Journal of Bridge Engineering

This paper discusses the calculations and presents the results of a detailed inelastic design of a three-span continuous plate-girder bridge, using suggested new inelastic design procedures. The characteristics of the calculations and the resulting design are compared to those of an elastic design of the same bridge by current AASHTO LRFD procedures. Various advantages, limitations, and implications of the recommended procedures are observed based on this study.

Steel curved I-girder bridge systems may be more susceptible to instability during construction than bridges constructed of straight I-girders. The primary goal of this research is to study the behavior of the steel superstructure of a curved steel I-girder bridge system during all phases of construction and to ascertain whether the actual stresses in the bridge are represented well by linear elastic analysis software developed for this project and typical of that used for design. Sixty vibrating wire strain gauges were applied to a two-span, four-girder bridge, and elevation measurements were taken by a surveyor's level. The resulting stresses and deflections were compared to computed results for the full construction sequence of the bridge as well as for live loading from up to nine 50-kip trucks. The analyses correlated well with the field measurements, especially for the primary flexural stresses. Stresses due to lateral bending and restraint of warping induced in the girders and the stresses in the cross frames were more erratic but generally showed reasonable correlation. In addition, it is shown that, for the magnitude of live load applied to the bridge, analyses in which composite behavior is assumed in the negative moment region yield better correlation than analyses in which just the bare steel girders are used (no shear connectors were used on the bridge in the negative moment region). It is concluded that the curved girder analysis software captures the general behavior well for these types of curved girder bridge systems at or below the service load level, and that the stresses in these bridges may be relatively low if their design is controlled largely by stiffness.

In 1992 the Federal Highway Administration, along with 13 states, began a project to study the behavior of horizontally curved steel bridges. The project is referred to as the Curved Steel Bridge Research Project and involves studying the behavior of curved members through theoretical, analytical, and experimental research. In this paper, detailed finite-element models representing a curved three-girder test frame, which is planned under the Curved Steel Bridge Research Project experimental phase, are used to evaluate the effects of curvature on the bending strength of curved I-girders. Linear-elastic static, buckling, and combined material and geometric nonlinear analyses are conducted using models that represent the test frame and component test specimens that will be inserted into it. The results are compared to various predictor equations developed from analytical work by the writers and to related work by other researchers, including Japanese research that is not readily available in the United States. It is demonstrated that the predictor equations developed by the writers are accurate in representing the behavior of the system. Limitations and needed improvements are described as well.

External posttensioning tendons in concrete box girder bridges are usually anchored in very massive end diaphragms. Several incidences of distress of such diaphragms have been reported in the literature. This paper presents the results of an analytical and experimental research program on design and behavior of end diaphragms when used for the anchorage of external tendons. Finite-element analysis results, strut-and-tie models, and results of a physical test series of half-scale specimens are included. The strut-and-tie model approach is shown to be an effective method to describe the flow of forces in the structure and for design. Ultimate strength predictions based on simple strut-and-tie models were safe but quite conservative. Refined strut-and-tie models based on finite-element analysis results were required to arrive at satisfactory reinforcement arrangements for crack control.

Elastomeric bearing pads are more appropriate than mechanical bearings to support bridge loads for many reasons. Two of the most significant reasons are cost and maintenance. Failure modes for bearing pads include crushing, delamination, and slippage. The most notable of these failure modes is slippage or “walking” out of place. The writers' objective in this paper is to establish a methodology to determine why some neoprene bearings are slipping under bridge girders. The methods used to investigate this phenomenon are discussed. A general survey of state DOTs was performed. The results are presented to show how some states have remedied this problem. During the investigation period, numerous bridge engineers and researchers gave their opinions as to why bearings slip. These theories are presented. This investigation has discovered that bearing slippage occurs on a daily basis.

Precast AASHTO concrete bridge I-beams are often supported at the ends by elastomeric bearing pads. The bearing pad-bridge beam interface defines support boundary conditions that may affect the performance of the bridge. In this study, finite-element modeling was used to validate AASHTO bearing stiffness specifications. Stiffness characteristics of the Florida DOT bearing pads were theoretically determined under varying elastomer shear modulus values. Finite-element models of AASHTO Types III and V beams were subjected to simulated static truckloads. Vertical and horizontal spring elements simulating new bearing pads were incorporated at the ends of the beam models. A full section of a bridge on U.S. Route 27 was also modeled, and the results were compared with field tests. In general, the restraint effects of the bearing pads are beneficial to the performance of the beams and the bridge. The beneficial effect, however, is small for new bearing pads and more pronounced under a drastic increase in bearing stiffness due to aging and colder temperatures. Such a dramatic increase in bearing stiffness must be justified if the beneficial elements are to be utilized. Current Florida DOT bearing pads are serving the main purpose of their application, which is to provide minimum horizontal restraint force to the beams while allowing horizontal movement.

