Narrow Results

Quantifying the higher-mode effects on the seismic demands of buildings may benefit not only the awareness of characteristics of the seismic responses of buildings, but also the development of rapid/simplified methods for the seismic assessment of buildings. This study proposes an approach that is applicable for quantifying the aforementioned effects, covering the full range of building heights and deformation types. The vehicle used in this proposed approach is the generalized building model, which has been modified from the conventional cantilever beam model. In addition to building height and deformation type, the strength ratio of each vibration mode and the site class of buildings are the parameters considered in this study. The higher-mode effects on floor displacements, inter-story drift ratios, floor accelerations, and base shears with relation to the aforementioned parameters are investigated. Finally, the proposed approach is verified via the investigation of the higher-mode effects of a 20-story exemplar building.

On September 7, 1999 an earthquake with magnitude M_W = 5.9 occurred close to the city of Athens in Greece. More than 80 buildings collapsed, about 150 deaths and hundreds of injuries were reported. Soon after the event a damage investigation was carried out by two of the authors in the most heavily struck areas. The most serious damages were observed in the northern suburbs of Athens, where reinforced concrete frames and masonry buildings represent the prevalent construction systems. The hysteretic energy demands imposed on RC buildings should have been rather severe considering the structural systems characteristics and the inadequate construction details. However, overstrengths, redundancy and especially the presence of infill walls, provided a significant increase of the seismic capacity and contributed to the survival of many buildings.

The objective of the present work is to reproduce and analyse the response of typical RC frames subjected to the 1999 Athens earthquake in areas where the observed damage was particularly severe but no recordings of the ground motion were available. After a general overview of the seismotectonic environment, seismological data, observed macroseismic intensities, structural typologies and observed building behaviour, an attempt is made to identify representative excitations in the meizoseismal area. Specifically, the required accelerograms are obtained by modifying available records so as to reproduce a given global energy content and to be consistent with the observed damage. To study the seismic response of RC models, the obtained accelerograms are used to perform nonlinear dynamic analyses.

A non-parametric multidimensional regression method is proposed for the prediction of seismic ground motion parameters. The main features which distinguish the method from standard regression procedures are: (1) The relationship between the input and output variables is not selected a priori by a prediction law, (2) an arbitrary number of input variables can be taken into account, provided that an appropriate data base exists, and (3) the computational procedure is very simple. The results can be easily updated when new information becomes available. The method has been applied for the derivation of attenuation relations by using a combination of databases compiled by other researchers. In the majority of the cases discussed in this paper, the method was used for the prediction of horizontal peak ground acceleration as a function of magnitude and distance. In some cases, ground conditions were also taken into account. Some results on the attenuation relations of peak ground velocity and displacement, as well as Arias intensity, are also presented.

A steel arch bridge originally designed against moderate earthquakes is retrofitted by installation of buckling-restrained braces (BRBs) to sustain severe earthquakes. Two retrofitting methods are considered to obtain good seismic performance of this arch bridge. The original model and retrofitted models subjected to the major earthquakes are investigated by dynamic analyses using 12 patterns of severe (level 2) earthquakes as input ground motions. It is found that the retrofitted models using BRBs can greatly improve seismic performance (displacement, section force, strain, reaction force, etc.) of the steel arch bridge. In addition, to investigate the influence of repeated earthquakes on the seismic responses of the main structure and the demands of BRBs, 12 patterns of earthquake ground motions are repeated by three times. Based on the analytical results, the seismic demands of BRBs against repeated earthquakes are obtained, and the required capacity of BRBs is recommended using a safety factor concluded by comparing the demands under the earthquake applied one and three times. Finally, the influence of the different yield stress on the demand of BRBs is examined by changing the steel grade of BRBs.

Intensity measure (IM) which describes the strength of an earthquake record plays an important role in the seismic performance assessment of structures. An improved IM that can reduce the variability in seismic demands helps reducing the number of records necessary to predict the seismic performance with sufficient accuracy. In this study, an improved RMS-based IM is developed based on the results obtained from incremental dynamic analyses of short-to relatively long-period frames under an ensemble of near-fault pulse-like earthquake records. It is observed that the root-mean-square value of pseudo spectral accelerations, (S_a)_rms, is generally superior to that of spectral velocities, (S_v)_rms, in seismic demand prediction under near-fault records. To compute (S_a)_rms as IM, two appropriate period ranges are suggested for short- and moderated-to relatively long-period frames, respectively. Comparing the efficiency of (S_a)_rms with several advanced IMs shows that (S_a)_rms is more efficient in predicting the inelastic response and collapse capacity of short-period frames. It is also found that intensity measure (S_a)_rms is sufficient with respect to the magnitude and source-to-site distance for all frames of various heights under near-fault ground motions.