Implications of impact experiments on honeycomb shielded exterior beam-column joint

https://doi.org/10.1016/j.engstruct.2020.110470Get rights and content

Highlights

  • Impact resistance of the beam-column joints could be enhanced by honeycomb panels as a shielding material.

  • Dynamic plastification of the honeycomb core is identified as the main energy absorbing mechanism.

  • Beam-column joints designed for seismic events show a better impact resistance.

  • Core stiffness of the honeycomb influences peak acceleration and displacement.

  • Influence of the variant honeycomb thicknesses on the impact mitigation is investigated.

Abstract

The response of building structures under impact loads is a challenging subject for both numerical and analytical studies, as the available experimental data is limited. In the current research, an attempt was made to study the experimental investigation on the exterior beam-column joint against accidental and intentional impact loading. The velocity regime of the projectile was considered in the range 30–34 m/s. The response of the structural member shielded with the Aluminum honeycomb sandwich panel was compared with the unshielded beam-column joint. The target damage, failure modes, energy absorption, and evolution of cracks were examined in terms of both analytically and experimentally. Results obtained from high-speed imaging were used to obtain the projectiles incident, residual, and re-bound velocities. The application of honeycomb shielding on the seismic group specimens shows that the ballistic limit was increased by its energy absorption characteristics. It was found that they have a 22% higher ballistic limit than that of the seismic group specimens without honeycomb shielding. The shielded honeycomb sandwich panel specimens show promising results as they absorbed 49% higher impact energy than that of seismic specimens without honeycomb shielding. The current study provides a valuable reference for designing of honeycomb panels as a sacrificial material to safeguard the engineering structures against impact loads in the intermediate velocity regime.

Graphical abstract

As the projectile penetrates the target specimen, the impact force damages the specimen by its perforation. An application of honeycomb energy absorber leads to the proposal that minimizes the damage of the structure during its impact. A Classification map of various failure modes illustrating the target morphology is proposed.

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Introduction

Numerous countries have become victims of wind-borne debris, rock fall, accidental, and intentional impact loading on structures on a grand scale. Engineers need to understand these detrimental effects on the design and assessment of building structures. Most of the existing structures are designed for ordinary loads and were not designed to withstand against such significant impact loadings. The designer should be conscious of the potential threat to the occupant's safety residing in the buildings. Understanding the importance of impact resistance of existing buildings against impact threats is worthwhile in designing and upgrading the critical infrastructure. Therefore, it is predominant to protect the structures from extreme events such as accidental, tornados and intentional impact loadings [1]. A lot of effort has been taken in this area to enhance the strength and ductility of structures to withstand from such extreme loading events [2], [3]. Before understanding the behaviour of building response under impact loading, it is essential to understand the response of the individual structural element, i.e., beams, column, beam-column joint. The objective of the current research is to study the implications of impact experiments on exterior beam-column joint protected with honeycomb panel shielding.

Seldom, important structures are likely to be situated in the highly seismic prone areas and their design is governed by respective country seismic codes (ACI 318-14, ASCE 7-10, EC 8, NZS 1170-2004). Furthermore, such important structures identified in the seismic prone area were designed based on the current seismic codes provided by the Indian Standard (IS: 1893-2002, IS: 13920-2016). An exterior Beam-Column joint plays a vital role in the RC framed buildings and are probably the most exposed structural components during the tornado and intentional impact loadings [3]. The shear failure, anchorage and bond failure within these vital beam-column assemblies may lead to the progressive collapse of the entire structure which ends with the huge loss of causalities and property [4], [5], [6], [7]. A significant amount of research have been done on structural members such as slabs, column, and beams under impact loadings [2], [3], [4], [5], [8], [9], [10], [11].

By reviewing the studies of dynamic load characteristics, the impact loading is divided into four categories, i.e., low-velocity impact, intermediate velocity impact, and high-velocity impact. A low-velocity impact occurs at a velocity below 10 m/s, an intermediate-velocity impact occurs between 10 m/s and 50 m/s. The high-velocity impacts occurs at a range of 50 m/s to 1000 m/s, and hyper-velocity impact have the range of 2 km/s to 5 km/s. Numerous studies had performed on the low-velocity, high-velocity, and hyper-velocity on the structural elements and composites [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. A limited study was conducted on the intermediate-velocity impact loading on structural members [23], [24], [25], [26]. In practical scenarios, the consequences of the wind-borne debris and accidental impact loadings can cause an intermediate velocity impact [27], [28], [29]. Hence, the key motive of the current research work is to study the intermediate-velocity impact behavior of the beam-column assemblage.

