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Soil behaviour is often an important consideration in the design of protective systems for blast and impact threats, as the properties of a soil can greatly affect the impulse generated from buried explosive devices, or the ability of a soil-filled structure to resist ballistic threats. Numerical modelling of these events often relies on extrapolation from low-pressure experiments. To develop soil models that remain accurate at very high pressures there is a need for data on soil behaviour under these extreme conditions. This paper demonstrates the use of a high-pressure multi-axial test apparatus to provide compressibility and shear strength data for four dry sandy soils. One-dimensional compression experiments were performed to axial stresses of 800 MPa, where the effects of particle-size distribution were observed with respect to compressibility and bulk unloading modulus. Each soil followed a bilinear normal compression line (NCL): more uniform soils initially had higher compression indices, but all four NCLs began to converge at void ratios below e ≈ 0.3. The failure surface of a sand was characterized to mean effective stress [Formula: see text] > 400 MPa using reduced triaxial compression experiments, removing the need to rely on extrapolation from low-pressure data.

Laser scabbling of concrete is a process by which the surface layer of concrete is removed through the use of a high power (low power density) laser beam. In order to understand how the age and treatment of structures may affect the laser scabbling process, the aim of the research presented in this paper was to establish a relationship between laser interaction time, surface temperature and volume removal for cementitious materials of different ages and different degrees of saturation. The investigation focussed on (i) the effect of age on saturated specimens and (ii) the effect of prolonged drying. The results show that drying of specimens had the largest effect on scabbling. The effect of age on saturated specimens was small for PFA + OPC pastes, mortars and concretes, but significant for OPC pastes, where the volume of scabbling dramatically reduced with age.

Friction-based structural control is an available strategy for reducing the seismic response of buildings. The friction dampers in such systems can be operated using passive and semiactive control. Passive dampers with constant, pre-defined capacity are effective and simple, but their adaptability to a broad range of frequency excitations is limited and their optimal configuration is complex. Semiactive control provides a means to vary the dampers’ capacity to their optimum levels in real-time, but time delays in the control action may affect their performance. In this investigation, a passive system is initially introduced in a multi-storey steel frame to identify a threshold of optimum control force demand related to the limits of the building’s elastic response. A new semiactive algorithm is then introduced to adjust the dampers’ capacity based on the current deformation state across the building. From simulations of the non-linear response of the frame, the semiactive system reduced the structural response to levels similar to the optimum passive control, with more uniform distributions of storey drift. The control system had optimum performance when a range of time delays was included to simulate different regulator mechanisms.

Friction-based dampers can be considered as one of the suitable passive control systems for seismic strengthening and rehabilitation of existing substandard structures due to their high adjustability and good energy dissipation capability. One of the main issues in the design of these systems is to obtain the magnitude of the maximum slip force and the distribution of slip forces along the height of the building. In this study, a practical performance-based optimisation methodology is developed for seismic design of RC frame buildings with friction energy dissipation devices, which allows for an accurate solution at low computational cost. The proposed method aims at distributing the slip loads of the friction dampers to achieve a uniform distribution of damage along the height of the building. The efficiency of the method is evaluated through the optimum design of five different low to high-rise RC frames equipped with friction wall dampers under six natural and six synthetic spectrum-compatible earthquakes. Sensitivity analyses are performed to assess the reliability of the method using different initial height-wise slip load distributions, convergence parameters and earthquake records. The results indicate that optimum frames exhibit less maximum inter-storey drift (up to 43%) and global damage index (up to 75%), compared to uniform slip load distribution. The method is then developed to obtain the optimum design solution for a set of earthquakes representing a design spectrum. It is shown that the proposed method can provide an efficient tool for optimum seismic design of RC structures with friction energy dissipation devices for practical purposes.

