Postgraduate research supervision

It has been well demonstrated that the impact loading resistance capacity of the concrete material can be effectively increased by adding fibres. Recent studies proved that compared to other conventional steel fibres, using steel fibres with spiral shape further increases the post-failure energy absorption and crack stopping capacities of concrete because of the better bonds in the concrete matrix and larger deformation ability. The present study conducts high rate impact tests using split Hopkinson pressure bar (SHPB) to further investigate the dynamic compressive properties of spiral fibre reinforced concrete (SFRC). SFRC specimens with different volume fractions of fibres ranging from zero to 1.5% are prepared and tested. The influences of different volume fractions of fibres on strength, stress-strain relation and energy absorption of SFRC specimens under quasi-static and dynamic loadings are studied. In SHPB compression tests, the strain rate achieved ranges from 50 1/s to 200 1/s. Highspeed camera is used to capture the failure processes and failure modes of SFRC specimens with different fibre volume fractions during the tests for comparison. Dynamic stress-strain curves under different strain rates are derived. The energy absorption capacities of the tested specimens are obtained and compared. Strain rate effects on the compressive strength are also discussed. The corresponding empirical DIF (dynamic increase factor) relations for SFRC are proposed.

Structures inevitably deteriorate during their service lives. To accurately evaluate their
structural condition, the methods capable of identifying and assessing damage in a structure
timely and accurately have drawn increasing attention. Compared to widely-used
frequency-domain methods, the processing of time-domain data is more efficient, but remains
difficult since it is usually hard to discern signals from different conditions. In fact, the signal
processing fields have observed the evolution of techniques, from such traditional fixed
transforms as Fourier, to dictionary learning (DL). DL leads to better representation and hence
can provide improved results in many practical applications. In this paper, an innovative
time-domain damage identification algorithm is proposed from a DL perspective, using
D-KSVD algorithm. The numerical simulated soil-pipe system is used for verifying the
performance of the proposed method. The results demonstrate that this damage identification
scheme is a promising tool for structural health monitoring.

Large-span steel frame structures prove to be an ideal choice for their speed of construction, relatively low cost, strength, durability and structural design flexibility. For this type of structure, the beam-column connections are critical for its structural integrity and overall stability. This is because a steel frame generally fails first at its connectors, due to the change in stress redistribution with adjacent members and material related failures, caused by various factors such as fire, seismic activity or material deterioration. Since particular attention is required at a steel frame?s connection points, this study explores the applicability of a comprehensive structural health monitoring (SHM) method to identify early damage and prolong the lifespan of connection points of steel frames. An impact hammer test was performed on a scale-model steel frame structure, recording its dynamic response to the hammer strike via an accelerometer. The testing procedure included an intact scenario and two damage scenarios by unfastening four bolt connections in an accumulating order. Based entirely on time-domain experimental data for its calibration, an Auto Regressive Average Exogenous (ARMAX) model is used to create a simple and accurate model for vibration simulation. The calibrated ARMAX model is then used to identify various bolt-connection related damage scenarios via R2 value. The findings in this study suggest that the proposed time-domain approach is capable of identifying structural damage in a parsimonious manner and can be used as a quick or initial solution.

Non-destructive measurement of stress can provide an effective way to explore the service life and performance degradation status of steel structures. In this paper, a measurement system is designed and developed, which includes both hardware and software systems. The hardware system consists of three modules: signal transmitting, signal conversion and signal receiving. The software system consists of four modules: signal storage, signal de-noising, calibration of stress to acoustic time difference factor, and stress calculation. To examine the performance of the system, a group of axial forces are applied on two steel members and axial stresses are measured on designed system. The strain gauge method is used for verification. The results show that the designed system is reliable and agrees with the results from strain gauge method. It has high potential to be applied in the field stress evaluation to monitor the structure, from pre-operation stage to service operation stage.

Guided-wave-based structural damage identification techniques have received more and more attention in the civil
engineering community. They not only have the capability of detecting smaller damages on a structure than vibration-based methods,
but also can cover a relatively larger magnitude, compared with other traditional non-destructive evaluation techniques. To realize
damage identification, features usually need to be extracted from the time domain responses. This is achievable for homogeneous
materials, including steel and aluminum. But for composite materials, such as concrete, the features are usually very difficult to be
extracted, because of the existence of small aggregates and the nature of uneven material properties which generate multiple
reflections. It is very difficult to simulate the time domain responses and to identify damages by using time domain responses directly
for such random material. Recently, a new damage identification scheme is proposed, named as DISC (Damage Identification based
on Sparse Coding). This method is essentially a pattern recognition technique, which avoids the traditional fixed transform process
but takes advantage of the existing data by dictionary learning techniques. This paper will review the DISC method and then apply it
to identification of de-bonding damage in concrete beam using guided wave test data. The results will demonstrate the effectiveness
of the DISC methodology.

