Abstract:To improve the accuracy of the extrapolated V_S30 values of boreholes on engineering sites with shallow overburden, this paper selects certain borehole data from KiK-net database for research. The correlation coefficient matrix was adopted to test the collinearity of parameters, and three parameters that can reflect the characteristics of borehole profiles were determined. Through the optimized subset method, the relationship functions between the residuals of V_S30 estimated by bottom constant velocity (BCV) model and the three selected parameters were established. On the basis of three feature selecting methods including adjusted R², Bayesian information criterion, and k-fold cross-validation, the optimal function was proposed. According to the distribution of mean and standard deviation of residuals between predicted and observed V_S30 values, a correction function of BCV model was proposed. Results show that the extrapolation model of V_S30 could improve the prediction accuracy of BCV model, and it was applicable in Xinjiang region. The method proposed in this paper can provide reference for developing empirical models for estimating V_S30 in other regions of China.

Abstract:To provide a basis for probabilistic seismic hazard analysis, this paper investigates the correlation between elastic input energy and other intensity measures of ground motions and the difference between offshore and onshore ground motions. A total of 271 earthquake events were collected from the K-NET of Japan, and 1 784 offshore ground motion records and 4 480 onshore ground motion records were selected. Seven representative intensity measures were determined through correlation analysis. Firstly, the correlation between the selected intensity measures and absolute and relative input energy under different epicentral distances was calculated, and the results were analyzed from the perspectives of period and epicentral distance. Secondly, the correlation between intensity measures and absolute and relative input energy under different damping ratios was analyzed. Finally, the difference in the correlation between offshore and onshore intensity measures and input energy was compared. Results show that the intensity measures that have the best correlation with the input energy of offshore and onshore ground motions are mostly the same in different periods, but there are differences in the extreme values of the correlation coefficient between the intensity measures and the input energy for offshore and onshore ground motions under different epicentral distances. Under different damping ratios, the correlation between the same intensity measures and input energy are different between offshore and onshore ground motions. Through preliminarily analysis, it was found that the difference might be caused by the rich low-frequency components of offshore ground motion. Therefore, the influence of damping ratio and epicenter distance should be considered when studying the correlation between input energy and intensity measures. The results can provide reference for the related research of offshore and onshore probabilistic seismic hazard analysis engineering.

Abstract:For estimating the response spectrum of offshore engineering structures with different damping ratios, the damping modification factor (D_MF) model of offshore engineering response spectrum was studied. Based on 5 680 horizontal offshore ground motion records from S-net network, we analyzed the effects of source parameters such as moment magnitude, fault depth, and earthquake type, as well as source distance and sediment thickness on D_MF, and proposed a D_MF model for horizontal acceleration spectrum from offshore ground motions considering damping ratio and spectral period, which was compared with onshore models. Results show that moment magnitude and source distance had significant impact on D_MF. D_MF with low damping ratios increased with the increase in moment magnitude and source distance, while D_MF with high damping ratios decreased with the increase in moment magnitude and source distance, and D_MF was not sensitive to the influence of fault depth and sediment thickness. Compared with the study of onshore area, the D_MF values of shallow crustal and upper-matle earthquakes in this study were slightly larger under low damping ratios and medium-long periods. When spectral periods were greater than 0.1 s or less than 0.1 s, the influence of damping ratio on D_MF could be simulated by cubic or quadratic logarithmic polynomials, and the influence of spectral period could be expressed by quartic or cubic polynomials. There were significant differences between offshore D_MF model and onshore models. The D_MF model for response spectrum from offshore ground motions proposed in this paper provides reference for the determination of seismic design spectrum of offshore engineering under various damping ratios.

