PSI - Issue 44

Vanni Nicoletti et al. / Procedia Structural Integrity 44 (2023) 371–377

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Vanni Nicoletti et al. / Structural Integrity Procedia 00 (2022) 000 – 000

1. Introduction Beam-Column Joints (B-CJs) are regions of intersection of structural elements in Reinforced Concrete (RC) structures that must be preserved from premature failure to allow structural members developing their full strength and ductility, especially in earthquake resisting design. Following the hierarchy principles, the B-CJ design and assessment is usually performed as the last step of the overall structural design, and this could be a trouble if verifications are not satisfied because of the need to revise and re-think the joints or even the whole structure. The research on the topic of B-CJs started during the late 1960s and nowadays has reached such a level as to make it possible the development of reliable and experimentally verified design methods and detailing rules, which are included in many technical codes worldwide (e.g. EC8, NZS 3101, ACI CODE-318-19). The first researches investigated the shear transfer mechanisms within the joint core (Paulay et al. (1978), Sarsam and Phillips (1985), Pantazopoulou and Bonacci (1992), Hwang and Lee (1999), Shiohara (2001), Lowes and Altoontash (2003)), performing both theoretical and experimental studies. Moreover, many experimental campaigns were performed to verify the faithfully of proposed failure mechanisms on which many verification formulae are based on, as well as to determine coefficient to be used in empirical formulations (Otani et al. (1984), Park and Milburn (1983), Kim (2007), Pauletta et al. (2021), Lima et al. (2012), Shen et al. (2022), Dabiri and Kheyroddin (2021)). At present, the research in this field is mainly focused on B-CJ modelling techniques for inclusion in structural models to be used for non linear analysis of buildings (Borghini et al. (2016), Yurdakul et al. (2021)). The modern seismic design of RC frame structures is based on hierarchy principles to exclude brittle or other undesirable failure mechanisms and to develop a ductile global failure mechanism. Hence, among the others, a basic principle is that the joints failure must be prevented for avoiding sudden and unexpected collapses of the structure (Collins and Mitchell (1980), Kotsovos and Pavlovic (1998), Calvi et al. (2002)). Moreover, the joint damage is also very difficult to be repaired. Consequently, obtaining a positive outcome from B-CJ verifications required by codes is of paramount importance in a structural design process; if this is not achieved, the designer can essentially decide to increase the amount of steel hoops within the joint, as well as to re-size and re-think the joint. Obviously, both solutions have a non-negligible impact on the structural design: such as an example, the amount of steel hoops is strongly related to construction issues concerning difficulties due to congestion of bars, while the joint re-size may involve changes of architectural details, thus lengthening the design process. Considering the aforementioned issues, a simple tool that supports the B-CJ design from the earliest stages of the design process of a structure could be very useful for practical engineers. This paper presents diagrams (called nomograms) that support the design process of B-CJs in framed structures. These diagrams consist in charts through which the B-CJ geometric dimensions and hoop amount that satisfy the code prescriptions can be individuated. In this work, the Eurocode 8 (EC8) prescriptions about RC joint verifications are considered, but nomograms can be also used with reference to the Italian technical code (NTC18 and Circ. 19). Indeed, as it will be shown in the sequel, the small differences in the joint assessment among EC8 and NTC18 are not substantial and, hence, do not lead to differences in the verification outcomes. Moreover, in this work a full explanation about the creation of nomogram is provided, so as to provide the reader the basis to realize customized nomograms adopting any code regulations, also the Italian technical one. Nomograms support the RC building design since they provide outcomes about joint verifications since the early stages of the design process; indeed, they can be also used for pre dimensioning issues because only forces acting on the structural members (deriving from seismic analyses), together with the beam rebar amount (calculated based on forces acting on beams, without manipulation due to hierarchy principles), are needed for their use. One of the main advantages in using nomograms is that they are suitable for all types of joints and with any kind of geometry since they are dimensionless. The paper is divided into two main parts: the first part (Section 2) is devoted to recall the EC8 principles and formulations about joint verifications, also proposing a comparison with verifications prescribed by the NTC18; in the second part (Section 3), the nomograms are introduced and described, and some indications for their use are provided.