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New Peru bridge: Spanning the seismic issue

Designing foundations to support Peru’s longest urban bridge and deal with the potential for 900mm seismically-induced vertical movements called for detailed analysis

Completion of construction work on the Chilina Bridge in the Peruvian city of Arequipa at the end of 2014 will help end years of traffic congestion in the north of the city. But building of the bridge is not just a victory for road users, it is also a success story for finite element analysis in proving the possibilities of modern engineering.

The concept for spanning the Chili River Valley was first conceived more than a decade ago but plans for the bridge were stalled by funding issues as well as the high seismicity. Design consultant Arenas and Asociados used software from Midas to prove that the structure could be built to cope with peak ground accelerations of 0.6g and within budget.

Nocturna2_03

The new bridge will help cut congestion but seismicity and limited budgets prevented the construction until now

The Regional Government of Arequipa commissioned Consorcio Constructor Puente Chilina – a joint venture of Metric, Incot and Isolux Corsan – to undertake the £56M PPP project and work is due to be completed at the end of 2014.

“The bridge is a segmental continuous pre-stressed concrete viaduct, 562m long with 157m main span and two 11.3m wide decks,” says Asociados and Arenas technical director Guillermo Capellán Miguel. “The typical box girder section has variable depths. Balanced Cantilever Method is being used for construction with 5.1m long insitu segments built using form travellers.”

“Seismic displacement demands are as high as 900mm in the transverse direction and 450mm in the longitudinal direction”

Guillermo Capellán Miguel, technical director, Asociados and Arenas

“The main challenges of the project are related to the large dimensions of the bridge and high seismicity at the bridge location. Deep foundation analysis was needed to understand the interaction of the structure with seismic behaviour and ground movements.

“We used Midas Civil software for the overall design of the structure and the package also allowed us to carry out the necessary seismic analysis studies.”

The analysis was done according to AASHTO and Caltrans specifications for a 1,000 year return period earthquake.

“Seismic displacement demands are as high as 900mm in the transverse direction and 450mm in the longitudinal direction,” explains Miguel. “We worked with seismologists Geoter to undertake the local seismic risk studies and Earthmechanics to understand the soil structure interaction of the 1.5m diameter piles deep foundation under seismic loading.”

Miguel’s team used the Midas software to understand the implications of the staged construction sequence and also to undertake modal elastic dynamic analysis, non-linear time history analysis and pushover analysis for the seismic calculations.

According to Miguel, the Midas package was used because of the complexity of the structure and the possibilities of the software to analyse the 95 different construction phases, the pre-stress loads analysis and AASHTO moving loads, as well as the foundation and seismic analysis.

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Midas software was used to analyse the bridge’s reaction during an earthquake as well as during the construction and normal operational phases

“Midas allows fast nonlinear time history analysis while considering different mass and stiffness matrices for foundations, and different acceleration time histories for each of the abutments and piers foundations,” says Miguel.

“AASHTO and Caltrans specifications led to the choice of an earthquake resisting system (ERS) type 1 with ductile behaviour of the substructure and essentially elastic superstructure. That means plastic hinges form in the columns’ bottom and top sections to limit the forces to be resisted and to dissipate energy in the process.

“Piled foundations remain essentially elastic and capacity protected by the adopting over-strength magnifiers in the Plastic Moment and forces applied. In any case it is very important to evaluate soil structure interaction in the Kinematic Motion Time series taken into account in the analysis, soil resistance in the axial pile analysis under seismic loads, and lateral pile capacity and stiffness using nonlinear p-y curves.”

Consorcio Constructor Puente Chilina is currently working on the construction of the 130 piles, which will support the bridge piers in the very dense matrix consisting of boulders, cobbles, gravel, sand, clay and silt that dominates the ground conditions at the site. The piles are all 1.5m diameter temporary cased, cast insitu and being constructed to 26m below ground.

Work on site is progressing well, but foundation work on site is being checked by dynamic load testing and cross-hole analysis to verify the soil parameters used in the design.

Miguel believes that having all the design work undertaken in a single software package will help fast track any design changes that result from the tests: “It was very helpful to have all-in-one software that allowed all aspects of the project to be accounted for at the same time as small changes can have a large effect on behaviour.

“Sometimes a small change would help to aid seismic analysis verification but meant another static verification wasn’t satisfied anymore. The software also made it possible to carry out very complex analyses of non-linear time history in a short period of time.”

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