Abstract:To simulate the nonlinearity of isolation layer and isolated structure simultaneously in seismic testing, this paper proposes a two-stage real-time hybrid testing method for isolated structures. In the first stage, the isolation bearing was tested separately, and the neural network was trained by the test data for fitting the dynamic characteristics of the isolation bearing. In the second stage, the trained neural network was used to build the numerical substructure model of isolation layer, which was then combined with the isolated structure to realize real-time hybrid testing (RTHT), so as to complete the dynamic performance test of the overall isolated structures. In numerical simulation, the peak error of the results between the proposed method and overall model simulation was within 3%, while in experimental verification, the peak error of the results between the proposed method and shaking table testing of overall structures was within 6%. Numerical simulation and experimental comparison show that the proposed two-stage real-time hybrid testing has a good accuracy. It avoids the influence of insufficient mass of isolation bearing caused by reduced scale in shaking table testing, and ensures the authenticity of its dynamic performance. Besides, it can solve the errors caused by inaccurate modeling of numerical substructure when using RTHT to test the isolated structures with strong nonlinearity in each part. The two-stage real-time hybrid testing provides a new method for seismic testing of isolated structures.

Abstract:Most existing hybrid test methods for building structures only consider the horizontal earthquake excitation. For further application, a hybrid test method of multi-story reinforced concrete (RC) frames under the combined excitation of horizontal and vertical earthquakes was developed. In this method, the frame structure calculation model and substructure boundary conditions were reasonably simplified. The horizontal and vertical vibrations of frame structure were simulated by series mass system model and series-parallel multi-mass system model respectively. Then, a complete set of hybrid test program was developed based on the proposed method. In the program, Matlab was adopted to form a motion equation solving module, OpenSees was used to simulate the numerical substructure, and loading equipment HNU-MUST was combined to develop a hybrid test control program. For verifying the proposed method and program, a full-scale RC frame column was designed and fabricated as the test substructure, and a seven-story three-bay RC frame was used as the overall structure for hybrid test. The smooth progress of the test and the stable test control effect verified the feasibility and effectiveness of the method. Test results show that the axial force of the RC frame column varied drastically under the combined excitation of horizontal and vertical earthquakes. The variation of axial force made the force and failure morphology of the specimen more complicated, and could reduce the strength and ductility of the specimen, which eventually resulted in an asymmetry and a certain “pinch” effect of hysteresis curve.

Abstract:The influence of vertical ribs on the local cladding pressure of high-rise buildings was investigated. The distribution characteristics of mean wind pressure and fluctuating wind pressure of high-rise buildings with different rib configurations were studied via wind tunnel tests. The variation of power spectra and proper orthogonal decomposition (POD) mode was analyzed. The non-Gaussian characteristics of wind pressure caused by the arrangement of vertical ribs were discussed. Finally, the extreme wind pressure coefficients in different cases were presented. Results show that there were no distinctions between the mean wind pressure, fluctuating wind pressure, and extreme wind pressure on the windward side when ribs were installed. While at the leading edge above the rib boundary on the side face, the mean value, fluctuating value, and extreme value of negative pressure increased, and those in other regions decreased. According to POD, the crosswind aerodynamic force contributed most significantly to the energy of the wind field. The installment of ribs decreased the power spectrum peak of first-order principal coordinate and increased the modal value of the leading edge regions above the rib boundary. The number of taps with non-Gaussian distribution on the side face decreased after the installation of ribs, while there was no evident change on the leeward side. The discrepancies were minor in the local pressure characteristics between half-distributed and full-distributed cases, indicating that ribs in the middle have little influence on the local cladding pressure.

Abstract:The effects of side ratio and incoming flow turbulence characteristics on base moment coefficients of rectangular high-rise buildings were investigated. Synchronization pressure wind tunnel tests for high-rise buildings with side ratios ranging from 1/9 to 9 under four terrain categories were conducted. The wind tunnel test results were compared with the data in previous literature. The effects of side ratio, turbulence intensity, and turbulence integral length scale on mean and standard deviation values of base moment coefficients in along-wind direction and standard deviation values of base moment coefficients in across-wind and torsional directions were analyzed. Results show that when the side ratio was not larger than 3, the mean and standard deviation values of base moment coefficients in along-wind direction first increased and then decreased with the side ratio. When the side ratio was larger than 3, the mean and standard deviation values of base moment coefficients in along-wind direction did not change with the side ratio. The standard deviation values of base moment coefficients in across-wind and torsional directions increased monotonically with increasing side ratio; the growth rate of the values in across-wind direction gradually decreased, while that in torsional direction increased with increasing side ratio. For the base moment coefficient in along-wind direction, increasing the turbulence integral length scale would increase the mean and standard deviation values, while increasing the turbulence intensity would decrease the mean value and increase the standard deviation value. The effects of turbulence integral length scale and turbulence intensity on the standard deviation values of base moment coefficients in across-wind and torsional directions varied with side ratio. On the basis of the analysis results, the fitted formulas for the base moment coefficients in the along-wind, across-wind, and torsional directions of the rectangular high-rise building under different terrain categories were proposed, which can provide reference for structural design and load code revision.

