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In this paper, fragility analysis for performance-based blast design of FRP-strengthened RC columns is carried out. The blast intensity levels, performance levels and performance objectives of the RC columns are defined. A simplified probabilistic risk assessment framework incorporating the performance-based design concept and fragility analysis is established. Since fragility analysis is the most important but time-consuming process of probabilistic assessment risk, an artificial neural network (ANN) based fragility analysis framework is proposed to improve its computational efficiency. Based on the rapid fragility analysis method, fragility curves of several typical RC columns with or without FRP strengthening are calculated to analyze their damage probabilities. This study provides avenues for engineers to estimate the failure probabilities of RC columns with or without FRP strengthening under blast loads and make decisions quickly.

Mild stroke is a known risk factor for dementia. The relationship between cerebral white matter (WM) integrity and cognitive impairment (CI) in mild stroke patients with basal ganglia region infarcts is unknown. Total of 33 stroke patients and 19 age-matched controls underwent diffusion tensor imaging scans and a formal neuropsychological test battery. CI was defined as having a performance score 1.5 SD below the established norm. We compared the differences in Z-scores and Fraction Anisotropy (FA) values among controls, stroke with no CI (NCI) and stroke with CI groups. Multiple linear regressions were performed between FA values in affected regions and neuropsychological tests in stroke patients. The majority of stroke patients were in their 50s (56.90 ± 9.23 years). CI patients exhibited a significantly decreased Z score in visual delayed memory and remarkably decreased FA values in the right external capsule and right fornix (FWE-corrected) compared with NCI patients and controls. In stroke patients, the FA value in the right fornix was positively correlated with delayed visual memory. Mild stroke with basal ganglia region infarcts may be related to widespread abnormality of WM integrity. The lower WM integrity in the right fornix may be a marker of impaired delayed visual memory.

© 2017 Elsevier Ltd This paper investigates effects of nanoscale materials and steel fibres on properties of ultra-high performance concrete (UHPC). Different types of steel fibres including twisted steel fibre (TF), waved steel fibre (WF), and micro steel fibre (MF) together with different kinds of nano materials including Nano-CaCO3, Nano-SiO2, Nano-TiO2 and Nano-Al2O3 are studied in the present research. Material compressive stress–strain relationships, strain energy absorption, the flexural strength and fracture energy absorption of UHPC with different nanoscale materials and steel fibres were compared and discussed. Laboratory static bending tests and field blast tests on structural components made of selected UHPC material composition were carried out, and the results highlighted the superior material ductility and blast resistant capacity of UHPC material developed in the present study.

© 2017 Elsevier Ltd Structural responses and damages under blast loading environments are critical to structural and personnel safety. The blast scenarios involving close-in detonations are attracting increasingly more attentions over the last few decades due to the rising of terrorism. Under close-in detonations, structural elements tend to fail in a brittle mode including shear, concrete crater and spall. In such loading scenarios, the structural designated loading capacity which is usually based on flexural deformation assumption is not fully developed. To provide high-level structural protection, high performance concretes with varying fibre additions are now widely investigated and used in blast resistance designs. In the present study, field blast tests results on reinforced concrete slabs under close-in detonations are presented. Performances of slabs made of normal strength concrete and steel fibre reinforced concrete are compared and discussed. Besides conventional steel rebar reinforcement, new reinforcement scheme i.e. hybrid steel wire mesh-micro steel fibre reinforcement is investigated through the laboratory static tests and field blast tests. Furthermore, a numerical study based on Multi-Material ALE and Lagrangian algorithm is carried out to further investigate the field tests’ phenomenon.

© 2014 American Society of Civil Engineers. A load-cladding-structure (LCS) model was used to study the mitigating effect provided by metallic foam cladding against blast loading on reinforced concrete (RC) structural members. The model considered the interactions between an external blast load, a protecting foam cladding, and a target RC structural member. The effectiveness of the LCS model was validated by field blast tests conducted in 2009. The validated model was then used to derive pressure impulse diagrams of the foam-protected RC members. Afterwards, two nondimensional parameters representing the relationship between the foam cladding and the target RC member were characterized. Using the suggested nondimensional parameters, normalized pressure-impulse (p-i) diagrams for the foam-protected RC members were generated. The effects of the two nondimensional parameters on the p-i diagrams were investigated by comparing the corresponding asymptotes. Based on the predicted results, an optimized design of the foam cladding for RC structural members was suggested.

