The authors discuss the conceptual plan, component design and reinforcement, structural analysis, together with the precasting and construction techniques used in renovating the famous Maria Cristina Bridge in San Sebastian, Spain. Renovation of the Historic Maria Cristina Bridge Javier Mainar Chief Civil Engineer San Sebastian City Council San Sebastian, Spain Juan J. Arenas Professor of Civil Engineering University of Santander Santander, Spain N estled in the picturesque Spanish Basque country, overlooking the Bay of Biscay near the French border, lies San Sebastian — a charming city of about 180,000 people. Because of its mild climate and beau- tifuI environment, over the centuries San Sebastian became a favorite seaside resort of Spanish royalty. Even today, it is patronized by a large segment of Spain's upper class. The urban architecture of San Sebas- tian evolved along traditional lines. The city has many historic buildings, monu- ments, and other structures which re- flect the uniqueness of Spanish culture and architecture. Therefore, when it came time to renovate the famous Maria Cristina Bridge, the San Sebastian City Council was ver y careful to preserve this tradition. But before discussing the renovation procedures, some back- ground is needed on how the original bridge came to be built. 20
Transcript
The authors discuss the conceptual plan, componentdesign and reinforcement, structural analysis,together with the precasting and constructiontechniques used in renovating the famous MariaCristina Bridge in San Sebastian, Spain.
Renovation of the HistoricMaria Cristina Bridge
Javier MainarChief Civil Engineer
San Sebastian City CouncilSan Sebastian, Spain
Juan J. ArenasProfessor of Civil EngineeringUniversity of SantanderSantander, Spain
Nestled in the picturesque SpanishBasque country, overlooking the
Bay of Biscay near the French border,lies San Sebastian — a charming city ofabout 180,000 people.
Because of its mild climate and beau-tifuI environment, over the centuriesSan Sebastian became a favorite seasideresort of Spanish royalty. Even today, itis patronized by a large segment ofSpain's upper class.
The urban architecture of San Sebas-
tian evolved along traditional lines. Thecity has many historic buildings, monu-ments, and other structures which re-flect the uniqueness of Spanish cultureand architecture. Therefore, when itcame time to renovate the famous MariaCristina Bridge, the San Sebastian CityCouncil was very careful to preservethis tradition. But before discussing therenovation procedures, some back-ground is needed on how the originalbridge came to be built.
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1-
BACKGROUND
The art and practice of using rein-forced concrete was introduced in Spainby the eminent civil engineer J.Eugenio Ribera (1864-1936). Beginningin 1897, Rihera built many reinforcedconcrete bridges using patents de-veloped by the Belgian contractorHennebique. It is interesting to notethat Ribera praised this "new material"for its "enormous advantages such aseternal durability, incombustibility andan economy of about 15 to 40 percent inrelation to steel constuction." This en-thusiasm also led Rihera to build thefirst Spanish portland cement factory inthe northern region of Asturias.
In 1902, Ribera began using a newconstruction method consisting of arigid steel lattice arch used as the corematerial for the concrete that wouldeventually cover it. After much delib-
eration, this method was chosen by theSan Sabastian City Council in 1904 asthe design alternative during a nationalcompetition for the erection of a newbridge over the Urumea River to link therailway station to the city. The juryawarded Rihera and architect Zapata thecontract despite contest rules statingthat any possible use of reinforced con-crete be covered to provide the appear-ance of a stone bridge because " .. .reinforced concrete seems to he thematerial that really is in accordance withthe spirit of progress ..."
The Maria Cristina Bridge is com-posed of three vaults with 24 m (79 ft)spans, two 3 m (10 ft) wide central piersand two massive abutments. The totalwidth of the bridge is 20 m (66 ft), in-eluding two 3.5 m (11 ft 6 in.) sidewalks,four traffic lanes totaling 12 m (39 ft),and heavily decorated parapets.
The lavishly ornate facade includes a
PCI JOURNALJMarCh-April 1986 21
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cornice with mythological lion heads, aswell as blue glazed tiles with gildeddragons and such city symbols as a pairof oars depicting the marine vocation ofSan Sabastian. The pier fronts areshaped in the form of a ship's bow, giv-ing emphasis to the fact that the bridgeis located only 500 m (1640 ft) away fromthe river mouth.
