Abstract:This study analyzed the influence of aerodynamic damping on frequency domain response of floating wind turbine. The 5 megawatt (MW) floating wind turbine model built by the National Renewable Energy Laboratory (NREL) of America was selected as an example. The aerodynamic damping matrix of the motion for the rigid body was established according to aerodynamic damping calculation method. Hydrodynamic coefficients were computed based on the three-dimensional potential flow theory, and the stiffness of mooring system was also taken into account as linear spring. Motion equations of the wind turbine with or without consideration of aerodynamic damping were then established and solved in frequency domain, respectively. The influence of aerodynamic damping on the motion of the floating wind turbine was examined in frequency domain through the response amplitude operators (RAOs) by solving the equations, and the response spectrum was derived from RAOs and JONSWAP wave spectrum. The results show that aerodynamic damping can effectively reduce the peak values of the surge and pitch RAOs under operating condition, and decrease the amplitude and zero-order moment of the corresponding response spectrum to a certain degree.

Abstract:The influence of the damper support stiffness on the damping effect of suspension bridge hangers was studied by means of numerical method. Firstly, governing equations of the support-damper-hanger system were established by simplifying the damper support as a spring-oscillator model. Secondly, the discontinuous Dirac Function in the governing equations was approximately handled to adapt the Finite Difference Method (FDM) utilized in this paper, and the governing equations of the support-damper-hanger system were numerically solved by FDM. The effect of the damper stiffness on the dimensionless curve of damping ratio, realizable maximum damping ratio and its corresponding damping coefficient, as well as mass of damper support was carefully studied. The results obtained in this paper agree well to the literature results. The results also show that the realizable maximum damping ratio and its corresponding optimal damping coefficient decrease with the decrease of the damper support stiffness, which affects the efficiency of the damper. As the dimensionless curves related to the damping ratio exhibit obvious differences at each mode, it is impossible to use a Universal Curve related to modal damping ratio to design the damper parameters. In addition, the mass of the damper support has little influence on the damping ratio.

Abstract:Field measurement and analysis of wind characteristics and wind-induced vibration responses of Chishi Bridge during construction stages were carried out, where Chishi Bridge is a large-span cable-stayed bridge located in Chenzhou, Hunan, China. The wind characteristic parameters at deck level and pylon top during observation period were analyzed, including the mean wind speeds, wind directions, wind attack angles and turbulence intensities. The wind-induced vibration responses of the bridge were also examined. The results showed that the wind attack angles changed within a rather wide range when the wind blew from the north, while the wind attack angles varied within a narrow range when the wind came from the south. The bridge with double cantilevers during construction stages mainly vibrated in the lateral bending and vertical pendulum modes under strong winds. The measured values of the natural frequencies of the bridge under construction stage agreed well with the analysis results by FEM.

Abstract:Based on the structural health monitoring system (SHMS) installed on Xijiang Bridge of Nanning-Guangzhou Railway, the whole-process data of the wind speeds and directions at the bridge site subjected to Typhoon Kujira in June 2015 were obtained. Utilizing the measured data, the mean wind speeds, mean wind directions, and fluctuating wind parameters including the turbulence intensity, gust factors, turbulence integral scales, power spectrum density (PSD) of fluctuating wind and spatial correlation in different observing points were summarized and statistically analyzed. The analysis results showed that 1-minute mean wind speeds fluctuated in a large margin in the typhoon periphery, and the distribution of measured turbulence intensity exhibited a low discreteness with a longitudinal value of 0.253. Both the turbulence intensity and gust factors decreased with the increase of the mean wind speed. Moreover, large differences between the measured turbulence integral scales and recommended ones in three principal directions were observed. Meso-micro scale vortexes dominated the flow field, whose dispersion slightly increased with the increase of the mean wind speed. Additionally, the Kaimal Spectrum obtained from the relative codes cannot match well the measured horizontal PSD functions, and the measured spectrum value of Typhoon Kujira was greater than those of the codes in both high and low frequency domains. The spatial correlation was apparent in the periphery, and the error was caused when the Davenport assumption was adopted.

Abstract:In order to investigate the characteristics of nonlinear aerodynamic system of a thin plate, flow field around the thin plate under forced unit impulse excitation was simulated by the unsteady Reynolds-averaged Navier-Stokes (RANS) equations and SST k-ω turbulent model, and time histories of aerodynamic force were obtained. The nonlinear aerodynamic system of the thin plate was then identified based on the Volterra theory. The investigation indicates that the present aerodynamic model can provide reasonable output under the excitation within the defined ranges of frequency band and amplitude. Moreover, under the present range of forced frequency and amplitude, no clear dependence of model response on the frequency and amplitude is observed. It is also found that the aerodynamic nonlinearity of the thin plate is not significant, and the flow around the thin plate can be considered as an aerodynamic system with weak nonlinearity. This study confirms that the CFD method is of remarkable advantage for the identification of aerodynamic system of bridge girders.

