Evangelos Katsanos

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.

Operational Modal Analysis based Stress Estimation in Friction Systems

It is possible to estimate the strain response of a structure in unmeasured points by the use of operational modal analysis and modal expansion. Both techniques are based on the assumption that the system is linear. However, this is not always the case since nonlinear elements often violate this assumption. In this paper, the precision of estimating the strain response of a nonlinear system is investigated using the operational response of numerical simulations. Local nonlinearities are introduced by adding friction to the test specimen and this paper finds that this approach of strain estimation can still predict the strains with high precision.

Structure-specific selection of earthquake ground motions for the reliable design and assessment of structures

A decision support process is presented to accommodate selecting and scaling of earthquake motions as required for the time domain analysis of structures. Code-compatible suites of seismic motions are provided being, at the same time, prequalified through a multi-criterion approach to induce response parameters with reduced variability. The latter is imperative to increase the reliability of the average response values, normally required for the code-prescribed design verification of structures. Structural attributes like the dynamic characteristics as well as criteria related to variability of seismic motions and their compliance with a target spectrum are quantified through a newly introduced index, δsv–sc, which aims to prioritize motions suites for response history analysis. To demonstrate the applicability of the procedure presented, the structural model of a multi-story building was subjected to numerous suites of motions that were highly ranked according to both the proposed approach (δsv–sc) and the conventional one (δconv), that is commonly used for earthquake records selection and scaling. The findings from numerous linear response history analyses reveal the superiority of the proposed multi-criterion approach, as it extensively reduces the intra-suite structural response variability and consequently, increases the reliability of the design values. The relation between the target reliability in assessing structural response and the size of the suite of motions selected was also investigated, further demonstrating the efficiency of the proposed selection procedure to achieve higher response reliability levels with smaller samples of ground motion.

Effect of Friction-Induced Nonlinearity on OMA-Identified Dynamic Characteristics of Offshore Platform Models

The identification of the modal characteristics of engineering systems under operational conditions is commonly conducted with the use of the Operational Modal Analysis (OMA), being a class of useful tools employed within various fields of structural, mechanical as well as marine and naval engineering. The current OMA methods have been advanced on the basis of two fundamental, though, restrictive assumptions: (i) linearity and (ii) stationarity. Nevertheless, there are several applications that are inherently related to various nonlinear mechanisms, which, in turn, violate the two cornerstones of OMA and hence, question its robustness and efficiency. Along these lines, the current study addresses the effect of friction-induced nonlinearity on OMA-identified dynamic characteristics of an experimental set up consisting of a pair of reduced scale offshore platform models that are connected through a friction-based mechanism. Both time-domain and frequency-domain methods were employed to assess the effect of the varying friction-induced nonlinearity on the OMA-identified modal characteristics. The findings of this study reveal that OMA-based methods provide reasonable identification results implying that nonlinear and nonstationary systems can be described by underlying linear systems, even though, in principles, the basic assumptions of linearity and stationarity are violated.

Damping Estimation of Friction Systems in Random Vibrations

Friction is one of the most efficient and economical mechanisms to reduce vibrations in structural mechanics. However, the estimation of the equivalent linear damping of the friction damped systems in experimental modal analysis and operational modal analysis can be adversely affected by several assumptions regarding the definition of the linear damping and the identification methods or may be lacking a meaningful interpretation of the damping. Along these lines, this project focuses on assessing the potential to estimate efficiently the equivalent linear damping of friction systems in random vibrations with the use of one novel method and two existing ones, modified, though, appropriately. Results of numerical simulations using the three procedures enabled their preliminary comparative assessment in terms of the related damping estimation potential. Indications from the current study showed that two of the methods estimate efficiently the equivalent linear damping, however, the equivalent linear damping seems to depend on the definition of the equivalence. Nonetheless, it seems that the variation of the equivalent linear damping estimates based on the three aforementioned methods becomes less significant when compared to their actual influence on the linear response.

On the Evaluation of EC8-Based Record Selection Procedures for the Dynamic Analysis of Buildings and Bridges

This chapter focuses on an assessment of the selection procedure for real records based on Eurocode 8 provisions, through a study of the performance of R/C bridges of the Egnatia highway system and of a multi-storey R/C building damaged during the Lefkada earthquake of 2003 in Greece. More specifically, the bridge was studied by using six alternative models and accounting for the dynamic interaction of the deck-abutment-backfill-embankment system as well as of the superstructure-foundation-subsoil system, while the building was studied in both the elastic and inelastic range taking into consideration material nonlinearity as well as the surrounding soil. Furthermore, different input sets comprising seven pairs of records (horizontal components only) from Europe, Middle-East and the US were formed in compliance with EC8 guidelines. The results of these parametric analyses permit quantification of the intra-set scatter of the seismic response for both structures, thus highlighting the current limitations of the EC8 guidelines. The chapter concludes with specific recommendations that aim at eliminating the dispersion observed in the elastic and more so in the inelastic response though appropriate modifications of EC8-proposed selection parameters.