SUMMARYThe paper presents a numerical investigation aimed at evaluating the improvements achievable through devices for passive seismic protection of buildings based on the use of shape memory alloys (SMA) in place of conventional steel or rubber devices. To get some generality in the results, di erent resisting reinforced concrete plane frames were analysed, either protected or not. 'New' and 'existing' buildings were considered depending on whether seismic provisions are adopted in the building design or not. Base isolation and energy dissipation were equally addressed for both conventional and innovative SMA-based devices. Fragility analyses were performed using speciÿc damage measures to account for comparisons among di erent damage types; the results were then used to estimate quantitatively the e ectiveness of the various protection systems. More speciÿcally, the assessment involved a direct comparison of the damage reduction provided by each protection system with respect to the severe degradation experienced by the corresponding non-protected frame. Structural damage, non-structural damage and damage to contents were used on purpose and included in a subsequent phase of cost analysis to evaluate the expected gains also in terms of economic beneÿts and life loss prevention. The results indicate that base isolation, when applicable, provides higher degrees of safety than energy dissipation does; moreover, the use of SMA-based devices generally brings about better performances, also in consideration of the reduced functional and maintenance requirements.
Abstract. The presence of long-period pulses in near-fault records can be considered as an important factor in causing damage due to the transmission of large amounts of energy to the structures in a very short time. Under such circumstances high-energy dissipation demands usually occur, which are likely to concentrate in the weakest parts of the structure. The maximum nonlinear response or collapse often happens at the onset of directivity pulse and fling, and this time is not predicted by the natural structural vibration periods. Nonlinear response leading to collapse may in most cases occur only during one large amplitude pulse of displacement. From the study of the response of both linear and nonlinear SDOF systems, the effects of these distinctive long-period pulses have been assessed by means of: (i) synthetic parameters directly derived from the strong ground motion records, and (ii) elastic and inelastic spectra of both conventional and energy-based seismic demand parameters. SDOF systems have first been subjected to records obtained during recent earthquakes in near-fault areas in forward directivity conditions. The results indicate that long duration pulses strongly affect the inelastic response, with very high energy and displacement demands which may be several times larger than the limit values specified by the majority of codes. In addition, from the recognition of the fundamental importance of velocity and energy-based parameters in the characterization of nearfault signals, idealized pulses equivalent to near-fault signals have been defined on account of such parameters. Equivalent pulses are capable of representing the salient observed features of the response to near-fault recorded ground motions.
Lateral displacements' control of structures subjected to earthquake ground motion has now been recognized as a key factor in the assessment of system performance, leading to design approaches that use displacements rather than forces as the starting point for the seismic evaluation of structures. In fact performance-based approaches offer significant advantages in comparison with traditional force-based approaches, since the former are capable of focusing on nonlinear behaviour and consequent damage to the structure, in contrast to the latter. Lateral displacement demand, particularly in structures that exhibit nonlinear behaviour, can be significantly affected by the features of strong ground motion, i.e., amplitude, frequency content and duration. Such characteristics are in turn profoundly influenced by the irregularity and changeability in earthquake ground motions, which should therefore be taken into account appropriately. The great number of strong motion records gathered throughout the last decades in the most widely varying soil-site conditions has made accounting for soil-site effects in the characterization of elastic and inelastic displacement demands feasible. The aim of this paper is to present the results of numerical investigations on the response of both single-degree-of-freedom (SDOF) and multiple-degree-of-freedom (MDOF) systems, through nonlinear time-history analyses performed on the basis of a wide data set of strong motion records. Constant ductility spectra of the ratios of the maximum inelastic displacement to the corresponding maximum elastic demand were derived for this purpose. In particular, the influences of earthquake magnitude, source-to-site distance, local soil-site conditions, ductility and hysteretic behaviour were quantified. Finally, simplified expressions for the ratio of the maximum inelastic to the maximum elastic displacement were established, in order to allow the evaluation of inelastic displacements for new or rehabilitated structures for which the global displacement ductility can be estimated, directly from the knowledge of the corresponding elastic demands. (c) 2007 Elsevier Ltd. All rights reserved
Artículo de publicación ISIThe development of a scientific framework for performance-based seismic engineering requires, among other steps, the evaluation of ground motion intensity measures at a site and the characterization of their relationship with suitable engineering demand parameters (EDPs) which describe the performance of a structure. In order to be able to predict the damage resulting from earthquake ground motions in a structural system, it is first necessary to properly identify ground motion parameters that are well correlated with structural response and, in turn, with damage. Since structural damage during an earthquake ground motion may be due to excessive deformation or to cumulative cyclic damage, reliable methods for estimating displacement demands on structures are needed. Even though the seismic performance is directly related to the global and local deformations of the structure, energy-based methodologies appear more helpful in concept, as they permit a rational assessment of the energy absorption and dissipation mechanisms that can be effectively accomplished to balance the energy imparted to the structure. Moreover, energy-based parameters are directly related to cycles of response of the structure and, therefore, they can implicitly capture the effect of ground motion duration, which is ignored by conventional spectral parameters. Therefore, the identification of reliable relationships between energy and displacement demands represents a fundamental issue in both the development of more reliable seismic code provisions and the evaluation of seismic vulnerability aimed at the upgrading of existing hazardous facilities. As these two aspects could become consistently integrated within a performance-based seismic design methodology, understanding how input and dissipated energy are correlated with displacement demands emerges as a decisive prerequisite. The aim of the present study is the establishment of functional relationships between input and dissipated energy (that can be considered as parameters representative of the amplitude, frequency content and duration of earthquake ground motions) and displacement-based response measures that are well correlated to structural and non-structural damage. For the purpose of quantifying the EDPs to be related to the energy measures, for comprehensive range of ground motion and structural characteristics, both simplified and more accurate numerical models will be used in this study for the estimation of local and global displacement and energy demands. Parametric linear and nonlinear time-history analyses will be performed on elastic and inelastic SDOF and MDOF systems, in order to assume information on the seismic response of a wide range of current structures. Hysteretic models typical of frame force/displacement behavior will be assumed for the local inelastic cyclic response of the systems. A wide range of vibration periods will be taken into account so as to define displacement, interstory drift and energy spectra for MDOF systems. Various scal...
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