Axial Curves Research Articles

Behavior of double-skin truss-reinforced composite walls under eccentric compression

This paper presented an experimental and theoretical studies on double-skin truss-reinforced composite wall (DSTCW) subjected to eccentric compression. Five full-scale DSTCWs with different loading eccentricities, height-width ratios and truss spacing were conducted under uniaxial eccentric compressive. The effects of those variations on the failure mode, load- axial displacement curve, lateral deflection, strain distribution were discussed. Subsequently, the finite element (FE) models were built and verified to discuss the working mechanism of DSTCWs under eccentric compression, and an extensive parametric analysis was performed to find the vital parameters on the mechanical behavior of DSTCWs subjected to eccentric compression. Finally, based on Eurocode standard EC4, the Interaction model for DSTCW was proposed to estimate the bearing capacity subjected to eccentric compression. The proposed Interaction model was validated against the results from FE model analysis and experiments.

Confinement mechanism and compressive stress-strain behaviours of FRP-interlayer-steel confined concrete tubes

A fiber-reinforced polymer (FRP)-interlayer-steel confined concrete (FISC) column is formed by bonding the FRP material and a concrete-filled steel tube (CFST) with the grouting material, and it can improve the corrosion resistance and bearing capacity of CFST columns and avoid the early cracking of the epoxy resin in FRP confined concrete-filled steel tubes (CCFT). However, there are few researches on the compressive behaviours of FISC columns and the dual confinement on core concrete from the FRP tube and steel tube has not been expounded clearly. In this paper, twelve FISC specimens were tested under axial compression. The ratio of FRP and steel confining indexes and the concrete strength were the main parameters and the loading conditions (core CFST loading and whole section loading) were included. Failure patterns, load–axial shortening curves, and the strain development were analysed. In addition, the confining stresses of the FRP tube and steel tube were obtained, and it was found that the confinement on concrete could be divided into three stages of the non-confinement stage, the steel-controlling stage, and the FRP-controlling stage. In the FRP-controlling stage, the FRP confining pressure increased at a constant growth rate while the steel confinement was gradually weakened owning to the restraints of the interlayer grouting material. Furthermore, by regarding the concrete and grouting material as a whole unity, the axial stress-strain model of the confined concrete-grouting was developed, and the proposed model can predict the compressive stress-strain behaviours of an FISC column with good accuracy and simplicity.

Axial compression behaviour of circular concrete-filled double-skin steel tubular columns with bolted shear studs: Numerical investigation and design

The concrete-filled double-skin steel tubular (CFDST) columns have recently attracted significant attention for practical engineering constructions. The axial resistance behaviour of CFSDTs can be greatly enhanced by installing bolted shear studs as pin supports for the outer. This is a companion paper that expands on the authors' work [1]. The previous paper concentrated on full-scale experimental testing to ascertain the influence of bolted shear studs on the axial resistance of CFSDT columns, while this study focused on finite element (FE) analysis and a simplified design approach for such composite structural members. A three-dimensional FE model for predicting the axial behaviour of circular CFDST columns without and with shear studs was developed and validated using the test results of the specimens' typical failure mechanisms and load-displacement responses. The model was then employed to perform structural analysis by comparing the axial load-displacement curve, internal force distribution, longitudinal stress distribution over the sandwiched concrete, and confinement mechanism of a circular column with bolted shear studs with that of its counterpart without bolted shear studs. Parametric studies were carried out to examine the effects of bolt longitudinal spacing, diameter, length, and yield strength. It was found that the spacing of the bolts-to-width (bs/Do) ratio had significant effects on the axial resistance and ductile performance of columns. Also, a bolt spacing corresponding to bs/Do<0.44 was suggested to attain an acceptable ductile performance in practical application. Verification of the design model revealed that it led to satisfactory predictions of the ultimate strengths of circular CFDST columns without and with shear studs.

