Anastasios Sextos

ISSARS: An integrated software environment for structure-specific earthquake ground motion selection

Current practice enables the design and assessment of structures in earthquake prone areas by performing time history analysis with the use of appropriately selected strong ground motions. This study presents a Matlab-based software environment, which is integrated with a finite element analysis package, and aims to improve the efficiency of earthquake ground motion selection by accounting for the variability of critical structural response quantities. This additional selection criterion, which is tailored to the specific structure studied, leads to more reliable estimates of the mean structural response quantities used in design, while fulfils the criteria already prescribed by the European and US seismic codes and guidelines. To demonstrate the applicability of the software environment developed, an existing irregular, multi-storey, reinforced concrete building is studied for a wide range of seismic scenarios. The results highlight the applicability of the software developed and the benefits of applying a structure-specific criterion in the process of selecting suites of earthquake motions for the seismic design and assessment.

Seismic assessment of bridges accounting for nonlinear material and soil response, and varying boundary conditions

Seismic assessment of bridges using the pushover analysis technique often ignores the effect of some sources of nonlinearity such as those associated with the foundation soil and the boundary conditions, that may significantly modify the overall performance of a bridge. In this context, the seismic response of a typical overpass is assessed herein using lumped plasticity models to account for the inelastic behaviour of the critical cross-sections of piers and piles, and nonlinear springs to consider foundation-soil compliance; in addition, a detailed solid finite element model of the abutment-embankment-foundation soil system is set up and compared with the simpler models. The results of the analysis show a markedly different seismic behaviour when the abutment—soil system is included in the analysis, rather than simply considering a pinned support (in the transverse direction) as usually done in previous studies. Furthermore, for stronger excitations, it is seen that as inelastic mechanisms (of piers, piles, pile caps, and soil) are introduced and boundary conditions change (i.e., joint /gap closure), the assumptions made on the foundation and soil compliance play an increasingly important role that can potentially modify the anticipated failure hierarchy, as well as the ensuing pushover curves in both directions of the bridge.

A knowledge-based software for the preliminary design of seismically isolated bridges

Seismic design of isolated bridges involves conceptual, preliminary and detailed structural design. However, despite the variety of commercial software currently available for the analysis and design of such systems, conceptual and preliminary design can prove to be a non-straightforward procedure because of the sensitivity of bridge response on the initial decisions made by the designer of the location, number and characteristics of the bearings placed, as well as on a series of broader criteria such as serviceability, target performance level and cost-effectiveness of the various design alternatives. Given the lack of detailed design guidelines to ensure, at this preliminary stage, compliance with the above requirements, a “trial and error” procedure is typically followed in the design office to decide on the most appropriate design scheme in the number and location of the bearing systems; the latter typically based on engineering judgment to balance performance with cost. To this end, the particular research effort aims to develop a decision-making system for the optimal preliminary design of seismically isolated bridges, assumed to respond as single degree of freedom (SDOF) systems. The proposed decision-making process is based on the current design provisions of Eurocode 8, but is complemented by additional criteria set according to expert judgment, laboratory testing and recent research findings, while using a combined cost/performance criterion to select from a database of bearings available on the international market. Software is also developed for the implementation of the system. The paper concludes with the application, and essentially the validation of the methodology and software developed through more rigorous MDOF numerical analysis for the case of a real bridge.

ICT applications for new generation seismic design, construction and assessment of bridges

Abstract This study focuses on the recent advances in information and communication technologies (ICTs) and their applications in the seismic design, construction and assessment of bridges. It aims to review and critically demonstrate advanced numerical analysis methods, open source finite element programs, web-based engineering tools, decision-making systems, collaborative on-site and remote research tools, frameworks for hybrid simulation, data and metadata dissemination and archiving, wireless data transmission and structural health monitoring, as well as earthquake-specific geographical information system applications; all developed and implemented recently, in order to enhance our understanding regarding the response of bridges under earthquake loading and thus, ultimately, mitigate seismic risk. The study concludes with the current research needs and challenging opportunities in integrating the current technological advancements in modern seismic codes and construction practice.

Multiple support seismic excitation of the Evripos bridge based on free-field and on-structure recordings

The 395 m long Evripos bridge in central Greece connects the island of Evia to the mainland. An accelerometer array of 43 triaxial sensors has been monitoring both the free-field excitation and the response of the superstructure in a series of seismic events since 1994. This paper focuses on the characteristics of the spatially variable earthquake ground motions (SVEGMs) recorded during two seismic events (1999 and 2013) and the corresponding bridge response. A model updating is performed to match the numerically predicted with the measured bridge response. Then, the nature of the recorded ground motions is studied and the incoherency patterns of the seismic waves are compared with empirical or semi-empirical models. It is observed that the loss of coherency at the site is isotropic. It is also documented, for the first time based on actual free-field and on-structure recordings, that the asynchronous excitation of a bridge excited higher modes of vibration while suppressing the oscillation on its fundamental frequency. The latter is in line with analytical predictions and is believed to be a key finding in understanding the nature of SVEGM and predicting its potential impact on the seismic response of bridges.

