Luis D. Decanini

FORMULATION OF ELASTIC EARTHQUAKE INPUT ENERGY SPECTRA

SUMMARY The object of this paper is to introduce a procedure for the determination of elastic design earthquake input energy spectra taking into account the influence of magnitude, soil type and distance from the surface projection of the fault. Firstly, an accurate selection of a large set of representative records has been realized. Secondly, the construction of the design input energy spectra has required determining the spectral shapes and a normalization factor which measures seismic hazard in terms of energy. This factor, denoted as the seismic hazard energy factor, has been defined as the area under the earthquake input energy spectrum in the period interval between 0)05 and 4)0 s. Finally, due to the importance of the source-to-site distance in the evaluation of the input energy, an investigation into the attenuation of the seismic hazard energy factor has been carried out. ( 1998 John Wiley & Sons, Ltd. The fundamental need to improve the reliability of the current procedures of earthquake-resistant design of structures has led to the recognition of methodologies based on energy criteria as e⁄ective tools for a comprehensive interpretation of the behaviour observed during recent destructive events. Energy-based design involves considering two essential aspects: the first is related to the establishment of Design Earthquakes, while the second concerns the evaluation of the actual energy absorption and energy dissipation capacities of structures. The aim of this work is to introduce a proposal which could contribute to the resolution of the first of the above-mentioned aspects, namely the definition of a design seismic action as a function of appropriate parameters providing a measure of the energy actually transferred from soil to structures during seismic shaking. The commonly adopted design approach in terms of forces, based on both elastic and inelastic response spectra, as results from the analysis and interpretation of the observed structural behaviour is open to criticism. The most controversial and uncertain aspect of the conventional design procedures specified by the di⁄erent codes is represented by the interpretation of the elastic design spectrum as a measure of destructiveness and, by the definition of the elastic response reduction factor as a function of a presumed inelastic behaviour. This factor, also known as the behaviour factor, although it is based on the comparison between elastic and inelastic response spectra and non-linear structural analysis, is still substantially assigned by the codes according to empirical criteria. Furthermore, uncertainty often arises in various steps of these

Recorded Motions of the 6 April 2009 Mw 6.3 L'Aquila, Italy, Earthquake and Implications for Building Structural Damage: Overview

The normal-faulting earthquake of 6 April 2009 in the Abruzzo Region of central Italy caused heavy losses of life and substantial damage to centuries-old buildings of significant cultural importance and to modern reinforced-concrete-framed buildings with hollow masonry infill walls. Although structural deficiencies were significant and widespread, the study of the characteristics of strong motion data from the heavily affected area indicated that the short duration of strong shaking may have spared many more damaged buildings from collapsing. It is recognized that, with this caveat of short-duration shaking, the infill walls may have played a very important role in preventing further deterioration or collapse of many buildings. It is concluded that better new or retrofit construction practices that include reinforced-concrete shear walls may prove helpful in reducing risks in such seismic areas of Italy, other Mediterranean countries, and even in United States, where there are large inventories of deficient structures.

The performance of churches in the 2012 Emilia earthquakes

In this study the damage suffered by churches during the 2012 Emilia seismic sequence in Italy is analysed, based on surveys and inspections carried out in the area. Similarly to what was observed after other Italian earthquakes, the damage to churches was severe. However, the Emilia churches present some characteristic features such as the use of unreinforced clay brick masonry. In order to appropriately address the performance of this class of buildings, typical architectural layouts and construction techniques are described. Such techniques are interpreted also in the light of the local seismic catalogue. Fifty churches are then selected and their damage is studied, with reference to typical local-collapse mechanisms of different macro-elements. The study highlights that the damage is often concentrated at the top section of the façade, in the clerestory walls, in the vaults and in the bell towers. Structural analyses are performed to explain some of the observations. The overturning of the top section of the façade is analytically addressed, modelling the friction interlocking. With reference to the case study of San Francesco in Mirandola, non-linear static and dynamic analyses allow us to correlate the directionality of damage to the higher seismic demand along the NS direction, to point out the negligible role of the large vertical component of ground motion and to emphasise the relevance of the buttresses for the seismic response of the façade.

Performance of Masonry Buildings During the 2002 Molise, Italy, Earthquake

The 2002 Molise, Italy, earthquake struck a relatively limited geographical area where the communities are mainly agrarian. While most buildings in the region are masonry, there are significant differences in the type of masonry construction, as material characteristics and construction practices had changed over the centuries. This paper focuses on the masonry buildings that predominate in domestic construction. The most significant features that contributed to the damage pattern appear to be (1) construction criteria, techniques, and details that were inadequate for seismically active areas, particularly in buildings constructed or substantially modified over the past 100 years, and (2) site effects resulting from differences in amplification and frequency of the vibrations that locally increased the destructiveness of the earthquake. The observed damage did not correlate to the vulnerability that would be assigned to the structures under the European Macroseismic Scale.

