Plenary lecture III: the effect of structural degradation on the dynamic behaviour of buildings
Authors: Tudor Sireteanu
Venue: Proceedings of the 9th WSEAS International Conference on Acoustics & Music: Theory & Applications (2008)

The paper is an analysis of buildings behavior during strong earthquakes, considering the modifications of the structural dynamic characteristics due to strong earthquake - generated damages. The acceptance of plastic hinges occurrence in a building according to the seismic design standards [1-3] leads to a degradation of the structural restoring force and to an increase of the structural damping. The first effect could be beneficial if the natural vibration periods of the building are longer than those of the main spectral components of the ground motion ("fast earthquakes"). In this case, by structural stiffness degradation the building is "pulled" from the resonance regime resulting in a reduction of seismic response. On the other hand, if the main spectral components of the seismic ground motion are longer than the building natural periods ("slow earthquakes"), then the structure could be "dragged" to resonance with a significant increase of the seismic response, which can result in important building damages or even in collapse. The increase of the building structural damping capacity due to the occurrence of plastic hinges is beneficial in both cases as more of the kinetic energy injected to the building by the seismic action is consumed as the structure experiences repeated stress reversals. However, this increase of structural damping is not so important such as to dramatically reduce the vibration amplification within the resonance range. The dynamic behavior of buildings during an earthquake must be analyzed for both undamaged and damaged conditions of the building. Figs. 1a,b illustrate the effect of damages produced by slow and fast earthquakes on the building dynamic behaviour, highlighting the increase or decrease of the structural seismic response, as function of the mode the initial value of the building vibration period is situated relative to the pre-amplification, resonance or attenuation frequency range of the normalized ground response spectra (curve). As one can see, for "slow earthquakes", such as Vrancea intermediate earthquakes or the earthquakes on soft soil sites, the controlled damage of buildings could result in most cases to an important increase of the seismic energy transfer from the foundation ground to the building. Therefore, the acceptance of plastic hinges occurrence, as specified by EUROCODE 8 and P100/22004, is not a solution to be adopted in all cases for earthquake protection of buildings. The study is focused on the difference in a building behavior, function of the mode the structure is situated relative to the seismic ground motion from the spectral point of view: above resonance or below resonance. Experimental and simulation results are presented for simplified mechanical models of building structures with stiffness degradation. As a convenient measure of the effect of duration and severity of the building seismic loads, the total energy dissipated through hysteresis is considered.