Earthquake Engineering Research - HKU Libraries - The University ...

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Embankment, Beanngs
& Fluid Dampers
Embankment, Bearings
& Fluid Dampers
Pile Foundation
(Equivalent Beam & Dashpot
Stick Model
Figure 4. Numerical model of 91/5 Overcrossing
In this paper, the dynamic response of three configurations of the 91/5 Overcrossing are compared: (a) the
as built configuration where the deck is supported at both ends by elastomeric pads and is equipped with
fluid dampers (Pads+Dampers); (b) same configuration as (a) but without fluid dampers (Pads only); (c)
the bridge-deck is rigidly connected to integral abutments (Integral Abutments).
Figure 5 summarizes the peak total accelerations and relative displacements of Point A (east end of deck)
for various earthquakes. The results are sorted according to the peak ground acceleration of the faultnormal
component of each earthquake record. The thin lines are the results for all three cases when soilstructure
interaction is neglected while the think lines are the results when soil-structure interaction is
included in the analysis. Along the longitudinal direction, the bridge with sitting abutment is more
flexible than that with integral abutments, so accelerations are smaller and displacements are larger.
Damping reduces both displacements and accelerations of the flexible configuration. However, along the
transverse direction, the case of bridge with integral abutment not only yields smaller accelerations but
also smaller relative displacements. This can be explained by looking at the transverse mode of the two
configurations. The transverse mode of the case of integral abutment is primarily a flexural mode,
whereas in the case of sitting abutment the transverse mode is primarily a translational mode where the
entire deck translates sideways without flexing appreciably. This results to larger displacements at the
deck ends and also larger accelerations. Supplemental damping reduces both displacements and
accelerations but the response of the bridge with sitting abutments appears to under-perform the response
of the bridge with integral abutments. Figure 5 also indicates that the soil-structure interaction increases
both accelerations and displacements. Similarly, Figure 6 plots the peak total accelerations and relative
displacements of Point B (middle of the deck). The trend of accelerations and displacements along the
longitudinal directions resemble the trend that one observes at Point A. Along the transverse direction the
results for accelerations and displacements of three configurations are mixed because the mid-span moves
sideways approximately the same amount regardless of whether the transverse movement is the result of a
primarily flexural mode or of a primarily translational mode. Again the bridge response is considerably
underestimated when the effect of soil-structure interaction is neglected.
Figure 7 plots the peak forces behind the end abutments. Clearly, the isolated configuration reduces the
longitudinal forces but not the transverse forces. Interestingly, the presence of fluid dampers yields
transverse forces that are higher than the forces when the bridge has integral abutments. Figure 8 plots the
peak shear forces at the base of columns. Along the transverse direction the isolated bridge transmits
approximately the same forces to the column base that the bridge with integral abutments transmits.
Along the longitudinal direction, the differences are dramatic since in some earthquakes the column base
shears of the isolated bridge are more than two times the column base shears of the bridge with integral
abutments.