Non-Stationary vs. Nonlinear Effects Recorded During Earthquakes on Monitored Buildings.

Analyses of changes 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, and 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). 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. Short term variations of fundamental frequency observed during a seismic event can be attributed to either non-linearity (i.e. damage) or non-stationary phenomenon (a 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. Practically, it is possible to confuse apparent fundamental frequency variation, due to non-stationary effects, with real fundamental frequency variations due to nonlinear effects. Then, it is necessary to provide simple criteria to discriminate non-stationary fromnonlinear phenomena. An innovative approach based on numerical and experimental experiences is proposed: simple thresholds based on the amplitude of the fundamental frequency variation. Further work must be performed in order to fully validate this kind of approach and to completely define these threshold for various structural forms and building typologies.