Abstract:For the further research on the bearing capacity of slab-column connections, the engineering accidents of slab-column structure collapse caused by the failure of slab-column connections at home and abroad were introduced, and the problems existing in the calculation methods of bearing capacity of slab-column connections were pointed out. The test results and prediction formulas of the failure surface inclination angle of punching or bending-punching failure of slab-column connections were analyzed. In view of the large deviation between the calculated results of the inclination angle of failure surface obtained according to the prediction formulas and the test values, a method was proposed to obtain the failure surface inclination angle by fitting the bearing capacity test results of slab-column connections in three-dimensional coordinate system, which takes the inclination angle of the punching and bending-punching failure surface as the ordinate and the longitudinal reinforcement configuration eigenvalue ρfy/fc as well as the punching span ratio λ=(M_i)/[(V_ih0i)] as the abscissa. Considering that the Chinese code GB 50010—2010 Code for Design of Concrete Structuresonly provided the punching bearing capacity formula, the existing calculation formulas of the bearing capacity of slab-column connections at home and abroad were reviewed, and the influences of the section height of the slab in each formula were analyzed. The analysis results show that the calculation of bearing capacity of slab-column connections should be treated differently according to punching failure, bending-punching failure, and direct punching failure, and the inclination angle of punching or bending-punching failure surface shall be reasonably considered in the calculation. The influence of key parameters such as punching span ratio and longitudinal reinforcement configuration eigenvalue should be comprehensively considered in the calculation of bearing capacity of slab-column connections, and the criterion and bearing capacity calculation method of direct punching failure of slab-column connections should be further improved.

Abstract:The Chinese code GB 50010—2010 Code for Design of Concrete Structures gives a formula to calculate the axial compression capacity of confined concrete columns by using the yield strength of spiral and welded ring stirrups, and it assumes that the stirrups have yielded under the peak compressive stress of confined concrete. In fact, the stirrups can yield only when the volume stirrup ratio exceeds a certain limit. Meanwhile, the code has not codified the case for square concrete columns confined by grid stirrups. To explore the tensile stress level in grid stirrups under peak loading, we conducted axial compression tests on 42 high-strength concrete square columns confined by grid stirrups. The axial compressive strength of the unconfined concrete ranged between 50.0 MPa and 68.0 MPa, and the stirrups were made of HRB400, HRB500, HRB600, PC800, PC1 270, and steel wire with standard tensile strength of 1 570 MPa. Results show that majority of the stirrups did not yield under the peak compressive stress of the confined concrete. The axial compressive strength of concrete, volume stirrup ratio, and hoop spacing had great impact on the stress level of stirrups. On the basis of the test data of unyielding stirrups under peak compressive load, a formula for calculating the actual tensile strain of stirrups under peak compressive load was established, and a method of calculating the lower limit value of volume stirrup ratio for stirrups capable of yielding under peak compressive load was proposed. By considering the effect of the actual tensile stress of stirrups on peak compressive stress and strain of the confined concrete, the full curve equation for the compressive stress-strain relationship of grid stirrups confined high-strength concrete was established, which provided reference for the engineering application and calculation of square concrete columns confined by grid stirrups.

Abstract:To study the shearing performance of concrete exterior beam-column joint with HRB400E reinforcement, we designed nine specimens based on orthogonal test method and conducted shearing test, which takes concrete strength, horizontal reinforcement anchorage arrangement, and reinforcement ratio as the main research parameters. Test results show that beam end bending failure and joint core shear failure were common failure modes of specimens, but bending anchorage arrangement could effectively reduce the number of cracks in the joint core. Among the three parameters, concrete strength had the most influence on the shearing deformation of the joint core, and as the concrete strength grade increased, the shearing deformation angle at the initial crack decreased. The anchorage arrangement of the horizontal bars had the most influence on the shearing deformation of joint core at the ultimate state, and shearing deformation angle reached the minimum value when 90° bending anchorage arrangement was adopted. The ratio of longitudinal reinforcement to beam had the greatest influence on the horizontal shearing force of the joint, and the horizontal shearing force increased with the increase in the reinforcement ratio. The primary and secondary relations of the parameters and corresponding changing trends were analyzed by using the orthogonal test principle combined with the mathematical statistics theory of range and variance.