Abstract:To study the seismic reinforcement method of prefabricated concrete pier with grouting sleeve (PCP-GS), this paper takes PCP-GS and PCP with grouting sleeve-prestressed tendon combined connection (PCP-GS-P) as research objects. Two 1/6 scale models of PCP-GS and PCP-GS-P were designed and manufactured. According to the damage characteristics of the two piers, a combined reinforcement method based on carbon fiber reinforced plastic (CFRP) and external metallic dissipater (CFRP-EMD) for the specimens was proposed. The near fault ground motions were selected as input. Shaking table tests for the model piers were carried out under unidirectional and bidirectional excitations. The reinforcement effects of CFRP-EMD on the two specimens were analyzed through test phenomena, basic dynamic characteristics, and seismic response laws. Results show that the main seismic damages of the two specimens were pier body cracking, opening and closing at the joint seam, and spalling or crushing of seam concrete. Compared with PCP-GS, the stiffness and self-reset ability of PCP-GS-P were improved, and the damage velocity and damage degree of the pier body and joint seam were reduced. The CFRP-EMD could effectively increase the stiffness, energy dissipation capacity, and anti-deformation capacity of the damaged fabricated specimens. The plastic deformation of PCP-GS-P strengthened with CFRP-EMD was more reasonable. The proposed design and implementation method can be used as reference for seismic reinforcement of prefabricated concrete piers.

Abstract:To study the seismic performance of L-shaped precast shear walls with grouted sleeve lapping connection, we carried out quasi-static test on a cast-in-place shear wall and two different precast shear walls in which vertical reinforcements were spliced by type I and type II grouted sleeve lapping connectors named APC connectors (all vertical members precasted in concrete structures). The failure modes, hysteretic properties, characteristic loads, deformations, reinforcement strains, and sleeve strains of the specimens were investigated. Results showed that the crack development of the precast wall was basically the same as that of the cast-in-place wall, and the failure mode was bending shear failure. The failure of the cast-in-place wall occurred at the root of the wall, while the weak section of the precast wall moved upwards due to the restraint of the sleeve. The concrete above the sleeve of the precast wall was crushed and the rebars were buckled at the ultimate failure state. The cracking load, yield load, peak load, stiffness, ductility, and energy dissipation capacity of the precast wall connected by type I connectors were comparable to those of the cast-in-place wall when the bottom stirrups and horizontally distributed reinforcements of cast-in-place wall and precast walls were densified, while the above indicators of the precast wall connected by type II connectors were superior to the cast-in-place wall. The two types of connectors could effectively transmit the stress of reinforcements in the precast wall, and the sleeves were basically in an elastic state during the loading process. Due to the high stiffness of the APC connectors, the absolute value of the maximum out-of-plane displacement of the precast wall was less than that of the cast-in-place wall. However, accumulated damage deformation in out-of-plane torsion appeared in the precast specimens due to the influence of horizontal grouting joint.

Abstract:To evaluate the seismic performance of large rupture strain fiber reinforced polymer (LRS FRP)-reinforced bridges under earthquake action, this paper takes a typical three-span continuous girder bridge as the analytical model. There are few studies on LRS FRP-confined concrete models under cyclic axial compression, which limits the seismic analysis of LRS FRP-reinforced bridges. Therefore, the simplified LRS FRP-confined concrete model under cyclic axial compression proposed by the authors’ research group was introduced. According to the simplified model, the seismic fragility of the continuous girder bridge before and after LRS FRP strengthening was analyzed based on the seismic fragility analysis theory and incremental dynamic analysis (IDA) method using OpenSees platform. Results show that the bridge columns were mainly in an undamaged linear elastic state under slight damage and moderate damage, and LRS FRP reinforcement had little effect on the seismic fragility. In extensive damage and complete collapse states, the exceedance probability of damage of unreinforced bridge was significantly greater than that of LRS FRP-reinforced bridge. The proposed simplified LRS FRP-confined concrete model can provide reference for seismic analysis of LRS FRP-reinforced bridges.

