Seismic Performance of Reinforced Concrete Nonconforming Columns

The estimation of deformation indices and strength capacities of reinforced concrete structural members is neither a simple nor a systematic unique approach. Several analytical models for such estimations have evolved over the years (Park and Paulay (1975), Lehman et al. (1998), Panagiotakos and Fardis (2001), FEMA 273 (1997), Priestley et al. (1996)) and some are an essential part of todays Seismic Codes of Assessment in the form of acceptance criteria (e.g. EC8-III 2020, ASCE/SEI 41-17). However, significant dispersion still exists between experimental data and code-based analytical estimates, especially when assessing structures built prior to the consolidation of capacity-design provisions in international codes (before the 1980s). These structures -referred to as nonconforming - consist primarily of structural members that possess deficiencies in the quality of concrete and steel, and in the transverse and/or longitudinal reinforcement detailing. Deficiencies in longitudinal detailing consists of presence of lap splice at the base of the column or inadequate development lengths, and sparse and poorly anchored transverse reinforcement. In the present work a comparative study was conducted using advanced nonlinear finite element simulations (using software ATENA, https://www.cervenka.cz/) for a benchmark set of columns modelling older detailing practices. Pushover analyses were conducted on the column models in order to determine the parametric sensitivity of the deformation indices, failure mode and associated strength of these benchmark cases to their basic design properties (such as longitudinal and transverse reinforcement ratios and arrangement, axial load ratio and anchorage/lap-splice geometries). It was found that deformation capacities of columns deteriorate significantly in the presence of lap splice or insufficient anchorage length in critical regions.Stirrup yielding and shear failure are suppressed in the case of adequate confinement and may only appear with higher axial compression. Yield penetration and pull out failure dominates the failure mechanism at low axial loads limiting the deformation capacity significantly in the absence of good confinement. The analytical results are calibrated against The design expressions used in the assessment codes illustrating the limits of the applicability and the discord between the different approaches used by current codes to define the so-called acceptance criteria in the seismic assessment practice. The simulation results are also used to vet the various assumptions that underlie the simple mechanistic models used in deriving seismic design expressions for yield and the ultimate deformation capacities, shear strength, and the rate of its degradation with increasing ductility.