Abstract:Two models of triangular guyed mast were tested by high frequency force balance (HFFB) technique under uniform and turbulent flow with different turbulent intensities, respectively. The mean and RMS force coefficients and the force spectra in along-wind, across-wind and torsion direction were obtained from wind tunnel test. Some existing codes and standards for the mean drag coefficients for guyed mast and ancillaries were employed and compared with test results, with emphasis on the influence of Reynolds number, turbulent intensity, wind incidence angle and ancillaries on wind force coefficients. The force spectra in along-wind direction are similar to the spectra of fluctuating wind velocity, while the force spectra in across-wind and torsion direction consist of the wind turbulence spectra in low frequency range and vortex shedding spectra in high frequency range. The vortex shedding spectra of the guyed mast without ancillaries have one peak, and the reduced frequency of the peak is close to 1.8. When the attack angle reaches 90°, the vortex shedding spectra of the guyed mast with ancillaries have two distinct peaks with the reduced peak frequencies closed to 0.9 and 2.2, respectively. The characteristics of the vortex shedding spectra for lattice mast and the influence mechanism of the ancillaries were discussed.

Abstract:A cellular Automaton (CA) model was introduced to simulate stochastic traffic, an 18 degrees-of-freedoms (DOFs) vehicle model was used to simulate a heavy truck, and a single DOF vehicle model was used to simulate other typed vehicles in traffic. The simulating stochastic traffic can be improved by considering the influence of the nearest neighbor vehicle. The coupled equations of bridge and vehicles in traffic flows were established by combining the equations of the motion of both the bridge and vehicles using the displacement relationship and interaction force relationship. The simulations show that the effect of considering the nearest neighbor vehicles on the maximal vertical displacement of bridge is significant, and the dynamic vibration of the main beam and tower under stochastic traffic flows cannot be neglected.

Abstract:In order to optimize the connection of precast composite shear wall, two local high damping concrete precast composite shear walls with or without concealed steel plate bracing were designed. Quasi-static tests of two specimens were carried out, and the bearing capacity, hysteretic curves, skeleton curves, displacement ductility, stiffness degradation, energy-dissipation performance, crack development and failure pattern were obtained, and compared with each other. The test results indicate that the test specimens exhibit great seismic structural behaviors, and the use of concealed steel plate bracing can improve the bearing capacity, deformation ability as well as its ductility of precast composite shear wall.

Abstract:For the particularity of design method of prestressed concrete frame beam, this paper took a prestressed concrete frame structure in the 8-degree (0.2g) seismic grade district as an example to study the rationality of moment amplification factors at column ends in the current code, and a design method that the prestressed concrete frame columns could be designed on the basis of the actual seismic flexural moment of the beam was also proposed. Eight prestressed concrete frames were designed according to the combined moment and actual seismic flexural moment. The static elasto-plastic analysis and dynamic elastic-plastic analysis were then carried out for the frame structures in OpenSees using fiber elements. Research shows that the prestressed concrete frame designed by using the current code has serious hinges at the first floor columns under rare earthquake. The seismic performance of yield failure mechanism of the structures was effectively improved with the increment of the factor. Moreover, as the value of moment amplification factors at column ends is low in 04 specification, it should be improved in the new specification. This paper recommends that for the prestressed concrete frame with second level seismic requirement, the moment amplification factors at column ends should be increased in the revision of code for seismic design of prestressed concrete structures, and on the basis of the combined moment at beam ends, the factor should be higher than 2.0, while on the basis of the actual seismic flexural moment, the factor should be 1.4.

Abstract:The accurate analysis of the stiffness and deflection of steel and concrete composite beams is complicated due to the possible interface slippage. Exact procedures for static linear-elastic analysis of composite beams with interlayer slip were presented. An efficient second-order algorithm using two second-order ordinary differential equations in terms of interlayer slip and deflection was established to calculate the interlayer slip and the deflection of composite beams, and exact closed-form solutions for four Euler boundary conditions were then derived according to their boundary conditions. The internal forces of composite beams with interlayer slip were also derived. Compared with the higher-order algorithm, the second-order algorithm can simplify the calculation of the deflection of composite beams with interlayer slip and give relatively comprehensive solutions.