Internal stress in structural steel members is an important parameter for steel structures in their design, construction, and service stages. However, it is hard to measure via traditional approaches. Among the existing non-destructive testing (NDT) methods, the ultrasonic method has received the most research attention. Longitudinal critically refracted (Lcr) waves, which propagate parallel to the surface of the material within an effective depth, have shown great potential as an effective stress measurement approach. This paper presents a systematic non-destructive evaluation method to determine the internal stress in in-service structural steel members using Lcr waves. Based on theory of acoustoelasticity, a stress evaluation formula is derived. Factor of stress to acoustic time difference is used to describe the relationship between stress and measurable acoustic results. A testing facility is developed and used to demonstrate the performance of the proposed method. Two steel members are measured by using the proposed method and the traditional strain gauge method for verification. Parametric studies are performed on three steel members and the aluminum plate to investigate the factors that influence the testing results. The results show that the proposed method is effective and accurate for determining stress in in-service structural steel members.

Waste management is becoming a major issue for communities worldwide. Glass, being nonbiodegradable,
is not suitable for addition to landfill, and as such recycling opportunities need to be investigated.
Due to the high material consumption of the construction industry, the utilisation of waste glass as a
partial replacement for fine aggregate in structural concrete is particularly attractive. This project aimed to determine
the level of glass replacement resulting in optimal compressive strength. Three concrete samples were
tested at 7 and 28 days, for glass replacement proportions of 15, 20, 25, 30 and 40%. Compressive strength
was found to increase up to a level of 30%, at which point the strength developed was 9% and 6% higher than
the control after 7 and 28 days respectively. This demonstrates that concrete containing up to 30% fine glass
aggregate exhibits higher compressive strength development than traditional concrete

Among many structural health monitoring (SHM) methods, guided wave (GW) based
method has been found as an effective and efficient way to detect incipient damages. In comparison
with other widely used SHM methods, it can propagate in a relatively long range and be sensitive to
small damages. Proper use of this technique requires good knowledge of the effects of damage on the
wave characteristics. This needs accurate and computationally efficient modeling of guide wave
propagation in structures. A number of different numerical computational techniques have been
developed for the analysis of wave propagation in a structure. Among them, Spectral Element
Method (SEM) has been proposed as an efficient simulation technique. This paper will focus on the
application of GW method and SEM in structural health monitoring. The GW experiments on several
typical structures will be introduced first. Then, the modeling techniques by using SEM are discussed.

Structures inevitably deteriorate during their service lives. Therefore, the methods capable of identifying and assessing various damages in a structure timely and accurately have drawn increasing attention. From a broader perspective, structural damage identification problem can be regarded as a pattern recognition problem by using sparse representation techniques. The unknown signal/feature from a damaged structure can be associated to a known type of signal/feature in a ?dictionary?, leading to damage identification. From this new angle, an innovative damage identification scheme has been proposed by the authors. In this paper, two important techniques of this scheme are further discussed, namely the construction of dictionary and the choice of parameters. The numerical simulated soil-pipe system is used for verifying the performance of the proposed method. The results demonstrate that this damage identification scheme will be a promising tool for structural health monitoring.

Since failures in sensors will degrade the performance of Active Mass Damper (AMD) control
systems, a dynamic filter design method, a state observer design method, and a robust control strategy are
developed and presented in this paper to overcome this deficiency. The filter design method will be
transformed into a H2/H? control problem that can be solved by Linear Matrix Inequality (LMI) approach.
Thus, it can be used to perform fault detection and isolation (FDI) for the control systems. And, the state
observer design method uses the acceleration responses as the feedback signal. The detected and isolated
fault signals in accelerometers are used to estimate the whole states that are used to calculate the control
force though a robust control strategy based on regional pole-assignment algorithm. Then, the active
fault-tolerant control (FTC) will be accomplished. To verify its effectiveness, the proposed methodology
is applied to a numerical example of a ten-storey frame and an experiment of a single span four-storey
steel frame. Both numerical and experimental results demonstrate that the performances of FTC controller
and the control system will be improved by the designed dynamic FDI filter and that it can effectively
detect and isolate fault signal.

