Analysis of non-stationary and nonlinear effects on the seismic behaviour of monitored structures.

Analyses of changing in the fundamental frequency of a building is considered the simplest way to detect the onset of structural and non-structural damage. Several authors in the past proposed that the difference in periods that can be observed among ambient noise, earthquake weak-motion measurements, can also be attributed to transient non-linearity due to reversible modification of the building characteristic (e.g. the degree of coupling between frame and infills in reinforced concrete buildings). The need of effective and efficient seismic protection of vast and aging structures and infrastructure has increased markedly the interest in the development of structural monitoring techniques. Damage to any structure alters its dynamic properties; dynamic monitoring techniques enable the identification of damage by comparing key-parameters before and after the seismic excitation. The main considered parameters are: building periods, damping factors and mode shapes. Recently, significant effort has been focused on the development of fast simplified methods to detect and localize damage using the above mentioned parameters. Non Destructive Evaluation (NDE) methods can be rank on four different levels, with the higher levels requiring increased quality and quantity of available information. The most common methods is therefore related to Level 1 assessment, due to their simplified and economic implementation. These methods are mainly based upon the variation of eigenfrequencies and/or relevant equivalent viscous damping factors. It is important to underline however, that although the presence of damage will lead to alterations in vibration modes the opposite does not necessarily hold true. Two types of frequency variations must be distinguished: long time period variations (due to variations of temperature, foundation soil moisture content etc.) and short period variations (for example observed during seismic event). For the latter variations, changes in frequency can be attributed to either non-linearity (i.e. Damage) or non-stationary phenomenon (the particular combination of input and response). This fact may lead to erroneous conclusions attributing variations in frequency due to non-stationary phenomena to the presence of structural damage. This article deals with the theoretical foundation of the analysis of non-stationary behaviour of structures, and then provides experimental evidence in order to distinguish non-linearity from simple non-stationary phenomena.