Time-dependent behavior of a new kind of precambered prestressed bridge deck for the high speed lines in Belgium

Abstract

A new kind of composite prefabricated prestressed and precambered bridge deck has been introduced in Belgium in the context of the construction of the railway High Speed Lines. The construction of this bridge deck is rather complex with precambering of steel girders, prestressing of a concrete slab and two-phases concreting. Large spans up to 26 m can be reached by using simultaneously precambered steel girders and prestressing forces with a construction depth of only 250 mm. In this paper, an instrumentation in situ is presented and the research carried out at University of Brussels on the time-dependent behavior of this structure is detailed. For a set of 36 decks, the long-term camber under permanent loading has been computed by two methods (the pseudo-elastic analysis used until now for the design and the step-by-step method) and has been compared with measurements taken one or two years after the construction of the bridge decks. At long-term, the camber computed by the step-by-step method shows a very good agreement with the measured camber.A new kind of railway bridge deck has been developed recently in Belgium for the replacement of old steel railway bridges with moderate spans and for the construction of multi-spans viaducts for the new high speed lines. Up to now, these bridge decks have been used for simply supported spans up to 26 m. The bridge decks are prefabricated in workshops and transported by train to the construction site where they are placed on their supports by cranes. These composite prestressed and precambered structures belong to the trough type with U shaped cross section (Fig.1,e). These composite steel-concrete trough shaped structures have a width that is limited to 4m due to transport restrictions. Two hot-rolled or welded steel girders are bent at the mill or in plant to produce an initial camber (Fig.1, a). Then, they are placed in a special equipment at the workshop. The upper flanges are maintained in order to prevent lateral buckling. The first step is the elastification phase of the steel beams (removing of the residual stresses by successive loading/unloading cycles). To remove the residual stresses, two local loads are applied on each steel girder at ¼ and ¾ of the span and removed and applied again until the camber does not change any more. Then, the construction begins in the workshop by applying two local loads on each steel girder at ¼ and ¾ of the span in order to straighten them and to obtain at this stage a camber equal to zero (Fig.1, b). In all cases, the stress level in the steel girders during this preflexion phase is lower than 80% of the yield strength. These two girders will be parts of the webs of the bridge. Then, the bottom slab of the deck is constructed: reinforcing bars (transversally and longitudinally) with characteristic yield strength at 500MPa and naked tendons (longitudinally) are disposed (and the tendons stressed) in the space that will be filled by the bottom slab (slab depth: 25 cm). No protection against corrosion is applied on the steel as it ...