The impact-hammer test is a widely known non-destructive vibration testing method to extract
the modal parameters of a dynamic system. Due to the fast development of computing, timedomain
analysis for data interpretation is now receiving increased attention. To provide
normalised and denoised data, appropriate signal processing is indispensable. This paper
presents a practical study on data normalisation and denoising. Specifically, discrete wavelet
transform (DWT) denoising filter and weighted moving average (WMA) filter are applied in
sequence for denoising. The signal processing results on the impact hammer test of a steel
frame are shown to demonstrate the effectiveness of the proposed methodology.

Corrosion has significant adverse effects on the durability of reinforced concrete (RC) structures, especially those exposed to a marine environment and subjected to mechanical stress, such as bridges, jetties, piers and wharfs. Previous studies have been carried out to investigate the corrosion behaviour of steel rebar in various concrete structures, however, few studies have focused on the corrosion monitoring of RC structures that are subjected to both mechanical stress and environmental effects. This paper presents an exploratory study on the development of corrosion monitoring and detection techniques for RC structures under the combined effects of external loadings and corrosive media. Four RC beams were tested in 3% NaCl solutions under different levels of point loads. Corrosion processes occurring on steel bars under different loads and under alternative wetting - drying cycle conditions were monitored. Electrochemical and microscopic methods were utilised to measure corrosion potentials of steel bars; to monitor galvanic currents flowing between different steel bars in each beam; and to observe corrosion patterns, respectively. The results indicated that steel corrosion in RC beams was affected by local stress. The point load caused the increase of galvanic currents, corrosion rates and corrosion areas. Pitting corrosion was found to be the main form of corrosion on the surface of the steel bars for most of the beams, probably due to the local concentration of chloride ions. In addition, visual observation of the samples confirmed that the localities of corrosion were related to the locations of steel bars in beams. It was also demonstrated that electrochemical devices are useful for the detection of RC beam corrosion.

Slab-girder structures composed of steel girder and reinforced concrete slab are widely used in
buildings and bridges in the world. Their advantages are largely based on the composite action
through the shear connection between slab and girder. In order to assess the integrity of this kind of
structures, numerous vibration-based damage identification methods have been proposed. In this
study, a scaled composite slab-girder model was constructed in the laboratory. Some removable
shear connectors were specially designed and fabricated to connect the girder and slab that were cast
separately. Then, a two stage experiment including both static and vibration tests was performed. In
the first stage, vibration tests were conducted under different damage scenarios, where a certain
number of shear connectors at certain locations were removed step by step. In the second stage, two
sets of hydraulic loading equipment were used to apply four point static loads in the test. The loads
are increased gradually until concrete slab cracked. The loading histories as well as deflections at
different points of the beam are recorded. Vibration test was carried out before and after concrete
cracking. Experimental results show that the changes of mode shapes and relative displacement
between slab and girder may be two promising parameters for damage identification of slab-girder
structures.

The reliable and efficient design of steel-fibre-reinforced concrete (SFRC) structures requires clear knowledge of material properties. Since the locations and orientations of aggregates and fibres in concrete are intrinsically random, testing results from different specimens vary, and it needs hundreds or even thousands of specimens and tests to derive the unbiased statistical distributions of material properties by using traditional statistical techniques. Therefore, few statistical studies on the SFRC material properties can be found in literature. In this study, high-rate impact test results on SFRC using split Hopkinson pressure bar are further analysed. The influences of different strain rates and various volume fractions of fibres on compressive strength of SFRC specimens under dynamic loadings will be quantified, by using kernel regression, a kernel-based nonparametric statistical method. Several kernel estimators and functions will be compared. This technique allows one to derive an unbiased statistical estimation from limited testing data. Therefore it is especially useful when the testing data is limited.

Monitoring of stress/strain is of particular importance to understand better the mechanical pa-rameters which underpin the condition of a wide variety of structures. Although they were initially developed some 30 years ago, optical fibre sensors have not been widely used in this field and the traditional strain gauges still dominate the market. During recent years, optical fibre sensors, especially Fibre Brag Grating (FBG) sen-sors, have received intensive attention with considerable research and development to allow them to be applied successfully in many engineering fields, including in a number of structural health monitoring systems. How-ever, ensuring the structural integrity of the FBG sensor and providing robust ?packaging? for the device remains a key issue, which becomes a major concern when applying these in various structural strain test methods. To protect FBG sensors in use, a number of packaging techniques have been proposed, including metal, fibre re-inforcement polymers and different types of epoxy. However, often such packaging is either insufficiently flexible or expensive. The recent development of 3D printing technologies provides a new and rapid solution for the packaging of FBG sensors, which allows the user easily to define the size and property of the sensor package tailoring that to the specific application. In this study, to examine the use of these techniques for struc-tural monitoring, several different printing materials were used with FBG-based sensors and their properties compared, with the optimal sizes for the packages used being identified. Experimental studies were conducted on a steel beam under repeated stepped loading and unloading processes with different sensor packages. To compare the performances of the traditional strain gauge and these FBG-based sensors, both types were attached on the beam to record the strain changes during the tests. The results demonstrate that the 3D printed packaged sensor designs are highly suitable for use in structural strain testing, and that the performance of FBG sensor thus configured is as stable and consistent in performance as is the familiar strain gauge counterpart.

