Abstract:The durability of underground concrete structures in corrosive environments is one of the hottest issues. The current research works on the durability of underground concrete structures in marine and offshore chlorine salt erosion environment and saline lake and inland saline sulfate environment were summarized based on concrete corrosion mechanism, erosive ion diffusion mechanism, rebar corrosion mechanism, concrete strength and stiffness damage and other aspects, respectively, and the damage characteristics and degradation mechanisms of underground concrete structures under chloride and sulfate corrosion environment were discussed. Considering the production technology of the PHC pipe pile and the bored pile, the deterioration mechanism of horizontally loaded PHC pipe pile in marine environment and vertically loaded bored pile in saline soil environment were discussed, respectively. The degradation laws of their load-bearing characteristic were investigated by considering the loading mechanism of bored piles and PHC pipe piles, and the methods for predicting the service life of horizontally loaded PHC pipe pile in marine environment and vertically loaded bored pile in saline soil environment were proposed. Finally, based on the current study, the further research ideas and directions were proposed to provide references for the durability research of reinforced concrete pile in corrosive environment and complicated stress conditions, and as well as the formulation of relevant standards.

Abstract:Mid-rise cold-formed steel (CFS) structure with reinforced end studs can promote the development of mid-rise CFS residential buildings from low-rise ones. To analyze the dynamic characteristics of mid-rise CFS residential structures with reinforced end studs under earthquakes, a simplified calculation model of CFS shear wall with reinforced end studs, which is the main lateral load-bearing structural component of the structure, was proposed. The proposed model could consider the real behavior of beam-column joint, and then the seismic calculation model of the whole structure was further obtained. Lateral design objectives for the structure under earthquake were proposed according to abroad specification and previous shear wall test results that performed by authors, and seismic analysis on a mid-rise CFS structure with reinforced end studs was carried out. The results show that equivalent-bracing model considering the mechanical behavior of beam-column joint can precisely predict the seismic performance of the structure. It is safe and reliable that 1/300 and 1/75 being taken as the elastic and inelastic storey drift limits of the structure under frequent and severe earthquake, respectively. Mid-rise CFS structure with reinforced end studs will not collapse under severe earthquake with a degree of 9. It is not suitable to adopt shear wall combination with large difference in stiffness between two directions, and the shear strength of the walls along the direction with larger opening rate should be strengthened emphatically. In areas of particularly high seismicity, both shear strength of the wall and vertical bearing capacity of end studs should be taken into account to avoid overall failure, which is caused by end studs' buckling.

Abstract:To study the deformation performance of I-section beam with longitudinally variable thickness flanges, 2 welded I-section variable thickness steel beams and 2 normal beams under static loading were conducted, and corresponding numerical simulation was performed using the ANSYS. The special material of the flanges was longitudinally profiled steel plates of Q345GJC, and single point loading and two point loading were adopted respectively. All of the four specimens occurred local buckling when the lateral displacement was constrained. The results show that the beam could not only ensure the bearing capacity with less steel but also safety margin, and also can meet the deformation requirements of compact cross section, compared with the beam with uniform thickness. The finite element results are in good agreement with the test results. The test results can be used for the plastic design of I-section beam with longitudinally variable thickness flanges.

Abstract:Seismic fragility analysis can accurately estimate seismic performance of a structure, to propose a method to study on the seismic fragility of space grid structure, the full-load field time-history analysis, parameter sensitivity analysis and seismic fragility analysis were carried out. The results show that space grid structure has a ductile damage with obvious signs. The responses of this kind of structure are more sensitive to steel yield strength, floor loads and ground motion records, which can be considered as random factors in subsequent analysis. The seismic damage index and damage grading standard, which are applicable to orthogonal pyramid space grid, were proposed based on the seismic response of whole process. According to the seismic fragility analysis, the probability of ensuring intactness at PGA=0.40g is 95%, while 79% at PGA=0.62g. The orthogonal pyramid space grid has a good seismic performance, which can largely ensure the seismic safety.

