A Predictive Regression Model for the Shear Strength of RC Knee Joint Subjected to Cyclic Load

Introduction

Due to the fact that they transmit forces from the member ends, reinforced concrete (RC) beam-column joints must be comprehensively understood. These primary joints of structural components typically experience failure (Akguzel & Pampanin, 2010, 2012; Anderson et al., 2008; Bakir & Boduroǧlu, 2002; Lowes & Altoontash, 2003a; Mitra & Lowes, 2007a; M. L. Moretti et al., 2014; Pampanin et al., 2002; Shafaei & Nezami, 2019; Wang et al., 2012). The geometrical and structural arrangement of the joint causes complicated stress condition, particularly when it is subjected to lateral stresses like earthquakes or wind. In contrast to other joints, the beam-column roof corner joints exhibit more complicated behavior. The opening moments, which produce compression outside the joint, or the closing moments, which create compression inside the joint, are both responsible for this complicated behavior of beam-column roof corner joints under cyclic loading. The erratic behavior of the roof corner joints is also related to the small to moderate axial stresses acting on the exterior columns of ordinary RC frames. The difference in cyclic response between the closing and opening actions of the roof corner joint causes this phenomenon. But there hasn't been enough research done on the roof corner joint before. The design of these joints is not even given any attention by the bulk of seismic construction regulations.

When the roof corner joint opens and closes, it travels through many load-resisting processes. Due to its unique behavior, the roof corner joint is more sensitive and sophisticated than other RC joints. An experimental study (Priestley et al., 1996) has determined the nominal joint shear strength to be MPa and MPa, respectively, is based on the principal tensile and compressive stresses. This study served as the basis for the recommendation (Meggeta, 2003) of reducing the roof corner joint strength from 0.2f c' to 0.1f c', which was also approved in New Zealand code of practice (NZS 3101, 2006). However, experimental tests have shown that the roof corner joint's shear strength for opening behavior is lower than for closing (Cote & Wallace, 1994; Mazzoni et al., 1991; McConnell & Wallace, 1995; Megget, 1998). This is due to a weaker diagonal concrete strut exposed to excessive axial stress from the neighboring tensile component reducing the opening shear strength (Mogili & Kuang, 2018; Zhang et al., 2017). Because of this, a modest opening shear stress is more dangerous for the RC roof corner joint's than closing shear stress of comparable size. Nevertheless, this aspect is mostly disregarded by the design code of practices.

The major structural design codes (NZS 3101, 2006; EN1998 Eurocode 8: Design of Structures for Earthquake Resistance, 2001; The People’s Republic of China National Standard, 2010; Architectural Institute of Japan, 1999) do not provide any explicit provisions for roof corner joints. As a result, the roof corner joints are typically subject to external joint provisions. In contrast, the nominal shear strength of the roof corner joint is suggested by a single expression by codes like (ACI-318, 2019) and (ACI352R, 2002). Whereas the need for opening and closing expression of the roof corner joint is made evident by the above discussion. Additionally, the axial load on a conventional roof corner joint's column might be reduced by using a discontinuous column with beams in one or two directions. However, the supported beams may result in a significant shear and moment transmission to the roof corner joints. Thus, the theoretical foundation of current design methodologies has to be changed in order to predict the behavior of the roof corner joint that is realistic.