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

Abstract

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.