PSI - Issue 64

2

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1050 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of SMAR 2024 Organizers Keywords: FRP; Reinforced Concrete Column; Seismic; Retrofit; Substandard. Nima Kian et al. / Procedia Structural Integrity 64 (2024) 1049–1056

1. Introduction Many RC buildings constructed prior to 2000s in Turkey, have key deficiencies that would face with structural vulnerability against probable future earthquakes. Particularly, having insufficient transverse reinforcement result in poor confinement of columns which thereby lead to insufficient lateral load capacity and ductility. In the past two decades, fiber- reinforced polymer (FRP) composites have been increasingly used for the elimination of these drawbacks (Fardis and Khalili 1982, Ilki and Kumbasar 2002, Ilki et al. 2009, Oucif et al. 2017, Ghatte et al. 2019, Dirikgil 2020, Kian et al. 2021). Among others, Teng et al. (2016) presented a numerical model for the seismic behavior of RC columns retrofitted with FRP jackets. It is worth mentioning that no stiffness degradation was considered for simplification. Also, the concrete compressive strength of the specimens was 36~40 MPa. It is worth noting that applied axial load to axial capacity ratios of columns were in the range of 5~15% which was very low value for the substandard concrete buildings made with very low concrete strengths. It should be noted that these studies were not intentionally carried out as to resemble substandard characteristics of existing building stock. The current knowledge strongly indicates that most of the columns of substandard aged structures in many countries bear axial stresses pretty close to their axial load capacities violating the specified code restrictions (Aydogdu et al. 2023) (e.g. the maximum allowed axial load is 0.4f c bh in Turkey Building Earthquake Code (2018)). Therefore, nonlinear analyses and modeling of such RC columns is a challenging matter. This would be even more challenging in case of unconfined substandard columns with very low concrete compressive strength ( ≈10 MPa ) and thus with under high axial stress. This situation necessitates specified modeling attempts to focus on the substandard column behavior under earthquake (or simulated) loads. With this in mind, for the first time in the literature, three previously tested columns (Authors, in prep.) consisting of one code-compliant (C), one substandard (S), and one retrofitted state of substandard (RS) columns were numerically modeled. Based on ACI 318-19 (2019), columns C, S, and RS had a factored shear force (V u ) to nominal shear strength (V n ) ratios of 0.5, 0.52, and 0.98, respectively. This is to reflect relatively high shear demand as a generally neglected common construction practice for the columns in Turkey. Also, the ℎ ⁄ value for S, C, and RS were computed to be 0.87, 1.43, and 1.64, respectively. In this regard, numerical and analytical evaluation of three cyclically tested specimens are carried out both in ATENA-GiD interface (Cervenka et al. 2005) and OpenSees platform (Mazzoni et al. 2006) to capture the hysteretic response. The privilege of ATENA is real-time visualization of crack propagation, stress, strain contours in cross section and reinforcements, etc. On the other hand, OpenSees requires less analysis effort in terms of time and computing model with respect to ATENA. Also, there are many capabilities in OpenSees to capture buckling of reinforcements, strain penetration in foundation due to longitudinal rebar slip, capturing distributed plasticity through defined fibers along the column length, etc. Results obtained in this numerical work show that hysteretic curves obtained from ATENA and OpenSees for lateral load-displacement relationships, and cumulative energy dissipation (CED) have a good agreement with the experimental ones. Section-wise strain limits defined in TBEC (2018) are obtained through numerical analyses. Results reveal that these limits could satisfactorily estimate the damage level of member in the section level. 2. Experimental program 2.1. Specimen specifications and material properties Three RC columns with square cross-sections were tested by the authors cyclically (Authors, in prep.) under a constant axial load corresponding to 0.75 ℎ for specimens S and RS and 0.37 ℎ for specimen C, where is the compressive strength of concrete, b and h are the cross-section dimensions of the columns. The control column