The massive concrete vaults have arise equal to 2 m (6 ft 8 in.) and a depthvarying between 70 cm (28 in.) at thebase and 60 cm (24 in.) at the crown.However, to avoid the need for scaf-folding during construction, Ribera em-bedded thirteen steel lattice arch struc-tures approximately 150 cm (59 in.) apartto integrate and strengthen the concretecomponents.
After the initial 25 cm (10 in.) thickconcrete layer was poured and hadhardened, the composite structure wasable to support the remaining deadweight of the bridge. The deck slab wassupported by longitudinal walls placedabove the steel lattice arches. Since theriver bottom consists of large amounts of
sand and other deposits, Rihera an-chored the entire structure with 5 in (16ft 5 in.) long precast reinforced concretepiles. The original design of the bridgecan be seen in Fig. 1.
As an engineer-constructor, Ribera'sboldness and abilities were exceptional:the entire Maria Cristina Bridge proj-ect was completed in only 6 months, in-cluding driving the piles into the sandyriver bottom. Loading tests of the bridgebegan December 21, 1904 and includedthe overload of an entire span with sandup to 500 kplm 2 (102 psf). Under suchloading, the crown of this vault sankonly 5 mm (0.2 in.). It is interesting tonote that the test took place on SaintThomas Day, a very auspicious holy daysince Ribera reportedly stated, "Seeingin order to believe ..."
As a result of the tests, the strength ofthe Maria Crist.ina Bridge was beyonddispute. Since its official inaugurationon January 20, 1905, the bridge hasbecome an important part of life forthe city's inhabitants and an essen-tial element of the San Sabastian
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Nw Fig. 1. Original drawing of Maria Cristina Bridge; (top) longitudinal section and detail of pier; (bottom) transverse cross section.
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Fig. 2. View of south facade of bridge in 1983.
urban landscape.Despite the bridge's initial strength,
time took its toll and, 80 years later, thephysical deterioration of the structurebecame alarming. The very aggressivemarine environment and lack ofmunicipal funds for maintenance hadled to the precarious structural deficien-cies shown in Figs. 2 and 3.
Fig. 2 shows the complete erosion ofthe south facade pier bows. The pierbows on the north facade have remainedrelatively undamaged. Still more dis-turbing, however, was the erosion of thevaults, including surface damage andinternal cracking. This damage con-sisted of several large openings alongthe vertical planes coinciding with theplacement of the steel lattice arches,some of which were completely missingdue to steel corrosion.
Thus, one can conclude that the sup-posed increase of strength Rihera soughtto achieve by using such steel rein-forcement had, in reality, been the main
factor in the bridge's structural dete-rioration. This problem was further ag-gravated by the use of concrete with arather open granularity, i.e., an excess oflarge gravel stones and a lack of mediumand small size aggregates.
RENOVATION OF VAULTSAND SUPERSTRUCTURE
Conscious of the irreparable damageto the bridge, the Municipality of SanSabastian launched a contest for thedemolition and reconstruction of theentire superstructure. The main designrequirement stated that the new bridgeshould offer exactly the same volumeand external form, including textureand color, as the original bridge.
The winning solution was offered bycontractor Fernandez Constructor, witha design alternative by engineers J. J.Arenas, C. Alonso and M. Pantaleon.The new design comprised a spandrel
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Fig. 3. Detail of north facade of bridge in 1983,
arch solution by using almost total pre-fabrication of the various load resistingelements.
Fig. 4 shows the transverse cross sec-tion of the new superstructure com-posed of five I section spandrel archesseparated 4.76 m (15 ft 7 in.) with trans-verse stiffeners or diaphragms spacedout 3.15 m (10 ft 4 in.). The web of theinternal arches is 24 cm (9.4 in.) thickwhile the external arch web (includingornamentation at the outer face) is 30 cm(11.8 in.) Such slender cross sectionsallowed the spandrel arches to supportthemselves and the rest of the deckduring the step-by-step construction se-quence.
Each spandrel arch rests on the pierheads through two plastic hinges as
shown in Figs. 5 and 6. The externalarch also includes the top cornice andthe lateral ornamentation (gilded dra-gons and projecting oars). Thesedecorative members, all prefabricated,serve as formwork and are anchoredwith projecting bars to the subsequentlypoured concrete.
