Abstract:
Flyover bridges have become an essential part of the transport system of modern
cities. They are constructed to ease the traffic congestion in mega cities and improve
the mobility of the people. Dhaka City, already a mega city, is one of the most
densely populated cities of the world with severe traffic congestion problems. There
are at present two flyover bridges. The Govt. is planning to construct several
additional flyover type structures in Dhaka City to improve its overall transportation
system.
Flyover bridges have some unique features including curved geometry which make it
different from conventional bridges. According to Bangladesh National Building
Code Dhaka city falls in the moderate seismic risk zone. Flyover structures built in
these areas need to be studied for seismic loading conditions.
In this study, response of flyover bridge structures subjected to earthquake motion is
evaluated using non-linear finite element method. Bridges with different angle of
curvature has been considered. Time history analysis is carried out as it is the widely
used method for assessing non-linear seismic response of structures. Both
conventional non-isolated and isolated bridges are analyzed. Bridge structure
including isolation bearings have been modelled with beam-column elements and
nonlinear spring elements. In the study, non linearity is restricted to the isolation
bearing while rest of the structure is assumed to remain elastic at all times.
Parametric studies are carried out to identify variations in different bridge responses
like pier and deck displacement, pier and deck forces due to change in bridge
curvature. The responses of curved bridges are compared with corresponding straight
bridge. The result shows that transverse responses (displacement, acceleration,
transverse shear) increase with increase in curvature and longitudinal responses
(displacement, acceleration and longitudinal shear) increase with increase in
curvature. Effectiveness of isolation is also evaluated by comparing results of nonisolated
and isolated bridge of identical geometric and material properties. Isolation
significantly reduces 0 the pier forces and responses (displacement and acceleration).
As a case study, the first flyover bridge of Dhaka City, Mohakhali Flyover Bridge
has been selected for seismic response analysis. Widely used earthquake record has
been used after scaling down its PGA (Peak Ground Acceleration) value to O.15g. It
is observed that the flyover bridge doesn't form any plastic hinge due to prescribed
earthquake excitation and behave elastically. Again, its seismic responses are
compared with an equivalent straight bridge and the influence of curvature is
carefully compared.