A simplified method is proposed for optimum design of friction dampers by considering the characteristics of design earthquakes. Optimum slip loads for 3, 5, 10, 15 and 20-storey RC frames with friction wall-dampers are obtained for a set of 20 near- and far-field earthquakes as well as artificial spectrum-compatible records scaled to different acceleration levels. Optimum solutions are shown to be more sensitive to Peak Ground Velocity (PGV) than Peak Ground Acceleration (PGA), especially for near-field earthquakes with high-velocity pulses. For identical PGA levels, far-field earthquakes on average result in 1.5 times lower optimum slip loads compared to near-field records, while they lead to 118% higher energy dissipation and 24% lower maximum inter-storey drifts. Empirical equations are proposed to predict optimum slip loads (as a function of number of storeys and PGA/PGV of design earthquakes) and their efficiency is demonstrated through selected examples.

The influence of strain rate and moisture content on the behaviour of a quartz sand was assessed using high-pressure quasi-static (0.001 /s) and high-strain-rate (1000 /s) experiments under uniaxial strain. Quasi-static compression to axial stresses of 800 MPa was carried out alongside split Hopkinson pressure bar (SHPB) experiments to 400 MPa, where in each case lateral deformation of the specimen was prevented using a steel test box or ring, and lateral stresses were recorded. A significant increase in constrained modulus was observed between strain rates of 0.001 /s and 1000 /s, however a consistently lower Poisson's ratio in the dynamic tests minimised changes in bulk modulus. The reduction in Poisson’s ratio suggests that the stiffening of the sand in the SHPB tests is due to additional inertial confinement rather than an inherent strain-rate dependence. In the quasi-static tests the specimens behaved less stiffly with increasing moisture content, while in the dynamic tests the addition of water had little effect on the overall stiffness, causing the quasi-static and dynamic series to diverge with increasing moisture content.

The concept of passive control of the seismic response of structures was introduced to improve the performance of structures by increasing their energy dissipation and reduce or eliminate damage in the structural elements. The key task in the design of passive systems is to determine the forces in the control devices (yield/slip or post-tensioning) at each floor, that will result in best performance (e.g. minimum inter-storey drift). This can be achieved by large parametric studies in which both the maximum control force (e.g. at ground level) and the distribution of forces along the height of the structure are varied. Alternatively, optimum forces in the devices can be achieved by semi-active control, where the structure self-adapts to the earthquake. Both solutions are expensive: the first requires hundreds of non-linear response simulations in the design stage; the second needs a system of sensors, controllers and electromechanical devices. Presented here is a new Self Adaptive Optimisation Approach (SAOA) in which the self-optimisation of a semi-active system is used in the design stage and the resulting distribution of control forces is adopted as a passive system. The new approach was evaluated through comparing the simulated dynamic responses of two relatively simple benchmark structures (braced and post-tensioned) with three sets of control forces: (1) passive system with forces obtained in parametric study, (2) semi-active system with self-adapting control forces, and (3) passive system with SAOA-optimized forces. The results show good performance of the SAOA systems, indicating that SAOA offers a simple and effective solution that can replace the existing optimisation approaches for the design of passively controlled earthquake resistant structures. This study presents a novel idea of using the semi-active control as a tool for optimising a passive control system. The passive control systems can be further improved by a larger study in which the semi-active control algorithms are also optimised.

Friction control systems have been widely used as one of the efficient and cost\ud effective solutions to control structural damage during strong earthquakes.\ud However, the height-wise distribution of slip loads can significantly affect the\ud seismic performance of the strengthened frames. In this study, a practical design\ud methodology is developed for more efficient design of friction wall dampers by\ud performing extensive nonlinear dynamic analyses on 3, 5, 10, 15, and 20-story RC\ud frames subjected to seven spectrum-compatible design earthquakes and five\ud different slip load distribution patterns. The results show that a uniform\ud cumulative distribution can provide considerably higher energy dissipation\ud capacity than the commonly used uniform slip load pattern. It is also proved that\ud for a set of design earthquakes, there is an optimum range for slip loads that is a\ud function of number of stories. Based on the results of this study, an empirical\ud equation is proposed to calculate a more efficient slip load distribution of friction\ud wall dampers for practical applications. The efficiency of the proposed method is\ud demonstrated through several design examples.\ud

In this paper an approach of real time measurement and analysis of magnetic field disturbances is described. Multiresolution based analysis appears as useful tool for feature extraction and identification of different signal phenomena. As a sensing element, a magentoresistive sensor is utilized together with virtual instrumentation. Experimental results are presented graphically.