ERECTION SEQUENCEErection of the Maria Cristina
Bridge superstructure began with theplacement of the precast spandrelarches - These members were then pro-visionally tied with two prestressingtendons tensioned between temporaryanchorages placed at the arch ends in
PCI JOURNALMareh-April 1986 25
0)
(1) Prefabricated external spandrel archincluding cornice and decoration
(1 a) Transverse stiffener(2) Prefabricated internal arch
(2) Prefabricated bottom slabs(3) Longitudinal in situ reinforced concrete strip
(4) Transverse in situ reinforced concrete stripbetween adjacent slabs
Fig. 7. Details of bar splicing at bottom plate joints,
order to produce a force equal to 53 and47 Mp (117 and 103 kips) for the exter-nal and internal arches, respectively.When a complete row of three archeshas been positioned and the corre-sponding zone of the pier heads havebeen concreted, the temporary ties areremoved and the arches rest on the piersand abutments in accordance with thefinal static scheme.
Next, the precast, curved bottomplates, 16 cm (6.3 in.) in depth, areplaced between the arches. Two longi-tudinal separation strips 40 cm (15.7 in.)wide are then poured between the platesand neighboring arches and the mildsteel reinforcement from both the plateand arch is crossed and anchored.
Transverse joints between adjacentplates are also crossed by some rein-forcement as shown in Fig. 7. By pour-
ing in place concrete along these longi-tudinal and transverse joints the initial Isection is transformed into a U orequivalent section.
In the next step, transverse precastreinforced concrete beams are placedover the transverse diaphragms of thearches at spaces of one-eighth of thearch span: 25.2018 = 3.15 m (10 ft 4 in.).Continuity is obtained between thetransverse girders and arch diaphragmsby pouring concrete at the joints wherethe projecting reinforcement crosses. Inthis manner, transverse frames arecreated which are tied at the bottom bythe lower plates (Fig. 8).
Next, 20 cm (7.9 in.) thick precast topslabs are placed over each cell of therectangular frame composed by thelongitudinal arches and transverse gird-ers. Longitudinal channels, 1.20 m (3 ft
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(1) Prefabricated external arch
(2) Prefabricated internal arch
(3) Prefabricated transverse girder
(4) in situ concrete (first stage)
(5) In situ top slab concrete
(a) Projecting bars to be spliced withbottom slab bars
(b) Projecting bars for positive bendingconnection of transverse girder
(c) Projecting bars and stirrups fornegative bending connection betweentop slabs and transverse stiffeners
Fig. 8. Detail of joint between transverse girder and arch diaphragm (reinforcement).
11 in.) wide, and transverse channels, 20cm (7.8 in.) wide, are formed over thearches and transverse girders, respec-tively.
Transverse top reinforcing bars pro-jecting from the top slabs are crossedover the first channel to insure resis-tance to continuity negative moments.Also, longitudinal top and bottom barsprojecting from the top slabs are crossedon the transverse channels. Without theneed of any formwork, the pouring of inplace concrete in this grillage of chan-nels makes the entire deck monolithic(Fig. 9).
Once the cast-in-place concrete hashardened, a very stiff and intercon-nected structure is created which actslike a box section spandrel arch in thelongitudinal direction. Local flexure isresisted by the rectangular top platesacting as continuous slabs in both direc-tions and general transverse flexure istaken by the rigid transverse frames de-scribed above. In this manner, an effi-
cient three-dimensional thin-wallstructure is created.
It may be noted that only 0.508 rns perm2 (0.555 yd9 per yd2 ) of concrete and 68kg per m2 (14 lb per ft2 ) of mild rein-forcing steel were used in the structure.Also, since bottom plates are providedfor architectural purposes, omittingthem would have reduced concrete con-sumption to only 0.38 m3 per m2 (0.415yd3 per yd2).
ANALYSIS OF STRUCTUREThe behavior of the newly designed
arches was compared using the finite ele-ment method and a mono-dimensionalanalysis. After acomparison ofthe results,each arch component was rep-resented by a polygonal one-dimen-sional element while a three-dimen-sional frame was used to depict theglobal behavior of the complete struc-ture. The transverse effect of the topgirder and slabs and of the bottom plates
PCI JOURNAL'March-April 1986 29
LONGITUDINAL SECTION
TRANSVERSE SECTION
(1) Prefabricated top slabs (4) Prefabricated arch
(2) Prefabricated transverse girder (5) In situ, reinforced concrete longitudinal(3) In situ transverse continuity channel continuity channel
Fig. 9. Typical top slab joint system (reinforcerneni).
was represented by transverse memberswith adequate shear flexibility.
