Case Study

Approach viaduct design for the Vasco Da Gama bridge

• reinforced concrete diaphragms

• prestress analysis and seismic loading

• 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 

See also: The Vasco da Gama bridge and approach viaducts