Seismic performance of existing hollow reinforced concrete bridge columns

Highway bridges can be considered as crucial civil structures for economic and social progress of urban areas. The damages to highway bridges due to earthquake events may have dramatic impact on the interested area, with or without life threatening consequences, since bridges are essential for relief operations. For these reasons, the assessment of seismic performance of existing bridge structures is a paramount issue, especially in those countries, such as Italy, where most of existing bridges was constructed before the advancement in earthquake engineering principles and seismic design codes. Several major earthquakes occurred throughout the world highlighted the seismic vulnerability of the bridge piers, due to obsolete design. If, for ordinary shaped reinforced concrete (RC) bridge columns the seismic assessment issue can be considered as almost solved, due to several analytical assessment formulations available in literature, and adopted by codes, the same cannot be said for columns with hollow-core cross section, despite their widespread use. None of the current codes addresses specialized attention to RC hollow core members, and only quite recently, attention has been paid to experimental cyclic response of hollow columns. Some critical issues for hollow RC columns are related to the assessment of their shear capacity, special focusing on degradation mechanisms, and the high shear deformation characterizing the seismic response of such elements. In the above outlined contest, a contribution in the seismic assessment of hollow bridges piers is provided by the present work: the investigation of cyclic lateral response of RC existing bridge piers with hollow rectangular and hollow circular cross-section is performed. Special attention has been focused on failure mode prediction and shear capacity assessment. A critical review of the state-of-the-art and of the theoretical background is firstly carried out: the review process has been focused on the past experimental and analytical research on seismic performance of hollow reinforced concrete bridge piers, both for hollow rectangular and hollow circular cross sections. The experimental campaign, conducted at Laboratory of the Department of Structures for Engineering and Architecture, University of Naples “Federico II”, is presented. The experimental program comprised tests on six reduced-scale RC bridge piers with hollow cross-section (four rectangular shaped and two circular shaped). The specimens were ad hoc designed in order to be representative of the existing bridge columns typical of the Italian transport infrastructures realized before 1980, by using a scaling factor equal to 1:4. The construction procedure is detailed, too. All the tests were performed in quasi-static way by applying increasing horizontal displacement cycles with constant axial load (equal to 5% of the axial compressive capacity) until collapse. The monitoring system is accurately explained: it was composed of two sub-systems, one used for global measures (forces and displacement), and the other to deeply investigate about local deformation. Experimental results for both hollow rectangular and hollow circular specimens are reported: for each specimen the results in terms of lateral load versus drift are shown and the evolution of observed damage with increasing displacement is described and related to the lateral load-drift response. An experimental analysis of deformability contributions to the top displacement is performed, mainly in order to better understand the relevance of taking into account shear deformations for bridge piers assessment. The energy dissipation capacity is also analyzed, evaluating the equivalent damping ratio and its evolution with ductility. For hollow rectangular specimens, the global response is modelled through a three-component numerical model, in which flexure, shear and bar slip are considered separately. The main goal of the numerical analysis is to reproduce the experimental deformability contributions. The last part of the work focuses on the definition of proper shear strength models for both hollow rectangular and hollow circular cross sections, and the definition of a deformability capacity model for hollow rectangular cross section. To this aim, two different experimental databases are collected and critically analyzed. The effectiveness in shear capacity and failure mode prediction of main existing shear capacity models is investigated, by applying these models to the database columns. Based on the obtained results, some modifications to existing shear strength models are discussed and proposed in order to improve their reliability for hollow columns. Finally, a new drift capacity model is developed and proposed to assess drift at shear failure of hollow rectangular columns