Characterization of displacement demand for elastic and inelastic SDOF systems

Abstract The results of a research concerning the characterization of elastic and inelastic displacement spectral demand as a function of magnitude, source-to-site distance, and soil type are presented. The displacement spectra were computed for single degree of freedom systems subjected to a large set of strong ground motion records. In the elastic case, design displacement spectra, modeled in a simplified way with a bilinear shape in the period range 0–4 s, are then proposed for the estimation of the displacement demand to structures located on different local soil condition, at different distance from the causative fault, and for different levels of magnitude. In order to evaluate the reliability of the proposed design displacement spectra, probabilistic displacement spectra corresponding to different levels of probability of non-exceedance were also carried out. The inelastic displacement demand to elasto-plastic systems was analyzed through the ratio between inelastic and elastic spectral displacements. Simplified relationships of the inelastic displacement ratio are then proposed as a function of displacement ductility, soil condition and period of vibration. Finally, as a comparison, the inelastic displacement ratios were also estimated considering other constitutive models.

Performance of Reinforced Concrete Buildings During the 2002 Molise, Italy, Earthquake

About 10% of the almost 20,000 buildings damaged by the 2002 Molise, Italy, seismic sequence were reinforced concrete (RC). The most frequent type of damage affected the infill masonry walls, but in some cases cracks in concrete columns were observed. Heavy damage to both infills and structural elements was restricted to a few cases in the meizoseismal area. Almost all the affected municipalities were only classified as seismic in May 2003, following this earthquake. Consequently, construction generally used vertical-load-bearing moment-resisting frames with no explicit design for seismic lateral forces. In particular, the reinforced concrete buildings typically consist of cast-in-place unidirectional RC slabs lightened with hollow clay tiles, supported by RC beams and columns. Usually no shear walls are present, except in some cases for the elevator shaft. This paper covers: a) an overview and statistical analysis of damage to RC buildings, and b) a detailed analysis of two damaged buildings.

Strength and stiffness reduction factors for infilled frames with openings

Framed structures are usually infilled with masonry walls. They may cause a significant increase in both stiffness and strength, reducing the deformation demand and increasing the energy dissipation capacity of the system. On the other hand, irregular arrangements of the masonry panels may lead to the concentration of damage in some regions, with negative effects; for example soft story mechanisms and shear failures in short columns. Therefore, the presence of infill walls should not be neglected, especially in regions of moderate and high seismicity. To this aim, simple models are available for solid infills walls, such as the diagonal no-tension strut model, while infilled frames with openings have not been adequately investigated. In this study, the effect of openings on the strength and stiffness of infilled frames is investigated by means of about 150 experimental and numerical tests. The main parameters involved are identified and a simple model to take into account the openings in the infills is developed and compared with other models proposed by different researchers. The model, which is based on the use of strength and stiffness reduction factors, takes into account the opening dimensions and presence of reinforcing elements around the opening. An example of an application of the proposed reduction factors is also presented.

Seismic Hazard and Seismic Zonation of the Region Affected by the 2002 Molise, Italy, Earthquake

In 1998, a new system of seismic classification promoted by the Department of Civil Protection identified the area in Italy hit by the 2002 earthquake in Molise and Puglia as a Zone 2 (moderately seismic). However, this classification was not adopted until March 2003, when an ordinance passed that partially closed the gap between scientific knowledge and official recognition of seismic hazard and that established a method for constantly updating the classification in the future. This paper reviews some of the methods available to assess the seismic hazard, particularly referring to the rich seismic history of Italy and using the “Associated Seismic Area” concept. This study confirms that the area affected by this earthquake should be considered as Zone 2. An appendix presents data on the seismic risk of existing buildings in the area and concludes that it is high for masonry buildings and that a strengthening program is needed.

Estimation of near-source ground motion and seismic behaviour of RC framed structures damaged by the 1999 Athens earthquake

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, over-strengths, 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 macro-seismic 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.

Repair and retrofit of a six storey reinforced concrete building damaged by the earthquake in south-east Sicily on the 13th December 1990

Abstract This paper presents the studies that led to the seismic retrofitting of a six storey reinforced concrete building damaged by the 1990 earthquake in south-east Sicily. A seismological study, available geological information and extensive geotechnical tests led to the definition of the ultimate design earthquake for the site. A detailed damage survey and a numerical damage simulation together with available information on the design and construction history denounced the highly deificient condition of the building against the design seismic action. After having established, through a campaign of destructive and non-destructive tests, the main structural characteristics of the materials and of the structural elements, joints included, seven different retrofitting systems are presented and discussed. Finally the recommended solution is presented in some detail and the methods of analysis and the conducted safety cheks are briefly described.