Seismic Vulnerability of Existing Buildings

Abstract

In order to assess the seismic risk for Switzerland, and particularly for the city of Basel, a joint project on the subject of “Earthquake Scenarios for Switzerland” was launched by the Swiss Seismological Service (SED) and the Institute of Structural Engineering (IBK) at the ETH Zurich. The goals of the study are to improve the assessment of seismic hazard, to investigate the vulnerability of the built environment and finally, to combine the results to elaborate risk scenarios as the first fundamental step in the mitigation process. The objective of this work is the evaluation of the seismic vulnerability of existing buildings with a focus on the residential building stock in the city of Basel. Since no major damaging earthquake has occurred in Switzerland in recent times, vulnerability functions from observed damage patterns are not available. A simple evaluation method based on engineering models of the building structures suitable for the evaluation of a larger number of buildings is therefore proposed. First, the general idea of the evaluation method based on nonlinear static procedures is introduced in Chapter 3 which briefly discusses the two key elements of a vulnerability analysis, the capacity (strength and deformation capacity) of a building and the seismic demand. The results are vulnerability functions expressing the expected damage of a building as a function of the seismic input. The application of the evaluation method to unreinforced masonry buildings and to reinforced concrete buildings is discussed in more detail in Chapters 4 and 5 respectively. Special attention is paid to the frame action due to the coupling of the walls by floors and spandrels. Comparisons with test results from model buildings in the case of masonry buildings and with a recently proposed and thoroughly checked deformation orientated method in the case of reinforced concrete buildings show that the proposed method suitably forecasts the capacity of a building. Finally, a comprehensive inventory of the buildings in a small target area in Basel was established based on plans and a street survey. The inventory comprised a total number of 87 buildings which were then assessed using the evaluation method. Based on the results of the assessment, building classes were defined depending on the type of structure and the number of storeys. Corresponding fragility curves were determined, expressing the probability of a building belonging to a certain building class of reaching or exceeding a particular damage grade given a deterministic estimate of the spectral displacement. The classification of the buildings allows an extrapolation of the results to a larger area or to the whole city. A statement on the actual seismic risk, however, is not possible without the knowledge of the local seismic hazard which is not yet available.