Abstract:Taking an 80-meters-span singer-layer reticulated dome which have been constructed in the Shenbei New District as an example, the failure mechanism of the singer-layer reticulated dome under severe earthquakes were analyzed with the FE software ANSYS. Based on the data of the numerical analysis and the test of the joints, the bending performance of bolt-ball joints were obtained and introduced into the numerical analysis considering the joint stiffness. Different structure models were established to investigate the effect of structure model changes, including the strengthening the local members and joints and installing maintenance materials on the surface of the structure. The analysis indicates that the failure mechanism of this structure is the collapse due to the dynamic instability of the structures while the local joints are damaged instantaneously and massively. The reinforcement of the reticulated dome merely improves the behavior of the structures in normal service condition, but it cannot improve the ultimate bearing capacity. The seismic response including displacement and support reaction increase obviously by installing maintenance panels on the existing surface, while the ultimate bearing capacity of the structures reduces by 57.1% and its seismic capacity decreases largely.

Abstract:To analyze the effects of Rayleigh damping coefficients reference frequency selection method on structural seismic response, some methods to estimate Rayleigh damping coefficients including the frequency contents-based second reference frequency, IDRISS method, traditional method and the optimization method were compared by the errors of top displacement and base shear with a 85.2 m length, 61.8 m width reticulated shell. As for the frequency contents of earthquake waves, the statistics ranges of the peak and centroid frequencies of pseudo-acceleration-, pseudo-velocity-and displacement response spectrum were discussed. The numerical results show that: the second reference frequency specified by the peak and centroid frequencies of the pseudo-velocity-and displacement response spectrum as well as peak frequency of the Fourier spectrum make the response results smaller than the exact ones. The estimation error is reasonable, which the second reference frequency is equal to the peak and centroid frequencies of the pseudo-acceleration response spectrum in the range below the fundamental frequency. It is suitable that the reference frequency is estimated by the smoothing peak frequency of the Fourier spectrum in the IDRISS method when the predominant frequency of earthquake wave is greater than the fundamental frequency of structure. Optimization method can get the Rayleigh damping coefficient directly, eliminate the arbitrariness of reference frequency and have high calculation accuracy.

Abstract:The bolt-ball joint, which have wide application in spatial lattice structure, have too many components, so the initial defect may be caused by the screw length insufficiency of the high-strength bolt during installation process. To study the effect of the defect on the mechanical properties of these joints, a sophisticated FEM simulation model of the bolt-ball joint with thread was calculated by ANSYS. The number of thread engagement was changed to simulate the screw length of high-strength bolt, and the simulation method was verified by existing experimental data. The flexural behavior of three common joints (M20, M24, M27) under different screw length was analyzed. The result shows that the bolt-ball joints mainly have three work and failure modes when the screw length is changed. The screw length insufficiency of the high-strength bolt significantly reduces stiffness and bearing capacity of the joint. The tensile property of three joints under different screw length was analyzed, and the deterioration law of the tensile bearing capacity and the possible failure modes were found. The curve which shows the relationship between the reduced proportion of the tensile bearing capacity and the screw length was given. This study can provide technical references for the construction control of the bolt-ball joint and the safety performance evaluation of the similar structure.

Abstract:Assembly light steel tube frame with recycled concrete-thin wall was proposed, which was suited for lower or multilayer floors farmhouse. Assembly frame was assembled by recycled concrete filled light steel tube beams and columns, and light wall adopted single row of steel bars and recycled concrete. Frames and walls were connected by bolts. The frame bears the vertical load and resists the horizontal load with walls together. Pseudo-static tests on 4 assembly light steel frame-thin wall and 1 single frame were conducted to study the influence of reinforcement spacing and the wall thickness on the joint work performance of frames and walls. Failure modes, hysteresis property, load-carrying capacity, ductility, stiffness and energy dissipation capacity of specimens with different bar spacing and wall thickness were analyzed. The results indicate that light steel frame-thin wall distinctly has two seismic defensive lines and works together well. The failure modes of walls are the shear failure and the frame are the axial-flexural failure, respectively, which indicate that the assemble components are safe and reliable and the structure has adequate ductility. Reducing spacing of bars or increasing thickness of the wall can improve ductility and dissipation capacity of specimens.

