Rehabilitation of the Figueira da Foz BridgeArmando Rito, Prof.,Proponte,Lda, Lisbon,Portugal, Julio Appleton, Prof.,A2P Consult,Lda, Lisbon,Portugal

Introduction

The Figueira da Foz Bridge over theRiver Mondego (Fig.1) has a totallength of 1421 m, inc1uding a 405 mlong cable-stayed bridge and two ap-proach viaducts with a length of 630 mon the left bank and 315 m on the rightbank. The bridge was designed by Prof.Edgar Cardoso and built in 1982. Thebridge is undergoing general rehabili-tation and strengthening.

Description of the Structure

The cable-stayed bridge has a mainspan of 225 m and side spans of 90 m(Fig. 2). The pylons, 85 m above thewater leveI, inc1ude four hollow con-crete inc1ined legs. The stays, spaced30 m along the deck, are made of gal-vanised wires passing over saddles atthe top of the pylons. The bridge deck

is a steel construction with two maingirders, where the cable-stays are an-chored, each one made of two 2 m-high I-beams interconnected by trans-verse beams spaced at 10 m. Thesetransverse beams support longitudinalstringers spaced at 8,20 m which inturn support a reinforced concrete slabwith a variable thickness of 0,13 m to0,20 m (Fig. 3).

The deck of the approach viaductshave a concrete slab 0,18 m to 0,22 mdeep supported by four longitudinalgirders, spaced 5,20 m apart with aspan of 45 m. The prestressed concretegirders have a rectangular section0,40 m by 2 m at mid-span and 0,60 mby 2,50 m at the supports. The slab isprestressed in the transverse direction(Fig.4). The deck is continuous foreach viaduct. The girders are fixed tothe transverse beams of the columnsby dowels and pinned plumb bearings.

Fig.1: General view ofthe bridge

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Fig.2: Plan and elevation

92 Structures in Portugal

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Right Bank Viaduct

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Fig.3: Transverse section at bridge pylon2060

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Fig. 4: Transverse section of the approachviaduct

Only in the transition pier to thecable-stayed bridge the support ofthe deck allowed relative horizontalmovements. The longitudinal beamsare also interconnected by transversebeams spaced at 15 m. Each supportalignment has 2 hollow rectangularcolumns 3 m by 1,60 m connected atthe top by a hollow rectangular beam 4m by 1,60 m, with a thickness of 0,25 m.The deck is fixed to the abutment forthe left bank viaduct.

Rehabilitation Work

A detailed inspection and assessmentof the safety of the structures accord-ing to the new codes showed that thebridge and the viaduct had sufferedsignificant deterioration and that seis-mic strengthening was also required.

The rehabilitation of such a construc-tion requires proper planning and theintroduction of working platforms(Fig. 5) which represent a significantcost of the works.

Cable-Stayed Bridge

For the cable-stayed bridge the mainmodifications were the following:

- Strengthening of the transverse topbeam of the towers for seismic actionby adding an external prestressingsystem

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Fig. 5: General view of the bridge underrepair and working platforms

- Replacing and strengthening the an-chorage system of the deck to thetransition piers by transferring thetension forces with prestressing bars

- General rehabilitation of the struc-

ture, including the saddles, the re-pairing and addition of a new corro-sion protection coating of the steelelements as well as the local repair-ing and concrete surface protectionof the pylons (Fig. 6).

Fig. 6: General rehabilitation of the saddles

Approach Viaducts

For the approach viaducts the mainmodifications were the strengtheningof the main girders, the introductionof earthquake damping devices be-tween the deck and the abutments, andgeneral rehabilitation of the concretestructures, including local repairingand a concrete surface protection.

Strengthening of the main girders wasmade by external prestressing (Fig. 7).This work was deemed necessary dueto the fact that the live load adopted inthe design was lower than the currentcode values as well as the damages ob"served in the beams .(cracking, con-

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crete defects). The prestressing systemincluded 4 cables for each beam, with atotal of 16 strands for each internalbeam and 14 strands for each externalbeam. A trapezoidallayout was adopt-ed with deviators under the two exist-ing transverse beams in each span. .

