M. De Stefano

Fragility Analysis and Seismic Record Selection

This paper explores the influence of spectrum-matched and amplitude-scaled ground motions on the development of fragility functions for structures. The quantification of the influence of these two types of ensembles on ground motions in predicting demands of structural and nonstructural systems is addressed. Moreover, the paper investigates the sensitivity of number of accelerograms in the ensembles, which produces consistent results in the nonlinear analyses. A multidegree-of-freedom (MDOF) inelastic shear-type building is used in the evaluations. The median and the dispersion of different types of damage measures are evaluated at each story and the effect of different levels of nonlinearity is investigated. Fragility functions are developed for structural and nonstructural components using the maxi- mum likelihood method from the response generated with the selected ground motions. The sufficient number of ground motions necessary in the estimation of the response parameters and on the evaluation of the fragility functions is presented herein. DOI: 10.1061/(ASCE)ST.1943- 541X.0000115.

Effect of the variability in plan of concrete mechanical properties on the seismic response of existing RC framed structures

A proper characterization of concrete strength is essential to correctly model existing RC structures, whose seismic performance is affected by the poor quality of materials. The purpose of this work is to evaluate the effect of incorrect assumptions for concrete strength and the adequacy of current Codes provisions (Eurocodes, FEMA). Even the effects of the non homogeneity of concrete strength within the building is considered due to its high variability; in fact, buildings can experience an irregular seismic response, both in plan and in elevation. In this work the effects of irregularity in plan due to the strength variability of concrete is analyzed on a case study, a four storey RC framed building, designed for vertical loads only. The variability of concrete strength has been evaluated using the data of an extensive investigation developed by REGIONE TOSCANA on a large sample of RC framed buildings.

On the variability of concrete strength as a source of irregularity in elevation for existing RC buildings: a case study

A proper assumption of the concrete strength is essential to model existing RC structures; their seismic performance, in fact, can be affected by the poor quality of materials, both in terms of low strength and high variability. This paper considers the effects of the variability of concrete strength within buildings. Due to the high variability of concrete strength, in fact, buildings can experience irregular seismic responses, both in plan and in elevation. This research investigates the effects of irregularity in elevation due to the strength variability of concrete in a case study, namely a four-storey RC framed building, designed for vertical loads only. The variability of the concrete strength has been evaluated on the basis of an extensive survey carried out by the REGIONE TOSCANA (Tuscany Regional Government) on a large sample of RC framed buildings. Special attention has been paid to the adequacy of current codes provisions (Eurocodes, FEMA) on how to quantify concrete strength.

A comparison of the present SEAOC /UBC torsional provisions with the old ones

Abstract The seismic response of single storey asymmetric structures modelled as two-degree-of-freedom elastic-perfectly plastic oscillators and designed using the design eccentricity formula of the present Uniform Building Code (UBC) torsional provisions, which first appeared in the 1988 edition, is compared with designs based on the earlier UBC 1979. Changes in the total strength requirement and its distribution among the resisting elements are examined, and the effects of these changes on the ductility demand of three-and five-element systems are evaluated. Jt is shown that for two levels of the strength reduction factor R (= 4;1) the ductility demands predicted for systems designed according to the UBC 79 provisions are in most cases similar to those for UBC 88 systems, with the exception of torsionally flexible systems - mainly mass eccentric and, to a much lesser extent, stiffness eccentric systems. In view of the relative complexity of the UBC 88 provisions, a modification of the simpler UBC 79 ones is proposed which results in the UBC 79 torsionally flexible systems behaving in a similar and often better manner than their UBC 88 counterparts.

The dispersion of concrete compressive strength of existing buildings

The correct estimation of the compressive concrete strength plays a key role in the evaluation of the structural performance of existing RC buildings. Both Italian (NTC 2008) and European (EC8) Standards define different levels of knowledge according to the number of tests carried out on a building. They indicate a reduced value to assume in the analysis, defined as the mean value of the compressive strength, divided by a confidence factor. However, such a procedure completely neglects the dispersion of the test data, as represented by the high values of the coefficient of variation. Instead, this aspect is treated by FEMA 356 where a limit to the coefficient of variation was introduced. In this paper, with reference to a significant number of existing buildings located in Tuscany, the coefficient of variation (cov) of concrete strength is evaluated and the frequency of high cov values is determined. The dispersion of compressive strength, obtained by SonReb method, using correlation curves calibrated ad hoc on single building, shows that increasing the number of data for each building the coefficient of variation does not necessary decrease. Moreover, the strength value considered by EC8 in the analysis for a single building, i.e. the mean value of compressive strength, is often not conservative, while the approach provided by FEMA 356 is safer since it dependent on the cov itself.

Seismic assessment of a real RC asymmetric hospital building according to NTC 2008 analysis methods

The seismic assessment of existing buildings is an essential issue of seismic engineering. This work is focused on the evaluation of the seismic performance of existing RC buildings according to the current Italian Technical Code, which almost coincides with the European Eurocode 8. Alternative types of analyses, all consistent to the Code provisions, have been performed with reference to a case-study, that is a real RC hospital building. An accurate knowledge of the building has been achieved, as a result of a collaboration between the University of Florence and the Regional Government. Both elastic and inelastic modeling, as well as static and dynamic ones, have been adopted in the analysis. The global response—with special attention to torsional effects—and the seismic performance of each single member have been found with all the performed analyses. The comparison among the analyses has been performed in terms of both global and local response parameters, and the reliability of each analysis has been pointed out.

Combined effects of axial load and concrete strength variation on the seismic performance of existing RC buildings

It is well known that the axial load plays an important role in the evaluation of the structural capacity of RC columns. In existing buildings this problem can be even more significant than in new ones, since the material can easily present poor mechanical properties. The paper is aimed at the investigation of the role of the axial load variation on the seismic performance of RC columns of a case-study, i.e. a doubly symmetric 4-storey RC building. The effects of the axial load variation have been checked on the first storey columns, by comparing the seismic response, measured in terms of chord rotation and shear force, with the corresponding capacity. The sensitivity of the seismic performance to the axial load is evaluated with special attention on the type of analysis adopted to determine the seismic response and on considering a wide range of values for the concrete strength. The study points out a non-negligible effect of the axial load variation on the seismic response of the case-study building, especially when combined to concrete strength variability.

Concrete Strength Variability as a Source of Irregularity for Existing RC Structures

Mechanical properties of concrete can consistently affect the seismic performance of RC buildings. A proper determination of the concrete strength is therefore essential for a reliable modeling of the structure. The current European Technical Code, Eurocode 8, provides a criterion for the strength assumption related to the knowledge level of the structure, which does not take into account the variability of the strength. If the concrete strength is affected by a large variability, the conventional value suggested by Eurocode 8 can be not conservative, since it does not consider the increase in the demand due to a not homogeneous strength distribution and the reduced capacity of the weaker members. In this paper the concrete strength variability is investigated as a source of in-plan and in-elevation irregularity. The effects of the strength variability on the seismic response and performance are evaluated on a case study, that is a 4-storeys 3D framed building. The seismic response of the case study has been represented by performing a nonlinear static analysis, while the seismic performance has been measured in terms of chord rotation. Results obtained from the analysis have been compared with the Eurocode 8 previsions.