Abstract:To study the fire performances of the simply supported fire-resistant steel-concrete composite beams (FRSCCB), the simply supported FRSCCB with fireproof coating and fire-proof board under standard fire were simulated by the finite element (FE) software ABAQUS. The effects of the load-ratio, the fire-insulation of the steel beam, the strength of materials, the section profiles and the diameter of longitudinal rebars in the concrete slab on the fire performance of the simply supported FRSCCB were analyzed. It is shown that the two most important factors, which influence the fire resistance for the simply supported FRSCCB, are the load-ratio and the fire-insulation thickness of the steel beam. The thicknesses of the beam plates and the width of the beam flange have favorable effect on the FRSCCB performance, but the strength of materials, the height of the beam, the thickness of the slab and diameter of the rebars have less influence. The fire-proof board has better fire-proof protection effect than the fire-proof coating under the same condition. When load ratio is 0.5~0.7, the fire-resistance limitation is increased by over 35% and the fire-insulation thickness can be decrease by 30% when the FRSCCB is used. The simplified calculation method for predicting the fire resistance limit according to Chinese code (CECS 200:2006) is conservative compared with the FE results when the load-ratio is small.

Abstract:To study the mechanical properties and design method of CFST columns with special-shaped cross-section, based on the special-shaped multi-cell CFST bifurcated columns of a super-high building under construction in Beijing, the FEM analysis on the seismic behavior of the special-shaped concrete-filled bifurcated steel tube columns was carried out. The test parameters were the different constructional measures, different steel and concrete grades. The results show that the developed FEM analysis method has adequate accuracy compared with the test results. The seismic behavior of the columns increased with increasing the number of the cells, the installation of the stiffener, L-type steel and steel tube. Increasing the number of the cavities has a better effect on seismic behavior than the increasing thickness of the steel tube in the case of the same steel consumption. The materials strength has a great influence on the seismic performance of the bifurcated column, and the strength matching between the steel and concrete should be considered in seismic design.

Abstract:To investigate the bond behavior and constitutive relationship at the interface between steel bars and recycled concrete, 15 beam-type specimens were designed to test the bond-slip behavior of reinforced recycled concrete. The design parameters included recycled coarse aggregate substitution, recycled fine aggregate substitution, steel bar shape, steel bar diameter and anchorage length. The influence of the above variables on the load-slip curves, bond strength and bond efficiency coefficient between steel bars and recycled concrete was analyzed synthetically, and the bond-slip constitutive relationship was proposed. The results show that: with the increase of recycled coarse aggregate replacement ratio, the bond strength between steel bars and concrete decreases; however, the anti-slid ability increases. The recycled fine aggregate addition makes the bond behavior become weaker significantly. The bond strength between deformed bars and recycled concrete is about 2 times of that between plain bars and recycled concrete. The interfacial bond behavior of reinforced recycled concrete becomes poorer with the increase of steel bar diameter and anchorage length. The proposed bond-slip constitutive relationship and parameters of reinforced recycled concrete fit the experimental results well.

Abstract:To provide theoretical data for the grout used in prefabricated structures and reinforcement reconstruction field. Pull-out tests were conducted on 27 specimens having grout with ribbed reinforced bars, and the failure mode and the variation of bond strength were studied. According to a typical bond-slip curve, the cracking, crushing and shearing process were analyzed. Based on the experimental data, the bond strength formula, the slip formula and the bond-slip formula were fitted. The empirical value of anchorage length of reinforcement in grout was presented. The result shows that the average bond strength increases with the increase of the cover thickness. While thickening the cover thickness, the increasing rate of bond strength is lower than that of concrete. In addition, increasing the bar diameter, the average bond strength decreases. Further-more, with the increase of the anchorage length, average bond strength decreases. The increase of anchorage length results in higher increment rate of the bond strength of grout compared to the self-compacting concrete. With the same compressive strength grade, the bond strength of grout is higher than that of ordinary concrete. In conclusion, the bond-slip behavior between ribbed steel bar and grout is different from that of concrete.

Abstract:To analyze the load-settlement response of tapered pile and tapered pile group, a hyperbolic model and a bilinear hardening model were incorporated in the proposed approach. Based on the two proposed models, considering the concordant displacement and property of multilayered soil, an iterative program was developed to calculate the settlement of the tapered single pile. A finite difference program was employed to analyze the interaction between two piles. Then considering the boundary condition and the proposed interaction between piles, the iterative program was proposed for the settlement prediction of tapered pile groups under rigid and soft pile caps. Finally, the pile settlement and the pile group settlement predicted by the iterative programs were compared with the load-settlement response of three well-instrumented tests. The results demonstrate the validity of the simplified calculation method adopted in the prediction of load-settlement response of the tapered pile and tapered pile group.

