Case Study
Approach
viaduct design for the Vasco Da Gama bridge
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reinforced concrete diaphragms
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prestress analysis and seismic loading
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reinforcement quantities calculated
Kvaerner Technology Ltd is one of a consortium of companies designing
the New Tagus Crossing in Lisbon, Portugal. It used the extensive solid modelling
capabilities of LUSAS Bridge for analysing reinforced concrete diaphragms on the
6.5km long central viaduct section of the crossing.
Each 77m long viaduct span unit consists of 8 precast box sections that
are assembled, prestressed and concreted together before floating out and jacking into
position onto piers initially on temporary supports. Once adjoining spans are in place,
continuity prestress tendons are fixed through the diaphragm walls into adjacent units and
a 1m wide 'working-room' gap between units is infilled with concrete to match web and
flange thicknesses of adjoining units. A pair of 1m x 1m permanent bearings are installed
directly under the infill concrete and all temporary supports are removed.
Three different diaphragm designs were required to
cater for standard span units; special 'lay-by' units - with wider decks and with thicker
walls; and for units with movement joints. Different LUSAS models were required for each
diaphragm type in order to analyse the stresses due to deck loadings, anchorage forces
from prestress and bearing reactions. Models of the end 6m of each span unit type were
built using 8-node enhanced strain solid brick elements. Spring supports represented
bearings and dead, supported, live, and seismic loadings all had to be considered.
LUSAS was used to analyse 3 stages of
construction: Stage 1 prestress in the casting yard; Stage 2 prestress once the units are
in final position with infill zones concreted; and Stage 3, to give bearing reactions
under normal and seismic loading. A linear static analysis was completed for the various
loadcases and, by use of the LUSAS slice section facility, moments and forces at critical
sections through the webs and flanges of the diaphragms were easily obtained to allow the
diaphragm reinforcement quantities to be calculated.
"This was the first
time I had used LUSAS. I found it easy to use and by using the slice
section facility we obtained a better understanding of the behaviour of the structure.
This helped us to produce a detailed design which could not have been done easily using
alternative simplified methods".
Walid Mahmoud,
Senior Bridge Engineer, Kvaerner Technology Ltd
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