Dynamic behaviour of concrete filled FRP tubes subjected to impact loading

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Numerous experimental investigations have demonstrated the advantages of Concrete Filled FRP Tubes (CFFTs) over reinforced concrete members under various loading conditions. None of the studies, however, investigated whether these advantages of CFFTs extended to resisting dynamic impact loads. This presented a fertile field for investigation as it was well known that the addition of the tube confined and protected the concrete core and increased its load carrying capacity, and by extension, its energy absorption. This investigation aimed at understanding the dynamic behaviour of CFFTs under impact loading and to develop a procedure for their analysis and design. These aims were achieved by testing six specimens, three of which were CFFTs and three were reinforced concrete. The specimens were tested in pairs to facilitate comparisons. The first pair were tested monotonically to act as a benchmark and tie into previous research. The remaining two pairs were tested under impact loading. The parameters investigated were the presence of the GFRP tube, the internal steel reinforcement ratio, and the input kinetic energy. The monotonic tests showed that the addition of the tube increased the flexural capacity and maximum displacement by 112%, which translated into a 487.5% increase in energy absorption, when compared to the reinforced concrete counterpart. The addition of the tube increased the impact specimens’ energy absorbing capacity by 467% when the steel reinforcement ratio was 1.2% and 1223% when the steel reinforcement ratio was 2.4%, compared to the reinforced concrete specimens. The GFRP tube also protected the concrete core and prevented its spalling and crushing during the impact tests. These encouraging experimental results led to the development of a nonlinear single degree of freedom (SDOF) model capable of capturing the behaviour of the system under multiple impacts to be used as a sophisticated analysis and design tool. Additionally, a simplified design method based on the conservation of energy was outlined.