Abstract:To evaluate the enhancement mechanism of ultimate bearing capacity and the bearing capacity properties, considering the loading process of tapered pile, two stages were adopted in the proposed approach: the elastic stage and the soil-squeezing plastic stage. Based on these two stages, a simplified analytical expression for the ultimate carrying capacity was deduced. The bearing capacity augmentation factor was introduced to calculate the enhancement of the ultimate bearing capacity for the tapered pile. Then the effects of the friction coefficient on the pile-soil surface, the taper angle and the coefficient of earth pressure at rest on the bearing capacity were analyzed. Finally, the bearing capacity predicted by the proposed expression was compared with the data from the available model test. The results demonstrate the validity of the augmentation factor adopted in the prediction. The bearing capacity augmentation factors increase with the increase of the coefficient of earth pressure at rest and friction coefficient between pile and soil, while decrease with the increase of the soil friction angle. The augmentation factors reach the maximum values with a specific taper angle.

Abstract:To comprehensively analyze the wedge stability whose parameters have various uncertainties, a method was proposed to analyze the stability of wedge probability based on blind data theory and Fisher optimal segmentation method. A blind data evaluation model of wedge stability considering a variety of wedge parameter uncertainties was created to determine the wedge stability. The evaluation model of the blind number of the stability of the wedge was given, and the model was applied to the classical double-wedge stabilization, in which the result was compared with that from the traditional mean safety coefficient, numerical simulation, and Monte Carlo sampling (MCS). The compared results show that the result of evaluation model of wedge stability based on blind data theory and Fisher optimal segmentation method is coincident with the generally accepted MCS method, which verifies the reliability and efficiency of the proposed method. The proposed method was applied to the stability evaluation of the wedge in Xingcheng to provide data support for the prevention and control of the wedge.

Abstract:To study the effect of moisture content on mechanical properties of soil subjected to freeze-thaw cycling, a series of triaxial tests were conducted under different confining pressures for samples which were made of silty clay from Qinghai-Tibet Plateau with different original moisture contents and had experienced various freeze-thaw cycles. The results show that the stress-strain curves of samples with different moisture contents tend to be similar after freeze-thaw cycling. The freeze-thaw cycling effect is uncertain because of the level of original moisture content. It is deteriorating for soil with low moisture content, and the deteriorating degree aggravates with the increase of moisture content within a certain range. When the moisture content is increased to a certain value which is generally close to the plastic limit, the effect is strengthening conversely. The water migration inside samples, which are subjected to freeze-thaw cycling in a closed system, leads to the redistribution of moisture content. The higher the original moisture content is, the larger the water migration amount is. The failure strength of soil decreases with the increase of moisture content in a nonlinear law, so the change amplitude of strength in the increase zone of moisture content is different with that of the decrease zone. The failure strength of sample may exhibit different tendencies including increase, decrease or remain unchanged. The mechanical properties of soil subjected to freeze-thaw cycling are affected by the dry density, moisture redistribution and soil structure simultaneously, but the dominant factor is changeable due to different moisture contents and number of freeze-thaw cycles, which makes the freeze-thaw cycling effect on soil diversified correspondingly.

Abstract:The flood current is one of the main sources of hydrodynamic conditions of debris flow. The formation mechanism of the hydraulics-erosion debris flow caused by the flood current is more complicated. To study the influence of different types of rainfall on the erosion and destruction process of debris flow in a flood process, flume experiments were conducted in Nanjiao gully. The big-scale flume was made, and the artificial rainfall device as well as the rear flow device was needed. The changes of precipitation as well as flood discharge were recorded in the process of surface erosion, slope erosion and trench erosion through the observation and description of the experimental phenomena. The erosion-destruction characteristics and mechanism of debris flow were studied based on the test results by moisture content and pore water pressure at different locations of the soils. The results indicate that in a flood process the gully erosion damage occurs before the arrival of the flood peak under the three types of rainfall. The antecedent rainfall reduces the cumulative precipitation of the soil damage, and gully failure occurs before the soil is saturated with "direct rainstorm type". The experimental results provide references for judging the rainfall and flood volume of debris flow occurrence, which has great significance for the study of rainfall pattern in the process of debris flow initiation.