A linear analysis was used to derivethe internal forces due to external loads.However, to realistically assess the sec-tional force values corresponding to im-posed deformations, i.e., shrinkage andthermal strains, a nonlinear analysis wasused.
It was important to carry out a non-linear analysis for this structure becauseshould all its cross sections remain un-cracked, any imposed shortening wouldgive rise to a significant reduction of theexternal thrust resulting in high positivemoment. As an example, the crown sec-tion of the central arch must resist amaximum bending moment due to liveload of 680 MN • m (500 kip-ft) and, inthe case of no cracking, must undergo abending moment due to shrinkage and atemperature drop of 15 deg C (27 deg F)equal to 1910 MN' m (1400 kip-ft) as-suming the favorable effect of creep isconsidered. Therefore, if cracking is al-]owed, the flexibility of the spandrelarch can he increased sufficiently to off-set its inherently large stiffness.
Fig. 10 shows the resulting moment-curvature diagrams of the crown section
of the central arch. Note that each curvecorresponds to a different value of theaxial force. In the figure, it may he seenthat elastic linear behavior is repre-sented by Line OL and that nonlinearbehavior branches out from Point Cwhere cracking can he expected.
The horizontal distance CD indicatesthe sudden increase of curvature at thesection where a crack appears. Line DE.shows the nonlinear behavior of thecross section after cracking. Note thatthe tension stiffening effect of concretebetween cracks is somewhat uncertainand, therefore for design purposes, LineCF is used instead which departs fromthe cracking points and parallels LineDE (see Fig. 10).
The actual value of the thrust drop,OH, due to the imposed deformation, isdetermined using a manual iterativeprocedure with the aid of the moment-curvature diagrams corresponding to thecross sections of the central half of thebridge, i.e., the zone where cracking ap-pears. Since the points representing thedead load of each cross section areknown (Point P) together with a chosenvalue of .H and the corresponding mo-ment increase at each cross section, it is
WFig. 10. Moment-curvature diagrams of central arch crown section.
possible to obtain the increase ofcurvature of each cross section andhence, by integration, the total horizon-tal movement at the arch hinges. Suc-cessive iterations lead to the AN valuewhich produces a relative movementbetween the hinges equal to the im-posed shortening of the arch.
In Fig. 10, Point P indicates deadload, Point Q denotes dead load plusimposed deformation, while Points Rand S represent service load conditionswith only frequent live load and full liveload. The linear calculated value of 1910MN • m (1400 kip-ft) for the imposeddeformation has been reduced to 1170MN • (860 kip-ft).
A larger reduction of this value wouldhave been obtained if a flatter branch ofthe moment-curvature diagram hadbeen used. For example, some authorsrecommend a hyperbola to represent thetension stiffening effect of the concrete.In Fig. 10, it should he observed thatPoints Q, R and S all lie on differentbranches of the moment-curvature dia-
gram because the axial force, N, in-creases from one end to the other.
Moment-curvature diagrams are alsouseful because they provide informationfor finding the actual stress of steel barsat cracked sections during service loads,from which a fairly accurate predictionof the crack opening can be made. Inthis case, more steel bars than needed toresist the loads have been placed at thebottom flange of the arches to achieve aservice stress with frequent live load notlarger than 1400 kp/cm2 (20 ksi). Undersuch conditions, the expected mean andcharacteristic crack opening reach 0.08and 0.14 mm (0.0031 and 0.00.55 in.) re-spectively.
RECONSTRUCTIONOF BRIDGE
The precasting yard was located about1.6 km (1 mile) from the bridge site.Using only one piece of wood formworkand simple scaffolding on the ground(Fig. 11), the contractor produced six
Fig. 11. Precasting yard for arch fabrication.
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external and nine internal arches. Aftertensioning its temporary tie, each archwas stripped from the formwork andreadied for transportation to the projectsite (Fig. 12).
The entire set of bottom curvedplates, transverse girders and top slabs,including also the various decorativepieces (Fig. 13), were produced by thecontractor in the same yard. The moldsfor fabricating the lion heads, oars, dra-gons, and other sculptures were ob-tained by the contractor from the origi-nals still remaining on the old bridge.