Piezoceramic-based smart aggregate has been widely used to evaluate early-age concrete strength and to detect damage in concrete structures. In these structural health monitoring systems, they are generally verified and calibrated through experiments under load-free condition. However, the stress levels of actual concrete members are different. The microstructures of concrete will change with the variation of external load, and the high-frequency waves used in the monitoring system may be highly sensitive to these changes. In this study, the effects of axial compressive loading on the monitoring results are investigated. Specifically, three loading cases, that is, single cycle load, cyclic load, and step-by-step load, are employed to stress the concrete specimens embedded with smart aggregates. The amplitude and velocity of monitoring signals were measured before, during, and after each loading case. The test results show that the axial load lower than 30% of failure load still have a significant impact on the received signals. The amplitude attenuation is dependent on both frequency and load history, while the velocity is highly stress-dependent. The results indicate that the baselines of monitoring signals obtained from the same concrete structure in its healthy state can vary under different stress levels. The axial load variation should be carefully considered during the monitoring process. This study also provides a potential method to assess stress state in concrete structures using smart aggregates.

Finite Element (FE) model updating has been attracting research attentions in structural engineering fields for
over 20 years. Its immense importance to the design, construction and maintenance of civil and mechanical
structures has been highly recognised. However, many sources of uncertainties may affect the updating results.
These uncertainties may be caused by FE modelling errors, measurement noises, signal processing techniques,
and so on. Therefore, research efforts on model updating have been focusing on tackling with uncertainties for a
long time. Recently, a new type of evolutionary algorithms has been developed to address uncertainty problems,
known as Estimation of Distribution Algorithms (EDAs). EDAs are evolutionary algorithms based on
estimation and sampling from probabilistic models and able to overcome some of the drawbacks exhibited by
traditional genetic algorithms (GAs). In this paper, a numerical steel simple beam is constructed in commercial
software ANSYS. The various damage scenarios are simulated and EDAs are employed to identify damages via
FE model updating process. The results show that the performances of EDAs for model updating are efficient
and reliable.

This paper presents an experimental study on circular stirrup-confined concrete specimens under uniaxial and monotonic load. The effects of stirrup volume ratio, stirrup yield strength and concrete strength on damage evolution of stirrup-confined concrete were investigated. The experimental results showed that the strength and ductility of concrete are improved by appropriate arrangement of the stirrup confinement. Firstly, the concrete damage evolution can be relatively restrained with the increase of the stirrup volume ratio. Secondly, higher stirrup yield strength usually causes larger confining pressures and slower concrete damage evolution. In contrast, higher concrete strength leads to higher brittleness, which accelerates the concrete damage evolution. A plastic strain expression is obtained through curve fitting, and a damage evolution equation for circular stirrup-confined concrete is proposed by introducing a confinement factor (C) based on the experimental data. The comparison results demonstrate that the proposed damage evolution model can accurately describe the experimental results

The analysis of rock slope stability is a classical problem for geotechnical engineers. However, for practicing engineers, proper software is not usually user friendly, and additional resources capable of providing information useful for decision-making are required. This study developed a convenient tool that can provide a prompt assessment of rock slope stability. A nonlinear input?output mapping of the rock slope system was constructed using a neural network trained by an extreme learning algorithm. The training data was obtained by using finite element upper and lower bound limit analysis methods. The newly developed techniques in this study can either estimate the factor of safety for a rock slope or obtain the implicit parameters through back analyses. Back analysis parameter identification was performed using a terminal steepest descent algorithm based on the finite-time stability theory. This algorithm not only guarantees finite-time error convergence but also achieves exact zero convergence, unlike the conventional steepest descent algorithm in which the training error never reaches zero.