Abstract:The energy dissipation contribution of wall-base rocking impact damping of post-tensioned prestressed rocking walls with ground motion energy input was analyzed. A refined finite element model of the rocking wall was established based on ABAQUS. The dynamic characteristics of the rocking wall structure was determined using modal analysis, and the free damping vibration of the rocking wall under large lateral displacement was simulated. The dynamic time history response of the wall top displacement, velocity, and acceleration was obtained. Taking the restitution coefficient of the rocking wall as a quantitative index of impact damping, the energy dissipation capacity of rocking walls with different aspect ratios and prestress levels was comprehensively evaluated, which contains multiple mechanisms of impact energy dissipation. Results show that the proposed model could accurately reflect the dynamic response of the rocking wall. The aspect ratio had great impact on the dynamic characteristics of the rocking wall. With the decrease in the aspect ratio, the proportion of impact energy dissipation increased significantly, up to 78.3%. The initial prestress level had no obvious influence on the dynamic characteristics and free damping vibration response of the rocking wall, but it significantly improved the energy dissipation capacity and increased the energy dissipation ratio to 36.8%.

Abstract:To study the mechanical performance of a new type of composite joint in a gymnasium, we designed three concrete-filled steel tube (CFST)-wrapped reinforced concrete (RC) composite joints using hot-bent steel tubes by taking the arch foot from the gymnasium project as the prototype, in which the long-span truss structural chord steel tube was bent and inserted vertically into the RC frame column. Load test and finite element simulation of the joints were carried out under complex loading with chord compression and web tension. The effects of residual stress, different bending angles, and whether the steel tube at the root of the chord is filled with concrete on the failure forms and bearing capacity of the hot-bent steel tube of composite joints were studied. Results show that the failure form of the hot-bent steel tube CFST-wrapped RC composite joints was mainly the buckling failure of the steel tube at the root of the chord. The bearing capacity of the joint depended on the compressive bearing capacity of the chord root. The residual stress generated by hot bending of the steel tube reduced the stiffness of the joint, and the greater the bending angle of the steel tube was, the less the stiffness of the joints was, but it basically had no effect on the bearing capacity. The stiffness and bearing capacity of the joints were significantly improved by filling concrete in steel tube, and were increased with the increase in the diameter of the steel tube.

Abstract:The flow mechanism around two tandem circular cylinders with different pitch ratios (P/D) was investigated. Numerical simulations were conducted on two tandem circular cylinders (P/D=1.1-5) at a low Reynolds number (Re=100). Dynamic mode decomposition (DMD) was adopted to decompose the flow around two cylinders. Reduced-order model was established based on the dominant DMD modes to reconstruct the vorticity field of two tandem circular cylinders. Results show that the typical wake flow regimes of two tandem circular cylinders were single bluff body (P/D=1.1-2), shear layer reattachment (P/D=3), and couple vortex shedding (P/D=4-5), which presented distinct wake flow features. With the increase in pitch ratio, the mode structure corresponding to the vortex shedding frequency moved from the wake of the downstream cylinder to the wake of the upstream cylinder. It indicates that the dominant mode characterizes the inherent flow mechanism of the transition of the three wake flow regimes for two tandem cylinders with different pitch ratios. The wake flow pattern of couple vortex shedding presented more complicated higher-order mode characteristics than that of single bluff body and shear layer reattachment. In terms of flow reconstruction, more sub-modes were needed for couple vortex shedding to achieve similar precisions with the other two regimes. The reconstruction error was mainly concentrated near the vortex formation region.

