Abstracts Category : Other

Seismic behaviour of cable-stayed bridges : design, analysis and seismic devices

by Casado Alfredo Cmara

The social and economical importance of long-span bridges is extremely large; cablestayed bridges currently span distances ranging from 200 to even more than 1000 m, representing key points along infrastructure networks and requiring an outstanding knowledge of their seismic response. The objective of the study is three-fold; (i) todiscern how project decisions affect the seismic behaviour of cable-stayed bridges;(ii) to shed light on appropriate analysis strategies in order to address their linearand nonlinear dynamic response; and (iii) to compare different control schemes with passive seismic devices disposed along the towers.The organization of the content follows a natural progression, starting with the motivation of the work and presenting the state of art on this topic, followed by thedescription of the framework where the research is developed; the studied structures and the simplifying assumptions employed throughout the document are clearlydefined. Modal analysis of these bridges precedes the presentation of the seismic action and the validation of the accelerograms which have been used. At this point,the discussion about the seismic results starts with a chapter completely devoted to the linear and nonlinear analysis procedures available to date, being followed bythe comparison of the elastic and inelastic response of cable-stayed bridges, focused on the effect of different project decisions. Finally, several control strategies withseismic devices are addressed in order to maintain the towers in the elastic range.The main conclusions are drawn to close the thesis, and new lines of research are suggested.Satisfying the proposed objectives by means of rigorous finite element models,validated through the comparison with experimental tests conducted by other authors,the main contributions of the present work are highlighted: - The localization of plastic strain deformation in finite element models representing hollow-section reinforced concrete structures has been studied, observing that the plastic hinge length obtained by classical expressions, is twice the optimum dimension for the linear 'beam'-type elements forming the towers.- A specific procedure to generate synthetic accelerograms for nonlinear analysis with Rayleigh damping has been introduced and validated, imposing actions coherent with the design spectrum when damping varies with the frequency.- The seismic consequences of key features like the tower shape, main span length, cable-system arrangement and type of foundation soil are analyzed.- Different calculation methodologies are validated in the linear range, and new 'pushover' procedures are proposed in order to study the nonlinear response of these flexible and strongly coupled structures when three-dimensional seismicexcitations are imposed.- Due to the large influence of the transverse seismic reaction of the deck against the towers, analytical models are proposed and validated in order to predictthis effect prior to the definition of finite element models representing theAdvisors/Committee Members: Astiz Surez, Miguel ngel.