Tagus river centenarian steel bridges’ rehabilitation

Abstract

This paper refers to the rehabilitation design projects and undertaken works of three centenarian steel/ iron bridges executed in Portugal and belonging to the national road network, namely Abrantes’ Bridge, Belver's Bridge and Chamusca's Bridge the three of them crossing the Tagus river.Their typology is basically a steel deck executed in steel or iron elements with rivets that are connected in trusses. These three examples cover upper and lower decks.The original platforms were generally made of wood and lately were changed to reinforced concrete slabs to meet traffic necessities. The decks are supported by piers and abutments made of masonry as well as wing walls when existing.In this paper each bridge is analysed in detail, highlighting its main anomalies which distinguish it situation from the other ones and the purpose of the repair and the reinforcement works that were prescribed.The existing situation before the intervention is characterized through bridge inspections both principals and special and also through the main conclusions obtained from laboratory tests over samples extracted from precise points of the structures.The hypotheses for the intervention in order to upgrade each structure to actual regulatory actions are discussed in terms of (i) present restraints, (ii) difficulties of execution due to the lack of resistance of existing materials (for instance non-weldable iron) or particular geometry of the elements or complex connections involving composite members and rivets or even (iii) aesthetics issues regarding patrimonial heritage.Important factors and curiosities observed during their rehabilitation design and works are also summarized. Namely the case of global reinforcement of a structure achieved with the reformulation of the external pre-stress and the elimination of joints in the upper slab and also the case of reinforcement of a non-ductile structure to the seismic action.The paper ends with a brief conclusion about the major problems that affect this kind of structures though their long existence: the nature of their constitution, the original conceptual design and the influence of its assumptions in the global behaviour of the structures, the difficulty to estimate quantities of repair in the early stages of the intervention (during project time) where no pickling process has yet started and real deficiencies are covered with paint, the thorough work necessary to add the compulsory resistance preserving its aesthetic and heritage, among other remarks.