Abstract:In order to promote the research and application of pseudo dynamic test method, based on the NetSLab's communication function, a remote collaboration pseudo dynamic test platform NetSLabOSR for building structures was developed. For the remote collaboration tests, using the communication model provided by NetSLab, long distance communication among computers located in different sites was realized, and timely transmission and feedback of operation information was achieved. NetSLabOSR provides test capabilities for conducting the substructure pseudo dynamic tests through both traditional local method and distributed remote collaboration with numerical simulation codes developed by the authors based on OpenSees. Using NetSLabOSR test platform, substructure pseudo dynamic tests of planar and space composite frames with steel tube concrete columns and steel beams have been carried out, which verifies the validity and stability of the NetSLabOSR test platform, as well as the efficient communication performance and the test accuracy.

Abstract:Technical solutions for sealing and covering seismic joints may induce dynamic interactions between as-built adjacent structures. Moreover, noise modes due to dynamic interactions make modal parameter identification complex. In this paper, ambient vibration tests were conducted to obtain the actual dynamic characteristics of a reinforced concrete office building, whose seismic joints were formed by cantilever members. The tested building was divided into the main building, eastern building and western building. Tests were conducted in each structural part separately. Four modes of the main building and eastern building, and two modes of the western building were identified using the frequency domain decomposition method. The results show that due to the dynamic interactions between adjacent structures caused by the infill of the seismic joints, natural frequencies of a structure were mixed together with those due to the interactions with the adjacent structures. Modes of each building from the finite element models agreed well with the identification results, except that the third natural frequencies of the main building and eastern building were smaller than the identified ones, because the integrity of the whole building was enhanced by the dynamic interactions. Therefore, it should not always be assumed that adjacent structures with seismic joint are independent.

Abstract:In order to examine the local buckling behavior of 960 MPa (yield strength standard) high strength steel (HSS) columns under axial compression, finite element models were developed by using the finite element (FE) software, ANSYS. Local buckling of four welded box section and four welded I-section columns under axial compression considering the initial geometric imperfections and welding-induced residual stresses was investigated. The FE model was then verified by comparing the ultimate bearing capacity and bearing capacity governed by local buckling of the existing test results. Using the validated FE modeling approach, parametric analyses were conducted on the local buckling behavior of 960 MPa HSS box section and I-section columns under axial compression. Moreover, the FE analysis results and the existing test results summarized in this paper were compared with the design curves in Chinese, American and European codes. It is found that the FE model established in this paper can accurately simulate the local buckling behavior of 960 MPa HSS columns under axial compression. The initial geometric imperfection and the residual stress have little influence on the ultimate stress of 960 MPa HSS columns, but exhibit a great influence on the local buckling stress. Compared with the design methods in the Chinese, American and European codes, the new design formulae proposed in this paper were recommended for the design of the local buckling stress and post-buckling ultimate stress of 960 MPa HSS columns under axial compression.

Abstract:Tests of three RC beams under impact loading and one under static loadings were carried out with the shear pan ratio of 3.58. The failure mechanism and the influence of impact energy on residual deflection of the RC beams under impact loading were investigated. The test results showed that cracks of the flexural beams under static loading were mainly attributed by the bending-shear failure mode under low-velocity impact, while they changed to the shear failure mode under high-velocity impact. The time history of impact loading, support reaction force, mid-span deflection and reinforcement strain in mid-span were recorded and presented. Based on the analysis of the time history curves, the failure mechanism of RC beams was discussed. The failure process under impact loading can be divided into partial response phase and overall response phase. Moreover, the impact test results in the existing related papers were collected, and based on the comparison of the relationship between residual deflection and impact energy, the applicability of empirical formula to estimate residual deflection under impact loadings was discussed.

Abstract:Glass fiber bundles with different gage lengths (25, 50, 100, 150, 200 and 300 mm) were tested under quasi-static loading at a strain rate of 1/600 s-1 using a MTS load frame, and samples with a gage length of 25 mm were tested under dynamic tensile loading at four different strain rates (40, 80, 120 and 160 s-1) and four distinct temperatures (25, 50, 75, 100 ℃) utilizing a drop-tower impact system. The experimental results show that the mechanical properties are dependent on the gage length, strain rate and temperature. Young's modulus increases with the increasing gage length and strain rate, but decreases with the increasing temperature. Tensile strength decreases with the increase of the gage length, but increases with the increase of the strain rate, while it decreases firstly and then increases as the temperature increases. Ultimate strain decreases with the increase of the gage length, but increases as the temperature increases. Finally, Weibull statistics were used to quantify the degree of variability in yarns' strength at different gage lengths, strain rates and temperatures, and the obtained Weibull parameters can be used for engineering application.

