French bridges experiences from prefabricated deck elements. Stockholm, march 4th, 2009

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1 French bridges experiences from prefabricated deck elements. Stockholm, march 4th, 2009 Born 1957, TPE civil engineering degree He joined SETRA in Experience in steel bridges, and their pathology. He has been involved in the design of many innovative composite bridges. J. BERTHELLEMY Senior Engineer at S.E.T.R.A. Bagneux, France. As a technical department attached to the French Ministry for Ecology, Energy and Sustainable development, SETRA implements an active policy supporting energy saving innovations, and quality of engineering structures. This paper describes different solutions recently developed in France to use prefabricated slab elements for composite bridges, in order increase durability by reducing tensile stress areas limiting concrete cracks of decks. 1- A TIED-ARCH BRIDGE TO CROSS THE RHÔNE. The bridge of St Gilles crosses the Rhône River in Camargue. With a main span of 120 meters, this tied arch bridge has been opened to traffic in Once the bridge structure is supported by its final bearings, the ready-made precast units of concrete slab were put in place using a mobile handling framework. Each unit is supported by two adjacent transverse steel beams. The 25 centimetres thickness concrete slab is thus not stressed by any traction due to the effect of its own weight. The second phase reinforced concrete is cast in place between the prefabricated elements. In this case, it isn t necessary to use longitudinal tendons to prestress the slab in order to avoid cracking. The second phase concrete must of course be relevantly cured and protected as for cast in situ slabs. Its longitudinal reinforcement ratio can be determined using the eurocode-4 rules. One particular point of this bridge is that the radiant hangers simplify and standardize anchorings on the archs. The design was performed by SETRA in association with the architect Philippe Fraleu. The steel contractor was RICHARD-DUCROS in Alès. - Views of the bridge - 1

2 2- THE VERRIERES VIADUCT. The same process was used for the Verrières bridge to face an analogue problem in the hogging moment areas on piers. The spans of the Verrières bridge are very large, reaching a maximum of 144 metres : and 80 m. In such a case, the slab would crack under permanent dead loads in hogging moment areas if it were conventionally cast in situ. This would also occur even with a sophisticated pilgrim steps sequence of concreting, using a mobile formwork. - Example of a very favourable pilgrim steps concreting sequence The total length of the Verrière bridge makes it impossible to reduce traction stresses in the slab by giving a pre-curvature to the steel frame, and adjusting bearing levels on piers during or after the sequence of concreting. - Verriere bridge crossing the 144 m span - -View of the precast slabs - So it was decided, in this other SETRA bridge design, to use prefabricated slabs for the lateral parts of the wide deck. On the contrary, the central part above the steel box can be cast in place. Some transverse tendons crossing the longitudinal reinforced joints help to make the slab monolithic. Just as for St Gilles bridge, the second phase reinforced concrete is cast in place between the prefabricated elements without longitudinal tendons. The longitudinal reinforcement ratio is determined using the eurocode-4 rules. 2

3 -Details of the steel structure before realisation of the concrete work The two first examples presented the use of reinforced transverse joints. The last one will illustrate the principle of match cast joints for prefabricated slab elements assembled and prestressed with longitudinal tendons. - Reinforced joint ( St-Gilles, Verrières ) - -Match-cast joint ( PS13 ) 3- INNOVATIVE PS13 FLYOVER WITH HIGH PERFORMANCE CONCRETE. For more common bridges, owning Authorities unfortunately often run away as soon as the mere word innovation is uttered. This reaction can be explained, as many innovations of the past are today causing the most serious maintenance problems. And it is a fact that in presence of traffic, corrective actions are more costly and difficult. Innovation must therefore today demonstrate to the Owning and regalian Authorities that they are not going to sign a diabolical contract, after which marginal savings during construction will lead in the end to highly maintenance and repair expenses. The flyover-bridge PS13 presented in the following article was proposed by contractors GTMConstruction and DUMEZ-GTM. It is innovating for many reasons and that is why it was subjected to a careful examination supervised by SETRA in partnership with LCPC within the framework of the Innovation Charter and in collaboration with the COFIROUTE Concession Company. The bridge is located on the east section of the A85 motorway between Vierzon and Tours in central France. It is a standard flyover with two spans of 17,70 m and 19,80 m long. The technical novelties of the PS13 flyover are intended to provide pertinent answers to the specific problems of crossing a motorway. 3

