Ricardo Dorado

Skewed steel I-girder integral abutment bridges (IABs) are increasingly used due to their structural efficiency and elimination of deck joints. Thermally induced behavior of such bridge systems, however, remains insufficiently understood. This report documents the live load and thermal behavior of skewed steel I-girder bridges, which are representative of current Illinois bridge practice, through long-term field monitoring and numerical parametric studies. Three-dimensional finite element models developed in ABAQUS showed good agreement with field-measured thermal response and highlighted the importance of realistic representation of support restraint and soil-structure interaction effects. Live load parametric studies demonstrated that skew angle, support restraint, cross-frame configuration, and bottom flange unrestrained length strongly influence flange lateral bending stresses, with significant stresses (up to 28 MPa [4 ksi] on interior girders) observed even below current AASHTO and IDOT skew thresholds. Long-term field monitoring further demonstrated that substantial thermally induced stresses develop in skewed steel I-girder bridges, particularly in IAB configurations, where lateral bending stress ranges up to 30 MPa (4.3 ksi) were observed near the abutment region on an exterior girder. Thermal parametric studies showed that thermally induced lateral bending stresses increase substantially with skew angle (from approximately 13–21 MPa [2–3 ksi] at low skew angles to 38 MPa [5.5 ksi] at high skew angles) and integral support restraint, with the largest stresses concentrated near abutment and pier regions. Overall, the results suggest that stresses due to thermal effects may warrant explicit consideration in the design and evaluation of skewed steel I-girder IABs.

Two skewed (around 45 degrees) two-span continuous bridges with stub abutments or integral abutments were monitored in the field during construction and after the bridges were in service. The goal was to establish a comprehensive understanding of load distribution in skewed steel I-girder bridges and to potentially refine their analysis and design procedures. Key girder cross-sections and cross-frames were instrumented with strain gauges, and global bridge movements were captured with displacement transducers and tiltmeters. Temperature was recorded simultaneously with structural response measurements. Numerical studies using 3D finite element analysis (FEA) were based on a field-validated simulation framework. The 3D FEA was conducted on the instrumented bridges to evaluate effects of bridge geometry, especially skew and abutment type, on skewed steel I-girder bridge superstructure response. Load distribution during deck placement and under live load was investigated, and standard design guidelines regarding girder strong-axis bending and flange lateral bending were evaluated. For a skewed bridge with a staggered cross-frame layout along its length under concrete dead load, standard design recommendations from the AASHTO “LRFD Bridge Design Specifications” regarding skew-related lateral bending stress might not be appropriate for both interior and exterior girders. Under isolated truck live loads, exterior girders are prone to larger strong-axis bending stress than interior girders when directly loaded, while interior girders are more critical regarding flange lateral bending stress than exterior girders. Distribution of lateral bending stress on a skewed bridge is dependent on the live load position, and limiting the distance between the first intermediate cross-frame and bridge ends is important to avoid lateral bending stress concentration near bridge obtuse corners. The dynamic load allowance of 33% used in design was found to be mostly conservative, except at a few monitored locations near obtuse corners of the integral abutment bridge.

Highly skewed steel I-girder bridges are used commonly across the US, especially in congested areas, despite complications in their analysis, design, and construction. Systematic investigation of skewed steel I-girder superstructure response, load distribution, and deformation through field monitoring and numerical simulation is needed, including analysis of bridges under construction, short-term live load, and long-term thermal and traffic loads. AASHTO’s load and resistance factor design specification allows for line girder analysis with defined live load distribution factors (considering effects of skew between 30° and 60°) when designing non-curved steel bridges. When considering lateral behavior of skewed bridges, AASHTO provides suggestions for design values of flange lateral bending stress in addition to line girder analysis when bridge skew exceeds 20° for certain cross-frame layouts. This skew limit is 45° in Illinois Department of Transportation’s “Bridge Design Manual.” When designing bridges for lateral bending during deck placement, AASHTO suggests equations to conservatively estimate flange lateral bending moments caused by eccentric loading from an overhang acting on an exterior girder top flange, in the absence of a refined analysis. Efficiency of the simplified design approaches and skew consideration in current standard design practice needs to be further evaluated through field monitoring and companion numerical studies. The research project partially described in this report was initiated in Illinois to investigate demands, load distribution, and static and dynamic responses of composite steel skewed I-girder bridge superstructures during construction and after bridges are in service. Two skewed steel I-girder bridges (41° and 45°), with stub and integral abutments, respectively, were studied through field monitoring and numerical simulations. This report documents the initial phases of the research: a survey to understand practices used and challenges faced by state transportation agencies when designing and constructing skewed steel I-girder bridges, information about and field instrumentation of the two monitored bridges, methods for 3D finite element analysis, and preliminary analysis conducted to guide field instrumentation planning of the bridges.