Abstract:The vortex-induced vibration (VIV) is a typical phenomenon of wind-induced vibration in low wind velocities, especially for the long-span bridges, and an important prerequisite for the evaluation and control of the vibration effects on bridges. Based on synchronously evolutionary characteristics analysis of distributed aerodynamic forces and structural effects during VIV, characteristic of distributed aerodynamic forces and their effects on structural behaviors were conducted to reveal the mechanism of VIV. Aiming at a traditional streamlined closed-box girder of long-span bridges, wind tunnel tests of synchronal measurement of force and displacement responses of spring-suspended sectional model were conducted. Pressure-measured tests were implemented to investigate the spatial aerodynamic distribution of the girder during VIV. Surface pressure distributions in different amplitude-developing period during VIV were compared, including pre-VIV period, ascent stage, amplitude extreme point, descent stage and post-VIV period. It is found that aerodynamic characteristics of the model has obvious changes during VIV, indicating that there are obvious differences between lock-in period and non-VIV period. The distributed aerodynamic forces and the amplitudes of aerodynamic forces at predominant frequency are positively correlated with the amplitude of VIV responses. The aerodynamic characteristics and the VIV response during VIV are synergistic, especially nearby downstream region of upper surface and the corner region of lower surface and tail wind fairing, which is the main cause of VIV. This study provides a new way for the research on the mechanism of VIV, and can be applied to other cross-sections.

Abstract:To investigate the characteristics of aerodynamic noise sources induced by flows around bluff bodies, the acoustic power level and surface acoustic power level of the bluff bodies were numerical simulated based on combination of the Realizable k-ε turbulence model and broadband noise sources model method. And the effects of oncoming wind speed, cross-section and characteristic size of the bluff bodies on the magnitude and distribution of aerodynamic noise sources were analyzed, and the influence mechanism of aerodynamic noise sources was further discussed. The results show that, the aerodynamic noise sources are quite significant in the regions where flow separation occurs with intensive turbulence, and they show a decreasing trend for the bluff body with a more streamlined cross-section. And the contribution of quadrupole sources to the total noise is much less than that of dipole sources, and thus the surface acoustic power level, which corresponds to the dipole sources, is used to analyze the characteristics of aerodynamic noise sources. Furthermore, the maximum surface acoustic power level is positively linear correlated with the logarithm of the oncoming wind speed, and is negatively linear correlated with the characteristic size. Finally, the proposed mathematical prediction model for the surface acoustic power level provides references for the acoustic environment design and aerodynamic noise control in engineering applications.

Abstract:Based on translation models, both Gaussian and non-Gaussian wind fields were generated using Monte Carlo simulation for investigating the influence of non-Gaussian characteristics of wind load on fatigue damage of wind turbine. With the blade aerodynamic model and multi-body dynamics, dynamic responses were calculated, and probability characteristics of the response were analyzed. Using linear damage accumulation theory and linear crack propagation theory, crack initiation life and crack propagation life were discussed in detail. The results show that the maximum responses under three types of wind fields are different, and non-Gaussian characteristics of wind load are more significant than those of the response. For areas with smaller annual mean wind speeds, non-Gaussian characteristics of wind load have less influence on fatigue life of wind turbine, but the influence becomes significant with the increase of the annual mean wind speed. When the annual mean wind speeds are 7 m/s and 9 m/s, the crack initiation lives under softening non-Gaussian wind decrease by 10% compared with Gaussian wind fields. It concludes that the influence of softening non-Gaussian characteristics should be considered for areas with higher annual mean wind speeds.

Abstract:To reduce wind loads on high-rise buildings, a new active aerodynamic control method named steady suction on the top of the building was introduced. The effects of free-end leading edge steady suction on the aerodynamic forces and free-end separation of a finite-length square cylinder were experimentally investigated in a wind tunnel. Flow visualization and flow field measurement were utilized to reveal the control mechanism. The dependence of aerodynamic forces on the suction ratio Q(=U/U_∞) was studied. The pressure distribution and the shear flow above the free end were compared in detail for Q=0, 1 and 3. The results show that the slot suction changes the flow separation on the free end significantly and also the aerodynamic forces on the whole cylinder span. The best control result appears at Q=1, with the fluctuation drag and lift reduced by 17.8% and 45.5%, respectively. At this suction ratio, the flow separation at the leading edge is weakened with reattachment and strong turbulence fluctuation presence on the free end, which results in strong momentum transport between the free-end shear flow and the wake and suppresses the vortex shedding and aerodynamic forces efficiently.

