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Abstract

The relatively few available practice-oriented proposals for performance-based seismic design of conventional and isolated bridges, aim primarily at a more consistent description of seismic demand and capacity of structures on the basis of simplified analysis, and to a lesser extent at the direct consideration of multi-level performance criteria using advanced analysis tools. In view of the type/device-specific existing methods, the present study presents a broad-scope methodology for the seismic design of bridges emphasising on (i) displacement-based principles, (ii) use of nonlinear dynamic analysis, and (iii) explicit consideration of multiple performance levels (PLs) and objectives (POs) in a practical design context, suitable for inclusion in design codes.

The deformation-based design (Def-BD) procedure, initially developed for seismic design of conventional (non-isolated) buildings, is first tailored to concrete bridges with energy dissipation in the piers. The key issues in this respect are the proper consideration of the intended plastic mechanism under the considered PLs, and the design of the bearings. The efficiency of the proposed design methodology is demonstrated by applying it to an actual bridge selected with a view to enabling comparisons among Def-BD, the modal direct displacement-based design (MDDBD), and a force-based code-type (Code-BD) method. Refined analysis along with the consistent performance-based design format within Def-BD, result in superior seismic performance. Significant cost reductions are achieved compared to MDDBD, whereas potential cost reductions may generally be obtained compared to ‘standard’ code design.

Considering the diversity of passive devices and their inherent weakness to optimise the bridge response under multiple PLs, a methodology is developed to enable the identification of the critical performance requirements and the comparative evaluation of different passive schemes at the early stages of design. Originating from an earlier study focusing on bilinear isolators, the method is extended with a view to developing generalised design equations (GDEs) capable of providing reliable estimates of peak response in linear/bilinear isolation systems with/without supplemental linear/nonlinear viscous damping under different PLs associated with code-based target spectra of different intensity. The Def-BD method is finally extended to address passive (isolation and energy dissipation) systems. Novel features are introduced, including (i) the use of GDEs for the preliminary ‘near-optimal’ selection of the basic system properties and the consideration of nonlinearity of viscous dampers, (ii) the enhancement of POs in line with the higher performance expected in the case of isolated bridges, (iii) specific conditions ensuring the effectiveness of the isolation system, and (iv) the proper consideration of the orthogonal component of seismic action under bidirectional excitation. The validity of the procedure is demonstrated by applying it to the bridge previously used to develop the Def-BD method for bridges with ‘ductile-pier’ behaviour. Alternative isolation schemes are investigated and compared with the design resulting from Eurocode 8 (Part 2), offering a useful insight into some pitfalls of modern code-based approaches. Assessment of the Def-BD designs reveals enhanced and controlled performance under multiple PLs, and significant cost reductions in the substructure design compared to the design for ‘ductile-pier’ response. On the other hand, further cost reduction observed in the case of the code-based design, results in reduced efficiency of the isolation system and improper performance of the piers.

In view of the previous remarks, Def-BD emerges as a rigorous methodology, applicable to most of the common concrete bridge configurations, albeit at the expense of additional computational effort associated with the use of nonlinear dynamic analysis and the design for multiple PLs. Nevertheless, minimum iterative effort is ensured by providing design ‘routines’ that facilitate the implementation and address implications resulting from the use of nonlinear dynamic analysis. Considering the suitable formulation of Def-BD, a framework of performance-based control principles for the future extension towards the integration of advanced structural control techniques, is finally set forth.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TG Bridge engineering
Departments:	Doctoral Theses School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses

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