© 2015 Elsevier Ltd. An investigation of shear transfer behavior in initially uncracked reinforced concrete members is conducted using finite element modeling method in this study. Although earlier experimental studies have been carried out to identify the role of different design parameters on the ultimate shear strength, there are no design provisions that are available to predict the relationship of shear stress to slip as a function of the basic parameters. One of the aims of this paper is to improve insight into the characteristics between the shear stress and slip for a range of design parameters, such as concrete strength, percentage of dowel and variation of lateral normal pressure on RC members. The other aim of this paper is to derive a set of simplified equations for evaluating the ultimate shear stress and relationship of shear stress to slip in practical structural design. High-fidelity finite element models are developed using LS-DYNA program to simulate push-off tests, and the models are calibrated using experimental results. Parametric studies are then carried out to generate data with the consideration of different combinations of the structural design parameters, i.e., concrete strength, percentage of dowel steel and lateral normal pressures. It is found that the numerical models are accurate in predicting the interface shear strength and slip occurring along the shear plane of the push-off test specimens. The study also shows that there is a good agreement in predicting the shear stress to slip relationship between the results calculated by simplified equations and numerical models, and experimental results.

© 2015 American Society of Civil Engineers. Concrete-filled double-skin tubes (CFDST) have been widely used in constructing high-rise buildings, arch bridges, and factories over the past years. Although a number of researches have been conducted to study the behavior of CFDST columns under a variety of loading conditions, their performance when subjected to lateral impact load is still lacking. In this paper, numerical models of CFDST columns with two different cross sections are developed: one is with a CHS (circular hollow section) outer and CHS (circular hollow section) inner, and the other one is with SHS (square hollow section) outer and SHS (square hollow section) inner. Conventional concrete is filled in double-skin steel tubes. Different blast loads are applied to the surface of these columns for dynamic analysis. In addition, different axial loads are also applied to simulate combined load conditions. The displacement-time history obtained from each simulation is recorded and then compared. The key parameters that affect the performance of CFDST columns under blast load are also investigated.

Concrete-filled double skin steel tubes (CFDST) have widely been used in constructing high-rise buildings, arch bridges and factories. Much research has been done to study its behaviour under axial compression. However, limited information can be found on its performance under lateral impact loading, especially under blast loading. In this paper, numerical models are developed for CFDST with two different cross sections: one is with CHS (circular hollow section) outer and CHS (circular hollow section) inner, and the other one is with SHS (square hollow section) outer and SHS (square hollow section) inner. Conventional concrete is filled in double skin steel tubes. Different blast loadings are applied on the surface of these columns for dynamic analysis. In addition, different axial loads are also applied on the columns to simulate the combined load condition. The displacement-time history obtained from each simulation is recorded and then compared. The key factors that determine the performance of CFDST columns under blast loading are discussed.

Through comparing the measured data of dynamic strains due to loading and temperature by the strain gauge and temperature sensor at the same location, the information in the strain data was divided into three parts in the frequency domain by using the defined index named PSD (power spectra density)- ratio. The three parts are dominated respectively by temperature varying, stresses and noises and can be distinguished from the determined values of the separatirix frequencies. Then a simple algorithm was developed to separate the three types of information, and to extract the strain caused mainly by structural stresses. As an application of the proposed method, the influence of strain deformation and noises. As an application of the proposed method, the influence of strain deformation and noises on the fatigue assessment was investigated based on the separated data. The results show that, the determined values of separatrix frequencies are valuable for the monitoring data from other bridges. The algorithm is a multi resolution and hierarchical method, which has been validated as a simple and effective method for data analyses, and is suitable for the compression and pre-processing of the great amount monitoring data and easy to be integrated in the SHM's (structural health monitoring)software system. The strain due to temperature varying attributes only a little to the errors of fatigue assessment. However, the noises or random disturbance existed in the monitoring data have much responsibility for the errors, the main reason is that the random disturbance shifts the real strain/stress amplitude picked up by real structural stress or strain.