Centralized semi-active control is a technique for controlling the whole structure using one main computer. Centralized control systems introduce better control for relatively short to medium high structures where the response of any story cannot be separated from the adjacent ones. In this paper, two centralized control approaches are proposed for controlling the seismic response of post-tensioned (PT) steel frames. The first approach, the stiffness control approach, aims to alter the stiffness of the PT frame so that it avoids large dynamic amplifications due to earthquake excitations. The second approach, deformation regulation control approach, aims at redistributing the demand/strength ratio in order to provide a more uniform distribution of deformations over the height of the structure. The two control approaches were assessed through simulations of the earthquake response of semi-actively and passively controlled six-story post-tensioned steel frames. The results showed that the stiffness control approach is efficient in reducing the frame deformations and internal forces. The deformation regulation control approach was found to be efficient in reducing the frame displacements and generating a more uniform distribution of the inter-story drifts. These results indicate that centralized semi-active control can be used to improve the seismic performance of post-tensioned steel frames.

This study aims to improve the computational efficiency and convergence rate of an already existing optimisation method for friction-based dampers based on the concept of Uniform Distribution of Deformation (UDD) and demonstrate the reliability of the results compared to Heuristic optimisation methods such as GA. In the proposed approach, the computational cost is considerably reduced by using a convergence factor that is modified in proportion to the level of performance violation. To investigate the efficiency of the proposed method, 3, 5 and 10-storey RC frames with friction dampers are optimised using adaptive UDD method, Genetic Algorithm (GA) and a coupled UDD-GA approach. The results indicate that the adaptive UDD method can lead to optimum design solutions with significantly lower computational costs (up to 300 times lower number of non-linear dynamic analyses) compared to both GA and coupled UDD-GA methods. It is shown that frames optimised under a single spectrum-compatible earthquake can efficiently satisfy the predefined performance targets under a set of synthetic earthquakes representing the design spectrum. Therefore, the proposed method should provide a reliable approach for more efficient design of friction-based dampers.

The Defence Science and Technology Laboratory sponsored, QinetiQ-led Force Protection Engineering Research Programme has two main strands, applied and underpinning research. The underpinning strand is led by Blastech Ltd. One focus of this research is into the response of geomaterials to threat loading. The programme on locally won fill is split into four main characterisation strands: high-stress (GPa) static pressure–volume; medium-rate pressure–volume (split Hopkinson bar); high-rate (flyer plate) pressure–volume; and unifying modelling research at the University of Sheffield, which has focused on developing a high-quality dataset for locally won fill in low and medium strain rates. With the test apparatus at Sheffield well-controlled tests can be conducted at both high strain rate and pseudo-static rates up to stress levels of 1 GPa. The University of Cambridge has focused on using one-dimensional shock experiments to examine high-rate pressure–volume relationships. Both establishments are examining the effect of moisture content and starting density on emergent rate effects. Blastech Ltd has been undertaking carefully controlled fragment impact experiments, within the dataspace developed by the Universities of Sheffield and Cambridge. The data from experiments are unified by the QinetiQ-led modelling team, to predict material behaviour and to derive a scalable locally won fill model for use in any situation.

The nonlinear inelastic stress-strain relationship of concrete is a result of changes to the fabric of the material caused by several damage mechanisms. Existing experimental research on concrete provides very little information on the evolution of these mechanisms. Instead, most experimental studies are limited to measuring the external loads and deformations of concrete specimens. The aim of the research presented here is to provide a direct link between macroscopically observed global stress-strain behavior and the development of damage mechanisms within the specimens. A novel experimental procedure is introduced to monitor the changes in the material during a conventional multiaxial compression test. This procedure, comprising small stress probes repeated at selected points of the stress-strain history, is designed to provide data on the tangent stiffness operator and the associated acoustic tensors. The acoustic tensor with the minimum value of its determinant provides information on both magnitude and orientation of the damage. The stress-probing methodology and some of the key practical aspects and limitations of the experimental procedure are illustrated by using five multiaxial compression tests as examples of different stress-strain histories. The results of the test methodology presented in this paper give an indication of the extent of damage and accurate information about its orientation. This paper therefore shows that the stress-probing procedure can be incorporated in multiaxial tests designed to monitor the relationship between damage evolution in the material and macroscopically observed stress-strain response of the specimen.

