PREBENT BEAMS USING VERY HIGH PERFORMANCE CONCRETE

Context:

The technique of prebent beams was invented in Belgium during the 50’s. It consists in encasing the lower flange of a metallic girder, previously bent, then releasing the prebending loads once concrete is set. Even if expensive, it is the far best solution for obtaining very stiff structures of very low height.

The research undertaken at LCPC within the frame of MIKTI national project, with the French Railways, Arcelor the steel producer, the Belgian precast concrete producer Ronveaux and Brussel’s University ULB as partners, is aimed to extend the technique of prebent beams to the use of very high performance concrete (VHPC). The fatigue resistance, the scientific accounting of creep have to be validated, so that a reliable design method consistent with the Eurocodes can be proposed.

Experimental research at LCPC:

Two 13 m-long prebent beams have been made at LCPC Structures Laboratory in Paris using steel profiles produced in the factory. The beams were then submitted to dead loads (40 kN spread on each beam), then to live loads representative of railway traffic (1,000 cycles represented the effects of conveys possibly loading the structure once a year, then 1 or 2 million cycles were applied representing more frequent heavy conveys). During 8 months, numerous strains and deflections were monitored, in parallel with concrete mechanical characterization, including Young’s modulus, creep and shrinkage determination. Finally both beams were loaded up to failure in order to reach the limit of composite behaviour between steel and VHPC, thus validating the design method following Eurocodes.

The project largely involved the technical staff and equipments of the Structures Laboratory. Moreover, co-operation of the team ‘Mix-design and Casting’ within the LCPC Division for Concrete and Cement Composites [BCC Division] (proportioning and producing a self-compacting concrete, 105 MPa of mean compressive strength, and characterizing its creep and shrinkage properties especially at early age) and of the LCPC Division for Metrology and Instrumentation (adaptation of the data acquisition programs, realization of two prototype miniaturised stress-meters) were necessary.

   Prebending of the steel profile                         Casting of self-compacting VHPC                   Characterisation
                                                                        (beam equipped with stress-meters)                     of shrinkage

Results :

The data (more than 140 channels) have been analysed. The possibility of optimising prebent beams design using VHPC is confirmed. Good fatigue resistance of this type of structure is demonstrated. The critical design issues have been confirmed: compressive stresses limitation at early age, tensile stresses limitation under service loads, which requires a precise determination of delayed strains effects. The satisfactory safety margin with respect to ultimate limit state has been validated.

Creep when submitted to dead loads                  Creep tests                       beam failure with structural instability     
                                                          under variable conditions                 (click on the photo to zoom on)

More information available:

• STAQUET S., TOUTLEMONDE F. (2006) Innovation pour les ouvrages ferroviaires en France : une poutre mixte préfléchie en BTHP / Innovation for the railway bridge decks in France : a precambered composite beam using VHPC, in La technique française du Béton, AFGC, 2ème congrès international de la fib, Naples, 5-8 juin 2006, résumé pp. 13-15, document complet pp. 12-24 sur CD-Rom.
• STAQUET S., MERLIOT E., N’GUYEN VAN PHU C., DERKX F., TOUTLEMONDE F. (2006) Détermination expérimentale, au moyen d’un contraintemètre actif, de l’évolution des contraintes dans une poutre mixte acier-BTHP préfléchie, Journées des Sciences de l’Ingénieur (JSI 2006), Marne-la-Vallée, 5-6 décembre, pp. I-17 à I-22.
• TOUTLEMONDE F., STAQUET S. (2007) Alternate fatigue response of VHPC for innovative composite pre-bended beams applied to railway bridges, 5th int. conf. On Concrete under severe conditions, CONSEC’07, Tours (France), 4-6 June, pp. 1181-1190, Toutlemonde et al. (eds).
• STAQUET S., TOUTLEMONDE F. (2007) Test up to ultimate limit state and failure of innovative pre-bended steel-VHPC beams for railway bridges in France, 6th int. conf. On Fracture Mechanics of Concrete and Concrete Structures, FRAMCOS-6, Catania (Italy), 17-22 June, Carpinteri et al. (eds), Taylor & Francis, vol. 2 Design, Assessment and Retrofitting of RC Structures, pp. 929-937

Contact: François Toutlemonde