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工程力学

Engineering Mechanics

Since 1984  Monthly

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Chief Editor: Xinzheng LU

Editor & Publisher: 《工程力学》杂志社

ISSN 1000-4750CN 11-2595/O3

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Articles online first have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
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2020 No. 11, Publish Date: 2020-11-25
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2020, 37(11).  
[Abstract](88) [PDF 12573KB](72)
Abstract:
METHODOLOGIES
STABILITY ANALYSIS OF EXPLICIT ALGORITHMS WITH VISCO-ELASTIC ARTIFICIAL BOUNDARY ELEMENTS
Shu-tao LI, Jing-bo LIU, Xin BAO, Xi-gui TAO, Yi-cun CHEN, Lan XIAO, Yi-fan JIA
2020, 37(11): 1-11, 46.   doi: 10.6052/j.issn.1000-4750.2019.12.0755
[Abstract](93) [FullText HTML](25) [PDF 1329KB](52)
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In the analysis of soil-structure seismic response or of near-field seismic wave propagations, the visco-elastic artificial boundary elements are often applied to transfer the infinite-domain problem into a finite-domain problem. Since the material parameters and the size of the visco-elastic artificial boundary elements are different from those of the internal medium elements, there are differences in the numerical stability conditions between the artificial boundary domain and the internal domain when the explicit time domain step-by-step integration algorithm is used. At present, however, there are no appropriate analysis methods and research results to determine the explicit numerical stability conditions and the stable integration time steps. In this study, we propose a stability analysis method for explicit time-domain stepwise integration algorithm when using the two-dimensional visco-elastic artificial boundary elements. Firstly, several typical local subsystems of the artificial boundaries are established. The transfer matrix of each subsystem is analyzed, and the analytical solutions of the stability conditions for each subsystem are obtained. By comparing the stability conditions of the subsystems and the internal medium system, a uniform stability condition of explicit time domain stepwise integration algorithm is obtained when using the visco-elastic artificial boundary elements. When the internal medium areas also satisfy this stability condition, this condition becomes a sufficient condition for the stability calculation of the overall system and can be used to deciding the stable discrete time step in the numerical analysis.
ANALYTICAL SOLUTIONS FOR FORCED VIBRATIONS OF TIMOSHENKO CURVED BEAM BY MEANS OF GREEN’S FUNCTIONS
Xiang ZHAO, Yang ZHOU, Yong-bo SHAO, Bo LIU, Ren ZHOU
2020, 37(11): 12-27.   doi: 10.6052/j.issn.1000-4750.2019.11.0708
[Abstract](180) [FullText HTML](45) [PDF 1287KB](53)
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This paper derives analytical solutions for the forced vibrations of Timoshenko curved beams and establishes the vibration equation of Timoshenko curved beams by analyzing the equilibrium equation for the intersection of curved beams. Green’s functions of Timoshenko curved beams are solved for different boundary conditions using the separation of variables and Laplace transform. Two characteristic parameters are introduced to measure damping effects on beam vibrations. Numerical calculations are conducted to validate analytical solutions, and the effects of various related physical parameters are investigated. The results show that by setting the radius R to infinity, it can be simplified to the Timoshenko straight beam vibration model, and on this basis, if the shear correction factor κ is set to infinity, it can be reduced to the Prescott straight beam vibration model. Finally, the moment of inertia γ is set to 0, which can be reduced to the Bernoulli-Euler straight beam vibration model. Numerical calculations are performed to validate the solutions.
A GEOMETRIC NONLINEAR PLANE BEAM ELEMENT BASED ON COROTATIONAL FORMULATION AND ON STABILITY FUNCTIONS
Ji-hua DENG, Jian-ping TAN, Ping TAN, Zhong-chu TIAN
2020, 37(11): 28-35.   doi: 10.6052/j.issn.1000-4750.2020.01.0012
[Abstract](59) [FullText HTML](24) [PDF 860KB](18)
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The establishment of an accurate and efficient geometric nonlinear beam element is significantly important for describing the nonlinear behavior of frame structures. This paper presents a geometric nonlinear plane beam element based on a co-rotational procedure and a stability function. The deformation is separated from the rigid body displacement during the formation of the element, and the stability function is used in the local coordinate system to consider the influence of the element P-δ effect. The co-rotational procedure method and the differential method are used to consider the geometric nonlinearity of the displacement transformation from the local coordinate system to the global one. The total equilibrium equation and tangent stiffness matrix of geometric nonlinear plane beam elements are developed in a global coordinate system. The expression of the element tangent stiffness matrix considering beam ends with hinges is derived according to the characteristics of zero bending moment at the end of the hinged beam. The accuracy and efficiency of the analytical method are verified by several typical examples.
