Angelo D’Ambrisi

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.

An energy-based approach for nonlinear static analysis of structures

Current codes and guidelines provide different methods to perform nonlinear static analysis of structures that require some non-intuitive assumptions in their application. In the present paper an energy-based method for nonlinear static analysis that allows to overcome these assumptions is proposed. In the method the capacity and the demand are both expressed in terms of energy. An energy capacity curve is computed considering that at each step the work of the lateral forces is equal to the structure internal work. The demand is represented in terms of an energy quantity, defined pseudo-energy, that is computed from both the maximum response displacement and the maximum response force. Constant ductility pseudo-energy spectra are introduced as energy demand design spectra, alternative to the input energy demand spectra. The definition of the performance point does not require iterative procedures for equating the internal dissipated energy to the demand energy. For the direct evaluation of the performance point two different operative procedures are proposed. The proposed method is evaluated comparing the earthquake-induced deformations of single degree-of-freedom systems resulting from the application of the presented nonlinear static analysis procedures with those obtained from the time-history analysis and from the application of the EC8 nonlinear static analysis procedure. The method is also applied in the case of a RC plane frame representing the inner frame of a six story building. The results obtained with the proposed method are in good agreement with those computed using nonlinear dynamic analyses, moreover they are characterized by a better accuracy with respect to the results obtained with the method provided by EC8.

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.

Design value estimate of the residuals of the seismic response parameters of RC frames

In the evaluation of the damage levels of reinforced concrete frames both the maximum values of the seismic response parameters and the residuals values are considered. The European seismic code (Eurocode 8) prescribes to calculate the design values of the seismic response of a structure with a nonlinear behavior as the mean value of the response to at least seven accelerograms or as the maximum value of the response to at least three accelerograms. The non-exceedance probabilities of the so calculated design values are very different, are not controllable and can depend on the number of accelerograms used in the analysis. In the present study a previously proposed probabilistic method is extended, with the appropriate modifications, to the definition of conservative design values of the residuals of the seismic response parameters of reinforced concrete frames. These values, characterized by predefined non-exceedance probabilities, are calculated using a limited number of generated accelerograms. The method utilizes a value of the response estimate modified with an amplification factor defined as a function of the scattering of the response and of the predefined non-exceedance probability. In the case of values of the response parameters, as the residuals, characterized by different scatterings the proposed methodology allows to obtain homogeneous non-excedance probability levels. The proposed method is used to evaluate the residuals of the response parameters of a reinforced concrete plane frame designed according to the Eurocodes. The results, compared with those obtained applying the Eurocode 8 recommendations, show the effectiveness of the method.

Seismic assessment of a historical tower with advanced numerical model tuned on ambient vibration data

This paper deals with the dynamic characterization and the evaluation of the seismic response of the medieval civic tower of Soncino (Cremona, Italy). The dynamic characteristics and the mechanical properties of the masonry tower are evaluated through ambient vibration tests, which provide results in a fast and non destructive way with respect to the traditional methods such as forced vibration tests. Nonlinear static and dynamic analyses are performed on a finite element model of the tower calibrated on the results of the dynamic identification. The damage levels and the seismic capacity of the structure are also evaluated. The obtained results allow to predict the seismic behaviour of the tower and to define possible strengthening and restoration interventions.