Contribution to methodology for developing fragility curves of shallow metro tunnels
Summary of undergratuate diploma thesis
Fragility curves express
the probability of structural damage due to earthquakes as a function of ground
motion indices, e.g., PGA, PGV. Based on the actual damage data of tunnels from various
earthquakes, a set of empirical fragility curves was constructed. However, the
stuctural performance (static and dynamic), divisions between the reasons that
cause damage and the shape of tunnels were not considered to construct the
empirical fragility curves. In this study, a numerical approach was adopted to
construct fragility curves for metro tunnels.
An one-dimension analysis
software and a finite element analysis program were chosen to approach the
aforementioned probem. Using the stong motion records from three earthquakes,
which response spectrum were close to the elastic response spectrum of soil A
(EC8), linear dynamic analysis were performed. Various kinds of soils with
different velocities of shear waves each, were simulated. Peak Ground Accelaration varied from 0.1g to
0.8g. The output of the one-dimensional analysis were the ground motion time histories. The ground displacement time
history, that corresponded with maximum shear deformation in the tunnel’s
depth, was imposed on the simulated model. An inelastic finite element
pseudo-static analysis was proposed for evaluating the response of tunnel due
to earthquakes. A numerical simulation of the seismic response of the tunnel
based on dynamic analysis is also used. Some typical cross-sections tunnels
were used. A wide parametric analysis was performed. The corresponding
variabilities were the tunnel diameter, the lining thickness and the cover
depth. The results of all the above analysis are presented in relevant
diagrams. In addition the construction of Peak Ground Accelaration - Shear
Strain diagram was attempted.
A methodology for constructing analytical fragility curves as a function
of PGA was developed utilizing moment values. A median and a standard deviation
value of the resistance of the lining were computed after statistical analysis.
Using the damage indices and ground motion indices, fragility curves were
constructed. Two damage states that depict minor and moderate damage were
represented. These curves were compared with the empirical ones (like those
from Hazus) and other damage data found in prior studies. The proposed approach
may be used in constructing the fragility curves for TBM and Cut and Cover
tunnels.