4 The motorway administrator tries to: Limit worksite risks during construction, especially above traffic lanes. The Contractor used a total prefabrication for that purpose, both for the framework and the slab. Limit the risk related with the impact on the deck of over-height vehicles. The traditional twin-girder must be improved with regard to the problem of impacts from oversize vehicles, then its lack of redundancy is otherwise obvious. The solution adopted for the PS13 crossover consists of exposing to impacts a box girder filled with concrete as shown in the following figure : Cross section of the deck Just for information, the equivalent static intensity of the impact, taken into account for the design, is 1 MegaN, including all the relevant ULS safety factors. The steel structure is a pure twin box-girder. Each box-girder is made of two hot-rolled 790 mm height I-sections with a high yield strength steel of 460 MPa and connected together by welds at the upper and bottom flanges levels. The girders were rolled and welded at ProfilArbed factories in Luxembourg. The motorway administrator also tries to: Carry heavy and eccentric located traffic loads, if need be. Thanks to distortion-free rigid boxgirders, the torsional stiffness of the bridge is greatly increased. As a result, the twin girder bridge will for instance better resist the 1st class loads of Eurocode 1. Increase the fatigue durability. The fatigue details which reduce the endurance classification have been avoided, with the suppression of intermediate cross beams. Cut maintenance costs. Another result of the suppression of cross beams is that most local water traps are avoided on the lower members. By finally reducing the most difficult surfaces to be repainted, it even plays a part in cutting maintenance expenses. Increase slab durability. At the PS13, the steel frame supports a full-width precast concrete slab of 80 MPa strenght, spanning over the longitudinal girders. The slab is composed of precast elements with glued match-cast epoxy joints and assembled with tendons before any connection with the steel frame. Owing to the absence of second phase concrete, it becomes possible to implement HP concrete on a composite bridge slab without fearing the creep and shrinkage effects. 4

5 -First precast panel installed- Steel studs welding - View of the intrados - The pertinence of this latter point is primarily based on an earlier experience. In 1988, the technique of prestress-assembling precast elements to simple glued joint faces, carefully fitted with keys, had already formed the subject of a first experimental bridge in Manosque on the A51 motorway belonging to the ESCOTA network. The structures built at that time were 2 independent twin-girder bridges carrying a motorway. Their maximum span is a little more than 50 meters for a total length of some 160 meters, with 4 spans. This experiment gave rise to a publication at the IABSE Brussels Congress in Professor Virlogeux quoted the Manosque bridge in a general article dated 1992 on composite bridges. But it was too near the date of construction to be able to evaluate the durability of the process. - Construction scenes - View of the finished work - The detailed inspections carried out for ESCOTA in 1995 by LRPC of Aix-en Provence, and the more recent checks, make it possible to note the absence of any cross-cracking of the prestressed slab of the composite bridge. The state of the keying joints likewise remains quite satisfactory. In the case of PS13, the joints are match-cast, in other words each end-edge surface of the previous precast element is used as a formwork to cast the next element. This method of construction avoids the finishing of imperfect joints and thus improves quality, compared with the Manosque experiment. The PS13 solution is patented by the contractors GTM-Construction and DUMEZ-GTM and gives a global answer to several key areas relating to the design of short- and medium-span composite bridges. The innovations of the PS13 flyover all tend to increase the durability of the structure. Its design is furthermore oriented towards implementation above traffic lanes. Owing to precasting, construction will only require short disruption of the traffic flow. It will therefore be possible to program them, thereby reducing the risks that works on site would otherwise cause to road users. Safety and rapidity of construction and functional durability are the principal advantages of the method. But the costs are high and only experimental bridges have been built. 5