CIVIL ENGINEERING
EXPERIMENTAL STUDY ON AND ANALYTICAL SOLUTION OF THE ELASTIC BENDING BEHAVIOR OF STEEL-UHPC COMPOSITE BOX GIRDERS
Jian-sheng FAN, Zhe WANG, Song YANG, Fan CHEN, Ran DING
2020, 37(11): 36-46.   doi: 10.6052/j.issn.1000-4750.2019.11.0700
[Abstract](114) [FullText HTML](21) [PDF 1024KB](45)
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The steel-ultrahigh performance concrete (UHPC) composite box deck system has a good application prospect in the construction of long-span bridges. Based on an actual bridge, two large-scale steel-UHPC composite box girder tests were conducted mainly to study the overall bending performance of the steel-UHPC composite box girders in the elastic stage. We focused on the deflection and strain distribution of the specimens. An analytical model of the composite box girder considering the shear lag, slip effect and shear deformation of the steel web was established and the analytical solution was derived. The results show that the shear lag effect was significant under concentrated loads. The ratio of the maximum to minimum values of the UHPC slab mid-span compressive strain was 5~7. With the increase in the UHPC slab depth, the stiffness of the steel-UHPC composite box girder increased and the strains on both the UHPC slab and steel girder decreased. Compared with the model without considering the shear deformation of the steel web, the analytical solution in this paper was generally in good agreement with the test results.
EXPERIMENTAL INVESTIGATION ON THE SEISMIC BEHAVIOR OF RC BEAMS STRENGTHENED WITH HIGH DUCTILITY CONCRETE
Ming-ke DENG, Li-ying GUO, Rui-zhe LI, Jia-li CHEN
2020, 37(11): 47-57.   doi: 10.6052/j.issn.1000-4750.2019.04.0237
[Abstract](247) [FullText HTML](38) [PDF 1004KB](26)
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To investigate the seismic behavior of reinforced concrete (RC) beams with high ductility concrete (HDC), eight RC beams were designed and strengthened with HDC jackets and carbon fiber strips. The failure modes, deformation capacity and energy dissipation capacity of the specimens were studied under low cyclic loading tests. The experimental results show that: The tensile strain hardening and multiple cracking of HDC can effectively control the development of shear cracks of the beams strengthened with HDC during the failure process and the HDC-strengthening system is effective to improve the brittle failure characteristics; HDC can provide obvious constraint for the core concrete. The shear strength, ductility and energy dissipation capacity of the strengthened beams are significantly improved. The HDC jacket could be more effective than the carbon fiber strips in enhancing the behavior of the beams; When the shear span is relatively large, using mesh reinforcements in the HDC jacket improves the ductility and energy dissipation capacity. However, it contributes little to the shear capacity. A calculation method for the shear capacity of strengthened beams is proposed based on a truss-arch model. The calculated results have good agreement with the test results.
EFFECTS OF WIND-WAVE LOADINGS ON THE SEISMIC RESPONSE OF OFFSHORE WIND TURBINES
Ren-qiang XI, Cheng-shun XU, Xiu-li DU, Kun XU
2020, 37(11): 58-68.   doi: 10.6052/j.issn.1000-4750.2019.12.0715
[Abstract](228) [FullText HTML](37) [PDF 2713KB](29)
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Considering the operational and parked states, a generalized single degree of freedom model was established to explore the influence mechanism of wind-wave loadings on the seismic response of offshore wind turbines (OWTs). The statistical relationship between the wind and wave was employed to determine the parameters of the power spectra of the wave. Subsequently, the seismic response of the NREL 5MW OWT was analyzed using FAST to validate the conclusions of the preliminary analysis and evaluate the effects of wind-wave loadings on the seismic response of OWTs. The results reveal that the aerodynamic damping and dynamic loading effects of the wind and wave had a significant influence on the seismic response of OWTs. For wind turbines in the operational state, the support structure is the most dangerous when the mean wind speed of hub-height equals to the rated speed. In addition, the wind-wave loadings reduce the internal force of the mudline and tower-top displacement under the excitation of strong earthquakes. However, they increase the internal forces and displacement amplitudes at these locations while the OWT is excited by a weak earthquake.
