Overall Conceptual Seismic Design and Local Seismic Capacity Design for Components of Bridges - Structural Seismic Design Optimization and Earthquake Engineering - page 280

Table 4. Seismic displacement comparisons

once the bridge installed with them, which may
change the overall seismic force distribution of
the bridge.
As a case study, consider a long span float-
ing cable-stayed bridge with spans 55 m, 165 m,
165 m and 55 m and the finite element model is
shown in Figure. 8. There are sliding bearings
at the subsidiary and side piers and no bearings
between the tower and the deck girder from the
original design scheme of the bridge.

Model Location Longitudinal

displacement
(m)

Transverse
displacement
(m)

(i)Original
inverted
Y shape
tower
model

Girder
end

1.030 -

Tower top 1.110 0.200

Span
center

1.040 0.950

Alterna-
tive spa-
tial tower
model

Girder
end

0.440 -

Tower top 0.390 0.410

Parametric Analyses

Span
center

0.450 0.870

Based on dynamic analyses and parametric simula-
tions, in which the elastic cables are simulated as
elastic bearing connection elements, the variations
of the bridge fundamental period corresponding
to the elastic cable stiffness are shown in Figure 9
and the relationship between the vibration modes
and the stiffness is shown in Table 6.
It can be seen that the fundamental period
decreases rapidly when the elastic cable stiffness
increases from 0 to 10×10 5 kN/m. At the stiffness
values larger than 10×10 5 kN/m, the period
variation is small. It is also shown in Table 6 that
the vibration modes do not change when the cable
stiffness increases from 0 to 5×10 5 kN/m. on the
contrary, at stiffness of 30×10 5 kN/m, the first
mode of vibration changes from a girder longitu-
dinal drift to a tower lateral bending in the same
direction. This may be due to the rational elastic
cable stiffness leading to a change in the dy-

Seismic Isolation Mechanism of
Elastic Cables in Cable-Stayed
Bridge

The excessive seismic displacement of the girder
end is one of the challenges to the cable-stayed
bridge designers, since it may exceed the deforma-
tion capacities of expansion joints. There may also
be excessive seismic bending moment at the tower
bottom. Isolation devices are usually adopted to
avoid excessive demands, including the system of
elastic cables installed between the tower and the
deck. For a floating cable-stayed bridge without
elastic cables, the seismic force is transmitted
from the deck to the tower mainly depending on
the inclined cables. However, the elastic cables
become one of the main force transmission paths

Table 5. Seismic force comparisons at the tower bottom

Model Column

number
(Figure 7)

Longitudinal direction Transverse direction

Shear force
(×10 3 kN)

Bending moment
(×10 6 kN.m)

Shear force
(×10 3 kN)

Bending moment
(×10 6 kN.m)

(i)Original inverted Y
shape tower model

1 (or 2) 30.90 2.290 47.60 1.670

(ii)Spatial tower
model

1 (or 2) 32.20 2.030 42.60 1.300

3 (or 4) 32.20 2.030 32.00 1.220

((ii)-(i))/(i) (%) 1 (or 2) 4.2 -11.4 -10.5 -22.2

3 (or 4) 4.2 -11.4 -32.8 -26.9