Bridge Structures

Construction stages of a cable-stayed bridge are characterized by a sequence in which geometric configuration, restraints and consequently stress and strain patterns vary many times till the final arrangement is achieved. When construction of concrete bridges is made by cantilever method the influence of time-dependent phenomena becomes significant. In this study an evaluation of stay stressing procedures is given by taking into account creep and shrinkage in cantilever construction of concrete cable-stayed bridges. A methodology of stay stressing is proposed with the final target of reaching the desired geometric configuration. Comparison with classical analyses performed by backward methodology is reported. A case-study is presented and discussed by performing analyses with different international creep models. Practical suggestions are given to designers, in order to minimize time-dependent effects on deck and stay internal forces and to reach an optimal final configuration.

Full-depth longitudinal cracks have been developed in many empirically designed bridge decks. Such cracks run along the entire length of the bridge over the top flange edges. Star City Bridge is among those bridges that developed longitudinal deck cracking. The bridge was instrumented with 750 sensors during its second phase of construction in November 2003 and the longitudinal cracks were visible on the deck surface in 2005. Therefore, additional twelve sensors were installed across the cracks in June 2007 to continuously monitor their opening and closure as well as the differential settlement across the cracks. The collected data indicate that some cracks have continued to open. In particular the crack along the first interior girder at the edge bay developed a permanent opening of 0.25 mm and a permanent settlement of 0.08 mm. The opening of the longitudinal cracks was correlated to the lateral bending of the steel girders, which adversely affect their flexural capacity as well as the axial stresses in the transverse steel rebar and diaphragm members. Additionally, the presence of the longitudinal cracks induce shear stress in the transverse steel rebar due to the passage of traffic loading that would affect their fatigue life.

This paper presents the results of a parametric study evaluating the effect of skew angle on the wheel load distribution in steel girder highway bridges. The finite element method was used to investigate the effect of various parameters such as the span length, girder spacing, and skew angle, on simply supported, one-span, two-lane, three-lane and four-lane steel girder bridges. A total of 270 bridge cases were analyzed and subjected to AASHTO HS20 design trucks positioned on each bridge to produce maximum bending in the interior steel girders. A combination of five typical span lengths, three girder spacing, and six skew angles were used in evaluating bending moments in skewed steel girder bridges. The finite element results were used to calculate the maximum bending moment in steel girders due to the various skew angles and compared to the reference straight bridges, and then compared to the reduction factors used in AASHTO LRFD Bridge Design Specifications. The finite element results showed the reduction in bending moment for all skewed bridges up to 30 degrees can be neglected and such bridges can be designed as straight bridges. These results are consistent with the AASHTO Standard Specifications and the LRFD procedure by not specifying any reduction factor for bridges with skew angles up to 30 degrees. For highly skewed bridges and span length less than 80 ft (24 m), the finite element results showed a reduction in moment ranging between 10% and 20% for skew angles up to 40 degrees, and between 20% and 35% for skew angle up to 50 degrees. For practical application, a conservative reduction in girder bending moment of 15% is suggested for skew angles between 30 and 40 degrees and another conservative reduction in girder bending moment of 25% for bridges with skew angles between 40 and 50 degrees. Furthermore, the AASHTO LRFD reduction factors, ranging between 15% and 5%, were more conservative when compared with the finite element results for short-span bridges with high skew angles.

Bridge owners, especially municipalities, are becoming overwhelmed with increasing maintenance costs and decreasing maintenance budgets. Limited available funds require that each maintenance dollar be efficiently allocated to the most critical bridge structure. Objectively determining the most critical bridge structure often requires nondestructive evaluation and testing, which can be costly in terms of equipment, traffic delay and personnel, all of which impact a limited transportation budget. The installation of traditional sensors, which typically need to be in physical contact with the bridge, can require special equipment for access to key bridge elements as well as wiring for power supply and data acquisition. Digital image correlation (DIC) can be used as an alternative to traditional bridge response measurement instruments such as strain gages or linear variable differential transformers, commonly referred to as LVDTs. DIC is an optical measurement technique that has the ability to capture displacement data in both two and three dimensions through high-resolution digital photography. Because it is a non-contact, nondestructive means of measuring bridge responses, it is an attractive choice for rapid testing of in-service structures. Researchers at the University of New Hampshire have conducted a series of laboratory and field experiments for verification of DIC application for civil structures in which the DIC system and LVDTs recorded displacement data simultaneously. Upon comparison, the two methods showed nearly identical results, with the DIC within 0.03 mm of LVDTs. With a high confidence in DIC, the field-collected deflection data was used to verify design and analytical structural models of two tested bridges; a newly constructed three span continuous steel girder bridge and a short concrete slab culvert with a fiber reinforced polymer reinforcement retrofit. The collected DIC measurements were used for model verification and evaluation of the innovative retrofit program, respectively. Results from both field tests are presented in this paper. The ability to capture a bridge's behavior with DIC and calibrate a structural model with the collected data provides bridge designers and managers with an easy-to-collect objective measure of bridge performance.