COMPLEX MODE SUPERPOSITION METHOD OF HYBRID STRUCTURE SEISMIC RESPONSES BASED ON DYNAMIC CHARACTERISTICS
Pan-xu SUN, Hong YANG, Qing-lin LIU
2020, 37(11): 69-82.   doi: 10.6052/j.issn.1000-4750.2019.12.0723
[Abstract](73) [FullText HTML](9) [PDF 2607KB](23)
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The damping matrix of a hybrid structure of different material damping characteristics is difficult to determine. By the aid of mathematical simplicity, Rayleigh damping model is widely used in the construction of damping matrix of a hybrid structure. But the calculation precision of Rayleigh damping model is affected by the choice of reference frequencies. Thusly, a calculation method of Rayleigh damping coefficients is proposed based on the characteristics of structural dynamic responses. The corresponding complex mode superposition method is also realized. The damping coefficients are determined based on the natural frequencies of the first two modes when the structural transient responses are calculated. The damping coefficients are determined based on fundamental frequency and certain frequency when the structural steady state responses are calculated. The certain frequency is the natural frequency close to external excitation frequency. According to the spectrum characteristics of seismic wave, a Rayleigh damping model is obtained based on the predominant frequency of seismic wave. Combined with the linear assumption of earthquake acceleration, a complex mode superposition method of hybrid structure is realized. The harmonic frequencies of seismic wave could be obtained by trigonometric series. Another Rayleigh damping model is developed based on the harmonic frequencies, and the corresponding complex mode superposition method is proposed. The cases show that the proposed method could avoid the arbitrariness of selective modes based on traditional methods. The calculation error of the proposed method is small. Compared with the complex mode superposition method based on the predominant frequency of seismic wave, the computational amount of complex mode superposition method based on the harmonic frequencies is larger, but with higher calculation accuracy and wider range of application.
STUDY ON THE MECHANISM OF TORSION INDUCED BY STEEL BRACE DYNAMIC BUCKLING AND THE MECHANISM OF TORSION REDUCTION SUPPLIED BY BRB
Xiang-yu GAO, Yang-long LI, Jian-qin LI, Ji-min XU
2020, 37(11): 83-96, 107.   doi: 10.6052/j.issn.1000-4750.2019.12.0748
[Abstract](97) [FullText HTML](36) [PDF 4835KB](26)
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The characteristics of the lateral and torsional seismic responses of steel braced concrete frame structures are studied. It is revealed that the dynamic buckling of steel brace will have an impact on the resistance torque during the repeated loading process, and the plastic hinge state of the structure will change imbalanced, resulting in inelastic torsion. By using D'alembert principle, the mechanism of the steel braced structure causing inertial force, inertial torque and inelastic torsional surge induced by the brace buckling is studied, and the similar performance is verified by shaking table test and finite element nonlinear analysis. The anti-torsion effect of buckling-restrained braces on structures and the working mechanism of preventing inelastic torsional burst of structures are studied. Finally, suggestions are proposed to use the ordinary steel braces as the first seismic defense line of CBF concrete frame structures, and the technical measures of using buckling-restrained braces to reduce of inelastic torsion are given.
EXPERIMENTAL STUDY ON THE SEISMIC BEHAVIOR OF RC BEAMS OF LOW SHEAR-SPAN RATIOS SUBJECTED TO FREEZE-THAW CYCLES
Shan-suo ZHENG, Jin-ming JI, pei PEI, Jin-chuan HE, Yi-xin ZHANG, Li-guo DONG
2020, 37(11): 97-107.   doi: 10.6052/j.issn.1000-4750.2019.12.0770
[Abstract](41) [FullText HTML](8) [PDF 1611KB](9)
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An artificial climate simulation laboratory was used to perform an accelerated freeze-thaw cycle test on six reinforced concrete (RC) beam specimens with a shear-span ratio of 2.6, which were then subjected to quasi-static loading. According to the test results, the effects of the freeze-thaw cycles and concrete strength grades on the seismic performance indicators of RC beam specimens such as the failure mode, hysteretic curve, strength, deformation capacity and energy dissipation were analyzed. The results show that after the freeze-thaw cycles, the compressive strength of the concrete decreased, the internal porosity of the concrete became larger, the micro-cracks were increased, and cracks appeared on the surface of the beam specimens. Six beam specimens exhibited a flexure-shear failure mode after the quasi-static test. With the increase in the number of freeze-thaw cycles, the strength and energy dissipation capacity degraded, and ductility first increased slightly and then decreased greatly. With the increase in the concrete strength grade, the degree of damage caused by the freeze-thaw cycle of the beam specimens was reduced, the yield, peak and ultimate strengths of specimens were increased, the energy dissipation capacity was enhanced, and the ductility was not significantly changed.
