Stefania Viti

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.

The Effect of Common Irregularities on the Seismic Performance of Existing RC Framed Buildings

This chapter deals with the seismic performance of irregular 3D RC existing framed structures subjected to seismic actions. More specifically, the effect of the noncoincidence between the mass and the stiffness centers on the seismic response of these structures is investigated. The analysis is performed on a 4-story 3D framed sample structure designed for vertical loads only. A very detailed nonlinear model of the structure is implemented under the computer code SeismoStruct. The seismic response of the structure is analyzed performing a nonlinear incremental dynamic analysis. The obtained response domain is compared with the limit values provided by FEMA 356 for the different limit states. The effect of the introduced irregularities on the seismic performance of the structure is not negligible despite the low value of the eccentricity. The performed analysis evidences that a particular attention has to be paid to the seismic behavior of RC buildings realized in the 1960s and 1970s, before the adoption of seismic codes, since even light irregularities can consistently affect their seismic performance.

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.

Seismic assessment of existing RC buildings under alternative ground motion ensembles compatible to EC8 and NTC 2008

This work investigates the effects of the choice of different ensembles of ground motions on the seismic assessment of existing RC buildings through nonlinear dynamic analysis. Nowadays indeed, all the main International Seismic Codes provide a soil classification which is based on the shear wave velocity, the soil morphology and the assumed distance from the fault source. Depending on the soil properties, a suitable elastic spectrum is provided as target, defined on the basis of average properties assumed for the soil. An ensemble of ground motions, compatible to the target one, must be selected to perform a nonlinear dynamic analysis. The ensemble can be made by artificial or natural ground motions, compatible with the Code spectrum for the assumed soil-type. Alternatively, the set of ground motions can be assumed as compatible with the bedrock Code spectrum and, subsequently, subjected to site response analysis, i.e. filtered through the specific stratigraphy of the site soil. In this work a comparison among these different approaches, all compatible to the European (Eurocode 8, EC8) and Italian (NTC 2008) Code provisions, has been made on a case-study, i.e. a real RC Italian building. The seismic response of the case-study under the assumed seismic inputs, expressed in terms of chord rotation and shear force, has been found by performing a nonlinear dynamic analysis under the different assumed seismic excitations. The comparison has been made in terms of seismic performance, expressed as the ratio between the seismic response found for each structural element and the corresponding capacity. The comparison among the seismic performance found by the application of the different ground motion ensembles pointed out significant differences, which underline the importance of the seismic input choice in the seismic assessment of RC buildings.

Influence of the Variability of Concrete Mechanical Properties on the Seismic Response of Existing RC Framed Structures

In Italy, many RC framed buildings have been realized in the 1960s and the 1970s before the adoption of seismic codes. These buildings were designed for vertical loads only and were realized with concrete having poor and nonhomogeneous mechanical properties. This last condition is usually neglected despite it significantly affects the seismic response of the structures. A sample structure is considered to represent a typical existing RC building realized in Italy before the introduction of seismic codes. The structure is a four-story RC framed building designed for vertical loads only. The variability of concrete mechanical properties is evaluated on the base of an extensive survey developed by the authors on a large sample of RC framed buildings realized before the adoption of seismic codes. The seismic response of the sample structure is evaluated performing a nonlinear static analysis. The seismic demand is defined following the EC8 prescriptions. In the analysis the concrete strength has been described through a probabilistic domain, and a different value of strength, consistent with the assumed domain, has been assigned to each storey of the sample structure. Therefore, an irregular distribution of stiffness and strength is assumed along the height of the building. The seismic response of the structure is expressed in terms of maximum interstory drift. The increase of the interstory drift due to the considered irregularity is analyzed following the instructions of EC8 and FEMA 356. The obtained results show that the EC8 approach can result not adequate to describe an irregular distribution of the concrete strength along the height of the building. The approach suggested by FEMA 356 provides a better representation of the seismic behavior of the sample structure, despite being not conservative.

The influence of axial load variation on the seismic performance of RC buildings

It is well known that the axial load can largely vary during a seismic event, playing an important role in the seismic performance 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 investigating the role of the axial load variation on the seismic capacity of RC columns, evaluated in terms of limit chord rotation and shear force, according to Eurocode 8. The research is performed with reference to a case-study, which is a doubly symmetric 4-storey RC framed building. The axial load variation affects both the seismic response and the capacity of the columns of the case-study, and, therefore, their seismic performance. Special attention has been paid to the role of the effective concrete strength of columns on the sensitivity of the seismic performance to the axial load variation.

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.

THE EFFECTS OF THE SEISMIC INPUT ON THE SEISMIC RESPONSE OF RC BUILDINGS

Abstract. This work is aimed at investigating the variation of the seismic response of RC structures subjected to different ensembles of ground motions in the nonlinear dynamic analysis for the seismic assessment of existing RC buildings. At the current time indeed, all the main International Seismic Codes provide a soil classification which is based on the shear wave velocity, the soil morphology and the assumed distance from the fault source. Depending on the soil properties, a suitable elastic spectrum is provided, defined on the basis of average properties assumed for the soil. To perform a nonlinear dynamic analysis, an ensemble of ground motions compatible to the elastic spectrum must be selected. The ensemble can be made by ground motions compatible to the Code spectrum for the assumed soil-type, or, alternatively, by assuming the ground motions to be consistent with the bed rock Code spectrum and by filtering them according with the specific stratigraphy of the site soil. In this work a comparison among these different approaches, all compatible to the European (Eurocode 8, EC8) and Italian (NTC 2008) Code provisions, has been made on a case-study, i.e. a real irregular RC Italian building. The comparison has been made in terms of seismic response, i.e. by checking the maximum displacement and interstory drift induced by the assumed ensembles of ground motions.

Influence of Infill Panels on the Seismic Response of Existing RC Buildings: A Case Study

An analytical evaluation of the seismic response of irregular RC buildings with infill panels designed without considering modern seismic provisions is presented in this chapter. Many RC buildings have been built in the 1960s and 1970s in Italy (and many other countries), before the seismic code was issued. Such buildings are usually irregular, due to mass and stiffness distribution, and they exhibit torsional effects when subjected to horizontal loads. One of the factors contributing to the torsional effects is the infill panels built on the perimeter of structures. Usually there is no discontinuity between the structure and the infill, so that the stiffness of the structure is largely affected by the distribution of the infill walls that can be irregular both in plan and in elevation. This work presents the effect of masonry infill in the seismic response of precode RC buildings. A numerical analysis has been performed on a case study, simulating the seismic response of the structure, including masonry infill panels. The case study of an existing building, rectangular in plan, having a mild mass eccentricity and an irregular distribution of infill panels, and, therefore, a different eccentricity at each storey is selected for the evaluation. The seismic performance of the building has been evaluated comparing the seismic response with the limit conditions provided by Italian Technical Code, both in terms of interstorey drift and shear capacity. The seismic response of the case-study building is satisfactory in respect to deformation parameters (chord rotation, top displacement, interstorey drift) but exhibits an unacceptable amount of shear in some of the elements, experiencing “short-column” failures. A possible solution for the problem is proposed, by removing some of the partial infill panels, in order to reduce the shear concentration demands and eccentricity.