Time-delays of control force calculation, data acquisition and actuator response will degrade the performance of Active Mass Damper (AMD) control systems. To reduce the influence, model-reduction method is used to deal with the original controlled structure. However, during the procedure, the related hierarchy information of small eigenvalues will be directly discorded. As a result, the reduced-order model ignores the information of high-order mode, which will reduce the design accuracy of an AMD control system. In this paper, a new reduced-order controller based on the improved Balanced Truncation (BT) method is designed to reduce the calculation time and to retain the abandoned high-order modal information. It includes high-order natural frequency, damping ratio and vibration modal information of the original structure. Then, a control gain design method based on Guaranteed Cost Control (GCC) algorithm is presented to eliminate the adverse effects of data acquisition and actuator response time-delays in the design process of the reduced-order controller. To verify its effectiveness, the proposed methodology is applied to a numerical example of a ten-storey frame and an experiment of a single span four-storey steel frame. Both numerical and experimental results demonstrate that the reduced-order controller with GCC algorithm has an excellent control effect, meanwhile can compensate time-delays effectively

Severe air pollution and its associated health impacts have become one of the major concerns in China. A detailed analysis of PM2.5 chemical compositions is critical for optimizing pollution control measures. In this study, daily 24-h bulk filter samples were collected and analyzed for totally 21 field campaigns at 17 sites in China between 2008 and 2013. The 17 sites were classified into four groups including six urban sites, seven regional sites, two coastal sites in four fast developing regions of China (i.e. Beijing-Tianjin-Hebei region, Yangtze River Delta, Pearl River Delta and Sichuan Basin), and two ship cruise measurements covered the East China Sea and Yellow Sea of China. The high average concentrations of PM2.5 and the occurrences of extreme cases at most sites imply the widespread air pollution in China. Fine particles were largely composed of organic matter and secondary inorganic species at most sites. High correlation between the temporal trends of PM2.5 and secondary species of urban and regional sites highlights the uniformly distributed air pollutants within one region. Secondary inorganic species were the dominant contributors to the high PM2.5 concentration in Northern China. However in Southern China, the relative contributions of different chemical species kept constant as PM2.5 increased. This study provides us a better understanding of the current state of air pollution in diversified Chinese cities. Analysis of chemical signatures of PM2.5 could be a strong support for model validation and emission control strategy.

Active mass damper/driver (AMD) control system has been proposed as an effective tool for high-rise buildings to resist strong dynamic loads. However, such disadvantage as time-varying delay in AMD control systems impedes their application in practices. Time-varying delay, which has an effect on the performance and stability of single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems, is considered in the paper. In addition, a new time-delay compensation controller based on regional pole-assignment method is presented. To verify its effectiveness, the proposed method is applied to a numerical example of a ten-storey frame and an experiment of a single span four-storey steel frame. Both numerical and experimental results demonstrate that the proposed method can enhance the performances of an AMD control system with time-varying delays.

Absolute stress in structural steel members is an important parameter for the design, construction, and servicing of steel structures. However, it is difficult to measure via traditional approaches to structural health monitoring. The ultrasonic time-of-flight (TOF) method has been widely studied for monitoring absolute stress by measuring the change of ultrasonic propagation time induced by stress. The TOF of the two separated shear-wave modes induced by birefringence, which is particular to shear waves, is also affected by stress to different degrees. Their synthesis signal amplitude spectrum exhibits a minimum that varies with stress, which makes it a potential approach to evaluating uniaxial stress using the shear-wave amplitude spectrum. In this study, the effect of steel-member stress on the shear-wave amplitude spectrum from the interference of two shear waves produced by birefringence is investigated, and a method of uniaxial absolute stress measurement using shear-wave spectral analysis is proposed. Specifically, a theoretical expression is derived for the shear-wave pulse-echo amplitude spectrum, leading to a formula for evaluating uniaxial absolute stress. Three steel-member specimens are employed to investigate the influence of uniaxial stress on the shear-wave pulse-echo amplitude spectrum. The testing results indicate that the amplitude spectrum changes with stress, and that the inverse of the first characteristic frequency in the amplitude spectrum and its corresponding stress exhibit a near-perfect linear relationship. On this basis, the uniaxial absolute stress of steel members loaded by a test machine is measured by the proposed method. Parametric studies are further performed on three groups of steel members made of 65# steel and Q235 steel to investigate the factors that influence the testing results. The results show that the proposed method can measure and monitor steel-members uniaxial absolute stress on the laboratory scale and has potential to be used in practical engineering with specific calibration.