A STRESS-INDUCED FRACTIONAL DILATANCY RULE FOR CLAYS
Hai-chao LI, Bo MA, Sheng ZHANG, Dai-chao SHENG
2020, 37(11): 108-116.   doi: 10.6052/j.issn.1000-4750.2019.12.0777
[Abstract](29) [FullText HTML](5) [PDF 1334KB](23)
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A fractional sub-loading surface model for clays is developed in the present study. The fractional plastic flow rule adopted in the proposed model is able to account for the non-normality of the flow direction with respect to the yield locus without introducing a plastic potential. Hence, a unified description of the associated and non-associated plastic flow rules is achieved. A stress-induced fractional dilatancy rule can be conveniently derived through the fractional plastic flow rule to consider the effect of the over-consolidation ratio on the dilatancy of clays. The analysis shows that increasing the over-consolidation ratio will reduce the dilatancy under a constant loading pressure. Compared with the modified Cam-clay model, the proposed model introduces only one extra dilatancy-related parameter and can describe the strain-softening and dilatancy features of over-consolidated clays. Model predictions show good agreement with the experimental results, indicating the capability of the proposed model in describing the behavior of clays.
STUDIES ON THE BOND PERFORMANCE OF FRP-TO-CONCRETE INTERFACES UNDER ENVIRONMENTAL TEMPERATURE DIFFERENCE
Kun DONG, Jian-wen HAO, Peng LI, Hai-yan GUO, Shu-tong YANG
2020, 37(11): 117-126.   doi: 10.6052/j.issn.1000-4750.2019.12.0783
[Abstract](81) [FullText HTML](12) [PDF 1336KB](36)
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To reveal the effect of environmental temperature difference on the interfacial bond behavior of reinforced concrete members strengthened with external bonded fiber reinforcement polymer (FRP) composites, an analytical method for the bond behavior of FRP-to-concrete joints was presented. Based on the cohesive zone model (CZM), a second order differential equilibrium equation was derived. The analytical models of interface slip, bond stress and FRP stress and strain were given by the superposition solution of the boundary condition. Based on the presented theoretical models, the calculation method of the maximum temperature difference that the FRP-to-concrete interface could bear was presented. The effects of the bond length, temperature difference and the number of FRP layers on the interfacial bond behavior was investigated. The results show that the presented theoretical models were in good agreement with the test results. The theoretical models could predict well the bond behavior of FRP-to-concrete interfaces under the temperature difference. When the bond length increased, the maximum temperature difference was increased until reaching an upper limit. With the increase in the environmental temperature, the maximum FRP stress occurred before the maximum temperature difference was reached. The interfacial shear stress was concentrated in the end of the bond interface, which was significantly affected by the temperature difference and the number of FRP bond layers. When the environmental temperature entered the glass transition area of the adhesive, the interfacial shear stress was greatly changed. These results can be helpful to calculate the temperature stress and to assess the interfacial capacity for the strengthened members in bridge or building structures under environmental temperature differences, such as strong sunshine or high temperature environments.
THE EFFECT OF SURROUNDING EMPTY HOLES ON THE BLASTING VELOCITY AND DAMAGED ZONES
Ke MAN, Xiao-li LIU
2020, 37(11): 127-134, 184.   doi: 10.6052/j.issn.1000-4750.2019.12.0790
[Abstract](18) [FullText HTML](5) [PDF 750KB](9)
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The blasting parameters of the Beishan Exploration Tunnel were designed based on the smooth blasting method. The smooth blasting which was influenced by the empty holes was studied in detail. From the blasting theory and the stress intensification principle, the distance of the surrounding empty holes was calculated to be 30 cm. Blasting was conducted under surrounding holes with empty holes to verify the calculated value. According to the hole utilization and the half hole ratio after blasting, the empty hole setting was considered to be more suitable. Moreover, the blasting vibration monitoring and the excavation damaged zone (EDZ) detection were separately approved. From the blasting vibration monitoring, the maximum blasting velocity of the empty hole setting cycle (12 cm/s) was much lower than that of the usual cycle (20 cm/s). From the EDZ detection, the damaged area of the empty hole setting side (22 cm) was also lower than that of the other side (27 cm). The blasting method includes many blasting parameters that interact with each other. The above blasting parameters were obtained for the empty hole effect of the surrounding hole. The results and the theoretical knowledge could be applied to the blasting and excavation of the deep geo-engineering and the HLW geo-disposal.