Special sculpturing techniques wereused to fabricate these ornaments. Ini-tially, a gypsum mold was made fromwhich a clay countermold was producedand covered with a thin layer of wax.Subsequently, the wax was melted andreplaced by an injected plastic mask.This mask served as the prototype forthe series of decorative pieces to bemade.
Because the highway leading from thefabrication yard to the bridge site was
Fig. 12. External arch taken out from itsprecasting base.
Fig. 13. Decorative pieces produced in casting yard.
PCI JOURNAL'March-April 1986 33
Fig. 14. Special truck rig for transporting precast arches.
narrow with many sharp curves, a majorproblem arose with regard to the trans-portation of the precast elements. Insome cases the transverse slope of thehighway was as much as 6 percent. Itwas imperative that no significant tor-sional moments would develop in thearch during transportation.
To counteract any undesirable stres-ses from developing, a specially de-signed steel device was used to act as aconnecting member between the fronttractor and the rear axle set. The archelement was suspended at both endsfrom a scaffold in order to allow it to ro-tate freely along its longitudinal axle inthe vertical plane. However, a small re-straint was considered necessary so as toavoid excessive movements of the archduring transportation.
To accomplish this, a set of springsconnecting the bottom flange of the archto the steel girders were designed inwhich the spring stiffness allowed a
small torsional moment to be inducedwithin the arch. Fig. 14 shows the spe-cial truck assembly used to transport theprecast arches.
The demolition of the vaults was car-ried out by longitudinal strips that, inorder to maintain equilibrium of hori-zontal forces over the pier heads,stretched along the three bridge spans.This became a dangerous operation be-cause the vaults and piers were alreadyin a damaged state and also because twoheavy cranes [weighing 80 Mp (88tons)] had to be placed over the pierheads in order to erect the precastarches. Fig. 15 shows a closeup of theexisting cracks in the old vault.
Fig. 16 shows one of the precast inter-nal arches being lifted shortly afterbeing transported to the bridge site. Thestructural aspects (stiffening ribs) of thiselement are visible in contrast to the ar-chitectural facade of the external arches(Fig. 17).
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Fig. 15. Existing cracks in old concretevault. Fig. 16. Erection of internal arch.
Fig. 17. Erection of external arch.
PCI JOURNAUMarch-April 1986 35
AkFig. 19. Arches and bottom slabs mounted.Fig. 18. Lateral arches in place.
Fig. 20. Inside view of bridge with conduit services installed.
Fig. 24. Oblique view of superstructure. Fig. 25. Front view of a bridge pier.
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Fig. 26. Panoramic view of newly renovated Maria Cristina Bridge.
Fig. 18 shows several arches po-sitioned in their final place prior to theinstallation of the bottom precast plates.In Fig. 19, the bottom slabs are in placeto await the cast-in-place concrete in thelongitudinal and transverse joints.
Fig. 20 shows a cell inside of thebridge with several city service conduitsalready installed. This picture also givesan inside view of the vault bottom, thetransverse diaphragms of the archeswith the corresponding transverse gird-ers and the soffit of the precast topslabs.
Note that the crown depth of thenew bridge was determined from theold structure [about 80 cm (31 in.) ].From this dimension, the depth of thetransverse girders was chosen to allow aclearance of 38 cm (15 in.) for mechan-ical services.
A closeup of the plastic hinge of anarch can be seen in Fig. 21. It should benoted, of course, that the concrete of thepier head was placed after positioning
the corresponding arches which weretemporarily placed over the horizontalconcrete pads. When the ties of the archwere removed, such temporary hearingswere similarly withdrawn to allow thehinges to transmit both vertical andhorizontal reaction forces.
Nearing completion, Fig. 22 showsthe three external arches alreadyerected. The actual geometry of the pierheads with openings can be seen. Thiswill make future inspection and anypossible repair work very accessible.
The pier openings are concealed withfinely sculptured ornaments as can beseen in Figs. 23, 24 and 25. Fig. 26shows a panoramic view of the newlyrenovated structure.
The reconstructed Maria CristinaBridge was officially opened to vehicu-lar traffic on f anuary 19, 1985, exactly 80years after the original bridge was builtby Ribera. It is anticipated that the ren-ovated structure will have a very longand useful life.
