In this thesis the propagation of seismic waves through a number of soft soil layers and
up into a bridge with a total span of 2 km is investigated. Only horizontal shear waves
(SH-waves) are modeled, as most buildings are often most vulnerable to excitations in
the horizontal direction. In seismic design the local site effects of soft layers is often
of big significance, as the amplification of seismic waves when they propagate through
soft soil layers is often quite significant.
The wave propagation through the soft soil layers are modeled using both a dynamic
linear viscoelastic semi-analytic Domain Transformation Method (DTM) and a dynamic
linear viscoelastic Finite Element Method (FEM) model. The models are compared in
order to validate the results, and the results fit quite nicely. The models are solved in the
frequency domain in order to use hysteretic damping, which is recognized as a good
damping model for soils. A partly linear model is introduced in the FEM soil model in
order to take into account decrease of the small strain shear modulus and increase of
the damping ratio as the shear strain amplitude increases.
Strong motion data for the 1995 Aqaba Earthquake, which had a moment magnitude
of 7.3, is used. The soil response is determined and used as an input to the bridge
model. It is found that the soft soil layers amplifies the seismic waves with several
magnitudes. The wave propagation through the bridge is modeled using a dynamic
linear viscoelastic three dimensional FEM beam model. Both compressional, torsional
and shear wave propagation through the bridge can be modeled. The angle at which
the the SH-waves hits the bridge can also be varied, which is used in a small parameter
study.
Another parameter, which is investigated is the delay at which the seismic waves hits
the different columns. The results of the parameter study show that the critical direction
of the earthquake is parallel to the bridge due to the fact that high normal stresses and
thus compressional waves occur in the bridge deck. The bridge is located in water and
hydrodynamic mass is therefore applied in the bridge model. Soil structure interaction
is also taken into account by using a lumped parameter model to model the rotational as
well as the torsional stiffness of the foundations which are located on the soft soils. For
the columns founded directly on bedrock, however, the rotational and torsional stiffness
is assumed infinitely stiff.