NUMERICAL SIMULATION ON SELF-CENTERING BEAM-COLUMN JOINTS REINFORCED WITH SUPERELASTIC SMA BARS
Hui QIAN, Zong-ao LI, Jin-zhao PEI, Li-ping KANG
2020, 37(11): 135-145.   doi: 10.6052/j.issn.1000-4750.2019.12.0791
[Abstract](72) [FullText HTML](18) [PDF 3238KB](34)
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In order to improve the energy dissipation capacity and self-centering capacity of the frame structure, an earthquake resilient beam-column joint based on super-elastic SMA bar was proposed. The numerical model of self-centering SMA reinforced concrete beam-column joints was established based on OpenSees finite element software platform, by using SMA material self-centering double-flag constitutive model, and nonlinear simulation under low-cycle reciprocating load was carried out to obtain the hysteretic curves and skeleton curves of the joints. The validity of the numerical model was verified by comparing the analysis data with the experimental results. The parameter analysis was carried out, considering the amount, length and yielding strength of SMA material, while the influences of SMA material parameters on the hysteretic performance and self-centering ability of the joints were analyzed. The results show that the concrete beam-column joints reinforced with super-elastic SMA bars have high energy dissipation and self-centering capacity. The numerical analysis model can well simulate the hysteretic behavior of self-centering SMA joints under low cyclic reciprocating loads. The mechanical parameters of SMA bars have a great influence on the seismic performance of joints. Under the condition of proper reinforcement, a larger SMA bars area leads to smaller residual displacement and stronger the self-centering capacity. Under the same condition, when the SMA bar exceeds the plastic hinge length, the length has little influence on the joint performance. Under the condition of proper reinforcement, increasing the yield strength of SMA can improve the bearing and self-centering capacity of the joint.
AEROSTATIC DIVERGENCE PROPERTIES OF SUSPENSION BRIDGES WITH LOW TORSIONAL STIFFNESS AND ANTI-WIND COUNTERMEASURES
Li-yang LIU, Zhi-tian ZHANG
2020, 37(11): 146-155.   doi: 10.6052/j.issn.1000-4750.2020.01.0002
[Abstract](43) [FullText HTML](12) [PDF 1866KB](6)
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Flexible suspension bridges are susceptible to wind-induced instabilities. The aerostatic stability properties of highway suspension bridges have been extensively investigated. It has been found that the torsional divergence ends up with the abrupt deck rotation of large amplitudes distributed in a symmetric manner along the bridge axis, and what accompanying to the instability is the complete unload of one of the two main cables to a stress state of the cable finish stage. In this work, however, it is found that the pattern of torsional divergence of small- to moderate-span bridges with very low torsional stiffness girders is significantly different from that of a traditional highway bridge due to the relative torsional stiffness of the bridge deck to that provided by the main cable system. In this case, the torsional divergence substantially differs from the traditional one and exhibits a twist-locking phenomenon. It ends up with asymmetric torsional deformation along the bridge axis with one or more inflection points, accompanied by the abrupt tightening up of the main cable system. Countermeasures are numerically investigated. The results show that improving the torsional stiffness of the main girder not only substantially enhances the threshold of divergence, but also changes the pattern of divergence from an asymmetric, twist-locking rotation to a symmetric rotation. Parametric investigations of wind-resistant cables are performed in terms of the cable section area, cable tension, installation angle and sag ratio. It is found that the best installation angle is between 15 to 45 degrees, and that the mitigation effect increases with the cable section area, cable tension and the sag ratio. The combination of the mitigation effect and material costs suggests that the best scheme is the one of wind-resistant cables to be of one third of the main cable’s area, of the amount of tension produced by a quarter of the dead load, and of a sag-ratio as large as allowed by the topography.
STUDY ON THE MECHANICAL PROPERTIES OF COLD-FORMED STEEL FRAMED SHEAR WALL WITH SLITS
Lu YE, Yu-hang WANG, Yu SHI, Wei LUO, Wei-bo KONG
2020, 37(11): 156-166.   doi: 10.6052/j.issn.1000-4750.2020.01.0005
[Abstract](49) [FullText HTML](13) [PDF 1759KB](15)
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In order to study the mechanical behavior of the cold-formed steel framed shear wall with slits (CFS-WS) which is suitable for low-rise and multi-story cold-formed thin-walled steel buildings, pseudo-static tests of 1 ordinary CFS-WS and 3 buckling-restrained CFS-WS were conducted to gain the mechanical properties of CFS-WS, including the failure modes, load-displacement hysteresis curves, skeleton curves and energy dissipation capacities, and the design value of shear capacity was put forward. The test results indicates that the CFS-WS relies on the "torsion-recovery-reverse torsion" of steel plate between vertical slits and the deformation of the steel frame to resist horizontal loads and dissipate energy. The steel plate tears and the end of the hat column buckles when the CFS-WS is broken. The CFS-WS has good bearing capacity, plasticity, ductility and energy dissipation capacity; however, the rheostriction of its load-displacement hysteresis curve is rather severe. Compared with ordinary CFS-WS, the buckling-restrained CFS-WS has higher shear stiffness, bearing capacity and energy dissipation capacity, and the rheostriction of its load-displacement hysteresis curve is mitigated. Furthermore, the stiffeners and the cold-formed steel beams and columns are connected to form a steel frame through the stiffener connectors, which can effectively enhance the early shear stiffness, bearing capacity and energy dissipation capacity of CFS-WS, and greatly improve the seismic performance of the structure.