Abstract:By adopting wind tunnel model tests with synchronous pressure measurement, the aerodynamic force and wind response characteristics of diagonal twin towers under different spacing and wind directions were investigated from the perspectives of structural wind load and wind-induced acceleration, which is expected to provide reference for the wind-resistant design of diagonal twin towers. Results show that there were two most unfavorable wind directions in terms of wind responses: diagonal direction (around 45°) and near tandem arrangement direction (around 80°). In the wind direction of around 45°, the diagonal twin towers experienced remarkable across-wind oscillations caused by vortex shedding. However, due to the aerodynamic interference between the two towers, these across-wind oscillations were smaller compared with the single tower under the same wind conditions. Moreover, the favorable aerodynamic interference was more evident on the upstream tower than on the downstream tower. In the wind direction of around 80°, the downstream tower was excited by the wake of the upstream tower, which might result in severe wake-buffeting in the across-wind direction. The across-wind oscillation due to vortex shedding in around 45° wind direction mainly occurred at a subcritical to critical wind speed of vortex-shedding, and the wind speed was relatively low. The critical wake-buffeting under a wind direction of about 80° mainly occurred at supercritical wind speeds, and the wind speed was relatively high. Since the design wind speeds for super-tall buildings are mostly lower than or close to the critical wind speed of vortex shedding, the control of the across-wind oscillation for wind direction of about 45° is the key to wind resistance design. In this case, it is beneficial to designing diagonal twin towers with smaller spacing.

Abstract:Long-span flexible photovoltaic support structures have been increasingly used because of their good site adaptability and economy. For improving the wind resistance design method of such structures, wind tunnel pressure test was conducted on a long-span flexible photovoltaic support structure with variable inclination angles, so as to investigate the distribution characteristics of mean and fluctuating wind pressure coefficients of photovoltaic panels under different wind azimuths as well as the extreme wind pressure change law of photovoltaic modules under full range wind azimuths. The power spectrum of the fluctuating wind pressure under typical wind azimuth was also given. On the basis of the wind pressure distribution characteristics of photovoltaic modules, the finite element software ANSYS was employed to simulate the wind-induced response of the flexible photovoltaic support structure, and the corresponding vibration coefficient was obtained. Research results show that at wind azimuths of 0° and 180°, the mean wind pressure coefficient appeared gradient distribution along the incoming flow direction and the absolute value decreased rapidly. With the increase in wind azimuth, the maximum absolute value of the wind pressure coefficient moved from the windward leading edge to the corner of the windward end. The trend of fluctuating wind pressure distribution on photovoltaic panels was similar to that of mean wind pressure distribution. Compared with structural displacement response, the cable tension response was not sensitive to the changes in wind speed. The wind vibration coefficients of the downwind and vertical displacements achieved maximum values at U=8 m/s, and the values were 2.11 and 1.98. The research can provide reference for wind-resistant design of similar photovoltaic structures.

Abstract:For the assessment of the safety level of the standing seam roof system with anti-wind clips (SSRS-AWC), an efficient method was proposed to analyze the reliability of wind-uplifted resistance of SSRS-AWC. Firstly, the mechanical model of SSRS-AWC was established and the corresponding failure criterion was obtained. Then, on the basis of the equivalent extreme-value event, the extended conjugate unscented transformation (ECUT) method, and the principle of maximum entropy, the analysis method for wind-uplifted resistance reliability of SSRS-AWC under multiple failure modes was proposed. Finally, the feasibility of the analysis method was verified by an engineering example. Results show that compared with the Monte Carlo simulation (MCS) method, the maximum relative error of reliability index of proposed method was 0.63%, and the calculation time was only 0.04% of the MCS method, which can accurately and efficiently analyze the reliability of SSRS-AWC under multiple failure modes. The failure probability of SSRS-AWC in descending order was the roof panel tearing damage, the clip rupture, and the clip separation from seam with a probability of 0. It indicates that the addition of anti-wind clips can effectively avoid the clip separation from seam of SSRS-AWC. The failure probability of SSRS-AWC under single failure mode was smaller than that under multiple failure modes. For the sake of structural safety, it is recommended to consider reliability index under multiple failure modes.

