Influence of the variability of concrete mechanical properties on the seismic response of existing RC framed structures

Abstract.. In Italy many RC framed building have been realized in the 60’s and the 70’ before the adoption of seismic codes. These buildings were designed for vertical loads only and were realized with concrete having poor and non homogeneous 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 storey 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 some of 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 capacity of the sample structure is evaluated considering the variability of the concrete strength for each storey. The concrete strength is assumed constant at each storey. 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 interstorey drift and compared with the limit values provided by SEAOC for the different limit states. The increase of the interstorey 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.