EXPERIMENTAL AND NUMERICAL STUDY ON THE NONLINEAR MECHANICAL PROPERTIES OF OBLIQUE 3D ISOLATION BEARINGS
Li-li ZHANG, Wen-guang LIU, Hao XU, Jie YANG, Wen-fu HE
2020, 37(11): 167-175.   doi: 10.6052/j.issn.1000-4750.2020.01.0006
[Abstract](84) [FullText HTML](20) [PDF 1375KB](13)
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An oblique three-dimensional (3D) isolation bearing was introduced based on the properties of lead rubber bearings (LRB) to meet the vertical strength and damping demands. A mechanical model considering the influence of the pressure and nonlinear stiffness degradation of LRB was established. The horizontal and vertical performance test was carried out on the 3D isolation bearings with inclination angles of 12 degrees and 15 degrees. The hysteretic behavior had non-parallel characteristics during the loading and unloading process. The vertical stiffness showed a nonlinear variation with the deformation. The variation of the vertical performance with the inclination angle and the vertical displacement was obtained. The proposed nonlinear theory and experimental results matched well. A numerical simulation of the 3D isolation bearings with different inclination angles was accomplished using the finite element method. The variations of the vertical nonlinear stiffness with the friction coefficient, shear strain, and inclination angle were discussed based on the simulation results.
RESEARCH ON THE SIMILARITY THEORY OF TEMPERATURE FIELD OF LIGHT STEEL MEMBERS (STRUCTURES) UNDER FIRE
Cang-hu QIU, ZHANG Geng-yuan, Bai-yu LI, Yu TONG, Qing LIU, Guang-yong WANG
2020, 37(11): 176-184.   doi: 10.6052/j.issn.1000-4750.2020.01.0009
[Abstract](61) [FullText HTML](16) [PDF 1948KB](19)
Abstract:
Based on heat transfer theory, the similarity theory of temperature field focusing on light steel members (structures) under fire is researched, and the simplified furnace temperature curves are proposed and proved by numerical and experimental methods. Meanwhile, the factors that influence the similarity theory of temperature field focusing on light steel members (structures) with or without non-bulgy fire retardant coating are compared. The results show that the temperature of experimental models under the simplified furnace temperature curves with time similarity constant "1" s basically the same as that of prototype structures under the standard ISO834 temperature curve. This implies that the similarity theory of temperature field focusing on light steel members (structures) under fire is valid. Furnace temperature curves are negatively correlated with steel density and specific heat, and positively correlated with specific surface area and size similarity constant. For light steel members (structures) with non-bulgy fire retardant coating, they are also negatively correlated with coating thermal resistance, and positively correlated with the coating thickness similarity constant.
STUDY ON THE INTERACTION MECHANISM BETWEEN SURROUNDING ROCK AND LINER WITH HIGHLY DEFORMABLE ELEMENTS IN SQUEEZING TUNNELS
Kui WU, Zhu-shan SHAO, Su QIN
2020, 37(11): 185-194.   doi: 10.6052/j.issn.1000-4750.2020.01.0013
[Abstract](52) [FullText HTML](12) [PDF 1043KB](22)
Abstract:
As a type of circumferential yielding supports, liner embedded with highly deformable elements can achieve the release of rock deformations through the compressible deformation of highly deformable elements. The paper aims to investigate the interaction mechanism between surrounding rock and liner with highly deformable elements. In this paper, the improved fractional Burgers model is established to describe the time-dependent behavior of rock, and then the analytical solutions for tunnel displacement and support pressure are derived taking into account the tunnel face advancement effect and the installation delay of support. In addition, the reliability and effectiveness of analytical solutions are well validated by comparing the monitoring data in Lyon-Torino Base tunnel. Furthermore, based on the analytical solutions proposed in this paper, a comprehensive parametric investigation including the influences of fractional order of rock constitutive model, supporting time and yield stress of highly deformable elements is carried out. The main conclusions are made as follows: The deforming ability of surrounding rock becomes stronger as fractional order of Burgers model increases, and the tunnel displacement and support pressure also exhibit an increasing trend; Supporting time has a great influence on tunnel performance. On the one hand, in order to keep rock stability it is recommended to install support structures as early as possible. On the other hand, the optimal number and length of highly deformable elements should be determined for ensuring that the support pressure varies within a range of bearing capacity of liner; The yield stress of highly deformable elements does not pose a significant effect on tunnel displacement and support pressure. However, this does not mean the yield stress of highly deformable elements can be determined randomly. There should be an appropriate range for this yield stress, where the purposes for both the tunnel stability and the release of rock deformation can be achieved.