Abstract:The axial tensile stress-strain curves and crack width control ability of three types of ultra-high performance concrete (UHPC) were investigated, including high strain-hardening UHPC, low strain-hardening UHPC and strain-softening UHPC, respectively. The tensile stress-strain curves and crack width-strain curves of the UHPC were attained from tests on dog-bone shape specimens. The results show that the tensile stress-strain curves of high strain-hardening UHPC and low strain-hardening UHPC include three stages (elastic, strain hardening, strain softening), while the strain softening UHPC has only elastic stage and softening stage. The turning point of strain hardening stage and strain-softening stage is the critical point of slow propagation and rapid expansion of the cracks. The ultimate tensile strain improvement of UHPC can improve the crack width control ability. When the tensile strain of high strain-hardening UHPC is below 0.42 %, the crack width is less than 0.05 mm. In comparison with C50 concrete (ultimate strain and ultimate tensile strength are 0.012% and 2.3 MPa, respectively), the impact of the crack width checking in serviceability limit state on high strain-hardening UHPC can be avoided by its excellent crack width control ability, and the high strain-hardening UHPC can work together with steel under tensile loading before steel yield. It is meaningful to use reinforced high strain-hardening UHPC in those structures with high demand of crack width control.

Abstract:To reveal the rule of high strength concrete stress under the influence of hygro-thermal coupling and the cracking risk, the calculation method of hygro-thermal coupling and the calculation method of early age restraint shrinkage stress were established taking the concrete prism as an example. The simulation of temperature and humidity field was realized by programming. The results show that the model of hygro-thermal coupling can predict the development of temperature and humidity change in the early age of high performance concrete from 1day to 28 days. Under the condition of watering, the hygro-thermal coupling effect has little effect on the temperature field, humidity field and stress of high performance concrete, and the restraint stress and cracking risk are growing rapidly after stoping curing. The curing and atmosphere have remarkable influence on the early ages stress and cracking risk for high strength concrete.

Abstract:To analyze the stress safety and shear force-transferring mechanism between steel and concrete in cable-stayed bridge with group aggregation anchor system, which is a new architecture of the cable-stayed bridge, a finite element model of anchorage zone was established by ABAQUS software based on an actual bridge project. The loading and boundary conditions of the sectional model were determined according to the equivalent beam element model. The stress distributions of the concrete pylon and steel box girder under three load cases were studied. According to the analysis of stress and displacement of the interface between concrete pylon and steel box girder, the force-transferring mechanical performances with different levels of pre-tension loss and friction coefficients were investigated deeply. The analysis results show that the shear force at the end of the steel girder is mainly undertaken by friction in design load case, indicating there is no relative sliding between the concrete pylon and steel box girder with enough safety margin. When the pre-tension loss reaches 60% or the friction coefficient of interface reduces to 0.2, the critical friction state is achieved and relative sliding is increased, which makes the structure unsafe.

Abstract:To study the effect of pointing mortar on the compressive behavior of masonry walls, according to different depth and strength of the pointing mortar, twelve wall specimens reinforced with pointing mortar and three unreinforced specimens were tested under uniaxial compression. Two test parameters, depth and strength of the pointing mortar, were investigated in terms of their effects on the cracking load, failure load, stress-strain relationship and elastic modulus of the reinforced specimens. Test results show that, compared to the unreinforced specimens, the specimens reinforced with pointing mortar exhibit a decrease in cracking load and an increase in both failure load and elastic modulus. And the increase range depends on the depth and strength of the pointing mortar, in which the largest reaches 30%. The cracking patterns and failure modes are similar for both reinforced and unreinforced specimens tested. Based on the test data and theoretical analysis, according to the formula from the code for design for masonry structures, equations that account for the depth and strength of the pointing mortar are proposed for predicting the compressive strength, stress-strain curve and elastic modulus of the pointed masonry walls. The test results also indicate that pointing can be used as a means of reinforcing masonry walls, especially for the historical buildings.