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Abstract
Construction of prestressed concrete cable-stayed bridges by cantilevering method involves major changes in configuration of the structure together with the applied loads. This arises from adding deck segments and stay cables to the partially constructed structure, besides adding or removing temporary prestressing force, and stay pretensioning forces. The final stresses and deformation of the completed bridge are highly dependent on both of the construction procedure and rate of construction. The judging of the performance of cable-stayed bridges during their service time after the application of subsequent loads, such as super imposed dead load, live loads, wind load, or any other kind of loads, is highly related to the construction method.
This research endeavours to establish a full static nonlinear analysis of prestressed concrete cable-stayed bridges constructed by the cantilevering method, including both the time-dependent and time-independent effects. The analysis covers the construction phase as well as the service life of the bridge. In addition, the analysis clarifies the effect of the construction method on the final behaviour of the bridge during its service life.
The considered time-dependent factors are creep, shrinkage, and aging of concrete. The considered time-independent factors are large displacements, P-delta effect in girders and pylons, and sag effect in the stay cables. Moreover, both time-dependent and independent losses in the prestressing forces are included.
With regard to the construction phase, the analysis is carried out through all the construction steps due to the application of construction loads. Subsequently, for the service life phase, the analysis is re-performed due to the application of subsequent loads. The final behaviour of the bridge is achieved through the superposition of the two phases.
A complete finite element formulation is implemented taking into account both of the geometric nonlinearity and time-dependent factors. The deck and pylons of the bridge are idealised as three-dimensional beam-column elements. The beam-column element used is a two-node element with seven degrees of freedom at each node; three displacements, three rotations, and warping of the cross section. Stays are idealised as straight cable elements based on the concept of equivalent modulus of elasticity. Prestressing tendons are idealised using a discrete modelling scheme. Alternatively, to account for the time-dependent factors, the equations recommended by ACI-209 are applied. Creep and shrinkage strains are transformed to nodal forces, while aging of concrete is accounted for by adjusting the modulus of elasticity of the concrete according to its actual age. With regard to prestressing losses, the recommended equations by ACI and AASHTO are applied.
Based on the sparse matrices technique, and applying the derived mathematical algorithms and models, NTDA computer software is designed. NTDA program is an innovative designed software which can perform full time-dependent geometric nonlinear analysis, together with stage analysis. The combination of all these factors is the most important advantage which gives NTDA the superiority over the other commercial softwares.
For the sake of numerical studies, a quite long-span cable stayed-bridge is chosen. The bridge is subjected to four types of analysis. Firstly, the bridge is analysed using an instantaneous analysis. Secondly, it is analysed using one-stage construction analysis, thirdly, using staged construction analysis, and finally, using time-dependent staged construction analysis. As an extent to the numerical studies, the bridge is re-analysed using time-dependent staged construction analysis to investigate the effect of the disagreement between the actual construction schedule and the designed one.
The research ends to five main conclusions. Firstly, time-dependent factors are verified to have a significant effect on the deck stresses as well as stay cable forces. Secondly, while time-dependent factors affect the displacements, they preserve the configuration of the deformed shape of the bridge. Thirdly, time-dependent factors have a grand effect on the displacement of the deck, and subsequently on the required camber to achieve the required profile. Fourthly, it is verified that any delay of the time schedule affects strongly the vertical displacements of the deck and subsequently the bridge profile. Finally, the construction method of the bridge has a great influence on its structural behaviour, and this effect is valid along its service life.