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 The Oyster
Channel Bridge near Yamba was
constructed in 1937 and was widened
to provide an improved carriageway
width and a pedestrian footpath. The
project was funded by the Roads and
Traffic Authority of New South Wales
in Australia.
During the project conceptual design
phase, a requirement to strengthen the
existing bridge superstructure was
identified. Strengthening using carbon
fibre reinforced polymer (CFRP)
laminates was proposed and included in
the preliminary project cost estimates.
Maunsell Australia Pty Ltd was
commissioned to undertake the detailed
design phase of the bridge widening.
Early estimates for deck strengthening
requirements were based on an elastic
analysis method which does not account
for the true post elastic behavior of
materials at the ultimate limit state.
The AS5100 Bridge Design Code
allows plastic redistribution of
moments, but this is rarely used
because of the computational
difficulty.
 Sam was used for
the detailed design analysis, and
effective use was made of the
“plastic member limit”
analysis feature. The analysis
involved setting “plastic
member limits” on the existing
girders equal to their ultimate
flexural strength. For this
analysis, when the specified member
limit is exceeded the load effect is
forced to remain constant while the
deflection increases by iteration
until structure equilibrium is
achieved for the applied load
condition. By this method, the load
was demonstrated to shed from the
overloaded existing members under
the traffic lanes to both the new
stronger girders on the outside and
to the lesser loaded existing girder
under the new footpath. The moment
curvature capacity of the deck
components was checked to ensure
that adequate ductility existed in
the members to accommodate the
plastic deflection with safety, and
then SLS checks were carried
out.
 The Sam section
design facilities were used for
accurate calculation of the beam
limiting capacities, for
determination of concrete and
reinforcement stresses, and for
crack width calculations at the
serviceability limit state. SLS
conditions were checked using an
elastic analysis taking advantage of
cracked section properties where
appropriate. The elastic analysis
was also used for establishing the
maximum shear forces (plastic
deformations necessary for shear
redistribution are less reliable and
worst case of the plastic and
elastic distribution of shear forces
was used).
 As a consequence
of the above analysis significant
CFRP strengthening to enhance
flexural strength for the Oyster
Channel Bridge was avoided. Shear
strengthening was still considered
necessary, and this was achieved
using CFRP laminates bonded as shear
stirrups in a zone adjacent to
supports.
The method of analysis adopted for the
Oyster Channel Bridge project
demonstrates the significant advantages
in adopting plastic methods of analysis
for the assessment and widening of
existing bridges.
Images courtesy of RTA and Maunsell
Australia Pty Ltd
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