Evgenia Anagnostou

This paper examines analytically the design criteria for the composite retrofit of reinforced concrete (RC) columns with a short lap splice length of steel rebars inside the critical region. The advanced potential of pseudo-dynamic three-dimensional (3D) finite element (FE) modelling is utilized to investigate critical design parameters for the required carbon fiber-reinforced polymer (FRP) jacketing of RC columns with a rectangular cross-section based on the experimental lateral force-to-drift envelope behavior of characteristic cases from the international literature. The satisfactory analytical reproduction of the experimental results allows for the systematic numerical investigation of the developed stress along the lap splice length. The maximum lateral force and the horizontal displacement ductility of the column, as well as the maximum developed tensile axial force on the longitudinal bars, their variation along the lap, the bar yielding, and the plastic hinge length variation, are considered to determine the seismic behavior of the columns. For the first time, cases of smooth bar slip together with delayed bar yielding or without bar yielding are identified that may be recorded through a “ductile” P-d seismic response. Such pseudo-ductile response cases are revisited through suitably revised redesign criteria for adequate FRP jacketing.

This study develops three-dimensional (3D) finite element (FE) models of composite retrofits in deficient reinforced concrete (RC) columns and frames. The aim is to investigate critical cases of RC columns with inadequate lap splices of bars or corroded steel reinforcements and the beneficial effects of external FRP jacketing to avoid their premature failure and structural collapse. Similarly, the RC-frame FE models explore the effects of an innovative intervention that includes an orthoblock brick infill wall and an advanced seismic joint made of highly deformable polymer at the boundary interface with the RC frame. The experimental validation of the technique in RC frames is presented in earlier published papers by the authors (as well as for a four-column structure), revealing the potential to extend the contribution of the infills at high displacement ductility levels of the frames, while exhibiting limited infill damages. The analytical results of the advanced FE models of RC columns and frames compare well with the available experimental results. Therefore, this study’s research extends to critical cases of FE models of RC frames with inadequate lap splices or corroded steel reinforcements, without or with brick wall infills with seismic joints. The advanced pseudodynamic analyses reveal that for different reinforcement detailing of RC columns, the effects of inadequate lap-spliced bars may be more detrimental in isolated RC columns than in RC frames. It seems that in RC frames, additional critical regions without lap splices are engaged and redistribution of damage is observed. The detrimental effects of corroded steel bars are somewhat greater in bare RC frames than in isolated RC columns, as all reinforcements in the frame are considered corroded. Further, all critical cases of RC frames with prior damages at risk of collapse may receive the innovative composite retrofit and achieve higher base shear load than the original RC frame without corroded or lap-spliced bars, at comparable top displacement ductility. Finally, the FE analyses are utilized to propose modified design equations for the shear strength and chord rotation in cases of failure of columns with deficiencies or prior damages in RC structures.

This study focused on characteristic cases of recently tested real-scale RC framed wall infilled structures with innovative seismic protection through polyurethane joints (PUFJ) or polyurethane-impregnated fiber grids (FRPU). The frames revealed a highly ductile response while preventing infill collapse. Herein, suitable 3D pseudo-dynamic FE models were developed in order to reproduce the experimental results. The advanced Explicit Dynamics framework may help reveal the unique features of the considered interventions. Externally applied double-sided FRPU jackets on OrthoBlock infills may maintain an adequate bond with the surrounding RC frame as well as with the brick infill substrate at up to a 3.6% drift. In a weak four-column RC structure, the OrthoBlock infills with PUFJ seismic joints may increase the initial stiffness remarkably, increase the base shear by three times (compared with the bare structure) and maintain a high horizontal drift of 3.7%. After this phase, the structure may receive FRPU retrofitting, reveal the redistribution of stress over broad infill regions, including predamaged parts, and still develop a higher initial stiffness and base shear (compared with the bare RC). The realization of a desirable ductile behavior of infilled frames through PUFJ of only 20 mm thickness, as well as through FRPU jacketing, may remarkably broaden the alternatives in seismic protection against the collapse of structures.