In this paper a Finite Element (FE) procedure is described for simulating hysteretic moment–rotation behaviour and failure deformations of bolted cold-formed steel (CFS) moment connections. One of the main challenges in modelling the response of bolted connections is the presence of bolt slip. A series of six beam–column assemblies comprising CFS curved flange beams, a support column and a through plate were tested under cyclic loading. The moment–rotation behaviour of the connections was dominated either by flexure in the beams or by bolt slip in the connections. FE models presented in this paper incorporate geometrical imperfections of the beams, material properties obtained from tensile coupon tests and bolt slip to address these two types of behaviour. The updated FE models result in an accurate prediction of the hysteretic moment–rotation behaviour of the connections dominated by a flexural behaviour in the beams. A simplified cyclic slip model which allows slip at a specified reduced slip resistance load simulates reasonably well the hysteretic behaviour of connections dominated by bolt slip. Using the updated FE model, the failure modes of the connections predicted by the simulations also agree well with those observed in the experiments.

Laser scabbling of concrete is the process by which the surface layer of concrete may be removed through the use of a low power density laser beam. Previous research has suggested that the driving force responsible for laser scabbling is developed within the mortar. The aim of this investigation was to establish the key parameters that influence laser scabbling of mortars. The results show that the removal of free water from mortars prohibits scabbling, but resaturation allows mortar to scabble. A reduced permeability, either due to a reduction in the water/binder ratio or the use of 25% PFA replacement, enhances the scabbling. A higher fine aggregate content increases volume removal and fragment sizes during laser scabbling.

Laser concrete scabbling is the process by which the surface layer of concrete may be removed through the use of a low power density laser beam. The main aim of this investigation was to establish relationships between laser interaction time and volume removal for a wide range of material compositions, including different w/b ratios, binder compositions (OPC/PFA), aggregate/binder ratios and coarse aggregate sizes. The results show that 25% replacement of ordinary Portland cement with pulverised fuel ash and/or a reduced water/binder ratio improves the efficiency of scabbling of cement pastes. Mortars and cement pastes were seen to scabble at a constant rate, whereas concretes experienced a peak rate, after which volume removal reduced dramatically. Basalt aggregate concrete was less susceptible to laser scabbling than limestone aggregate concrete. The effects of composition on the mechanisms which drive laser scabbling are discussed. It is suggested that pore pressure spalling governs behaviour in cement pastes, and thermal stress spalling is more dominant in mortar specimens. The driving force responsible for laser scabbling of concretes is developed within the mortar.

This paper presents an experimental investigation on the use of thin-walled cold-formed steel (CFS) sections as energy dissipative elements for earthquake resistant moment frame multi-storey buildings. The tests were performed on six bolted beam-to-column connections, using through plates and curved flange beams with different types of out-of-plane stiffeners in the connection region. The hysteretic behaviour of the CFS connections shows high seismic energy dissipation capacity and sufficient ductility to satisfy code requirements for seismic design. The use of out-of-plane stiffeners inside the beams in the connection region results in improvement of the moment–rotation behaviour of the connection by up to 35% in strength and 75% in ductility. Mobilising connection slip after the elastic cycles provides highly stable hysteretic behaviour and an increase of up to 240% in energy dissipation capacity. The tested connections can be classified as rigid with partial or full strength depending on the connection stiffeners.