The main mechanics to amend the impact resistance of the structure are (i) energy absorption, (ii) dissipation, (iii) defragmentation and disintegration, (iv) impact trapping, (v) reflection/divergence and, (vi) structural strengthening. The impact mitigate strategies included employment of deformable sacrificial protective layer that can dissipate or absorb impact energy and reflect/deflect the projectile. In this paper, it is focused on the development and implementation of protective sacrificial layered material that mitigate the impact damage by virtue of energy absorption. When a structure is wrapped or strengthened with composite materials, its ductility behaviour gets enhanced and will exhibit enhance resistance to extreme loadings. From the literature, it was acknowledged that Aluminum foam and honeycomb sandwich panel have shown promising results in resisting impact loads [17], [30]. Several experimental studies on impact found that the application of Aluminum foam panels increases the energy absorption and impulse transfer to the protected structure [31]. Sharath et al. [30] have proposed a probabilistic quasi-static energy balance model with modified nonlinear contact interaction to predict contact force, indentation, and global deflection in a sandwich structure. It is evident that extensive numerical and analytical investigations are essential to optimize the design of protective devices for improving the effectiveness of the dissipative protective device [17], [30], [31], [32].

It is worthy to mention that significant progress has been made to understand the behavior of isolated composite sandwich panel to resist impact loadings. However, limited research literature is available on the performance of beam-column joints subjected to impact loading, which is most vulnerable to such threats [33], [34], [35], [36], [37], [38], [39]. It was noticed that there is a lack of systematic research on the beam-column joints protected using composite shielding material to resist impact loading. The current research aims to study the endurance and sustainability of gravity design (IS: 456-2016) and seismic design criteria (IS: 1893-2002, IS: 13920-2016) of exterior reinforced concrete beam-column joint subjected to impact loadings. Experiments at reduced scales are adopted to investigate the structural behavior and dynamic response of beam-column joint. Impact studies are conducted on half-scale models of exterior reinforced concrete beam-column joint shielded with and without honeycomb protective system. A comparative study has been conducted on the results obtained from Aluminum honeycomb sandwich panel with pristine specimens. In addition, honeycombs with different thicknesses and beam-column joint specimens that were designed for seismic and non-seismic were also investigated. The results obtained from the impact test demonstrated the efficacy of utilizing the Aluminum honeycomb sandwich panel as sacrificial shielding material that absorb the impact energy by progressive plasticization of core.

Section snippets

Aluminum honeycomb core

In the current investigation, two variants of hexagonal Aluminum honeycombs manufactured from the Aluminum alloy ACG-3003 grade were employed as cores. These honeycombs are varied in terms of their cell size and cell wall thicknesses. The Aluminum honeycombs are represented based on the configurations of Density - Cell size - Cell wall thickness and all other dimensions are expressed in SI units. For example, 56-9-70 designates an Aluminum honeycomb with a density of 56 kg/m3, the cell size of

Experimental observations

High-Speed imaging is employed to capture the perforation process during the impact along with the dynamic response of the beam-column joint. A high-speed video camera (Phantom VEO 640L) was employed to record the impact phenomenon and also to measure the projectile’s normal incidence and residual velocities. The imaging is recorded at 7300 frames per second (FPS) with a pixel resolution of 512 × 1024 to maintain a proper visualization of the perforation process. To facilitate a good

Experimental results and findings

The dynamic behavior of the exterior reinforced concrete beam-column joint was diagnosed using the following experimentally recorded data and observation:

  • 1.

    Effect of impact velocity and energy absorption characteristics

  • 2.

    Evaluation of target damage using crater analysis technique

  • 3.

    Study of target morphology after impact

  • 4.

    Evolution of cracks near beam-column joint

  • 5.

    Transient displacement response

  • 6.

    Acceleration time histories

The evaluated data was processed in order to determine the dynamic characteristics

Summary and conclusions

The aim of the current study is to address the effectiveness of shielding honeycomb panels on exterior beam-column joint specimens under impact loading. A series of Aluminum honeycomb sandwich panel with variant thicknesses were tested under static compression as well as under impact loading. This includes both seismic and non-seismic detailing beam-column joint specimens shielded with honeycomb energy absorbers. Furthermore, a parametric investigation on the honeycomb with a variant thickness

CRediT authorship contribution statement

Anupoju Rajeev: Data curation, Writing - original draft, Visualization, Investigation. Damith Mohotti: Software, Validation, Writing - review & editing. Amit Shelke: Conceptualization, Methodology, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The financial support of the Defence Research and Development Organization, Government of India under Grant No. ARMREB/CDSW/2017/192 is greatly acknowledged. The authors are also grateful to Dr. Anugrah Singh (IIT Guwahati) for providing high-speed camera system.

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