Abstract:To study the effects of parameters such as the position of the post-installed rebar on the mechanical behaviors of a new type of grouted sleeve lapping connector APC (all vertical members precasted in concrete structures), a total of 45 specimens were designed for tensile test. The failure mode, ductility, ultimate bearing capacity, and sleeve strain of the specimens were studied, and ABAQUS was used for numerical simulation and parametric analysis. Test results showed that the initial stiffness and ductility of the connector decreased with the increase in the distance between two rebars. The decrease in the distance between the rebars as well as the contact between the rebar and the sleeve both reduced the bearing capacity of the connector, and the contact between the rebar and the sleeve played a dominant role in the reduction of bonding strength. Under ultimate load, the middle section of the sleeve in the deflection direction (the direction of the line connecting the centers of two rebars) was longitudinally compressed, and the compressive strain increased with the increase in the rebar diameter for specimens with tensile failure of rebars. Under ultimate load, the circumferential strain of the middle section of the sleeve was dominated by tensile strain, and the average circumferential tensile stress increased with the increase in the rebar diameter. Refined numerical simulation of connectors was carried out based on ABAQUS, and the results were in good agreement with the test results. The simulation parameter analysis showed that the deflection reduced the ultimate bearing capacity of the specimen, and had a greater influence on the ultimate bearing capacity of the specimen with pull-out failure of rebar than the specimen with tensile failure of rebar. With the increase in the lap length, the peak value of the bonding stress curve first increased and then decreased, while the fullness of the bonding stress curve first decreased and then increased. The ultimate bonding strength calculation formula obtained from current and previous tests has been verified to be applicable and safe, which can be used as a reference for engineering applications.

Abstract:To investigate the engineering applicability of bimetallic steel bars and sea water sea sand concrete, concentric pull-out tests were carried out on sea water sea sand concrete and sea water sea sand concrete containing polyoxymethylene (POM) fibers. Considering different concrete ages and ratios of concrete cover c to bar diameter d(c/d), the influences of the two variables on bond performance were studied to quantify the bond-slip relationship. Test results show that bimetallic steel bar was suitable for the reinforcement of sea water sea sand concrete because of its good performance. The peak bond stress τ_u between the early-age concrete and the steel bar was significantly affected by the age, and the sea water sea sand concrete with POM fibers added had better ductility and bonding properties. The influence of concrete age on the slip value s_u corresponding to the peak bond stress and the shape parameter α of the ascending section was generally manifested as the larger the age was, the smaller the slip value s_u was, and the smaller the parameter α was. With increasing concrete age, the shape parameter k of the descending section was increased. Based on the test results, a bond-slip model of bimetallic steel bar and sea water sea sand concrete was proposed considering the effect of concrete age, and the model was in good agreement with the test results. The research indicates that bimetallic steel bar and pure sea water sea sand concrete can be used in engineering applications, and the concrete improved by adding POM fibers has better performance.

Abstract:Eight beams of reinforced concrete were prepared to investigate the deterioration law of non-uniform corrosion on the bond performance of steel bar and concrete. First, the corrosion of the steel bar was obtained by applied-current. The electricity was stopped when the specimen reached the designed mass loss rate, and the development of corrosive cracks on the concrete surface was recorded. Then, three-point bending load was applied to the beam specimen to test its bonding performance. Finally, the corroded steel bar in the bonding area was taken out for 3D scanning test after the bond failure of the beam specimen. Test results show that the volume expansion of the corrosion products of the steel bar caused the corrosive cracks in the concrete protective layer along the axis of the longitudinal tensile bar, and the maximum corrosive crack widths on the concrete surface had a logarithmic relationship with the mass loss rate of the steel bar. There were rust pits on the surface of the corroded steel bar, and as the mass loss rate of steel bar increased to 10.3%, the total length, width, and depth of the rust pits increased by 68.7,0.21, and 9.98 mm, respectively, suggesting that the total length of the rust pits had the most significant increase. On the basis of the test results, a relative bond strength degradation model of steel bar and concrete was established in consideration of the influence of non-uniform corrosion. According to the correlation between the mass loss rate and the maximum corrosive crack width on the concrete surface, a calculation model of relative bond strength based on the maximum corrosive crack width on the concrete surface was proposed and verified.

