The Ingenieur Vol 59 July-Sept 2014 The Ingenieur Vo. 59, July-Sept 2014 | Page 33

The result

Figure 6 shows the synthetic response
spectrum for Penang in 500 and 2500 return
period (TR) (Azlan, 2005) attached together with
longitudinal period for all types of bridge. Longer
periods provide less response and produce better
overall bridge performance. Normally this value is
taken as the design value for designing a bridge
subject to seismic events.

Modal Analysis
The modal mass participation depends on the
number of degrees of freedoms. The more degrees
of freedom, the more frequencies, and likely the
more modes are required to get the precise
percentage of mass participation. This analysis
uses 65 total number of modes for the modal
analysis to get the modal load participation ratio
of more than 90% of the dynamic analysis for both
horizontal directions. Table 2 shows summary of
the bridge periods in three directions (UX,UY and
UZ). The result clearly shows that more flexible
bearings in shear direction (especially for seismic
isolator bearings) provide bigger period for the
bridge in horizontal directions (longitudinal and
transverse).

Figure 6: Response of a bridge due to ground
acceleration in longitudinal direction

Table 2: Summary of bridge Period
Model
Type

UX
(Longitudinal)

UY
(Transverse)

UZ
(Vertical)

Fix-based

0.2250 Sec

0.1704 Sec

0.2646 Sec

Rubber
Bearing
Pad

0.9725 Sec

0.9764 Sec

0.2621 Sec

Seismic
Isolator
Bearing

1.6980 Sec

1.7019 Sec

Table 3 shows the result extracted from
Figure 6 together with the calculated amplification
percentage based on original ground acceleration
(ag) for both TR. For TR=500 the ground acceleration
is 0.0555g and it is amplify 98% for fix-based model
to 0.110g and 23% for rubber bearing pad model
(0.068g). For TR=2500 with 0.10781g in ground
amplify 96% for fix-based model (0.212g) and 21%
for rubber bearing pad model (0.130g). However,
the performance of the bridge was enhanced by
using seismic isolator bearings with 23% decrease
of ground acceleration (0.042g at TR=500 years)
and 33% decrease to 0.072g (TR=2500).

0.2768 Sec

Table 3: Summary of bridge response due to ground acceleration in longitudinal direction
Model
Type

Longitudinal
Period

Response (Spectral Acceleration)
TR=500 years
(ag=0.0555g)

TR=2500 years
(ag =