Case Study
Seismic
design check of the Vasco da Gama bridge and approach viaducts
- cable stayed road bridge and viaduct structures
- pile/soil interaction
- seismic analysis
Hyder Consulting used LUSAS Bridge
to perform a design check of the Vasco da Gama bridge and associated crossing of the Tagus
at Lisbon, Portugal. The 18km long crossing comprises a series of concrete viaduct
structures together with a 420m span cable stayed bridge over the main shipping channel.
All structures are supported on piles extending through extensive alluvial deposits to a
depth close to bedrock 60m below river bed level. Lisbon was devastated by an earthquake
in 1774 so all parts of the crossing are designed to ensure minimum seismic damage in the
event of another severe earthquake.
LUSAS was used to calculate the seismic response of
the foundations and various parts of the superstructure. Models of each section of the
approach viaducts were created for seismic response analyses and an initial 2D pile/soil
interaction model was developed. Pile/soil interaction is of primary significance with
seismic displacements induced at pile cap level affecting the entire bridge and viaduct
structures.
In the substructure analysis, soil degradation was a key consideration
and this required an iterative analysis procedure. 2D plane strain finite elements were
used to model the soil, together with constraint equations to ensure that only shear
deformation was considered. An eigenvalue analysis was done to determine the natural
frequencies and mode shapes of the soil profile. This was followed by a spectral response
analysis whereby an idealised seismic event was applied at bedrock level via
acceleration/frequency curve input to give soil displacements throughout the soil model.
From the results of soil displacement with respect to soil depth Hyder were able to
determine forces and moments in piles at any depth and perform checks with the original
design. Using the soil profile model, Hyder have carried out additional soil profile
analyses for other locations on the crossing.
In
the cable stay bridge and viaduct analyses, displacements at movement joints due to
seismic loading were just one of many checks that had to be made. To model these
structures 3D beam elements were used throughout, with the exception of the decks which
were modelled with 3D shells. Joint elements were used to model sliding and pinned
connections and spring supports represented upper and lower bound pile stiffnesses.
A nonlinear static dead load analysis was undertaken to determine the
deflected shape and stiffness of the structure, followed by an eigenvalue analysis on the
stress-stiffened model to determine natural frequencies and mode shapes. A spectral
response analysis using seismic data for upper and lower bound pile group stiffnesses
produced the dynamic displacements, accelerations, forces and moments required for Hyder
Consulting to complete the check.
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