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.