YIC2012 — Universidade de Aveiro First ECCOMAS Young Investigators Conference A. Andrade-Campos, N. Lopes, R.A.F. Valente, H. Varum (editors) 24–27 April 2012, Aveiro. Portugal. Nonlinear modeling of the cyclic response of RC columns J. Melo a,* , H. Varum a , T. Rossetto b , C. Fernandes a , A. Costa a a UA – University of Aveiro Department of Civil Engineering Campus de Santiago 3810-193 Aveiro, Portugal b UCL – University College London Department of Civil, Environmental & Geomatic Engineering Gower Street, 101 WC1E 6BT London, UK *Corresponding author: josemelo@ua.pt Abstract. Cyclic load reversals (like those induced by earthquakes) result in accelerated bond degradation, leading to significant bar slippage. The bond-slip mechanism is reported to be one of the most common causes of damage and even collapse of existing RC structures subjected to earthquake loading. RC structures with plain reinforcing bars, designed and built prior to the enforcement of the modern seismic-oriented design philosophies, are particularly sensitive to bond degradation. However, perfect bond conditions are typically assumed in the numerical analysis of RC structures. This paper describes the numerical modeling of the cyclic response of two RC columns, one built with deformed bars and the other with plain bars and structural detailing similar to that typically adopted in pre-1970s structures. For each column, different modeling strategies to simulate the column response were tested. Models were built using the OpenSees and the SeismoStruct platforms, and calibrated with the available tests results. Within each platform, different types of nonlinear elements were used to represent the columns. Bond-slip effects were included in the OpenSees models resorting to a simple modeling strategy. The models and the parameters adopted are presented and discussed. Comparison is established between the most relevant experimental results and the corresponding results provided by the numerical models. Conclusions are drawn about the capacity of the tested models to simulate the columns response and about the influence of considering or not considering the effects of bars slippage. Keywords: non-linear modeling; RC columns; bond-slip mechanism. 1 INTRODUCTION The hysteretic behavior of reinforced concrete (RC) structures is highly dependent on the interaction between concrete and steel. Cyclic load reversals (like those induced by earthquakes) result in accelerated bond degradation, which leads to significant relative slippage between the reinforcing bars and the surrounding concrete. Plain reinforcing bars, which are present in a large number of existing RC structures that were designed and built before the 1970s, thus prior to the enforcement of the modern seismic-oriented design philosophies, have poor bond properties between concrete and steel. Therefore, RC elements containing this type of steel reinforcement are particularly sensitive to the effects of bar slippage. The numerical analysis of RC structural elements is usually conducted under the assumption of perfect bond conditions, which may lead to predicted lateral deformation significantly smaller than the real element deformation or to predicted lateral stiffness larger than the existing element stiffness [1]. Bond-slip effects should therefore be included in the numerical models of structural analysis in order to represent more accurately the