STUDY ON AXIAL COMPRESSION PERFORMANCE OF CONCRETE STUB COLUMN STRENGTHENED BY PRESTRESSED STEEL HOOP
Jian XIE, Li YANG, Fu-quan XU
2020, 37(11): 195-208.   doi: 10.6052/j.issn.1000-4750.2020.01.0020
[Abstract](142) [FullText HTML](28) [PDF 1230KB](23)
Abstract:
Both thirty-three concrete stub columns reinforced by prestressed steel hoops (PSHC) and nine unreinforced columns were tested under axial compression. The studied parameters were the hoop ratio (the spacing of hoops and the concrete diameter), the prestressing level and the initial axial compression ratio. The influences of these parameters on the carrying capacity and deformation capacity of PSHC were obtained and discussed. The results show that the bearing capacity and deformation capacity of the PSHC can be improved by 107% and 540%, respectively, compared with the comparison column. The reinforcement effect will become better with the increase of the hoop ratio. When the prestress ratio increases gradually, the carrying capacity of PSHC increases gradually and the peak strain decreases gradually. The initial axial compression ratio has no obvious influence on the reinforcement effect, the PSHC can be used as a non-unloading reinforcement method. Based on the experiment results, a prediction equation was proposed and verified by the comparison of prediction values and test data in this investigation and other references.
EXPERIMENTAL STUDY ON THE VIBRATION CHARACTERISTICS OF UNTOPPED PRECAST RC FLOORS UNDER BOUNCEAND JUMPING LOADS
Rui PANG, Wen-kang WANG, Tian-peng ZHANG, Ke XU
2020, 37(11): 209-218, 256.   doi: 10.6052/j.issn.1000-4750.2020.03.0192
[Abstract](105) [FullText HTML](19) [PDF 1706KB](43)
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To study the dynamic characteristics and vibration responses of discretely connected precast RC diaphragms (DCPCD) under human-induced excitation, six full-scale DCPCD specimens and one cast-in-place specimen were tested by hammer modal test and human-induced excitation test under bounce and jumping loads. The effects of plate seam and connector layout on DCPCD vibration characteristics were examined. The results show that the low-order vibration mode of DCPCD and cast-in-situ floor was basically the same, but the natural frequency of DCPCD was slightly lower. The DCPCD had the same vibration transmission mechanism as the cast-in-situ floor. The vibration response of DCPCD was slightly larger than that of the cast-in-situ floor under the same excitation. The peak acceleration value of the cast-in-situ floor appeared at the edge of the mid-span, and the peak acceleration of DCPCD appeared at the edge of the slab joint near the mid-span. The natural frequency of DCPCD increased with the increase of the slab joint connectors and with the decrease of the slab joint connections, where the effect of the connector on the frequency was greater than that of the slab joint connection. Methods for calculating the vertical natural frequency and acceleration of the DCPCD in the orthogonal slab laying directions with two pairs of sides simply supported were proposed. The accuracy of the proposed methods was verified by the experimental results.
NONLINEAR TORSIONAL ANALYSIS OF A SINGLE PILE IN AN EXPANSIVE SOIL FOUNDATION
Jie JIANG, Shun-wei WANG, Xiao-duo OU, Zhi WANG, Di YANG
2020, 37(11): 219-227.   doi: 10.6052/j.issn.1000-4750.2020.04.0243
[Abstract](40) [FullText HTML](7) [PDF 1007KB](21)
Abstract:
The pile-soil interaction in an expansive soil foundation is complicated, and the influence of the inundation swelling deformation on the torsional bearing characteristics of the pile foundation remains unknown. In this paper, the displacement governing equations for the pile shaft were proposed considering the load transfer method. The distribution of the vertical resistance of the shaft was calculated by the finite difference method considering the influence of the swelling deformation of the expansive soil on the shaft resistance and the boundary condition of the pile tip. A new nonlinear analytical method for the torsion of a single pile due to the inundation swelling deformation was proposed incorporating the boundary element method. The distribution law of the circumferential frictional resistance of piles was revealed and the effectiveness of the proposed method was verified by model tests. It was shown that the ground heave induced by the swelling of the expansive soil foundation would significantly reduce the torsional bearing capacity of the single pile. In addition, the safety is threatened because the conventional calculation method overestimates the ultimate torque of the pile and the loading stiffness of the pile-soil system. The torque of a single pile approximately decreases linearly along the pile shaft prior to the inundation swelling deformation. However, the torque of the pile shaft is close to that of the pile top and the torque of the pile shaft decreases rapidly in the “unslip section”. The length of the interface “slap section” increases and torsional force capacity of the single pile decreases with the increase of the swelling ratio.
