Journal of Bridge Engineering

This paper presents an evaluation of flexural live-load distribution factors for a series of three-span prestressed concrete girder bridges. The response of one bridge, measured during a static live-load test, was used to evaluate the reliability of a finite-element model scheme. Twenty-four variations of this model were then used to evaluate the procedures for computing flexural live-load distribution factors that are embodied in three bridge design codes. The finite-element models were also used to investigate the effects that lifts, intermediate diaphragms, end diaphragms, continuity, skew angle, and load type have on distribution factors. For geometries similar to those considered in the development of the American Association of State Highway and Transportation Officials Load and Resistance Factor Design Specifications, the distribution factors computed with the finite-element models were within 6% of the code values. However, for the geometry of the bridge that was tested, the discrepancy was 28%. Lifts, end diaphragms, skew angle, and load type significantly decreased the distribution factors, while continuity and intermediate diaphragms had the least effect. If the bridge had been designed using the distribution factors calculated with the finite-element model rather than the code values, the required concrete release strength could have been reduced by 6.9 MPa (1,000 psi) or the live load could have been increased by 39%.

Existing restrainer design methods recommended by the American Association of State Highway and Transportation Officials (AASHTO) and the California Department of Transportation (Caltrans) to prevent unseating in simply supported bridges subjected to strong earthquakes were evaluated. Three new methods with different levels of complexity were developed: W/2, modified Caltrans, and the equivalent linear static design for restrainers. The adequacy of different design methods was assessed using a nonlinear response history analysis computer program. Effects of different earthquakes, substructure stiffness, bearing strength, seat width, and skew angle were studied. It was found that in bridges with reinforced concrete pier caps the inherently wide supports help prevent unseating, and simple methods such the AASHTO and W/2 method are satisfactory. In bridges with steel pier caps that provide narrow bearings the modified Caltrans method is recommended. The relatively involved equivalent linear static design for restrainers method is a more rational design procedure and leads to considerably fewer restrainers than others.

In current AASHTO LRFD bridge design specifications, the nominal flexural strength of I-girders made from steel with a yield stress >345 MPa (>50 ksi) is limited to the yield moment rather than the plastic moment and inelastic design procedures are not permitted. With the recent development of high performance steel (HPS) for highway bridges, the need for these restrictions should be revisited. This paper focuses on I-girders made from HPS-100W steel. Two I-girders were designed with HPS-100W steel according to the AASHTO LRFD specifications, neglecting current restrictions related to the use of high strength steels. The I-girders were tested to failure under three-point loading, which simulated the condition of negative flexure at the pier of a continuous-span bridge. The flexural strength and ductility of the HPS-100W I-girders are compared with the strength and ductility anticipated by the AASHTO LRFD specifications for conventional steel I-girders. In addition, the results of relevant previous tests of conventional steel I-girders are summarized and compared with the HPS-100W I-girder test results.

On the basis of a statically admissible moment redistribution in rectangular and skew slab bridges with free edges parallel to the bridge axis, an alternative positioning of the prestressing tendons concentrated at the outer edge regions is studied, with regard to the fulfillment of the respective strength conditions. The moment redistribution is adapted to the available bending resistances across the slab and is accomplished not only through the inherent statically indeterminate plate response, but also through the use of an additional fictitious loading achieving a self-equilibrating state-of-stress condition. It is shown that with the selection of appropriate values for some nondimensionalized design parameters that are introduced herein, a substantial economy in the use of prestressing steel can be attained as compared to the uniform arrangement of prestressing tendons across the whole slab, resulting in a clear savings in total material cost. The partial prestressing that has been implicitly adopted shows an acceptable performance at the service level.

For the construction of composite steel-concrete decks of cable-stayed bridges, methods of erection and analysis have to be applied that, upon completion of the deck, accurately yield the prescribed dead load configuration of the deck regarding geometry and forces. During deck erection, no unwanted bending moments should be locked into the composite sections when the concrete slab is connected to the steel substructure. Such locked-in moments would bend the deck, cause concrete creep that is difficult to predict, and introduce the risk of deviations from the desired deck alignment and the corresponding distribution of forces. This paper presents a simple and practical method of erection and erection analysis for composite decks with precast concrete slabs. A two-step tensioning procedure of the stay cables is proposed that minimizes the effects of unwanted locked-in moments, making it easy to predict the geometry of the erection stages and to yield the desired dead load configuration of the deck. The method was successfully applied for the erection of the Ting Kau bridge in Hong Kong, a cable-stayed bridge of 1,200 m in length having a composite deck with a precast deck slab.

A one-fourth scale outrigger beam-column frame with as-built details was tested to assess its performance under reversed cyclic loading and to develop a retrofit procedure suitable for moderate seismic regions. The ductility of the as-built frame was limited due to pullout of poorly embedded positive moment reinforcement in the joint and shear inadequacy in the joint and beam. Strut-and-tie truss idealizations were developed to aid in predicting the failure mechanism and failure loads. Sectional and nonlinear finite-element analyses were used to predict the performance of the as-built outrigger frames. The retrofit procedure involved fiber-reinforced concrete sleeving of the beam and the joint, together with column jacketing, to enable plastic hinging and energy dissipation to occur in the column. This retrofit solution increased the strength, ductility, and energy absorption of the system. The provision of high-performance fiber-reinforced concrete in the beam sleeve was very effective in controlling the cracks and hence would improve durability.

The objective of this study is to investigate the strength and ductility aspects of reinforced concrete (RC) beams strengthened with an externally bonded carbon fiber reinforced polymer (CFRP) laminate and to examine how such retrofitting affects strength, deflection, curvature, and energy, as exemplified by the area under the load-deflection curve of the strengthened composite beam. Three series of tests on eleven RC beams were carried out and their ultimate load capacities and ductilities evaluated. The variables in the experimental program were longitudinal steel ratio, volume of internal stirrups, and the location and configuration of external anchorages. The results show that both deflection and energy absorption are drastically reduced when beams are strengthened with bonded CFRP plates without external anchorages. Suitably designed and positioned external anchorages allow much of this lost ductility to be regained; however, even then the ductility of the strengthened beam cannot be restored to its original level. It is shown that definitions of ductility based on deflection and energy are able to give a good and rational representation of the physical aspects of ductility of RC beams strengthened with bonded CFRP laminates with or without external anchorages. The results show that the effects on ductility arising from strengthening existing RC beams with CFRP laminates cannot be ignored, even if it is not clear at this stage how to apply the concept of ductility indices and ductility ratios developed in this paper in design practice.

Full-scale ultimate load tests were conducted in March 1995 on the bearings of the Strawberry Park Bridge, a two-span, 45 m (148 ft) long ballasted railway bridge typical of many such bridges around the country. The bridge, located in Los Angeles, was scheduled for removal in spring 1995, a fact which allowed for the performance of failure push tests on the bridge, with the pushing loads applied at the bridge abutments. The main objective of this experiment was to identify the extent to which the rails, ties, and ballast and the plate containing the ballast (ballast pan or tie plate) help to strengthen the bridge during a seismic event. The rails, ballast, and ballast pan were completely removed at one end, while the other end was left in its as-built condition. The bridge was then pushed laterally to failure at both ends. Comparison between the failure loads at each end indicated that the as-built end was 45% stronger than the free end.