers, the swift reopening of this structure was required for their continued supply. The condition of the remaining spans was evaluated to be capable of the expected traffic. A restoration of the damaged spans to a comparable level was quickly designed in the field and was subsequently checked with finite-element analyses. The construction, which involved the extension of piers with mass concrete and the erection of panel bridging, was accomplished in 20 days by an international team of civilian and military engineers. Following a proof test, the bridge has performed reliably with traffic control and structural inspection safeguards. While conventional procedures and equipment were used on parts of the project, several unique problems arose and shortcomings in current military methods were uncovered. The project further serves as an interesting example for practicing engineers faced with the rapid restoration of an important lifeline.