Chapter 4. Experimental and analytical investigation of an exterior reinforced
4.1 Introduction
Chapter 4
joint subjected to shock loading
failure within the joint; ii) anchorage failure of rebars and; iii) bond failure of the beam or column rebars (ACI guidelines and, IS 13920:2016). Structures subjected to earthquake loading undergo large inelastic deformations and require adequate ductility to dissipate energy (MacGregor et al. 1997; Park and Paulay 1975). The ductility of the RC structural member is enhanced by additional detailing and confinement of the concrete with transverse reinforcement (Park and Paulay 1975).
For the low seismic regions, RC moment resisting (MRF) frames are primarily designed to resist gravity loads. For RC-MRF under gravity loading, demands for flexure and shear are higher at the internal beam-column connections. As such, the detailing and anchorage of internal beam-column connections must be superior to external connections. Also, internal connections are laterally confined in both directions by their neighboring joints. On the contrary, the external beam-column joints are laterally confined only with two or three beams, and are thereby less efficient in resisting lateral loads. The behavior of an external beam-column joint has been studied under quasi-static lateral loading (0.025 Hz - 2 Hz) (De Risi et al. 2015; De Risi et al.
; De Risi et al. 2016; De Risi et al. 2017; De Risi and Verderame 2017; Murty et al.
2003; Nie et al. 2008; Pantelides et al. 2017; Ramanjaneyulu et al. 2013; Ricci et al.
2016; Roehm et al. 2015; Sasmal 2009; Sasmal et al. 2011; Sharma et al. 2011;
Verderame et al. 2018; Yurdakul and Avsar 2016; Yurdakul and Avsar 2015). Due to inadequate confinement, external beam-column joints are vulnerable to damage under lateral loads, which may arise from an earthquake, impact, shock and blast (Ngo et al.
2007; Parisi and Augenti 2012; Sezen et al. 2000). Moreover, existing reinforced concrete buildings are mainly designed for gravity loads and which are typically built in seismic prone countries before the introduction of suitable seismic design provisions and the implementation of capacity design concepts. From the literature studies, it is confirmed that the expected inherent weakness of these systems is observed in the past earthquake events and terrorist attacks (Agbabian et al. 1994; Dhakal et al. 2005;
Hakuto et al. 2000; Marthong et al. 2013; Schofield et al. 2006) . Furthermore, the current chapter is emphasized in the study of the dynamic response of the external beam-column assemblies with typical structural weaknesses/deficiencies involved in the pre-1990s’ buildings. However, limited studies have been conducted on the effect of the loading rate on performance of beam-column joints whilst considering various deficiencies such as weak beams in flexure, weak beams in shear and weak columns in shear (Agbabian et al. 1994; Dhakal et al. 2005; Hakuto et al. 2000; Marthong et al.
2013; Schofield et al. 2006). The external beam-column joint, being the weakest link in the structure, requires a comprehensive study for localized shock loads.
Studies have been conducted to evaluate the performance of RC structural members such as beams, columns and slabs subjected to blast and impact loads (Adhikary et al.
2017; Aoude et al. 2015; Cui et al. 2017; Gombeda et al. 2018; Kong et al. 2016;
Krauthammer 2017; Lee et al. 2018; Li et al. 2017; Luccioni et al. 2018; Luccioni et al.
2004; Masi et al. 2018; Nystrom and Gylltoft 2011; Peng et al. 2016; Shi et al. 2008;
Thomas et al. 2018; Xu et al. 2016; Yuan et al. 2017). Fujikake et al. (2009) incorporated the high strain rate effect to estimate the flexural capacity of a RC beam subjected to impact. Carta and Stochino (2013) modified their model to develop a theoretical moment-curvature relation for a RC beam under blast loading. Saatcioglu and Baingo (1999) observed that the structures designed and detailed for seismic loading also perform satisfactorily in terms of ductility and drift demands when subjected to blast loading. Lim et al. (2016) conducted finite element analysis to investigate the influence of the shear and diagonal reinforcement detailing on the blast resistance of beam-column joints.
Aoude et al. (2015) conducted a simulated blast loading test on a steel fiber reinforced column (SFRC) and demonstrated its superior blast resistant characteristics.
Due to restrictions on the use of explosives, blast simulators such as shock tubes and hydraulic blast actuators are used to characterize the response of a structure subjected to blast loading (Stewart and Durant 2016; Subramaniam et al. 2009; Vivek and Sitharam 2018). Magnusson and Hallgren (2003) conducted an air blast loading test on a RC beam and studied the influence of the steel fibers and strength of concrete on its performance under blast loading. The air blast test was conducted with the TNT explosive charge detonated inside a shock tube. It was observed that the inclusion of fibers enhances the ductility of the member. It was also observed that the beam with a high steel reinforcement ratio fails under shear while beam with low a reinforcement ratio fails in flexure. Li et al. (2012) conducted a series of blast simulator tests on columns designed with seismic considerations. They observed that such columns showed blast resistant characteristics in terms of high ductility and enhanced damage tolerance.
Krauthammer (Krauthammer 2008) studied the influence of blast overpressures on the ductility of a RC structural member. The magnitude of damage in the structure is quantified using a performance indicator based on the support rotation and ductility.
ASCE manual 42 and UFC 3-340-01 consider the damage to be severe when the member rotation exceeds 4° for a doubly reinforced beam and column. However, Krauthammer (2017), argued that the member level ductility and maximum support rotation are not sufficient to evaluate the level of damage, and overall stress and strain behavior in the member should be investigated. Krauthammer (2017) and Abedini et al. (2018) have proposed simplified design charts such as shock response spectra and pressure inpulse (P-I) diagrams for predicting the structural behaviour and quantifying the damage level of RC structures subjected to blast loading. These popular design charts are derived from a single degree of freedom (SDOF) approximation using an elastic-perfectly plastic resistance function UFC 3–340–02 (2014).
Based on the literature review, most of the investigations are focused on far-field shock loading (Adhikary et al. 2017; Kyei and Braimah 2017; Ngo et al. 2007; Parisi and Augenti 2012). There is lack of systematic research on the structural response of members subjected to near-field shock loading. Also, limited investigations have been conducted on the performance of beam-column joints under blast loading, which are
joint subjected to shock loading
most vulnerable to such threats (Lim et al. 2016; Parisi and Augenti 2012; Remennikov et al. 2017; Syed et al. 2018). The key motivation of the current research is to study the behavior of a beam-column sub assemblage subjected to shockwave loading. The aim of this chapter is to study the dynamic response of an external beam-column joint under shock loading using experimental and numerical investigations. Most of the earlier investigations have focused on shock, blast and impact loading on individual structural members i.e., beam, column and slab elements (Abedini et al. 2018; Aoude et al. 2015; Kyei and Braimah 2017; Li et al. 2012; Lim et al. 2016; Magnusson and Hallgren 2003; Ngo et al. 2015; Syed et al. 2018; Zhang et al. 2013). Conducting full- scale blast tests in the field is expensive, and also requires experience and licensing in handling explosives. Experiments at reduced scales are adopted to investigate the structural behavior and dynamic response. In this investigation, the dynamic response of the external beam-column joints with deficiencies such as weak beams in flexure, weak beams in shear, and weak columns in shear, are studied in detail. The beam column sub-assemble is idealized as a single degree of freedom system and its dynamic response is evaluated. The peak dynamic response for a bilinear resistance system is evaluated for different amplitudes and durations of shock loading to generate a design aid using the shock response spectrum.