The strands are placed inside a 0 75mm High Density Polyethyleneduct and injected with cement grout.A double High Density Polyethyleneduct was used at the deviators. Nearthe active anchorage a free unboundedlength was provided to enable futurecable replacement. Along that lengththe strands are protected by individualducts and grease. At the deviators andat the anchorages the High DensityPolyethylene duct is inserted into ao88,90 mm steel pipe.

The deviators include a curved platesegment from a 0 101,60 mm steelpipe connected to a steel box fixed tothe main girders and filled with groutto provide a rigid support to the cable(Fig.8a). A prototype test was madeto check the behaviour of the deviatorsand the cables.

The anchorages are standard activeanchorages with galvanised steel cov-ers. The anchorages are fixed to an-chor concrete blocks which are fixedto the transverse beams and deck(Fig. 8b).

The cables were stressed to 65% oftheir ultimate strength to achieve theeffective prestressing force of 2480 kNfor the internal beams and 2170 kN forthe external beams.

New energy dissipating devices wereintroduced and provided between thedeck and the abutments. The dowelbars used to fix the deck to the abut-ments were found not to be robustenough and the connection to thebeams did not guarantee the requiredstrength. A three dimensional modelalso showed that the shorter columnbents did not have the requiredstrength. To install the viscous dampers

Fig. 7: External prestressing of the approachviilducts

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Passive anchprage by co~g Actiye anchorage

b) Anchorage External cables

Fig. 8: External prestressing - Details

(one in each of the four beams) theabutment had to be modified to ac-commodate the dampers and new slid-ing bearings between the girders andthe abutment (Fig. 9).

To establish the characteristics of theviscous dampers, a three-dimensionalnon-linear time-dependent dynamicanalysis was performed with 10 artifi-cial accelograms to simulate the Eu-rocode 8 design earthquake action inPortugal. The viscous dampers, one foreach beam, were defined by the consti-tutive law F = cV«.A parametric studywas performed with C = 500; 1000;1500 kN (slm)« and a =0,1;0,2.For the left bank viaduct the selec-ted dampers have a constitutive lawF =1000 VO.I.The efficiency of this so-lution was the following:

- The earthquake design longitudinaldisplacement was reduced from79,90 mm (obtained in a model witha free longitudinal displacement atthe abutment) to 32,00 mm at theabutment and from 88,60 mm to39,50 mm at the transition pier

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J_ ~b:ttFig. 9: Details of the abutment changes to instaUthe dampers

- The force transferred to the abut-ment was reduced from a total valueof 11268 kN (obtained in a modelwith a rigid connection of the deck tothe abutment) to 4 X 802 = 3208 kN

- Yielding at the shorter columns forthe design earthquake is avoidedwith the introduction of dampers.

The dynamic behaviour model waschecked with the experimental evalua-tion of the vibration modes and fre-quencies on site.

Conclusions

Although the structure presented con-siderable deterioration, the inspec-tions of the stays of the cable-stayedbridge showed that the galvanisedwires and the anchorages were in goodcondition. That information was a ma-jor contribution to the decision ofmaintaining the stay cable system.

The design concept of modifying theviaducts - installing external prestress-

ing and dampers - is straightforwardbut the execution required very care-fuI and detailed preparation of thestructure due to the complex geometryof the bridge.

SEI Data Block

Owner:

Estradas de Portugal E.P.E., Almada,Portugal

Rehabilitation Design.'Proponte, Lda, Lisbon, Po(tugalA2P Consult, Lda, Lisbon, Portugal

Contractor:

Soares da Costa, SA, Portugal

Prestressing steel (ton):

Surface protection

Steel structure (m2):

Concrete (m2):

Concrete Repair (m2):

Total cost (EUR million):Service date after

rehabilitation:

762

22500

93 500

11 300

9

June 2005

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