Overall Conceptual Seismic Design and Local Seismic Capacity Design for Components of Bridges - Structural Seismic Design Optimization and Earthquake Engineering - page 272

BACKGROUND Generally the ductility approach and the seis-

mic mitigation and isolation approach are two
common strategies in the conceptual design of
a bridge. The ductility method requires detailed
reinforced concrete design for shear resistances
and confinement design at the potential plastic
hinges of piers in order to resist earthquake ac-
tions (Fan & Zhuo, 2001). The seismic mitigation
and isolation approach protects the substructure
through additional means of energy dissipation
by shifting the structural fundamental frequency
away from the dominant frequencies of the ground
motions. However, the above two methods are
not satisfactory for a long span floating cable-
stayed bridge in view of the significant excessive
displacement demands at the tower top and the
girder ends under earthquakes. Hence a new con-
ceptual design strategy is formulated to improve
the seismic performance of a long span floating
cable-stayed bridge through rational configuration
of bridge tower forms. Accordingly a new-type
spatial tower is proposed to replace the generally
used inverted Y shape tower for an optimal bal-
ance between the seismic force and displacement
demands of the bridge.
Cable-stayed bridges are classified into differ-
ent categories by the connection types between the
tower and the girder, such as the floating system
(no restraint between the girder and towers), the
semi-floating system (partial restraint between the
girder and towers), the fixed system (full restraint
between the girder and towers), and so on. They
are widely constructed in seismic zones for their
good seismic performance (Walther, Houriet,
Isler, Moïa, & Klein, 1999). However, the floating
connection may cause excessive seismic displace-
ment demands at the girder ends while the fixed
connection may lead to excessive force demands
at the tower bottom (Ye, Hu & Fan, 2004). So the
seismic isolation connection types between the
tower and the girder, especially the elastic cable
connection, are adopted by more cable-stayed
bridges. It is important to study the seismic iso-
lation mechanism of the elastic cables, since the

For the bridge engineers, the conceptual seismic
design is the first but most important stage in a
bridge design that involves numerous complex
and time-consuming tasks. Any optimization in
the conceptual design phase will lead to greater
cost savings than those at the detail design stage
when decisions become more restricted. Though
in strict terms, the structural seismic design opti-
mization follows rigorous formulations and needs
definite algorithms, the choices and adjustments
of structural systems from the perspective of
“overall conceptual seismic design” will inno-
vationally give meaningful enlightenments and
correct directions for seismic design optimization
of structures. Instead of deferring seismic analyses
to the detail design stage, it is emphasized that
adequate seismic planning should be implemented
early in the conceptual design.
For continuous girder bridges, past conceptual
design used to focus on the longitudinal seismic
behavior, perhaps because most previous damage
reports referred to longitudinal support unseating
or structural collapse (Yang, 1999). However,
some transverse damages have been caused in
Wenchuan Earthquake because of the lack of
transverse earthquake resistances (Yuan & Sun,
2008). Pier's vertical torsion under earthquakes
also increases the transverse deformation of the
girders. Although a bidirectional ductility design
will be effective to increase pier's torsional stiff-
ness and transverse ductility capacity so as to
improve the transverse seismic performance, but
it may decrease the shear resistance significantly
(Fan, 1997). Therefore it is necessary to study the
transverse seismic performance of a continuous
girder bridge to select the best structural form
under different site situations. Accordingly the
structure's dynamic characteristics should adapt to
the site conditions and the demands should be uni-
form and rational along the structural components
so as to decrease the overall seismic demands.