Among different Structural Health Monitoring (SHM) systems applied on bridges, Bridge Weight-in-Motion (BWIM) is probably the one with widest applications worldwide. Briefly, BWIM uses on-structure sensors that are able to acquire signals sensitive to traffic load events, which can be used as an indirect indicator of the load magnitude. The sampling rate required for this is relatively high (at least 10 Hz), which usually lead to databases with sizes that might reach the order of gigabytes. It is impractical to process this volume of information in the context of infrastructure asset management. Hence, an effective and efficient method for the compression and storage of BWIM data is becoming mandatory. In this paper, sparse representation algorithms have been innovatively applied to the BWIM data compression. A comparative study is performed based on measurements collected from a real bridge, by exploring different methods including Discrete Fourier Transform (DFT), Discrete Cosine Transform (DCT), Discrete Wavelet Transform (DWT), and two dictionary learning methods, i.e. Compressive Sensing (CS) and K-means Singular Value Decomposition (K-SVD). It has been found that the K-SVD method shows the best performance when applied to this specific type of data, while the DWT method using Haar wavelet is the most computationally efficient. Nearly lossless reconstruction of the signal is achieved by using K-SVD with less than 0.1 % reserved coefficients, which gives evidence that dictionary learning technologies are feasible to guarantee the same level of information even with much smaller databases. Therefore, the utilization of dictionary learning is a clear step forward towards higher levels of efficiency in the compression and storage of data collected by SHM systems.

The absolute stress in structural steel members is an important indicator of the performance of steel structures. Among the existing non-destructive testing (NDT) methods, ultrasonic methods have received the most research attention. The existing ultrasonic methods can evaluate the average stress in a fixed acoustic path but cannot easily measure the stress field within the tested objects. We present a non-destructive method to evaluate the absolute stress field in a structural steel member using longitudinal critically refracted (Lcr) waves. Specifically, a theoretical expression is derived for absolute stress measurement. A measurement system is developed to demonstrate the performance of the proposed method. A sensor group, which contains one transmitter and two receiver transducers connected by a Vernier calliper, is designed to transmit and receive Lcr waves. The proposed method is applied to two steel members with variable cross-sections. The traditional strain gauge method is used for verification. The results show that the proposed method can efficiently evaluate the stress distribution and stress extremum in structural steel members.

This paper proposes a novel vibration-based damage identification method, named the probability distribution of decay-rate (PDDR). By introducing a statistical framework, the PDDR method estimates damage-induced changes in overall damping behaviour of a free-vibration dynamic system. Utilising free-vibration impulse-response (IR) data, a one-dimensional dataset of local maxima-minima points is constructed. A statistical analysis of this dataset is then performed to derive damage-sensitive parameters. It is demonstrated that through the use of a statistical analysis framework, a number of enhancements are attained in terms of both robustness and leniency in estimating the significantly nonlinear property of overall damping. An impact hammer test is conducted in the laboratory to verify the efficacy of the proposed PDDR method. The test was performed on a scale-model steel Warren truss bridge structure, subjected to bolt-connection failures. The comparison results between the PDDR method and the standard experimental modal analysis (EMA) method confirm that the former is effective for damage identification of complex structures, particularly with real experimental data and steel-frame structure assemblies.

This paper introduces a novel vibration-based damage identification method, named the
probability distribution of decay-rate (PDDR). The PDDR method is a time-domain statistical damping
estimate for damage detection and identification applications. Suitable for both SDOF and MDOF
dynamic systems, a mode-superposed damping estimation is determined based on time-domain based
observations of an impulse response (IR) decay rate. Firstly, a detailed literature survey and discussion
are presented to show the limitations of existing methods. Secondly, the overall process for PDDR
implementation is succinctly introduced. Lastly, a hypothetical MDOF model was formulated and used
to demonstrate the first phase implementation of the PDDR method. The results show the proposed
method is sensitive to the change of damping parameters, which is usually more sensitive to structural
damage, and therefore can be used complimentarily with traditional vibration-based methods.

The effect of mega braces on structural stiffne
ss has been comprehensivel
y discussed for various
mega-braced frame-core tube structures. However,
few studies have considered how mega braces
affect the failure mechanism of mega-structures
exposed to seismic action, which is a nonlinear
process. To address this issue, we
present a study on the effects of diff
erent brace patterns on the failure
mechanism and seismic performance of mega-braced
frame-core tube structures. Specifically, the
yield order of components, the distribution of plasticity, the distribution of internal forces, the
degradation of structural nonlinear stiffness, and the
behavior factor have been investigated. This study
reveals that the yield of mega braces will change the deformation mode of adjacent mega columns,
and thus affect the plasticity distribution of adjacent sub-structures. The enhancement of mega braces
improves the exterior tubes (thereby increasing their capacity to serve as the second line of seismic
defence), mitigates the rate at which system stiffness degrades, and improves the overstrength of the
structural systems. In addition, after mega braces
yield, the maintenance of a higher-amplitude axial
force changes the proportion of internal force components in mega columns, reducing their ductility
and further affecting the overall ductility of the structural system.

