Load Distribution Factors for Skewed Composite Steel I-Girder Bridge

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.;Recently mandated North American bridge code specifications include provisions considering angle of skew for slab-on-girder bridges applicable within certain ranges of the design parameters. These ranges are often found too narrow and thus frequently exceeded in routine design check. When one of the design parameter exceeds its corresponding limit, refined analysis is suggested. Unfortunately many bridge design engineers are not fully aware or adequately skillful with these refined analysis techniques. In addition, the analysis equations in current design code specifications are developed using the regression of grillage analysis results that is not always recommended for skewed bridges. Further, these design guidelines are developed by ignoring the contribution of diaphragms in a skewed bridge structure, which may not be realistic and leads to inaccurate prediction of load distribution for a skewed bridge structure.;In order to address the shortcomings in the current code specifications, this research was initiated to address these concerns by better understanding the skew bridge behavior and developing design guidelines for rational and accurate assessment of load distribution factors for composite skewed slab-on steel I-girder bridges. For this purpose, a parametric study was conducted using three-dimensional finite element modeling of a composite bridge structure under dead and CHBDC live loads for ultimate, serviceability and fatigue limit states by considering different design parameters including: skew angles, girder stiffness and cross-frame layout, span length, girder spacing, number of girders, and number of design lanes. Based on the results obtained from a parametric study, a set of empirical expressions were developed for the girder moment and shear distribution factors for rational prediction of the girder load distribution. Further, the load distribution factors for girder moment and shear obtained by FEA for both straight and skewed bridge was correlated with the proposed empirical equations and the CHBDC design guidelines. The results showed that the proposed equations for girder moment and shear distribution factors were in good agreement with the FEA results for both straight and skewed bridge configuration. However for straight bridge, the CHBDC equations proved to be ineffective to capture the behavior of most of the straight slab-on-girder bridge geometries. For skewed bridges, the CHBDC equations gave conservative response for certain bridge configurations and for others it produced highly underestimated response, yielding to an unsafe design. Finally, the applicability of the proposed equations for moment and shear distribution factors developed for simply supported straight and skewed slab-on-girder bridge geometry under dead and live load conditions to the multi-span continuous bridge structures was also investigated. The results showed that both the proposed equations and the CHBDC simplified equations proved to be unsafe for some cases, and for other situations resulted in conservative estimates. Based on the limited set of data selected for this study, a new set of design equations for a skewed continuous bridge were proposed, adequately conforming the results obtained from finite element analysis. Design guidelines for bridge engineers were proposed to treat a skewed bridge as an equivalent straight bridge. The findings of this design-oriented dissertation would enable bridge engineers to design composite skewed slab-on steel I-girder bridges more reliably and economically.