The importance and structural characteristics of the Vasco da Gama Bridge led to the development of some special studies to achieve a more detailed analysis of its structural behavior. The studies reported in this paper concern the following subjects: (1) Analysis of the behavior of the deck under a “design ship fire”; (2) study of an elastoplastic damping system to control the seismic response; and (3) analysis of the aerodynamic behavior of the bridge (computational fluid dynamics analysis of the deck and dynamic analyses of the structure to assess the effects of wind gusts). These studies gave technical support to some important decisions that were undertaken, such as the installation of two vertical baffles under the deck to improve the aerodynamic stability and eight hysteretic damping devices in the connections between the deck and the towers to reduce the seismic displacements.

This paper presents a computer-aided approach for the design of integral-abutment bridges. An analysis procedure and a simplified structure model are proposed for the design of integral-abutment bridges considering their actual behavior and load distribution among their various components. A computer program, for the analysis of integral-abutment bridges, has been developed using the proposed analysis procedure and structure model. The program is capable of analyzing an integral-abutment bridge for each construction stage and carrying the effects of applied loads on the structure members from a previous construction stage to the next. The proposed analysis methods and structure models are compared with the conventional analysis method and structure model currently used by many structural engineers for the design of integral-abutment bridges. The benefits of using the proposed analysis method and simplified structure model for the design of integral-abutment bridges are discussed. It was concluded that it may be possible to obtain more sound and economical designs for integral-abutment bridges using the proposed analysis method and structure model.

This paper presents an investigation on the effects of construction loads and vibrations on typical newly poured reinforced concrete bridge decks. The main objective of this paper was to perform dynamic finite-element analyses of concrete bridge decks under construction loads and vibrations. Two types of bridges were modeled by effectively simulating the loads encountered during construction. Separate analyses were performed to simulate one truck as well as two side-by-side trucks. Additional analyses were performed to simulate the loads during construction and pouring sequence. The loads imposed by concrete mixer trucks, including the full concrete load and the load imposed by a trailer unit carrying the pump, were taken into consideration during the modeling of the bridges. The results obtained consisted of vibration modes and frequencies for each structure in addition to top and bottom stresses in each principal direction. A separate analysis was performed that took into account the sequence of pour to determine the effect of moving loads from adjacent traffic and construction loads on fresh concrete. The time history and dynamic analysis indicated that the sequence of pour has a significant effect on the deformation of the concrete at early ages. As a result, the first three mode shapes obtained by the finite-element analysis will be used to optimize sensor locations for future dynamic field testing.

This paper presents a proposed rating model that incorporates the fatigue damaging effects of overloads. This is achieved by introducing a “fatigue index” in the rating equation. The index, which appears in the form of a correction factor in the rating equation, is intended as a means to reduce the rating value computed for a bridge in cases where the damage from overloads is expected to be significant. The use of this index by itself does not impose any upper limit on the total number of overloads that may annually be permitted on a bridge. However, because the use of the index will result in a lower rating value than those from current equations, it is expected that a certain number of overloads will ultimately be disallowed. This provides for a built-in mechanism that will eventually result in lower fatigue damage to highway bridges resulting from overloads. In developing the model, typical records of overloads were acquired and used in bridge structural analyses to determine the damaging effect of overloads. The study on five bridges showed that fatigue damage from overloads can use up about 3.5% of fatigue life over a 25-year period if the overload occurrences remain at the current level. The use of the proposed index is in line with this amount of fatigue damage. This percentage is rather low and may not, in fact, be critical for most bridges over a 25-year period. However for older bridges, this percentage of fatigue life consumption may become important. Many such bridges were designed for a lower gross truck weight than what is used today for bridge design. Some of these bridges are located along feeder ramps and must carry loads in excess of 356 kN (80 kip) in an overload situation. For this group of bridges, it may be important to consider imposing a limit on the amount of fatigue damage resulting from frequent overloads. However, additional studies on a larger pool of bridges will be needed to establish a baseline for a maximum percentage fatigue life that can be used for overload permits.