There are many issues and challenges to deal with when designing a curved I-girder bridge. These challenges primarily deal with the many performance stages that curved I-girder bridges have such as the erection, construction, and in-service stages. When design engineers assess the stability of a bridge system, they typically evaluate the system in its final configuration with all cross frames attached and the hardened concrete deck placed. The evaluation of girder stability during erection and early stages of construction stages is difficult because of the limited presence of bracing in the system. Due to a lack of readily available analytical tools, many contractors do not conduct detailed analytical evaluations of the bridge behavior during early stages of the construction when stability is often critical. Instead, many contractors use rules of thumb and experience to ensure stability during erection. Erection and construction practices typically vary among contractors and consistent erection methods are a rarity. Although some rules of thumb may be quite conservative, others are much less so. Therefore, coming up with design guidelines based on parametric studies rather than rules of thumb are desirable to help allow the contractor and the designer to work together to prevent issues that may occur due to the lack of communication between the two professions. Lastly, many challenges arise due to the complex geometry of curved I-girders. To prevent excessive rotation in erected girders, three points of vertical support are often provided. Two of these points usually consist of permanent supports in the form of bridge piers or abutments. The third point of support may consist of a temporary support in the form of a shore tower or holding crane. Cases where a holding crane may be satisfactory over a shore tower are also not well understood. To improve the understanding of lifting practices and temporary support requirements, parametric studies were conducted using the finite element program ANSYS. Field data consisting of displacement, stress, and girder rotations gathered from two tests were used to validate both the linear and geometric non-linear three-dimensional FEA models. Upon validation, the finite element model was used to conduct linear and geometric non-linear analyses to determine critical factors in curved I-girder bridges during construction. Specifically, serviceability limit states were studied for the lifting of curved girders. For partially constructed states, parametric studies were conducted to determine optimal locations to place temporary supports as well as to investigate stability differences between using a shore tower and a holding crane. Recommendations are presented to provide guidance for the lifting of curved I-girders as well as to maximize stability of partially constructed bridges. / text

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AASHTO LRFD requires the use of longitudinal stiffeners in plate girder webs when the web slenderness D/tw is greater than 150. This practice is intended to limit the lateral flexing of the web plate during construction and at service conditions. AASHTO accounts for an increase in the web bend buckling resistance due to the presence of a longitudinal stiffener. However, when the theoretical bend buckling capacity of the stiffened web is exceeded under strength load conditions, the Specifications do not consider any contribution from the longitudinal stiffener to the girder resistance. That is, the AASHTO LRFD web bend buckling strength reduction factor Rb applied in these cases is based on an idealization of the web neglecting the longitudinal stiffener. This deficiency can have significant impact on girder resistance in regions of negative flexure. This research is aimed at evaluating the improvements that may be achieved by fully considering the contribution of web longitudinal stiffeners to the girder flexural resistance. Based on refined FE test simulations, this research establishes that minimum size longitudinal stiffeners, per current AASHTO LRFD requirements, contribute significantly to the post buckling flexural resistance of plate girders, and can bring as much as a 60% increase in the flexural strength of the girder. A simple cross-section Rb model is proposed that can be used to calculate the girder flexural resistance at the yield limit state. This model is developed based on test simulations of straight homogenous girders subjected to pure bending, and is tested extensively and validated for hybrid girders and other limit states. It is found that there is a substantial deviation between the AISC/AASHTO LTB resistance equations and common FE test simulations. Research is conducted to determine the appropriate parameters to use in FE test simulations. Recommended parameters are identified that provide a best fit to the mean of experimental data. Based on FE simulations on unstiffened girders using these recommended parameters, a modified LTB resistance equation is proposed. This equation, used in conjunction with the proposed Rb model also provides an improved handling of combined web buckling and LTB of longitudinally stiffened plate girders. It is observed that the noncompact web slenderness limit in the Specifications, which is an approximation based on nearly rigid edge conditions for the buckling of the web plate in flexure is optimistic for certain cross-sections with narrow flanges. This research establishes that the degree of restraint at the edges of the web depend largely on the relative areas of the adjoining flanges and the area of the web. An improved equation for the noncompact web slenderness limit is proposed which leads to a better understanding and representation of the behavior of these types of members.