MECHANICAL ENGINEERING
RESEARCH ON VERTICAL MECHANICAL BEHAVIOR OF WJ-8 FASTENER UNDER CLAMPING FORCE FAILURE
Liang GAO, Wen-qiang ZHAO, Bo-wen HOU
2020, 37(11): 228-237.   doi: 10.6052/j.issn.1000-4750.2020.12.0004
[Abstract](81) [FullText HTML](27) [PDF 1393KB](24)
Abstract:
In order to study the effects of fastener clamping force failure on dynamic performance of vehicle-track system, a detailed fastener model was established in the paper, and the vertical nonlinear stiffness behavior of the WJ-8 fastener in different stages was analyzed. An improved bilinear tension stiffness spring model and a no-tensioned spring model were proposed to represent the invalid fastener. The influence of different failure types of fasteners on dynamic response of vehicle-track system was also analyzed. The results show that, the vertical stiffness of the fastener can be divided into two parts: uplift stiffness and compressive stiffness. When the uplift force exceeded the clamping force, the vertical uplift stiffness of the fastener would decrease rapidly and the fastener would lose its constraints on the rail uplift deformation. Both clamping force failure and complete failure of fastener would weaken rail constraints and increase vibration. Clamping force failure mainly increases the vibration of rail in 8 Hz-50 Hz central frequency band, while the complete failure brings an increase in the whole frequency band.
OTHER ENGINEERING DISCIPLINES
FRACTURE BEHAVIOR OF WELDED JOINTS OF SHIP STEEL PLATES AND NUMERICAL ANALYSIS WITH GTN MODEL
Zi-chao ZHUO, Qing-ya ZHANG, Jiang-chao WANG
2020, 37(11): 238-247.   doi: 10.6052/j.issn.1000-4750.2019.12.0746
[Abstract](75) [FullText HTML](5) [PDF 1072KB](11)
Abstract:
Ship steel structures are always fabricated by welding, while mechanical performance and fracture strength of welded joints significantly influence the strength and service life of the whole ship considering the impact of actual working condition and external loading. In this study, base materials of ship steel plate (Q345 and Q690) were examined by uniaxial tension test to establish strain-stress curves, which can be employed for the assessment of fracture performance. Aiming at Gurson-Tvergaard-Needleman (GTN) failure mode, computational code was programmed and a series of numerical analyses were carried out to investigate the fracture behavior of ship steel plate, while constitutive relations of the examined ship steel plates were proposed with optimized parameters of GTN failure mode. In addition, computation results of strain-stress curves have a good agreement with experimental data. Concentrating on the butt welded joints of ship steel plate with high welding quality, uniaxial tension test was also conducted to obtain stress-strain curves of welded joints. In order to consider the influence of micro welding defects and welding residual stress on fracture performance of welded joints, modified initial void volume fraction and plastic hardening parameters were proposed, and good agreement between computation results and measurements was observed for fracture performance of welded joints.
DEVELOPMENT OF A CRYOGENIC FATIGUE TEST FACILITY FOR HIGH TEMPERATURE SUPERCONDUCTING TAPES
Yuan-zhou PAN, Tian-hu HE, Can-jie XIN, Ming-zhi GUAN
2020, 37(11): 248-256.   doi: 10.6052/j.issn.1000-4750.2019.12.0797
[Abstract](46) [FullText HTML](7) [PDF 842KB](13)
Abstract:
Because of the high critical current density in the intense magnetic field, wider temperature margin, resistance irradiation and good mechanical properties, high temperature superconducting tapes (BSCCO and ReBCO taps) have a huge potential application in accelerator magnets, high field magnets and superconducting electric power systems. Some expectable opportunities will be created for modern high-tech electromagnetic equipment. High temperature superconducting tapes with substrate layers usually have high tensile strength. Thus, they can be applied under an intense electromagnetic field. However, in its application process and under the operation conditions, many kinds of inevitable fatigue loads will occur, which have an impact on the current-carrying capacity of the high temperature superconducting tapes and can destroy the high temperature superconducting magnets. A cryogenic fatigue test facility for high temperature superconducting tapes is introduced, the system can be used to measure the mechanical, thermal and strain-field-dependent behaviors of fatigued superconducting wires and tapes under variable cryogenic temperatures. Based on the cryogenic fatigue test facility, the mechanical behaviors and current-carrying characteristics of tension-compression fatigued YBCO tapes were experimentally studied. The preliminary test results show that their mechanical behaviors and current-carrying characteristics showed a nonlinear dependence on the stress ratio during the fatigue test. The successful development of the cryogenic fatigue test facility will provide a basic fatigue test platform for the design and development of accelerator high temperature superconducting magnets.

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