Nonlinear inelastic analysis of eccentrically loaded square high-strength concrete short columns incorporating steel equal-angles

Steel-Equal Angle (SEA) has recently been utilized as longitudinal reinforcement in high-strength concrete columns to improve their strength and ductility. However, investigations have been very limited on the responses of eccentrically loaded SEA Reinforced Concrete (SEARC) columns. This paper presents the development of a computer simulation model based on fiber element discretization for determining the structural behavior of eccentrically loaded SEARC short columns. The inelastic buckling of SEA sections and the concrete confinement induced by SEA sections are explicitly taken into account. The modeling procedures are given for axial load moment-interaction curves and moment-curvature curves of SEARC columns. The accuracy of the developed fiber model is verified against the available test data. The performance of eccentrically loaded SEARC columns incorporating various design parameters is extensively investigated. It is shown that the developed computer simulation model captures well the experimentally measured responses of short SEARC columns and can be utilized in the analysis and design of such columns in practice. It has been found that the behavior of SERAC columns under eccentric loads is significantly influenced by the longitudinal steel ratio, the compressive strength of concrete, and the axial load level. The width of SEA sections and tie spacing considerably influence the ductility of the columns.

Test on compressive performance of hollow concrete filled circular steel tube connected by thread through inner lining tube

A method of lengthening the steel tube of concrete-filled circular steel tube by inner lining tube and threaded connection is proposed. Taking the length, depth and position of the thread as the basic parameters, 12 hollow concrete-filled circular steel tube specimens connected by thread through inner lining tube are designed, and the axial compression test is carried out. The axial compressive loading-longitudinal compressive displacement curves, axial compressive loading -strain of steel tube curves and failure mode of the specimens are analyzed, and the effects of different parameters on the axial compressive bearing capacity and stiffness of the specimens are studied. The results show that the axial compressive loading-displacement curves of the specimen can be divided into linear elastic stage, elasto-plastic stage and descending stage. The bearing capacity and stiffness of the specimens connected by thread through inner lining tube are basically the same as those of the unconnected specimen or the specimen connected by weld. In the range of parameters studied of this paper, bearing capacity and stiffness of the specimens connected at end section by thread through inner lining tube are higher than those of the specimens connected at middle section by thread through inner lining tube, and the axial compressive bearing capacity and stiffness of the specimen increase with the increase of thread length and depth. Based on the comparison between the calculation results calculated by existing formulas of the axial compression bearing capacity of hollow concrete-filled circular steel tube and the test results of specimens connected by thread through inner lining tube, the calculation method of the bearing capacity of hollow concrete-filled circular steel tubes connected by thread through inner lining tube are suggested.

Experimental and numerical studies on the axial compression performance of hexagonal stiffened CFDST stub columns

In this paper, the authors conducted sixteen axial compressive tests on the hexagonal ultra-high performance concrete-filled steel tubular stub columns. The parameters studied in this research program included the cross-sections, section width, the steel tube thickness, and the hollow ratio. Based on the experimental results, the failure modes, axial load-shortening curves, composite strength, and ductility coefficient of the column were illustrated and discussed. The results indicated that stiffeners can significant delay the local bucking of steel tube and improve the interaction between the ultra-high performance concrete (UHPC) and the steel tubes. To simulate the mechanical response of these new types of columns, the authors developed a fiber-beam model (FBM) including the local buckling of steel tubes and the confinement effect. Then, the authors conducted a parametric study to discuss the influence of the hollow ratio, the steel tube thickness, and the strength of steel and concrete. Finally, the authors proposed a new method for predicting capacity and verified it against the test/numerical results. With the comparison of the design rules recommended by the current design codes, the new method proposed in this paper showed a good agreement with the test/numerical results. It can provide a reference for related engineering design.

Out-of-Plane Compression Behaviour of Aluminum Alloy Large-Scale Super-Stub Honeycomb Cellular Structures.

The out-of-plane compression behaviour of 6061-T6 aluminum alloy super-stub honeycomb cellular structures without and with friction stir welding (FSW) facesheets are presented in this paper. A total of twelve axially compressed experiments on large-scale specimens, six with square hollow section (SHS) cores and six with hexagonal hollow section (HHS) cores, were conducted, with failure modes, ultimate resistances and axial load-end shortening curves analysed. The accuracy of finite element (FE) models was validated in accordance with test results. The numerical data obtained from extensive parametric analyses combined with test data were subsequently used to evaluate the applicability of existing design rules in Chinese, European and American aluminium alloy specifications. The results showed that the three specifications generally yielded very conservative predictions for the out-of-plane compression resistances of SHS and HHS super-stub honeycomb cores without and with FSW facesheets by about 30-37%. Design recommendations on the cross-section effective thickness are finally proposed and shown to provide much more accurate and consistent predictions than current design methods. The research results are beneficial to the application and development of large-scale super-stub honeycomb structures in structural engineering, such as the helicopter landing platforms, the base of fluid and gas tanks and ship decks.