Udgivelsesdato: 5-1

The dynamic response of the Tsing Ma Bridge under running train is simulated with finite element model. The results show that the train speed affects the girder delormaltlon distinctly, but takes less effect on the stress level in structural elements. The faster the running train, the higher the main girder deformation. The simulated dynamic responses under two running train loadings show that the main girder deformation is nonlinear.

The methodology and strategy for evaluating fatigue damage and possible critical locations of local fatigue on bridge-deck sections are developed, using the real-time monitoring data and the fatigue damage model based on continuous damage mechanics. A local damage model for evaluating the fatigue damage of bridge-deck sections is proposed, which allows fatigue damage to be considered at the continuum scale as a deteriorating process with its physical mechanism such as fatigue crack initiation and growth. Therefore the fatigue damage variable associates not only cycles of stress range on the accumulated process of fatigue, but also relates directly to the state of deterioration and the mechanics behavior in supporting the net stress. The effective stress range for the variable-amplitude stress spectrum due to traffic load is used to evaluate the fatigue strength of the bridge-deck section at different locations of strain gauges, by which the possible location of critical fatigue failure can be primarily determined. The method has been successfully applied to the Fatigue Damage evaluation for a suspension bridge under Typhoons and Overload Effect.

The methodology and strategy for concurrent multi-scale modeling of large structures such as long span bridges were studied in order to perform the multi-scale numerical analysis on structural, dynamic response and local damage. The multi-scale finite element modeling was first studied by using substructure method and the multi-point constraints. A multi-scale model of Tsing Ma Bridge (TMB) was then developed by using the design drawings and the proposed strategy on the basis of the software ABAQUS and the developed programs via the user interface. The model updating and verification were carried out based on the online data from the structural health monitoring system installed on TMB. The procedure for damage analysis using the developed model was proposed, in which locally deteriorating process at welded details of interconnects is able to account for the major effect of the damage and nonlinear influence on structural response. The calculated result shows that damage features can be successfully performed via multi-scale modeling and analysis; and fatigue damage accumulation on the weld details shows significantly local effect. The fatigue life obtained from the fully coupled calculation on stress and damage is shorter than that from the uncouple computation, which suggests that the significant interaction between the damage and the structural response should be considered in the structural damage analyses using the multi-scale numerical analysis

This paper studies the finite element (FE) modeling oriented to structural health monitoring (SHM) of long cable-stayed bridge and the optimal sensor configuration based on the structural dynamic response. Firstly , the FE model oriented SHMof Runyang Cable-Stayed Bridge (RYCB) is developed using ANSYS. Modal analysis of RYCB is carried out using the developed FE model . Taking the non-diagonal elements of the MAC matrix as target function , and increasing or decreasing the number of degree of freedom (DOF) , the optimal sensor placement of the bridge is obtained by minimizing the target function. Validation of the sensor placement is checked with the results obtained from the ambient vibration test of RYCB. It is shown that the sensor placement obtained from the measured data of RYCB is coincident with the result calculated from the FE model . The sensor placement obtained from the FE model of RYCB is valid from the experimental point of view.

This paper studies how to develop the multi2scale FE models in which the feature of local detailed such as the stress concentration on the welded components can be considered and to make the simulation on the structural response as well as local stress on hot-spots . Therefore , the multi-scale FE model including welded components of the longitudinal fortified truss of bridge steel box girder is developed based on substructure and coupling equations respectively , then modal and static analysis are carried out by using the developed FE model in this paper. The results of the dynamic properties and stresses calculated by the multi-scale FE model are compared with those from the experiment and those calculated by traditional FE models , which suggests that the developed multi-scale FE model of the studied truss based on substructure would be valid in the analyses on both structural and local hotspot behavior. The proposed approach for FE modeling can be used to develop the multi-scale models of long-span bridges as the basic model of the structure in the further nonlinear static simulation such as the hot spot stress analysis.

