Many box girder bridges are constructed with doubly unsymmetrical precast beam units. The aim of this paper is to present special considerations for the optimum arrangement of pretensioned strands in a simply supported precast pretensioned beam with a doubly unsymmetrical section. The goal of the optimum arrangement is to minimize the distortion (lateral sway phenomenon) caused by the doubly unsymmetrical characteristic of the beam section at transfer. The study shows that the distortion of the unsymmetrical section is minimized when the resultant stresses at transfer are constant over all of the top flange and over all of the bottom flange of the precast beam for most of the sections within the beam length. In this case, the neutral axes of these sections will be horizontal with respect to the beam section. This approach is verified with the help of two different finite element models. In the first model, the beam is modelled as one‐dimensional space frame elements; in the second model, the beam is modelled using three‐dimensional solid elements. A practical example of a box girder bridge made from doubly unsymmetrical precast pretensioned beams in the APM (Automated People Mover) elevated bridge project in Saudi Arabia is also presented.
The automated people mover (APM) bridge project constructed at Princess Nora University (PNU) IntroductionPrecast prestressed beams have been increasingly used in recent decades for the construction of simple and multispan bridges as well as in buildings such as multi-storey car parks, factories, etc. The main advantages of precast beams can be summarized as the considerable savings in costs, labour and construction time. Moreover, the precast beam alternative offers a solution in congested areas where formwork is unacceptable due to traffic requirements. However, in the general case, the precast multigirder deck solution has the disadvantages of exhibiting inferior aesthetics and lower torsion stiffness capacity compared with box girder bridge solutions. In the light of this, the optimum solution -combining good aesthetics and high torsion stiffness capacity -is the use of precast box girder bridges. In most cases the precast box girders, with their considerable weight, require special lifting equipment, transport and handling which may not be readily available on the construction site. Therefore, splitting the box girder into two or more precast beams and then using stitching concrete can be the answer for a simplified box girder construction. The precast beams required for the box girder construction can be unsymmetrical in one or both directions. Further, precast segmental techniques can represent another solution for the construction of box girder bridges.Many publications are found in the literature dealing with precast prestressed beams. Rodriguez et al. [1] presented the design of a precast concrete light rail system for JFK international airport using precast prestressed concrete segments with symmetrical sections. Yee [2] discussed the structural and economic benefits of precast/ prestressed concrete construction. He, too, was dealing with precast symmetrical sections. Tadros and Sun [3] discuss the behaviour and advantages of unsymmetrical precast beams used for box girder construction but do not emphasize the effect of the beam's self-weight and the pretensioning strand arrangement on the distortion that may occur at transfer due to the doubly unsymmetrical section behaviour.This article presents the design and construction of the major elevated automated people mover (APM) bridge at Princess Nora University (PNU) in Saudi Arabia, which is based on precast concrete techniques. Fig. 1 shows the layout of the APM project, which in terms of geometry can be divided into straight parts, curves and bifurcations, whereas from the structural point of view the bridge project combines simply supported systems for straight and bifurcation parts, with a maximum span of 36 m (pier to pier), continuous systems for curved parts, with a maximum span of 36 m, and straight parts spanning 42.5 m. The total length of the current elevated APM is 12 km; it has 400 piers founded on shallow footings. A box girder deck with 2.25 m total depth was adopted for all parts of the project.
The double‐track elevated viaducts of the Doha Metro Green Line, which is currently under construction, are 2.7‐km long and consist of in situ and precast segmental simply supported spans ranging from 20 to 35 m and in situ two‐ and three‐span continuous (39.5–57, 50–51–44, and 37–68–37 m) U‐trough decks. The nontypical configuration of the continuous spans was imposed due to utilities and infrastructure requirements. All viaduct decks (in situ and precast) are posttensioned to minimize any concrete cracking. To ensure passenger comfort and traffic safety during train operation, performing a dynamic analysis was vital. The dynamic analysis focused on the vertical accelerations and vertical displacements. The actual train of the project, comprising six vehicles with a total length of 120 m and with actual axle loads (max. axle load = 160 kN, with four axles per vehicle), was adopted in the dynamic analysis. The analysis was carried out using both direct time integration of the equation of motion and modal time history analysis for different train speeds ranging from 60 km/hr to the maximum permissible speed along the metro line (160 km/hr). The maximum vertical accelerations and maximum vertical deflections were monitored for each train speed using the CSiBridge software used in the current project and compared with the allowable values given in EN 1991‐2 and EN 1990, Annex 2. According to relevant Eurocodes requirements, the vertical accelerations and the vertical deflections were found to be acceptable.
<p>Skew bridges are basically required in highway and railway engineering. Unlike straight bridges, the behavior of skew bridges under seismic loading is not quite simple due to the interaction which exists between longitudinal and transversal bridge directions when subjected to in-plane loading such as earthquake forces. The aim of the present paper is to study the behavior of skew bridges when subjected to longitudinal and transversal earthquake loading. The response spectrum analysis as presented by AASHTO LRFD is considered in the present study. Straight, moderate and sharp skew bridges are included. The effect of pier stiffness is also discussed. The present study is based on two span skew bridges resting on pot bearings. Two types of finite element models are adopted for the seismic analysis of the skew bridges. In both models shell elements are used to represent the bridge superstructure; while, frame and shell elements are considered for the pier representation of model 1 and model 2 above, respectively.</p>
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