The development of thin-walled cold-formed steel (CFS) sections as energy dissipative elements for seismic moment-resisting multi-storey frame buildings is presented through FE analysis and experimental work. Studies on different structural levels are undertaken. At the element level, increasing the number of flange bends enhances both the elastic and inelastic behavior, and beams with an infinite number of bends (with curved flanges) show the highest strength, stiffness and ductility. At the connection level, different configurations of CFS beam-to-column connections using through plates are investigated numerically and verified experimentally. In web bolted connections without out-of-plane stiffeners, premature web buckling results in early loss of strength. A minimum of two pairs of vertical stiffeners are identified as essential in the connection region to delay web and flange buckling and produce relatively high moment strength and ductility. This investigation is validated by beam-to-column connection tests using through plates and curved flange beams with different types of out-of-plane stiffeners in the connection region. The results show that the envelope of the hysteretic curves obtained in the tests of the CFS connections can be predicted by the FE analysis. The use of out-of-plane stiffeners can increase the seismic energy dissipation capacity by up to 90%, the moment strength by up to 35% and the ductility by up to 75% when compared with connections without stiffeners. Correspondingly the use of the two minimum pairs of the vertical stiffeners can increase the seismic energy dissipation capacity by 30%, the moment strength by 28% and the ductility by 50%.

This paper presents an investigation on the potential use of cold-formed steel sections (CFS sections) in moment-resisting frames (MRFs) for seismic applications. The main limitation of CFS sections is the low out-of-plane stiffness of their thin-walled elements which leads to low ductility. The main components of MRFs are beams, beam–column connections and columns. In earthquake resistant MRFs, the beams are designed to provide considerable ductility, whereas the other elements are mainly limited to their elastic range. The performance of a new shape of CFS beam with curved flange is examined analytically and compared with that of conventional shapes. The proposed beam–column connections include through plates which potentially limit the out-of-plane action of the forces transferred through the connections. The behaviour of both individual CFS beam sections and CFS beam–column connections is studied by means of finite element analysis (FEA). The results of the analyses show that the new beam cross sections and connections exhibit a good ductile behaviour, something which cannot be achieved by conventional cold-formed frame elements.

Concrete in structures exposed to high temperatures is practically always heated under stress. Yet, there are few experimental studies in which the concrete was heated under stress and then loaded to the peak, and most of these were performed under uniaxial compression. This paper reports on an experimental study of the effects of different heat–load regimes on the stress–strain behaviour of partially sealed concrete under multiaxial compression, at elevated temperature. The specimens were first heated (stressed/unstressed), then loaded to the peak in multiaxial compression. In contrast with previous experimental research, the results show that concrete heated under relatively low compressive stress has lower strength and stiffness than concrete heated without load. The results suggest that the presence of stress during first heating produces a specific damage, which could be the cause for a major component of the load induced thermal strain (LITS) in concrete.

The increased popularity of cold-formed thin-walled steel sections in housing construction has lead to an increased interest in the development of thin-walled frame buildings in accordance with seismic requirements. This paper investigates appropriate details for the main frame elements of a thin-walled building structure. Some of the proposed details are tested by two full scale one-storey frames under gravity and lateral cyclic loads. The results show that this type of structure offers a good potential for earthquake resistant construction, but more thorough studies are needed.

Previous experimental research has shown that the compactive strains in concrete subjected to a load-then-heat regime exceed those measured in heat-then-load tests under compression. This excess in strain is known as transient thermal creep or load-induced thermal strain (LITS). All previous experimental research on LITS in mature concrete has been conducted in unsealed conditions, mainly under uniaxial compression (with a few biaxial compression tests, but no multiaxial tests) on specimens subjected to monotonic heating to high temperatures (> 500 °C). This paper presents the findings from a novel laboratory investigation of LITS under uniaxial, biaxial and hydrostatic compression in partially sealed conditions, at transient temperatures of up to 250 °C. The results from 49 experiments show that LITS in the sub-250 °C range is highly dependent on the moisture flux conditions and, consequently, on the relationship between heating and drying rates.

Laser scabbling of concrete is the process of removal of surface material using a high power laser beam. The main aim of this investigation was to establish an experimental procedure for assessing the effects of various parameters that may be critical for the effectiveness of the process, such as material composition and initial moisture content. The study shows that the key characteristics of the process can be detected by monitoring surface temperature variations. This experimental procedure is used to provide data on the effects of each parameter to explain the mechanisms that drive the process. The results suggest that scabbling is mainly driven by pore pressures in the cement paste, but strongly affected by other factors. Reducing permeability by adding PFA to the cement paste resulted in significant increase in volume removal; but reducing moisture content by air-drying of the material did not result in the expected reduction in volume removal.