This paper aims to develop an approach for multi-scale numerical analysis on structural dynamic response and local damage in long-span bridges. The strategies of finite element modeling of long-span bridges for multi-scale numerical analysis was first investigated. A multi-scale model of the Tsing Ma Bridge (TMB) was then developed based on the design drawings and proposed strategies for multi-scale modeling. The procedures for multi-scale numerical analysis on the dynamic response and local damage are proposed, in which local deterioration processes at welded details of interconnects are able to account for the major effect of the damage and nonlinear influence on structural response. The multi-scale computations are implemented based on the software ABAQUS and the developed programs via the user interface provided by the software for the purpose of easier application to large civil structures. The accuracy of the calculated procedures is addressed with two smaller controlled examples before the approach can be applied to the Tsing Ma Bridge. Based on the online monitoring data acquired from the Structural Health Monitoring (SHM) system installed on TMB, the global dynamic response of the bridge and local damage accumulation of two typical weld details in the longitudinal truss of TMB under traffic loading are numerically analyzed by using the developed model and computational procedures. The maximum value of the accumulated damage is found on the toe of the weld of the upper and/or bottom chords with the diagonal and gusset plate intersection. The fatigue life obtained from the fully coupled calculation is smaller than that from the uncoupled computation, which suggests that the significant interaction between the damage and the structural response should be considered in the structural damage analyses using the multi-scale numerical computation.

Typhoons are very common in Hong Kong during May to November each year, so within this typhoon prone regions the safeties and reliability of the large civil infrastructures such as long-span cable-supported bridges are some of the most important issues that should be considered. This paper aims to evaluate the effects of typhoons on fatigue damage accumulation in steel decks of long-span suspension bridges using Tsing Ma Bridge in Hong Kong as a case study. The strain-time histories at the identified critical locations of deck sections of the bridge during different typhoons passing the bridge area are investigated using on-line strain data acquired from the structural health monitoring system installed on the bridge. The fatigue damage models based on Miner’s Law and Continuum Damage Mechanics (CDM) with the correspondingly developed finite element model are applied to calculate the increment of fatigue damage due to the action of a typhoon. Accumulated fatigue damage during the typhoon is also calculated and compared between Miner’s Law and the CDM method. It is found that for the Tsing Ma Bridge case, the stress spectrum generated by a typhoon is significantly different from that generated by normal traffic and its histogram shapes can be described approximately as a Rayleigh distribution. The influence of typhoon loading on accumulative fatigue damage is more significant than that due to normal traffic loading. The increment of fatigue damage generated by hourly stress spectrum for the maximum typhoon loading may be much greater than those for normal traffic loading. It is, therefore, concluded that it is necessary to evaluate typhoon induced fatigue damage for the purpose of accurately evaluating accumulative fatigue damage and structural health assessment of the bridge for the bridge administration to make cost-effective decisions regarding inspection, maintenance and repair.

A structural health assessment (SHA) methodology is developed using data acquired from structural health monitoring (SHM) system installed on long-span bridges. A set of fatigue criteria has been proposed for pre-determining the global state of the bridge structure failure due to fatigue. This involves finding the threshold of fatigue initiation, below which the rate of fatigue damage may be undetectable under current technology or it is economically unfeasible. The state-of-art for large structures corresponds to the initiation of macro-cracks caused by the accumulation of damage generated by actual service loads for the case of bridges. In what follows, consideration is given to developing fatigue crack growth criterion based on the concept of the continuum damage mechanics (CDM). Fatigue accumulative is included in the model where a fatigue limit for multi-axial stress state is considered. The proposed criterion advocates the evolution of micro-crack growth up to the stage of macro-crack formulation. Considered are the loading histories that correspond to normal traffic loading for highways and railways, incidental or accident loadings such as those caused by typhoons and effective environmental loadings. The potential sites of damage are determined are discussed. The proposed criterion is applied to analyze the fatigue damage of the Tsing Ma Bridge with online strain history data acquired by the SHM system that is permanently installed in the bridge.