Abstract:Considering that conventional shear connectors cannot be applied to steel-ultra-high performance concrete (UHPC) composite deck systems as the UHPC layer is too thin, a novel shear connector consisting of specified designed pre-embedded sleeve and high-strength bolt was proposed, which can realize the rapid assembly and disassembly of prefabricated UHPC and steel member. Push-out tests on six groups of specimens were conducted, including five sets of high-strength bolt connectors and a set of negative Poisson’s ratio bolt connector. The failure pattern and load-slip curve characteristics of the novel shear connectors were analyzed. The effects of different parameters (diameter, aspect ratio, and type) of bolts on the mechanical properties (ultimate slip and shear stiffness) of the novel shear connector were discussed. Results show that the failure mode of the novel shear connectors was the shear-off failure of the bolt shank, and there was no apparent local collapse of UHPC under the pre-embedded sleeves. The shear capacity and the relative slip of the high-strength bolt connector increased with the increase in the diameter of the high-strength bolt. The shear capacity of the novel high-strength bolt connector was approximately 55.8% of the tensile strength of the bolt. Thus, bolt connectors with larger diameters are recommended in steel-UHPC composite structures to effectively reduce the number of shear connectors. The shear capacity and shear stiffness of the negative Poisson’s ratio bolt connector were significantly smaller, but the ultimate slip was significantly increased, showing good ductility. Therefore, it is suggested to apply negative Poisson’s ratio bolt to the negative moment section of steel-UHPC composite structure, so as to avoid cracking in the negative moment section.

Abstract:Long-term tests under axial compression were carried out on 16 steel tube-confined concrete-filled steel tube columns and two concrete-filled steel tube columns for 350 d to investigate the long-term deformation performance. The main research parameters were stress ratio of the core concrete (0.5,0.50, and 0.65), total steel ratio of the inner and outer steel tubes (5.8%, 10.5%, and 15.0%), and the steel ratio between the inner and outer steel tubes (0.0,1.25, and 2.23). The working mechanism of the steel tube-confined concrete-filled steel tube columns under long-term axial compression was analyzed, and a theoretical calculation method for long-term deformation of steel tube-confined concrete-filled steel tube columns was established. Results show that the long-term deformation increased with the increase in the stress ratio of the core concrete, and decreased when the total steel ratio and the ratio between the inner and outer steel tubes increased. The comparison between the theoretical and test results shows that the long-term deformation increased with the decrease in relative humidity. When different concrete shrinkage and creep models were used, the influence of relative humidity varied, and the CEB-FIP 2010 model had the best performance. Therefore, the CEB-FIP 2010 model with 100% relative humidity is recommended for the calculation of steel tube-confined concrete-filled steel tube columns under long-term deformation.

Abstract:The critical loads for flexural buckling of axially loaded fiber reinforced polymer (FRP) members were investigated. A test database containing data of axial compression tests of 176 FRP members that failed in the flexural buckling mode was established. Five existing models proposed by Euler, Engesser, Haringx, Strongwell company, and Fiberline company for predicting flexural buckling loads were summarized and evaluated. An equation to determine the stability factor for global buckling of FRP members under axial compression was developed based on the observed initial crookedness of FRP members. Then, a theoretical model of flexural buckling loads of FRP members was proposed considering the influence of initial crookedness of FRP members. On the basis of the proposed database, an empirical model of flexural buckling loads of FRP members was derived through regression analysis. The two models were verified by the test database and numerical simulations. Finally, the performance of the two proposed models was analyzed. Results show that by using the proposed test database (section width of members is between 25.4 mm and 254 mm, section height is between 25.4 mm and 254 mm, and effective length is between 203 mm and 6 300 mm), the error of the model proposed by Strongwell company was the highest, and models proposed by Euler, Engesser, and Haringx overestimated the flexural buckling loads of FRP members, while that proposed by Fiberline company underestimated the flexural buckling load. The proposed theoretical model and empirical model both obtained more accurate results than the comparison models in predicting axially loaded FRP members within the range of section dimension and effective length covered by the test database. The proposed models were applicable to axially loaded FRP members with I-shaped, L-shaped, square, and circular sections, and could accurately predict the flexural buckling loads of axially compressed FRP members within the above parameter range.