Infrastructure performance is of great importance for a nation?s economy and its
people?s quality of life. For efficient and effective infrastructure asset management,
structural health monitoring (SHM) has been researched extensively in the past 20-30
years. With an increasing number of SHM systems being installed, the interpretation of
the large volume of monitoring data, i.e. often manifested as condition identification,
becomes essential in asset integrity management. This paper provides an appraisal of
existing literature reviews on SHM, considering both reviews on different types of
structures and those focused on different approaches for data interpretation. It explores
the evolution of research interests in this field and identifies the need for an integrated
physics-based and data-driven structural condition identification approach.

The various elements that are affecting the Earth's climate have brought climate change to the top of the priority list amongst scientists and policy-makers. Expected changes to local climatic conditions impact directly on the surrounding environment and potentially lead to changes in the degradation processes of building materials, affecting the durability and service life of infrastructures. The aim of this paper is to investigate the effects of future climate projections on concrete structures in Malta, in particular on carbonation-induced corrosion resulting from increasing temperatures and CO2 concentrations. Thirteen reinforced concrete structures in Malta were chosen for a retrospective analysis in order to validate two carbonation depth prediction models. The validated prediction models were subsequently used to evaluate the varying climate change scenarios in order to determine the effects on concrete carbonation depth for several concrete grades. The age of the structures used for the retrospective analysis ranged from 10 to 60 years. The field data verified the validity of both prediction models for structures with carbonation depths less than 50mm. Although both models proved valid for the retrospective analysis, a difference was noted between the models with regards to the predicted carbonation depth in relation to different climatic scenarios. An increase in carbonation depth of up to 40% is being predicted, by 2070, when considering the worst case climatic scenario. The findings prove that climate change plays a major role on the carbonation depth of concrete, which in turn reduces the service life of concrete structures.

The absolute stress in the in-serviced steel members is a critical indicator employed for the evaluation of structural performance. In the field of structural health monitoring, the stress is usually monitored by the stress monitoring system. However, the monitored stress is the relative value, rather than the absolute value. The longitudinal critically refracted (Lcr) wave has shown potential for use in absolute stress measurement. The accurate measurement of the Lcr wave time-of-flight (TOF) is the core issue with this method. In this study, a cross-correlation-based algorithm is presented for stress evaluation using the Lcr wave. Specifically, a cross-correlation theoretical formula is derived and a five-step framework is proposed for the Lcr wave TOF measurement. Four steel members are employed to investigate the parametric calibration using the Lcr wave to measure the stress. On this basis, the proposed cross-correlation-based algorithm is used to evaluate the stress of a steel member. The results indicate that the cross-correlation-based algorithm can measure the Lcr wave TOF without filtering the noise signal, and the stress measurement results are better than those of the traditional peak value method. The proposed method provides a potential way to measure the absolute stress in practical engineering applications.

The absolute stress of steel members is a key parameter for determining the performance of steel structures. Compared with other non-destructive evaluation methods, ultrasonic methods, which correlate material stress with ultrasonic velocity, have received the greatest amount of research attention. In this study, we investigated the measurement of the absolute stress distribution of steel members using two ultrasonic methods: a longitudinal critically refracted (Lcr) wave method and a shear wave method. The Lcr wave is generated from the longitudinal wave mode conversion and exhibits the greatest sensitivity to stress. The shear waves are generated by the birefringence effect, and their synthesis signal spectrum exhibits a minimum that varies with stress. A comparison of the two absolute stress evaluation methods is performed. Specifically, four steel members with identical dimensions and materials are used to investigate the discreteness of the calibrated parameters. The uniaxial absolute stress distributions of two steel members with variable cross-sections are measured using the two methods and verified using the traditional strain gauge method. The results show that the uniaxial stress distributions of the two steel members can be evaluated by both the Lcr wave time-of-flight (TOF) method and the shear-wave spectrum method, although the latter is more accurate for the measurement of stress distribution. Furthermore, the measurement principles, parametric calibrations, sensitivity, accuracy and repeatability of the two methods are compared, and their applicability is discussed.

To obtain actual conditions of infrastructure assets and manage them more efficiently, extensive research efforts have been placed on structural health monitoring (SHM), especially those using data-driven methods. Recently, deep learning becomes a research hotspot in many application areas, including the SHM domain. Their performance largely relies on the quality and quantity of the training data, obtained either experimentally or numerically. Due to the time and expense restraints, field or laboratory test data are normally limited by the variation of structural conditions, while the quality of numerical simulation data is subjective to experts' modelling skills. Therefore, the actual performance of deep learning algorithms with limited training data needs to be studied, and the alternative ways to generate more training data need to be developed. In this work, we develop a new one-Dimensional Convolutional Neural Network (1D-CNN) for structural condition identification. A laboratory case study is conducted to evaluate the performance of the algorithm. A steel Warren truss bridge structure is constructed and instrumented with accelerometers and impact hammer. The vibration tests under seven different scenarios are conducted, and each scenario has five repeated test data. The algorithm is trained with different quantities of training data (from one test data to four test data for each scenario). The results show that condition identification results become reliable with at least three repeated test data. To overcome the challenge of limited monitoring data, we propose the potential application of Generative Adversarial Networks (GANs) to generate more reliable training data.