Probabilistic Assessment of Abutment-Embankment Stiffness and Implications in the Predicted Performance of Short Bridges

This article investigates the bi-directional stiffness of abutment-embankment systems in highway overpasses, considering the contribution of the abutment foundation in the force-transfer mechanism and soil material uncertainty. Stiffness is probabilistically assessed through refined, three-dimensional nonlinear static analyses, after validation with large-scale experimental results. The force-displacement relationships as well as the corresponding variance are derived separately for different abutment configurations along the transverse and longitudinal direction. Application of the above expressions for the case of a well-studied highway overpass leads to distinctly different probability of failure at the system level compared to the prediction made based on conventional code-prescribed procedures.

Soil-bridge system stiffness identification through field and laboratory measurements

Despite the major advances in finite-element (FE) modeling and system identification (SI) of extended infrastructures, soil compliance and damping at the soil-foundation interface are not often accurately accounted for due to the associated computational demand and the inherent uncertainty in defining the dynamic stiffness. This paper aims to scrutinize the effect of soil conditions in the SI process and to investigate the efficiency of advanced FE modeling in representing the superstructure-soil-foundation stiffness. For this purpose, measured, computed, and experimentally identified natural frequencies of a real bridge were used. Field measurements obtained during construction were reproduced both in the laboratory and by refined FE modeling. In addition, to understand the physical problem more thoroughly, three alternative soil conditions were examined: rock, stabilized soil, and Hostun sand. Discrepancies on the order of 3-13% were observed between the identified and the numerically predicted natural frequencies. These discrepancies highlight the importance of reliable estimation of soil properties and compliance with the SI framework for extended bridges under ambient and low-amplitude vibrations

Field Experiments for Monitoring the Dynamic Soil–Structure–Foundation Response of a Bridge-Pier Model Structure at a Test Site

AbstractSummary results from a series of field experiments at a test site in Greece are presented, involving an in situ instrumented bridge-pier model built on realistic foundation conditions, to study the dynamic behavior of structure-foundation-soil system. It was attempted to link the variation of its dynamic characteristics to certain changes in its structural system, including the development of structural damage. This measured response was next utilized to validate numerical tools capable of predicting influences arising from such structural changes as well as from soil–foundation interaction. This bridge-pier model was supported on soft soil deposits allowing the study of structure–foundation–soil interaction effects during low-to-medium intensity artificial excitations. The in situ experiments provided measurements that were used to verify fundamental analytical solutions for soil–structure interaction. They were also used to validate numerical simulations that were developed to predict the respon...

Eurocode-compliant seismic analysis and design of R/C buildings:

This chapter provides a concise qualitative overview of the philosophy for earthquake resistant design of ordinary structures adopted by relevant international codes of practice, including Eurocode 8. The aim is to facilitate practicing engineers with the interpretation of the code-prescribed design objectives and requirements for the seismic design of ordinary reinforced concrete (r/c) building structures which allow for structural damage to occur for a nominal design seismic action specified in a probabilistic manner. In this regard, the structural properties of stiffness, strength, and ductility are introduced along with the standard capacity design rules and requirements. Further, the role of these structural properties in the seismic design of r/c building structures following a force-based approach in conjunction with equivalent linear analysis methods is explained. Emphasis is placed on delineating the concept of the behaviour factor, or force reduction factor, which regulates the intensity of the seismic design loads and ductility demands. Moreover, the development and current trends in the emerging performance-based design approach for earthquake resistance are briefly reviewed. Lastly, practical recommendations to achieve higher-than-the-minimum-required by current codes of practice structural performance within the force-based design approach are provided.

A paperless course on structural engineering programming: investing in educational technology in the times of the Greek financial recession

This paper presents the structure of an undergraduate course entitled ‘programming techniques and the use of specialised software in structural engineering’ which is offered to the fifth (final) year students of the Civil Engineering Department of Aristotle University Thessaloniki in Greece. The aim of this course is to demonstrate the use of new information technologies in the field of structural engineering and to teach modern programming and finite element simulation techniques that the students can in turn apply in both research and everyday design of structures. The course also focuses on the physical interpretation of structural engineering problems, in a way that the students become familiar with the concept of computational tools without losing perspective from the engineering problem studied. For this purpose, a wide variety of structural engineering problems are studied in class, involving structural statics, dynamics, earthquake engineering, design of reinforced concrete and steel structures as well as data and information management. The main novelty of the course is that it is taught and examined solely in the computer laboratory ensuring that each student can accomplish the prescribed ‘hands-on’ training on a dedicated computer, strictly on a 1:1 student over hardware ratio. Significant effort has also been put so that modern educational techniques and tools are utilised to offer the course in an essentially paperless mode. This involves electronic educational material, video tutorials, student information in real time and exams given and assessed electronically through an ad hoc developed, personalised, electronic system. The positive feedback received from the students reveals that the concept of a paperless course is not only applicable in real academic conditions but is also a promising approach that significantly increases student productivity and engagement. The question, however, is whether such an investment in educational technology is indeed timely during economic recession, where the academic priorities are rapidly changing. In the light of this unfavourable and unstable financial environment, a critical overview of the strengths, the weaknesses, the opportunities and the threats of this effort is presented herein, hopefully contributing to the discussion on the future of higher education in the time of crisis.