6 4- CONTINUOUS CONNECTION IN THE PAST. Several examples of big bridges with continuous shear connection were for instance built in France during the 60s. ( Pontoise, Cergy, Conflans, described in OTUA bulletin n 6 by Henri Grelu ) mobilizing friction as principal favourable effect achieving a really continuous connexion which avoids local transversal cracks. All these bridges slabs are always in very good condition today. Their cutting line was one very simple sinusoidal cut because other effects than friction as precobeam type discontinuous forces - were even regarded as negligible at that time, and were not allowed. 6

7 EXAMPLE OF THE A 15 MOTORWAY BRIDGE AT PONTOISE. Transversal tendons were constituted at this time with 18 strands of 7mm diameter. The transversal tendons were disposed every 0,80m in most usual sections. In local areas near the abutments, they were disposed every 0,60m. 7

8 The slab was cast in place with a pilgrim's step type of concreting sequence. The transverse pre-stressing cables were then tensioned. Level changes reaching 2,20m were imposed at supports! At the end, longitudinal tendons were also acting on the composite structure. Pictures of the A15 motorway bridge over River Oise in 2008 : good structural condition (slab condition of the second bridge on the left - built more recently without tendons - is not so good ) Friction for bridges in former National Codes was taken into account. Friction coefficients exist and in the British standards and in the French code of 1966 for composite bridges. Coefficient 0,40 was obtained by beam tests that were required by Setra before construction of the first A15 motorway bridge over River Oise. A 6,30m long element was tested with two Megacycles reproducing the most severe cases of characteristic slipping force. This endurance test was then followed by a rupture test. Beam Tests achieved in CONTINUOUS CONNECTION TODAY. The economic conditions in term of cost of the workforce on the building site leads to the use of prefabrication in order to achieve today the quality allowed by a continuous connection. For instance the connectors studied in the Precobeam European Research program offers the possibility to connect a slab in a second phase after launching. With prefabrication, the continuous connection is more economic, and therefore realizable again. The slab can be realized by elements or can be ripped as a longitudinally prestressed whole over the steel skeleton before connection. In both cases, the durability of the slab is achieved with less difficulties, than with a cast in place slab requiring longitudinal prestressing as it was the case in the sixties. Connection would be realized with rebars and adapted cement grout injection from inside the box. It also has to resist forces due to distortion of the box. 8

9 The following system as been proposed for evaluation at the step of preliminary design for a large bridge carrying a motorway. It can save transverse beams and simplify slab construction. schematic view of the proposed system General view of the cross section Details 6- CONTINUOUS CONNECTION TODAY WITH THE RAZEL SYSTEM. In this patented system the continuous connection is achieved by a central reinforced concrete tunnel including the studs. The concreting of this tunnel is achieved in a second phase after launching the rest of the slab. At service limit state, the slip forces are resisted both by friction on both sides of the concrete tunnel, and by reinforced openings. The height of the tunnel is generally of about 200 mm. At ultimate limit state, the slip forces are resisted by the reinforced openings only. Many composite bridges have been recently built with this system. No openings can be detected with this system between the upper flange and the slab. In addition the effects of thermal and endogenous shrinkage after concreting are avoided, so that shrinkage is significantly reduced. 9

10 General view of the slab launching operation for a viaduct on the Reunion Island in the Indian Ocean Formwork of the friction surface on the prefabrication area of an other bridge. References Berthellemy, J.; Composite construction, innovative solutions for road bridges. Puentes mixtos, composite bridges 3rd international meeting - Madrid, January Placidi, M.; Virlogeux, M.; Berthellemy, J.; Prefabrication and prestressing of concrete slabs in composite bridges. IABSE international Congress - Innsbruck, September