Abstract:In the seasonal frozen regions, the pore water pressures and water contents in soil are influenced by freeze-thaw cycles and external loads. In the model test, the sensors of pore water pressure and water content were used to measure the variations of pore water pressures and water contents of the loess under the freeze-thaw cycles and static load, and the changing process of pore water pressures and water contents at different depths of soil was obtained. The relationship between static stress and pore water pressures in the space was then examined. It is found that the pore water pressures of soil increased rapidly at the beginning of freeze-thaw and static load actions; and the pore water pressure then changed periodically with the temperature. In a freeze-thaw cycle, soil pore water pressure and moisture content increased with the increase of temperature, and decreased with the decrease of temperature. In addition, the pore-water pressure and moisture had hysteretic quality with the changing of temperature. Moreover, three concentrated areas of pore water pressure from the longitudinal section with the increasing number of freeze-thaw cycles were observed: one was directly below the loading position, and the other two were located between the edge of loading area and test chamber. Meanwhile, a high water content area below the static load and low water content areas on both sides of the static load were also found. Under the static load, the distribution of pore water pressure resembled the stress field calculated by the corner-points method. It is also considered that the present of these concentrated areas was related to the stress field of soils produced by the static load.

Abstract:Based on the deformation characteristics of the composite foundation reinforced with encased stone columns under vertical loads, a new calculation formula to estimate the pile-soil stress ratio of geosynthetic-encased stone columns was proposed with consideration of pile-geosynthetic-soil interactions. In the derivation process, initial stresses in soil and a column were taken into account, the column was considered as an elastic-plastic material with constant dilatancy angle and satisfying both the Mohr-Coulomb yield criterion and non-associated flow rule, and the soil and geosynthetic encasement were considered as linear-elastic materials. The proposed formula was validated by comparison with the results of elasto-plastic limit analysis method. Finally, a parametric study was conducted to investigate various parameter effects of encasement stiffness, deformation modulus of soil, and replacement ratio on the behavior of the pile-soil stress ratio. The parametric study shows that the pile-soil stress ratio of the composite foundation with the encased stone column increases with the increase of encasement stiffness, replacement ratio of composite foundation and friction angle of the column, but decreases with the increase of deformation modulus of soil and dilation angle of column. Moreover, among all these parameters, the encasement stiffness hasd a significant influence on the pile-soil stress ratio.

Abstract:The drying shrinkage, compressive strength, fracture strength, dry density and thermal conductivity of vitrified microsphere insulation mortar were systematically studied when the water to binder ratio are 1.2, 1.3, 1.4 and 1.5. This study reveals the reason that the performance index of vitrified microsphere insulation mortar depends on the different water to binder ratio by mercury test and SEM scanning electron microscope analysis. The results indicate that the drying shrinkage of the vitrified microsphere insulation mortar increases obviously with the increase of water to binder ratio. When the water to binder ratio is fixed, the growth rate of drying shrinkage of vitrified microsphere insulation mortar is rapid in the early stage but becomes slow in the later stage. When the water to binder ratios are 1.3, 1.4, and 1.5, the compressive strength, flexural strength, dry density and thermal conductivity change obviously compared with those of the vitrified microsphere insulation mortar with the water to binder ratio of 1.2. The compressive strength correspondingly decreases by 13.1%, 40.0% and 73.8%, respectively; flexural strength decreases by 18.8%, 35.7%, and 77.7%, respectively; dry density decreases by 8.3%, 19.4%, and 33.3% respectively; and thermal conductivity also decreases by 4.6%, 11.3% and 21.4%, respectively. The structural performance of the vitrified microsphere insulation mortar varies obviously with the change of water to binder ratio. It is also found that the porosity of vitrified microsphere insulation mortar increases with the increase of water to binder ratio by mercury test and SEM analysis.

Abstract:Since Municipal Solid Waste Incineration (MSWI) bottom ash can be used for a new roadbed material, in order to study the influence of water contents on the strength properties of MSWI bottom ash, the material characteristics were firstly examined. The ultrasonic wave velocity tests, saturated consolidated drained triaxial compression tests, and unconfined compressive strength tests were carried out within the water contents range of 10% to 20%. The results show that the main components of MSWI bottom ash is SiO2, and the best moisture content is about 15.5% with the largest dry density of about 1.52 g·cm-3, which are for well graded gravel soils. Meanwhile, the water content shows significant influence on the strength of MSWI bottom ash, where the ultrasonic wave velocities, uniaxial compressive strength and maximum principal stress difference firstly increase with the water increasing contents, and then gradually decrease, which reaches the maximum values with a water content of approximately 15%. Based on the elastic theory, the relationship between the Poisson's ratio, cohesive force and uniaxial compressive strength of MSWI bottom ash under condition of different water contents was obtained, which provides theoretical basis for further analysis on the strength of MSWI bottom ash.