The behavior of crack growth for the fatigue damage accumulation near tip where damage is most severe is analyzed. Fatigue life is assessed for the welded members of bridges under traffic loading. Two parts are considered. They consist of the development of a fatigue damage accumulation model for welded bridge members and a method for calculating the stress intensity factor that is needed for evaluating the fatigue life of welded bridge members with cracks. Based on the concept of continuum damage accumulation and fatigue and fatigue crack growth relations, results are obtained to describe the relationship between the cracking count rate and the effective stress intensity factor. Crack growth and fatigue life are found for two types of welded members assisted by using fatigue experimental results. The stress intensity factors are modified by correcting for the geometric shape of the welded members in order to reflect the influence of the weldment and geometry. This is accomplished via the stress intensity factor. The calculated and measured fatigue lives were generally in good agreement for the initial cracking conditions of two types of welded members widely used in steel bridges.

The hot spot stress approach is usually adopted in the fatigue design and analysis of tubular welded joints. To apply the hot spot stress approach for fatigue evaluation of long span suspension bridges, the FEM is used to determine the hot spot stress of critical fatigue location. Using the local finite element models of the Tsing Ma Bridge, typical joints are developed and the stress concentration factors are determined. As a case for study, the calculated stress concentration factor is combined with the nominal representative stress block cycle to obtain the representative hot spot stress range cycle block under traffic loading from online health monitoring system. A comparison is made between the nominal stress approach and the hot spot stress approach for fatigue life evaluation of the Tsing Ma Bridge. The comparison result shows that the nominal stress approach cannot consider the most critical stress of the fatigue damage location and the hot spot stress approach is more appropriate for fatigue evaluation.

Structural health monitoring is becoming a hot research direction in civil engineering. Considering the importance of long cable steel bridges in local transportation and field economic development, strong attention has been paid to structural health analysis to monitor the integrity, durability and reliability of the bridges. Long cable steel bridges are vulnerable to fatigue damage. Local test method, adopted in a number of fatigue damage analysis for short and/or medium bridge, could not be used for long cable bridge fatigue analysis as long cable bridges have many structural properties making inspection, instrumentation and testing of them very difficult. Finite Element Method (FEM), recommended as a rigorous method for fatigue damage analysis by British Standard, is proposed in this paper to analyze fatigue damage for a practical long cable steel bridge under traffic loadings. To this end, an effective Finite Element (FE) model of the Tsing Ma Bridge (TMB) in Hong Kong, embodying the properties of almost all the structural members is developed. Verification of the efficiencies of the model for bridge modal analysis and bridge dynamic response modeling are presented. The computed results are compared with, and found to match closely, online measured data. It shows the constructed bridge finite element model is efficient for structural health analysis.

This paper aims to evaluate the effect of typhoons on fatigue damage accumulation in steel decks of long-span suspension bridges. The strain-time histories at critical locations of deck sections of long-span bridges during different typhoons passing the bridge area are investigated by using on-line strain data acquired from the structural health monitoring system installed on the bridge. The fatigue damage models based on Miner's Law and Continuum Damage Mechanics (CDM) are applied to calculate the increment of fatigue damage due to the action of a typhoon. Accumulated fatigue damage during the typhoon is also calculated and compared between Miner's Law and the CDM method. It is found that for the Tsing Ma Bridge case, the stress spectrum generated by a typhoon is significantly different than that generated by normal traffic and its histogram shapes can be described approximately as a Rayleigh distribution. The influence of typhoon loading on accumulative fatigue damage is more significant than that due to normal traffic loading. The increment of fatigue damage generated by hourly stress spectrum for the maximum typhoon loading may be much greater than those for normal traffic loading. It is, therefore, concluded that it is necessary to evaluate typhoon induced fatigue damage for the purpose of accurately evaluating accumulative fatigue damage for long-span bridges located within typhoon prone regions.