Xihong Zhang

Laboratory Test on Dynamic Material Properties of Annealed Float Glass

In this study, laboratory tests were conducted to investigate the dynamic material properties of annealed float glass, which is widely used in building applications. The influence of strain rate effect on glass strength and Youngs modulus is studied. Quasi-static tests were performed first to determine the glass static strength and Youngs modulus; then dynamic compressive tests were carried out at the strain rates from 98/s to 376/s using a modified Split Hopkinson Pressure Bar. Tensile tests were performed in the strain rate range of 35/s to 990/s through splitting tensile test (Brazilian test). Test results reveal that the compressive and tensile strengths of annealed glass are very sensitive to strain rate. Dynamic increment on glass compressive strength is found more significant than its tensile strength, a phenomenon different from other brittle materials such as concrete. The Youngs modulus is found relatively insensitive to strain rate in the testing range, and is slightly larger in compressive tests than in tensile tests. Based on the test data compressive and tensile dynamic increment factors (DIF) of annealed glass with respect to strain rate are formulated. The glass fracture process is also investigated in this paper based on the images taken by high-speed camera during the tests. The fracture images and glass fragments are discussed and used to explain the testing results.

Experimental Investigation on Monolithic Tempered Glass Window Responses to Blast Loads

Monolithic glass is one of the most commonly and widely used materials for structural glazing in buildings. Due to its relatively low strength and brittle nature, monolithic glass window is often the most fragile part of a structure when subjected to air blast wave. The breakage of glass window under explosion always leads to enormous injuries and fatalities as a result of ejecting glass sharps flying at high speed towards people in the occupied area. For better protection of building occupants, it is necessary to fully understand monolithic glass responses under blast pressure. In this study, a series of full-scale field blasting tests were carried out to investigate monolithic glass window responses to blast loads. Typical windows with tempered glass panels and steel strip boundaries were mounted onto a reinforced concrete (RC) frame purposely constructed to support the window specimens for the tests. TNT explosives of different weights were detonated at different stand-off distances in front of the window. Window responses were monitored with high-speed cameras and linear variable displacement transducer (LVDT). Pressure sensors were used to measure the reflected pressure. Glass window failure patterns and associated glass fragments were recorded and analyzed. The tested window performances were compared with the predicted results based on ASTM and UFC standards, as well as previous testing results. Based on the testing data, criteria for tempered glass crack and fracture under blast loadings were formulated.

The effect of concrete shear key on the performance of segmental columns subjected to impact loading

Conventional precast segmental columns with seismic resistance design consist of only flat concrete segments with prestress tendon. This is because friction between adjacent segments is sufficient to resist the lateral forces from earthquake-induced actions. However, the friction between segments is not necessarily sufficient to resist lateral impact loads such as vehicle impact the column might experience during its service life. This article investigates the effectiveness of using concrete shear key in segments of precast segmental column in resisting the lateral impact loading. The precast reinforced concrete segments were designed with concrete shear keys to improve the column shear resistance capacity and minimize the relative displacement between adjacent segments. Two groups of segmental columns with and without shear key were designed and tested using a pendulum impact system. The effectiveness of shear key in resisting lateral impact loads was analysed by comparing the performance of the two groups of segmental columns. The testing results revealed that by introducing concrete shear key to segmental column, the relative displacement between adjacent segments could be effectively reduced. However, the large concrete shear key increased stress concentration in the concrete segments. Relatively, more severe damages to concrete segments were found on the columns with shear key. Further improvements on shear key designs should be made for better performance of segmental columns against impact loading.

Experimental study of precast segmental columns with unbonded tendons under cyclic loading

Precast segmental column has attracted a lot of interests over the past decade. It has become more and more popular in construction industry to achieve fast construction, improve construction quality and reduce environmental impact. Many factors including the energy dissipation bar, number of segments, prestressing force and shear keys could affect the performance of segmental column, but each of previous studies normally focused on investigating one or two of these factors. Since different previous studies used different materials, different structural dimensions, different axial loads and different testing methods, it is difficult to have comprehensive comparisons of the influences of these parameters on the performances of segmental columns. Moreover, limited studies indicated that shear keys had insignificant influences on performances of segmental columns subjected to cyclic loadings, while other studies revealed shear keys contribute to resisting impact loads but could lead to more severe damage due to stress concentration. In this study, cyclic tests were carried out to systematically assess the performance of segmental columns. Special attentions were paid on the influences of shear keys on column responses. Five scaled segmental columns with different designs were tested to study their damage mode, hysteretic behaviour, residual drift and energy dissipation capacity. One monolithic column was also tested as a reference. Test results showed that comparing with the monolithic column, the segmental columns exhibited better ductility and smaller residual drift, indicating better seismic performance, but segmental columns had less loading capacity and absorbed less energy. The influences of different designs including energy dissipation bar between adjacent segments, number of segments, prestressing force level in the tendon and concrete shear key on the seismic performance of segmental column were investigated and discussed.

Multi-hazard resistance capacity of precast segmental columns under impact and cyclic loading

Using precast segmental concrete columns in structures improves the construction efficiency and site safety, leads to better construction quality, and reduces the construction cost, site disruption and environmental impact. The performance of segmental columns to resist earthquake and impact loads is not well studied yet. As a structure might be subjected to such loads during its service life, understanding its resistance capacities is essential for structural safety. This article reports the findings of our recent studies on the response of precast segmental columns with unbonded prestress tendons. Pendulum impact test and uniaxial cyclic test were conducted on quarter-scale segmental columns. The seismic performance and impact-resistant capacity were evaluated experimentally and compared with a reference conventional monolithic column. Test results showed that under cyclic loading, segmental columns exhibited better deformation ability and smaller residual drift; under impact loading, segmental columns also showed better self-centring capacity and less residual displacement. By introducing concrete shear key, the shear resistance at segmental joint could be improved; however, shear key would also result in more concrete segment damage owing to stress concentration.

A special issue on protective structures against blast and impact loading

Recent decades have witnessed an increasing number of terrorist bombing attacks, accidental explosions, vehicular impacts, and rockfall hazards. Such blast and impact loads are normally not considered by the engineers. Therefore, structural analysis and design against such highly dynamic loadings have become an important issue which requires comprehensive understanding and studies on a variety of topics including the dynamic material properties, structural responses, and mitigation retrofits. The 24th Australasian Conference on the Mechanics of Structures and Materials (ACMSM) was held by Centre for Infrastructural Monitoring and Protection, Curtin University (in Perth, Australia), in December 2016. Authors of six selected papers were invited to extend their ACMSM conference papers for publication in this special issue of International Journal of Protective Structures (IJPS). The topics of these invited papers cover dynamic material properties, reinforced concrete, plymetal structure, FRP mitigation retrofit, glass windows, and precast structures, which all specifically represent state-of-the-art development in the area of protective structures against shock and impact loadings. The papers published in this special issue have undergone a strict peer-review process. We are grateful to all the authors and reviewers for the contribution and support during the course of editing this special issue. We hope this special issue will serve as an initiative to encourage more researchers and engineers to develop new and advanced materials/structures and mitigation technologies in this important and promising area.