Grouted splice sleeve (GSS) connectors are mainly used in precast concrete structures. However, errors in manual operation during construction cause grouted defects in the GSS connector, which can lead to a negative effect on the overall mechanical properties of the structures. Owing to the complex structure of precast concrete members with a GSS connector, it is difficult to detect grouted defects effectively using traditional ultrasonic parameters. In this paper, a wavelet packet analysis algorithm was developed to effectively detect grouted defects using the ultrasonic method, and a verified experiment was carried out. Laboratory detection was performed on the concrete specimens with a GSS connector before grouting, in which the grouted defects were mimicked with five sizes in five GSS connectors of each specimen group. A simple and convenient ultrasonic detection system was developed, and the specimens were detected. According to the proposed grouted defect index, the results demonstrated that when the grouted defects reached certain sizes, the proposed method could detect the grouted defects effectively. The proposed method is effective and easy to implement at a construction site with simple instruments, and so provides an innovative method for grouted defects detection of precast concrete members.

Failure of bolted connections in steel structures may result in catastrophic effects. Many algorithms in existing literature use
modal information of a structure to identify damage in that structure, based on the data acquired from accelerometers which record the vibration
time histories at different points on the structure. The location of these points may have significant effects on the quality of the acquired data,
and thus the identified modal information. In this paper, a distance measure based Markov chain Monte Carlo algorithm is proposed to
determine the optimal locations for the accelerometers, and the optimal location of the impact hammer if need. Different damage cases with
various combinations of bolt failures are considered in this study. Failures at various levels are simulated by loosening the bolts in a predefined
order. To compare the efficiency of the proposed method, the total effect of various damage cases on the accelerations at the optimal locations
are calculated for the proposed method and a state-of-the-art method from the existing literature. The results demonstrate the efficiency of the
proposed strategy in locating the accelerometers, which can produce data that are more sensitive to the bolted connection failures.

The deep learning technologies have transformed many research areas with accuracy
levels that the traditional methods are not comparable with. Recently, they have received
increasing attention in the structural health monitoring (SHM) domain. In this paper,
we aim to develop a new deep learning algorithm for structural condition monitoring
and to evaluate its performance in a challenging case, bolt loosening damage in a frame
structure. First, the design of a one-Dimensional Convolutional Neural Network (1DCNN)
is introduced. Second, a series of impact hammer tests are conducted on a steel
frame in the laboratory under ten scenarios, with bolts loosened at different locations
and quantities. For each scenario, ten repeated tests are performed to provide enough
training data for the algorithm. Third, the algorithm is trained with different quantities
of training data (from one to seven test data for each scenario), and then is tested with
the rest test data. The results show that the proposed 1D-CNN with three convolutional
layers provide reliable identification results (over 95% accuracy) with sufficient training
data sets. It has the potential to transform the SHM practice.

The offshore wind turbines (OWTs) are dynamically sensitive, whose fundamental frequency can be very close to the forcing frequencies activated by the environmental and turbine loads. Minor changes of support conditions may lead to the shift of natural frequencies, and this could be disastrous if resonance happens. To monitor the support conditions and thus to enhance the safety of OWTs, a model updating method is developed in this study. A hybrid sensing system was fabricated and set up in the laboratory to investigate the long-term dynamic behaviour of the OWT system with monopile foundation in sandy deposits. A finite element (FE) model was constructed to simulate structural behaviours of the OWT system. Distributed nonlinear springs and a roller boundary condition are used to model the soil-structure-interaction (SSI) properties. The FE model and the test results were used to analyze the variation of the support condition of the monopile, through an FE model updating process using Estimation of Distribution Algorithms (EDAs). The results show that the fundamental frequency of the test model increases after a period under cyclic loading, which is attributed to the compaction of the surrounding sand instead of local damage of the structure. The hybrid sensing system is reliable to detect both the acceleration and strain responses of the OWT model and can be potentially applied to the remote monitoring of real OWTs. The EDAs based model updating technique is demonstrated to be successful for the support condition monitoring of the OWT system, which is potentially useful for other model updating and condition monitoring applications.