The European research project BANDIT investigated the effectiveness of a novel Post-Tensioned Metal Strapping (PTMS) strengthening technique at improving the seismic performance of deficient RC buildings using shake table tests. A full-scale two-story structure was designed with inadequate reinforcement detailing of columns and beam-column joints so as to simulate typical deficient buildings in Mediterranean and developing countries. Initial shaking table tests were carried out until significant damage was observed in the beam-column joints of the bare frame. Subsequently, the damaged building was repaired and strengthened using PTMS and additional tests were performed. The results of this study show that the adopted strengthening strategy improved significantly the seismic performance of the substandard RC building under strong earthquake excitations.

X-ray computed tomography (X-ray CT) has been applied to nondestructively characterise changes in the microstructure of a concrete used in the pressure vessel structure of Advanced Gas-cooled Reactors (AGR) in the UK. Concrete specimens were conditioned at temperatures of 105 C and 250 C, to simulate the maximum thermal load expected to occur during a loss of coolant accident (LOCA). Following thermal treatment, these specimens along with an unconditioned control sample were characterised using micro-focus X-ray CT with a spatial resolution of 14.6 microns. The results indicate that the air void pore structure of the specimens experienced significant volume changes as a result of the increasing temperature. The increase in the porous volume was more prevalent at 250 C. Alterations in air void size distributions were characterized with respect to the unconditioned control specimen. These findings appear to correlate with changes in the uni-axial compressive strength of the conditioned concrete. © Owned by the authors, published by EDP Sciences, 2013.

Intensive investigations in the field of the control of structural behaviour have been conducted at the Institute of Earthquake Engineering and Engineering Seismology in Skopje. Part of them, relating to energy absorption elements are presented in this paper. In order to provide a more sophisticated presentation of the characteristics and capabilities of these energy absorption elements, a short review of the currently available and most frequently applied passive control systems is included. To prove the efficiency of the energy absorption elements quasistatic tests on single-span models CR10 and CR20 of the characteristic frame have been performed. The results proved that the CR20 models are of considerably higher strength, even six times higher at ultimate state, ductility and energy dissipation capacity. A mathematical model has been formulated for the nonlinear behaviour of the main elements of the system, whereby a trilinear P-Δ diagram has been used, as developed at the Institute. The analysis of the hypothetical prototype, a five-storey building, with energy absorption elements incorporated in six spans, subjected to the seismic excitations of two PGA levels, 0.12 g and 0.15 g, has shown very good results. Further investigations in this field resulted in the development of new energy absorbing elements (through friction), SBC, which when placed at the joints of the frame introduce hysteretic damping to the structure in the case of low level excitations, when the rest of the system is in the linear range.

In this paper we explore Chaikin's algorithm for generation of arbitrary curves to reconstruct non-uniformly sampled signals. The sampling is adapted to the signal shape. For the sampling rate estimation the discrete Haar wavelet transform is used. The results of reconstruction are presented graphically and compared with those obtained by reconstruction based on multiresolution.

Friction-based supplemental devices have been extensively used for seismic strengthening of existing substandard structures, however, the conventional use of these dampers may not necessarily lead to an optimum structural performance. Conventionally designed friction dampers usually follow a uniform height-wise distribution pattern of slip load values mainly for simplicity of implementation. This can lead to localizing structural damage in certain storey levels, while the other storeys accommodate lower relative displacement demands. In this study, a practical performance-based optimisation methodology is developed to tackle with structural damage localization of RC frame buildings with friction energy dissipation devices under severe earthquakes. The proposed optimisation is aiming at redistributing the slip loads of the friction wall dampers so as a uniform height-wise distribution of inter-storey drifts is achieved. The efficacy of the method is evaluated through the optimum design of five different low to high-rise RC frames equipped with friction wall dampers under six real spectrum-compatible design earthquakes. The effects of different design parameters including number of storeys, convergence factor and design seismic excitations are also evaluated on the efficiency of the adopted optimisation approach. The results indicate that compared to the conventional design, using the suggested methodology to design friction wall systems can result in up to 40% reduction of maximum inter-storey drift and considerably more uniform height-wise distribution of relative displacement demands under the design earthquakes.