Axial compressive behavior of built-in carbon-steel marine concrete-filled stainless steel tubular columns

To investigate the axial compressive behavior of marine concrete-filled stainless steel tubular (MCFST) columns with built- in carbon steel, a total of 10 specimens were tested under axial loads. The mechanical failure process and morphology of the specimens were observed, and the axial load-displacement curve and the strain curve of each steel were obtained. Based on the test data, the influence of various variation parameters on the ultimate bearing capacity, axial ductility, energy dissipation capacity and damage development of kinds of specimen was analyzed. The three-dimensional nonlinear finite-element (FE) models were established and verified with the experimental results. The test results indicated that the failure modes of specimens are local buckling and wrinkle of stainless steel tube and oblique shear failure of concrete. The built-in carbon steel can improve the ultimate bearing capacity, axial ductility and energy dissipation capacity of the specimen, and inhibit the damage development of the specimen. Under the condition of increasing the unit carbon steel ratio, MCFST column with built-in circular steel tube achieved the axial compression performance best. Considering the positive effect of the constraints of built-in carbon steel, the calculation methods of bearing capacity were proposed, and the calculated value reflected the measured data well.

Performance of double-skin truss-reinforced composite wall under axial compression

In order to improve the buckling capacity of double steel concrete composite wall (DSCW), a new-type of double-skin truss-reinforced composite wall (DSTCW) with steel truss used as stiffeners was proposed. In total, 7 DSTCWs were loaded axially, and the variables included steel truss joint spacing and rebar diameter. The effects of those variations on the axial load-displacement curve, ultimate strength and failure mode were discussed. The results showed that the steel truss significantly improved the ability of steel plate to resist local buckling, and enhanced the composite action between steel plate and concrete. The larger steel truss joint spacing reduced the ultimate bearing capacity of the DSTCWs under axial compression, and the rebar could effectively resist the deformation of steel plates on both sides. Based on the test results, the finite element (FE) models were established and verified to discuss the working mechanism of DSTCWs, and an extensive parametric analysis was performed to find the key parameters on the mechanical behavior of DSTCWs. Formula for predicting the axial load capacity of DSTCWs was proposed.

Material synergy and parameter optimization of axially-loaded circular UHPC-filled steel tubes

This paper aims to investigate the material synergy and parameter optimization of circular ultra-high performance concrete (UHPC) filled steel tubes towards excellent axial bearing capacity and material utilization efficiency. 12 stub columns of UHPC-filled steel tubes (UCFST) were tested and analysed combined with 293 literature data sets under axial compression. The results show that increasing confinement index ξ and steel ratio α can enhance the ductility of the axial load-displacement curve and the failure mode of the column. Threshold values of ξ = 1.0 and α = 0.25 are proposed to distinguish the failure modes. Appropriately higher steel strength grade, lower D/t, larger α and ξ are recommended to ensure the good axial bearing capacity of the UCFST columns. To produce excellent material synergy and axial bearing capacity, the optimum steel and UHPC strength grades are recommended around C100 ∼ C160 and Q345 ∼ Q690 for the axially-loaded UCFST short columns, respectively. Furthermore, the optimum ranges of diameter-to-thickness ratio D/t, steel ratio α and confinement index ξ are <90·(235/fy), 0.10–0.30, 0.7–2.0, respectively. The current normal CFST design codes are inappropriate for the UCFST columns due to the parameter values exceeding limit values. Therefore, a reliable analytical model is proposed and well validated to predict the axial bearing capacities of circular UCFST short columns.

