Geology Reference
In-Depth Information
Optimal Design for Layout and
Detail of Continuous Girder Bridge
portance coefficient and the synthetic influence
coefficient are both assumed to be 1.
For an example continuous girder bridge with
25 spans, each span being 29 m long, the finite
element model is shown in Figure 2. The structure
is separated into 5 parts by 4 expansion joints.
There are three types of piers, the solid single-
column pier (SSP), the hollow single-column pier
(HSP) and the solid twin-column pier (STP), with
heights ranging from 5.5 m to 48 m. The bridge is
analyzed based on response spectrum method by
a combination of transverse and vertical seismic
action. The peak ground acceleration equals 0.41g.
For the original bridge, the response spectrum
analysis results including the pier top displace-
ments, the pier bottom shear forces and normal
stresses are displayed in Table 1 for both soil Type
I (rock and stiff soils) and soil Type III (soft to
medium clays and sands). It can be seen that both
the response magnitudes and variations along
these piers are relatively large due to the uneven
distribution of the piers' stiffness. New strategies
are suggested in the following sections to adjust
the piers' stiffness distribution adapt to the site
conditions and to avoid the uneven seismic force
demands caused by the transverse rotation of
girders so as to make the demands more uniform
and rational along the structural components.
The continuous girder bridge is one of the most
commonly used bridge structural forms. The
substructure has major effects on the bridge's
dynamic properties and the seismic demands
in different site conditions. Considering both
the longitudinal and transverse responses of the
bridge, many efforts of the optimal design are put
on the substructures, especially on piers. In view
of the design acceleration response spectrum for
normal bridges in Chinese code shown in Figure 1
(Ministry of transport of the People's Republic of
China, 1989), the acceleration response spectrum
varies according to the structural fundamental
period and the type of site. So in designing a
continuous girder bridge as a complete structure,
the stiffness distribution of some piers can be ad-
justed to decrease seismic forces and improve the
seismic performance for different site conditions.
In the design acceleration response spectrum,
the amplifications of ground acceleration are
respectively
2 25
×
.
T
for soil type I and soil type III according to Chi-
nese code (Ministry of transport of the People's
Republic of China, 1989), which is only used as
seismic inputs for the bridge. So the seismic im-
×
.
T
and
2 25
)
.
0 95
.
(
0 2
/
)
.
(
0 45
/
Conceptual Seismic Design Strategies
for Soil Type I
Figure 1. Design response spectrum (Ministry of
transport of the People's Republic of China, 1989)
From the code design response spectrum (Figure
1), the specific period
T
g
=
0 . s
for soil Type I
is relatively small, beyond which the acceleration
amplification decreases rapidly. This means that
any increase in the structural period can decrease
the seismic force demands significantly, if the
structural fundamental period exceed the spe-
cific period. Therefore, reducing the lateral flex-
ural stiffness (i.e. shifting to larger fundamental
period) can efficiently enhance the seismic per-
formance in the transverse direction. Based on
Search WWH ::
Custom Search