Abstract:The hysteretic behavior and collapse mechanism of steel two-tiered braced frames (STBF) under rare earthquake was studied. Quasi-static test was conducted on a 1/2-scale STBF, and numerical simulation was carried out for 31 validated models of STBF. The failure mode, deformation, and internal force of STBF under cyclic load were analyzed. The influences of parameters on the collapse mechanism of STBF were investigated, including the load on the top of the column, slenderness ratio, diameter-thickness ratio, and tier-height ratio of braced frames. Results show that the bracing failure of STBF under cyclic load was primarily in one tier. The tier underwent a larger buckling deformation outside the brace plane, which might develop to fracture failure. The column experienced in-plane bending moment due to the asynchronous failure of upper and lower braces. With the increase in the tier-height ratio, the energy dissipation capacity of STBF increased. When the axial compression ratio was larger than 0.5 or the tier-height ratio of the braced frame was less than 0.5, STBF was damaged prematurely due to the instability of columns. Therefore, it is recommended that the axial compression ratio of the column should not be larger than 0.5, and the tier-height ratio should not be less than 0.5.

Abstract:To investigate the feasibility of timber-framed masonry structure reinforced with ultra-high ductile concrete (UHDC) layer and explore the influence of UHDC layer on the seismic performance of timber-framed masonry structure, shaking table tests were performed on 1/2 scaled timber-framed masonry structure model under conditions of original un-strengthened structure and UHDC strengthened structure. The seismic properties of un-strengthened structure and UHDC strengthened structure including dynamic characteristics, failure modes, displacement response, torsional effect, and base shear were compared and discussed. Test results show that the un-strengthened structure could not meet the seismic fortification requirements of the current code under the selected table excitation, and it could only be used after seismic reinforcement. The UHDC layer significantly improved the stiffness and reduced the stiffness degradation of the structure, so as to effectively control the structural damage under large earthquake and improve the seismic performance of the structure. Under rare earthquake of 9 degree, the x-direction inter-story drift of UHDC layer strengthened structure was 3.96‰. The UHDC layer reduced the torsional effect of the structure caused by structural asymmetry. The seismic performance of UHDC layer strengthened structure was better than that of CFRP strengthened structure. The structure strengthened with UHDC layer could meet the requirements of 8 degree seismic fortification intensity as stipulated in GB 50011—2010, which preliminarily proved that UHDC was feasible to repair the earthquake damaged structure and strengthen the existing buildings.

Abstract:Uniaxial compression tests on cementitious grout specimens and ordinary concrete specimens with same strength at different ages (1 d≤t≤28 d) were carried out to investigate the compressive constitutive behavior of early-age cementitious grout. The effects of age on the characteristic parameters of stress-strain curves of cementitious grout specimens and concrete specimens under uniaxial compression were analyzed. The characteristic parameters of cementitious grout specimens were compared with those of concrete specimens. Results show that with the increase in specimen age, the peak stress, peak strain, ultimate strain, elastic modulus, peak secant modulus, strain ductility coefficient, and energy dissipation coefficient of the specimens gradually increased. The peak stress, peak strain, ultimate strain, strain ductility coefficient, and energy dissipation coefficient of cementitious grout specimens were greater than those of concrete specimens, while the elastic modulus and peak secant modulus were smaller. Considering the characteristics of cementitious grout, a modified segment-based compressive stress-strain model was proposed based on the existing compressive stress-strain models, and a statistical damage constitutive model was derived according to the statistical damage theory. The calculated results of the proposed models were in good agreement with the experimental results, which indicates that the proposed models can accurately describe the deformation characteristics of early-age cementitious grout under compression.