Shake table test and numerical analysis of the seismic response of buried long-distance pipeline under longitudinal non-uniform excitation

To investigate the seismic response characteristics of buried pipelines under longitudinal non-uniform seismic excitation, a three-dimensional finite element overall analysis model is established on finite element analysis software based on the shaking table test of buried oil and gas pipelines under longitudinal non-uniform excitation. The calculated results are compared with the experimental analysis results to evaluate the correctness and reliability of the established model. The acceleration response, strain response, and displacement response of buried oil and gas pipelines and surrounding soil under longitudinal traveling wave excitation at different seismic intensities are compared and analyzed using finite element models to supplement the test conditions. The test and analysis results show that the longitudinal displacements at each measurement point under longitudinal non-uniform excitation follow the same pattern except for differences in displacement magnitude and the vibration pattern of the tube and soil when the peak input acceleration reaches 1.00 g. As shown by both experimental and numerical simulation results, the soil acceleration amplification coefficients along the soil height fluctuate between 0.5 and 1.5. The pipe acceleration time curves and Fourier spectrum exhibit the same simulated and experimental pipe acceleration waveforms, and they both show significant amplification in the low to medium frequency band of 5–20 Hz. The testing and numerical simulated pipeline axial strain peak curves are similar and in good agreement, indicating the high reliability of the experimental results.

Test on Compressive Performance of Concrete Filled Circular Steel Tube Connected by Thread through Inner Lining Tube

The connection method of lengthening the steel tube of concrete filled circular steel tubes by inner lining tube and threaded connection is proposed. Taking the length, depth, and position of the thread as the basic parameters, 12 concrete filled circular steel tubes connected by thread through inner lining tube were designed and manufactured, and an axial compressive test was carried out. The axial compressive loading-longitudinal compressive displacement curves, axial compressive loading-strain of steel tube curves, and failure mode of the specimens were analyzed, and the effects of different parameters on the axial compressive bearing capacity and stiffness of the specimens were studied. The results show that the axial compressive loading-longitudinal compressive displacement curves of the specimen can be divided into the elastic stage, elasto-plastic stage, and plastic reinforcement stage in the range of parameters studied in this paper. The bearing capacity and stiffness of the specimens connected by thread through inner lining tube are no worse than those of the unconnected specimen or the specimen connected by weld. Bearing capacity and stiffness of the specimen increase with the increase of thread length. The calculation method of the axial compressive bearing capacity of concrete filled circular steel tubes connected by thread through inner lining tube are suggested.

A scheme for capturing the kinetic energy of the flow liquid in a ship's cabin

The study proposes a solution for the energy capture of a sailing liquid cargo ship, which is essentially an organic combination of an Savonius turbine and a ship's rolling cabin. A model of the cabin turbine was simulated in ANSYS FLUENT from the perspective of the ship's roll and the rotor's rotational motion. Specifically, three cabin with opening lengths of 0.5, 1.0 and 1.5m were simulated to illustrate the sinusoidal trend of the axial velocity curves of the liquid at the cabin's opening position where the turbine is installed; When the burden of ship increases, the velocity at the opening decreases; when the burden is 90% and the ship's rolling angle is 20°, the peak axial velocity can reach 2.2 m/s, the average kinetic energy reaches 2.2 kJ and the flow rate reaches 1.38 m3/s. Then, the maximum values of the turbine's power coefficients are 0.21, 0.25 and 0.33 respectively, and their corresponding tip-speed rates are 1.0, 1.4 and 1.0. The torque coefficients decrease as the tip-speed rate increases. The velocity and pressure in the vicinity of the turbine present the non-stationary gradient images. Finally, the prediction of the power of turbine is discussed.

Cyclic axial compressive performance of the RC columns reinforced with FRP confined concrete core encased rebar

During an earthquake, the collapse of a structure is generally controlled by the failure or damage of its columns. It is important to enhance the seismic performance of reinforced concrete (RC) columns, including load capacity, ductility, and energy dissipation capacity. Column configurations with several small confined concrete components, namely, the confined concrete cores in this paper, are regarded as an effective way to improve the mechanical performance of square RC columns based on previous studies. The external confining material of a confined concrete core can be steel spirals, steel tubes, or FRP tubes. In this study, a new component, namely, FRP confined concrete core encased rebar (FCCC-R), and a new member, i.e., an RC column reinforced with FCCC-R were investigated. Eight RC columns with/without FCCC-R were cast and tested under cyclic axial compressive loading. The test result indicates that the RC columns reinforced with FCCC-R generally have a much higher axial load capacity. The axial load-displacement curves of all the tested specimens are compared, and the influences of the steel hoops, FCCC-R, and center FCCC are discussed. Moreover, the behaviors of different parts in a hybrid column, including steel hoops, and FRP tubes, are analyzed according to the strain development during the test. Finally, a section superposition method is proposed to predict the axial load-axial deformation behavior, whose results agree well with the experimental results.