Integral abutment bridges (IABs) exhibit complex and evolving structural behavior due to their interaction with the foundation and surrounding soil. Long-term effects, such as backfill soil ratcheting and cumulative in-plan superstructure rotation, can lead to unexpected structural responses that are not fully understood. This study analyzes more than 3 years of superstructure displacement, abutment rotation (tilt), and temperature data, collected at 0.5 Hz, from an in-service two-span skewed (45°) steel I-girder bridge with integral abutments and staggered-X-cross-frames. The study advances the understanding of IAB behavior, including behavioral anomalies, which could assist in the interpretation of stress deviations reported in previous studies. Findings show that the monitored IAB presents changing behavior, as well as an accumulation of transverse displacement and abutment tilt over time. Analysis indicates that boundary condition representation in finite element models should incorporate the flexibility provided by soil and pile deformation to accurately reflect field behavior. It is further observed that abutment tilt data displays different trends over short and long periods. Discrepancies between these trends underscore the complexity of IAB behavior under varying temperature conditions. Deep learning techniques, particularly long short-term memory models, assisted in identifying these behavioral patterns. This application demonstrates their potential for detecting subtle deviations in bridge response.

As part of a long-term monitoring project, a two-span continuous steel I-girder bridge (skewed 41° with seat-type abutments) was instrumented and evaluated in the field during construction and after the bridge was in service. This paper discusses data from when the first stage (of a three-stage sequence) was constructed and initially opened to traffic. Girders and cross-frames were instrumented with strain gauges using a data acquisition system with a high sampling frequency (up to 20 Hz). Data collection began just before deck placement, and a series of live load tests was then conducted on the completed Stage I (half of the bridge) using a loaded truck. Three-dimensional finite element analyses were carried out to provide enhanced understanding of bridge behavior. Good agreement was observed between the numerical simulation results and the field monitoring data for both deck placement and live load testing. A slight inconsistency between numerical simulation and field measurement data at one girder section and an adjacent cross-frame indicated an unexpected local site condition, which was confirmed through a targeted field inspection. Live load distribution factors used during design conservatively overestimate bridge girder strong-axis bending response under live load (by around 50%). Maximum flange lateral bending stresses of 6.3 MPa (0.9 kips per square inch [ksi]) and 2.5 MPa (0.4 ksi) were observed during deck placement and truck tests, respectively. In addition, out-of-plane response of 10.6 MPa (1.5 ksi) was observed at girder web plates under concrete dead load.

El objetivo del estudio fue diseñar un programa computacional que automatice la verificación de corte por punzonamiento en losas planas, tanto macizas como aligeradas, para columnas rectangulares, circulares, u otra geometría. Además, se propuso un procedimiento para calcular esfuerzos de punzonamiento en columnas situadas en la posición de “esquina invertida”. El código fuente fue desarrollado y compilado en el entorno de programación Visual Studio, utilizando el lenguaje de programación VisualBasic. Para el análisis de la columna en esquina invertida, procedimientos similares a los descritos por las normas internacionales de concreto reforzado vigentes fueron utilizados. Como resultado del proyecto se obtuvo el programa computacional en formato ejecutable para el entorno de Windows, acompañado de la formulación y descripción del procedimiento para realizar la verificación de esfuerzos por punzonamiento en la posición mencionada. El desarrollo de dicha formulación queda sujeto a un análisis más detallado, respaldado por modelos matemáticos y físicos.