Search

The rigidity of a six-story shear-wall building, damaged by the May 21, 2003 Boumerdes earthquake (Algeria) has been evaluated before the event, in its damaged state, and after retrofitting, by estimating its natural frequency, damping factor, and modal shape. Pre-event parameters are estimated through a numerical analysis, based on the structure drawings, complemented by an ambient vibration investigation of an identical non-damaged building located on the same site. Parameters of the damaged and retrofitted states were obtained from ambient vibration recordings. Variations of the modal parameters, for the three states of the building, allow evaluating rigidity changes, and thus checking of both the retrofitting and strengthening of the building. [ABSTRACT FROM AUTHOR]

The Bovec basin, which is filled with glaciofluvial sediments, has recently been struck by two strong earthquakes (1998 and 2004) which caused extensive damage (VII–VIII EMS-98). Strong site effects resulted in large variations in damage to buildings in the area, which could not be explained by the surface variations in Quaternary sediments. The microtremor horizontal-to-vertical-spectral ratio (HVRS) method was therefore applied to a 200 m dense grid of free-field measurements to assess the fundamental frequency of the sediments. Large variations in the sediments frequency (3–22 Hz) were obtained, with most of the observed values in the range 6–12 Hz. The observed frequencies cannot be related to the total thickness of Quaternary sediments (sand, gravel), but can be explained by the presence of conglomerate or lithified moraine at shallow depths. Microtremor measurements performed in 20 two, three and four storey houses (masonry with RC floors), which prevail in the Bovec basin, have shown that the main building frequencies in the area are in the range 7–11 Hz. This indicates that damage to houses in both earthquakes in some parts of the basin was enhanced by site amplification. Areas of possible soil-structure resonance were identified which correspond good to the distribution of damage in both earthquakes. [ABSTRACT FROM AUTHOR]