Corroded reinforced concrete columns strengthened with basalt fibre reinforced ECC under axial compression

This study presents an experimental investigation aimed at evaluating the effectiveness of basalt fibre reinforced engineered cementitious composite (FRECC) in strengthening corroded reinforced concrete (RC) columns. In this strengthening technology, ECC acts as both a matrix for bonding basalt fibre and a new cover replacing the damaged concrete cover. A total of 34 specimens were tested under axial compression. The studied parameters were the number of basalt fibre layers, corrosion ratio of steel reinforcements and stirrup spacing. The experimental behaviour was characterized in terms of axial load-axial shortening curves and tensile strain development in stirrup and basalt fibre. The test results showed that FRECC composites significantly improved the performance of corrosion-damaged columns, and the strengthened columns gained peak load and ductility enhancements of 5%–62% and 160.0%–415.2%, respectively. The monitored strains revealed a possible interaction between basalt fibres and steel reinforcements and the influence of steel corrosion, and the fibre efficiency ratios at peak load and at 85% peak load in the decreasing branch were 0.10–0.48 and 0.25–0.62, respectively.

Fire performance on steel-reinforced concrete-filled steel tubular columns with fire protection

Steel-reinforced concrete-filled steel tube (SRCFST) is widely used as an innovative composite structure to provide better performance in construction. This study focuses on comprehensive experimental and numerical investigations of the fire performance of SRCFST columns with fire protection under the ISO-834 standard fire test procedures. The fire resistance tests were conducted on eight SRCFST columns with various thicknesses of fire resistive coating. The fire test results, including the thermal conductivity of protection material, failure modes, temperature evolution in time, axial and lateral deformation curves, and fire resistance were reported in detail. The results demonstrated that fire protection delayed the degradation of bending stiffness and significantly increased the fire resistance. Further comparisons of fire resistance between SRCFST columns with and without fire protection indicated that the fire resistance of SRCFST column with protection thickness of 12 mm is 3.32 times higher than that of that without fire protection under an equal load level. A sequentially coupled thermal-stress finite element (FE) analysis model was develped and validated against the tested results. The measured thicknesses of fire resistive coating were compared with the predicted results to evaluate the applicatibility of the existing design method of concrete-filled steel tubular (CFST) columns to SRCFST columns with thick fire-resistive coating. The results indicated that the current design method should be adjusted or modified further to provide safe and accurate predictions of the thickness of fire protection for SRCFST columns.

Effects of nitrogen dilution on flame characteristics and stability of lifted propane fire whirl

Nitrogen dilution effects on flame characteristics and stabilization mechanism were experimentally investigated in lifted propane fire whirls. A small rotating screen facility with finely controlled imposed circulation (Γ) and propane heat release rate (Q˙=2.0–4.0 kW) was used for experiments. The nitrogen volume fractions (φ) in the diluted propane flow ranged from 0 to 90%. The stereoscopic particle image velocimetry (SPIV) technique, as well as the synchronized color imaging technique, was utilized to obtain 3-D instantaneous velocity distributions of fire whirl. It was found that the initially anchored fire whirl (φ=0) could be lifted after attaining a critical value of φ in the propane flow. Three regimes, including the attached flame, the lifted flame, and extinction, were identified in φ-Г coordinates. The fire whirl could exhibit flame liftoff and flame attachment under the same φ and Г, depending on the initial flame state in the φ-Г plot. The flame liftoff height and the flame diameter increased, and the flame length decreased with increasing φ. SPIV measurement results showed that the axial velocity followed an S-shaped distribution in the vertical direction. The mean axial velocity conditioned at the instantaneous lifted flame base decreased with φ and increased with Γ. The stable equilibrium was achieved at the locations where the flame propagation velocity and the axial velocity were roughly balanced, and the axial velocity curve had a negative slope with height. The variations of φ and Γ could change the flame liftoff height by breaking the local balance between flame propagation velocity and gas flow velocity. In addition, the peak value in the axial velocity curve, which was related to Γ, strongly affected the transitions among the different regimes.

