Understanding the behavior of construction materials under combined effect of extreme loadings
Engineers endeavour to design protective structures that can withstand extreme loading scenarios. One such extreme is the post-impact fire load caused by explosion or high-speed collision and potential subsequent fire. Through this thesis, a fundamental understanding of the behaviour of conventional construction materials namely steel, concrete and steel-concrete composite under the combined actions of impact and subsequent fire has been achieved experimentally. Based on the results, a unified and versatile material model, capable of accurately reflecting this coupled effect, has been developed which can easily be incorporated in software and prospective codes of practice for rational engineering for extremes designs.
Abstract: Concrete-filled double-skin tubes (CFDST) have been increasingly popular in the field of engineering in recent years. A lot of research has been carried out to investigate the behaviour of CFDST members under a variety of loading conditions. This paper presents an experimental investigation on ultra-high performance concrete infilled double-skin tube columns subjected to close-in blast loading. Two types of CFDST columns were investigated in the experiments – one with both inner and outer tubes circular and the other one with both square. The main test parameters included the explosive charge weight and the magnitude of axial load. After the blast tests, there was no visible buckling nor ruptures found on the steel tubes and only minor cracks, of no more than 1mm width, were observed in the core concrete when the outer steel tube was removed. Based on the findings of the experiments, it is evident that CFDST column has excellent blast resistance. This feature has the potential to be used in high-value structures which may be the targets of terrorist attacks, such as embassies, government buildings and critical infrastructures.
Pub.: 14 Mar '16, Pinned: 30 Aug '17
Abstract: In recent years, a large number of studies have been carried out to investigate behaviours of concrete filled double skin steel tube (CFDST) members due to its increasing popularity in the construction industry. This paper firstly presents an experimental study on ultra-high performance concrete filled double-skin tubes subjected to close-range blast loading with cross section being square for both inner and outer steel tubes. It is evident that the proposed CFDST column was able to withstand a large blast load without failure so that it has the potential to be used in high-value buildings as well as critical infrastructures. Then, to further investigate the behaviours of the proposed CFDST column, a number of parametric studies were carried out by using a numerical model which was developed and calibrated based on the data acquired from the blast test along with some laboratory tests. Parameters that affect the behaviours of concrete filled double skin steel tube (CFDST) members against blasts are characterised.
Pub.: 26 Jul '16, Pinned: 30 Aug '17
Abstract: With the increasing trend towards using concrete-filled steel tubes (CFST) in civil structures, understanding their mechanical properties under impact loads has attracted the interest of researchers. The dynamic properties of concrete confined by steel tubes are size-dependent. An experimental program was carried out to investigate the relation between the size and impact response of CFST sub-samples. High-strain-rate tests were conducted on specimens made from self-consolidating normal concrete confined by mild steel tubes. To take into account the stress uniformity and confinement effects in the specimens, various height-to-diameter ratios (H/D) and diameter-to-tube-wall thickness ratios (D/t) were considered. Dynamic Increase Factors (DIFs) were derived as the ratio of the material strength at high strain rate to those of the same size under quasi-static loading conditions. The results were compared to two sets of reference tests, namely unconfined concrete and hollow steel tube specimens of the same size and with the same boundary conditions. The results indicate the influence of H/D ratio, D/t ratio, and end-friction coefficient on the stress-strain distribution, dynamic compressive properties and failure modes of sub-scale concrete-filled steel tubes under impact load. The size-dependent behaviour of the CFST is found to be a function of the level of confinement the circumferential steel tube imposes on the concrete filling. Two expressions are proposed for predicting the DIF of yield stress for CFSTs: one considering the concrete-steel interaction relationship presented in Eurocode 4, and an empirical expression based on the Cowper-Symonds model for steel. The proposed rate- and size-dependent expressions show close correlations with experimental results.
Pub.: 03 Nov '16, Pinned: 18 Aug '17
Abstract: The present research investigates the effect of high temperatures on the mechanical properties of plain concrete as well as steel-concrete composite samples which have previously sustained partial damage under high-strain-rate loading. With the rise of interest in investigating extreme loading events such as post-impact-fire scenarios, this study will help in evaluating whether partially damaged concrete and composite elements can further sustain additional stresses in case of a subsequent fire outbreak. Unconfined self- compacting concrete (SCC) and SCC-filled mild steel tube (CFST) samples are subjected to a dual-phase testing procedure where they undergo interrupted compressive loading at impact rates of strain, controlled locally at pre-defined damage levels to account for different deformation states. Damaged specimens are subsequently exposed to elevated temperatures and the residual mechanical properties of the samples are measured under quasi-static compression test conditions. Results indicate that for concrete and CFSTs, variation of residual properties is dependent on the level of pre-induced damage as well as exposed temperature, with the effect of pre-deformation losing significance at very high temperatures. Residual characteristics of CFSTs are shown to be reliant on rate and temperature dependency of both constituent materials. Furthermore, X-ray imaging has been utilized to investigate the extent of cracking and crack propagation at different damage levels.
Pub.: 22 Feb '17, Pinned: 18 Aug '17