Abstract:To investigate the tensile properties of high strength steel T-stubs at high temperature, we carried out tensile tests on 11 T-stubs at ambient temperature and 17 T-stubs at high temperature, including T-stubs made of Q355, Q460, Q690, S690, and S960 with different strengths under various high temperature conditions, i.e. 0,0, 500, and 600 ℃. The initial tensile stiffness, tensile capacity, and failure modes of T-stubs under different test conditions were obtained. The test results were compared with the theoretical calculation results of European and Chinese codes to verify the applicability of the codes for high strength steel T-stubs. Moreover, the effects of flange strength, flange thickness, bolt strength, bolt diameter, bolt position, and high temperature on the tensile properties of T-stubs were analyzed. Results showed that the European code and Chinese code overestimated the initial tensile stiffness of T-stubs at high temperature, and were conservative in terms of tensile load-bearing capacity. Compared with ordinary steel T-stubs, the high strength steel T-stubs with thin-walled flange had better deformation capacity while maintaining the same load-bearing capacity. When the ratio of the distance between bolt and web to the distance between bolt and flange was less than 1, increasing the distance between the bolt and the web could significantly reduce the initial tensile stiffness and tensile load-bearing capacity of the T-stubs. The increase in the bolt diameter improved the initial tensile stiffness of T-subs, while the bolt strength had little influence on the initial tensile stiffness. The research results can provide experimental basis for the fire-resistance design of high strength steel end-plate connections and their equivalent T-stubs, which are reference for the revision of relevant design codes.

Abstract:Considering the cooperative working performance of negative Poisson’s ratio rebar (NPR rebar) and ultra high performance concrete (UHPC), central pull-out tests on 27 pull-out specimens were conducted, and the effects of UHPC type, rebar shape, embedded depth, and protective layer thickness on the bonding performance between NPR rebar and UHPC were studied. The damage and failure modes of bond slip failure between NPR rebar and high-strain strengthened UHPC were analyzed by acoustic emission technology. Results show that the bond performance between high-strain strengthened UHPC and NPR rebar was better than that between low-strain strengthened UHPC and strain softened UHPC. With the increase in embedded depth, the ultimate load of the pull-out test gradually increased, and the trend of load-slip curves did not change obviously. When the thickness of protective layer was small, the failure of the specimen was brittle. The bond slip failure between NPR rebar and high-strain strengthened UHPC could be divided into initial, failure, and residual stages. Acoustic emission test could better characterize the evolution process of internal damage in the matrix during the failure process.

Abstract:To study the mechanical behaviors and failure modes of the push-on joints of ductile iron water pipelines under both tension and bending, this paper conducts a series of quasi-static tests on push-on joints to assess their ultimate bending capacity. Four-point bending tests were performed on the push-on joints of DN150 and DN200 pipelines with internal water pressure of 0.2 MPa and without soil coverage. Different loading protocols including coupled tensile and bending tests and monotonous and cyclic loading tests were adopted in the experimental program to investigate the mechanical behaviors and failure modes of the push-on joints. Results show that for push-on joints with different initial openings, the “bending moment-angle” curves of pipeline joints could be roughly divided into three stages: linear growth stage, plastic deformation stage, and the stage corresponding to ultimate load-carrying capacity. When the ratio of the assembling depth to the socket depth was about 0.67, the maximum angles of DN150 and DN200 pipeline joints were 15.9° and 12.8°, and the corresponding bending capacities were 7.8 kN·m and 9.2 kN·m respectively. The pipeline joints with larger assembling depths exhibited higher effective bending stiffness under cyclic loading, but the stiffness degraded significantly under reloading cycles. Buckling deformation occurred at the pipeline joints when the joint reached its ultimate load-carrying capacity, with the maximum residual strain of the pipe bottom being 5.7×10^-4 and the ellipticity of joints being 4.2%. The relevant results can provide reference for the research of seismic damage, seismic checking calculation, design, and measures of buried ductile iron pipelines.