Effect of Filling Phosphogypsum on the Axial Compression Behavior of Cold-Formed Thin-Walled Steel Walls

To study the effect of filling phosphogypsum (PG) on the axial compression behavior of cold-formed thin-walled steel (CFS) walls, four full-scale test specimens were designed and fabricated, in consideration of the filling regions of PG as well as measures with or without wall sheathings. The fabricated specimens were tested under monotonic vertical loads, and the failure processes and failure modes of specimens were elaborated. Each specimen’s axial load-displacement curve, bearing capacity, strain curve, and energy dissipation capacity were investigated in detail. Furthermore, the internal force distributions of wall components and failure mechanisms were revealed. The test results indicated that the failure characteristics of specimens include the buckling of the steel tubes, cracking of wall sheathings, crushing of PG, and distorting of tracks. Compared with the cavity wall specimen, the axial bearing capacity of the specimen filled with PG in the studs only increased by 37.4%, and the bearing capacity of the specimen filled with PG in and between the studs increased by 115.7%. This indicates that filling PG can effectively improve the axial bearing capacity of CFS walls. The bearing capacity of the specimen without wall sheathings is lower than that of the specimen with wall sheathings, indicating that the wall sheathing has a beneficial effect on the bearing capacity of the specimen. In addition, the internal forces of components during the loading process were analyzed. It found that the steel tube and PG made a great contribution to the bearing capacity of the wall. Specifically, the steel tube played a leading role in the early loading stage, while the PG played a leading role in the later loading stage.

Strength Laws and Crack Evolution Mechanism of Slurry Grouting under High-Permeability and High-Stress Curing Conditions

The grouting method is often used to strengthen the bearing capacity of building foundations. In the process of grouting, the stress structure of slurry and rock and the soil mass under the condition of high permeability and high stress (“double height”) is complex, and the strength mechanism of slurry stone under different double-height conditions is not clear. Therefore, stone samples of grout under the curing pressures of 2 MPa, 4 MPa, 6 MPa, 8 MPa and 10 MPa were prepared for the uniaxial compression test to analyze the influence of curing pressure on compressive strength. The microstructure of the slurry stone was observed by scanning electron microscope, and the development law of microcracks was further studied using the particle flow program. The research shows that the strength of the slurry stone body increases with the increase of curing pressure. When the curing pressure increases from 2 MPa to 10 MPa, the compressive strength of the stone powder-cement slurry stone body increases from 8.3 MPa to 22.7 MPa, an increase of nearly 2.7 times, and the compressive strength of the clay-cement slurry stone body increases from 5.7 MPa to 16.8 MPa, an increase of 2.9 times. According to the axial compressive stress-strain curve of the specimen, the failure process goes through three continuous stages: continuous elasticity, crack propagation, and strength failure. When the stress peak is reached, the number of cracks increases slowly; when the stress peak is reached, the cracks expand rapidly, the number increases exponentially, and a penetrating main crack is finally formed, which destroys the specimen. This study provides a reliable basis for the selection of grouting parameters and grouting materials in stratum-grouting engineering.

Simulation Study on Mechanical Characteristics of Rock Bolt in Rock Mass with Bedding Separation Based on the Nonlinear Bond Slip Relationship

The mechanical equation on the mechanical characteristics of rock bolt in rock mass with bedding separation is established, based on the bond slip relationship of anchoring interface by the load transfer method. The validity of the numerical simulation is verified by the analytical solution of mechanical equation, when the bond slip relationship is linear. Further, the nonlinear bond slip relationship of anchoring interface is input into Flac3D by FISH. The mechanical characteristics of rock bolt are studied with the single and multiple bedding separation by numerical simulation. The results show that the axial force and the interfacial shear stress distribution curves on both sides are symmetrically distributed with the single bedding separation in rock mass, when the bedding separation is located in the center of the anchoring segment. When the bedding separation positions are not located in the center of the anchoring segment, the bedding separation value at different positions versus the axial force curves shows a single peak distribution. The axial force of rock bolt is mainly controlled by the side with the shorter anchoring length. Compared with the single bedding separation in rock mass, the additional stress of rock bolt occurred by the multiple bedding separation will superimpose each other. This paper leads to a better understanding for the load transfer mechanism for grouted rock bolt systems in rock mass with bedding separation and provides a reference for scientific support design and evaluation method.