Larry A. Fahnestock

This report evaluates the potential role of camera-equipped unmanned aerial systems (UAS) as well as other forms of reality-mapping solutions in supporting bridge inspection practices for the Illinois Department of Transportation (IDOT). Traditional bridge inspections can require substantial labor, specialized access equipment, traffic control, and inspector exposure to hazards such as working at height, near live traffic, and over waterways. Camera-equipped UAS offer opportunities to improve visual access, reduce reliance on access-intensive equipment, minimize traffic disruption, and enhance inspection documentation. This report synthesizes federal and state case studies, regulatory requirements, emerging research, cost considerations, and IDOT bridge inventory characteristics to evaluate where UAS-based inspection can provide the greatest practical value. The findings indicate that camera-equipped UAS are most beneficial for routine visual inspection, rapid assessment, bridge deck screening, and documentation of difficult-to-access components, but are not a universal replacement for conventional methods, particularly for inspections requiring hands-on evaluation. The report also reviews emerging technologies, including autonomous navigation, computer vision-based 3D reconstruction with images and videos, artificial intelligence-assisted damage detection, automated reporting, and complementary sensing tools such as 360-degree cameras. A preliminary benefit-cost analysis and inventory-level prioritization framework are presented to identify bridge scenarios where camera-equipped UAS deployment may offer the greatest benefits in safety, accessibility, efficiency, cost-effectiveness, and inspection quality. Overall, the report recommends that reality-mapping solutions be integrated into a broader digital inspection program for IDOT, supported by targeted pilot studies, updated regulatory and cost assumptions, quality assurance protocols, and careful selection of high-value bridge inspection scenarios.

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.

Bridge strikes caused by over-height vehicles frequently damage steel girders and can reduce their load-carrying capacity, requiring rapid and reliable evaluation to ensure structural safety. Current evaluation practices rely on manual measurements that often require traffic disruptions and simplified assessment approaches that do not fully capture the effects of damage. In addition, finite element simulations are time-consuming and computationally intensive to develop for practical rapid evaluation. This study developed an integrated, data-driven framework for the inspection and assessment of damaged steel bridge girders. A semiautomated procedure was developed to process laser-scan point cloud data and extract displacement profiles along the girder span. Machine learning models were trained and validated using a dataset generated from finite element simulations informed by field measurements of actual bridge strike incidents to predict the residual capacity of damaged girders. Explainable artificial intelligence techniques were incorporated to interpret model predictions and identify key parameters influencing structural performance. Results demonstrated the framework’s capabilities in effectively capturing girder displacements, and for accurately and efficiently estimating the residual capacity, to support post-strike bridge evaluation. Measured girder deformations obtained from laser scanning can be used as inputs to the machine learning models for quantifying a girder’s capacity. The predicted capacity can then be incorporated into bridge rating software to compute rating factors, enabling rapid decisions on traffic restrictions and load posting. Additionally, the interpretability results can guide engineers in identifying critical design parameters and prioritizing repair actions.

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.