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The Partially Concrete Filled Steel Tube (PCFST) structure has gradually been applied to bridges as the main load carrying components in recent years, owing to the advantages of light weight and high stability. The failure mechanism and load carrying capacity of PCFST structure highly depends on the concrete filling ratio. For this purpose, the failure mechanism and parametric analysis of a single arched PCFST cable stayed bridge tower are investigated by using both refined solid element simulation and quasi-static experiment. The results shows that the PCFST arched tower experiences asymmetrical instability failure subjected to quarter span symmetrical loading, and the plastic hinges appears firstly at the tower arch closely above the tower columns, and then the bottom of the tower columns, forming a rotating mechanism leading to the instability failure. Filling concrete or not will result in two opposite deforming propagations of the plastic hinges, and when the concrete filling length exceeds the length of plastic hinge, the initial rigidity and ultimate load carrying capacity of the PCFST structure will sharply increase. Finally, according to the results of the extended parametric analysis based on the real size simulation, an improved initial stress reduction coefficient of PCFST structure is then suggested. [ABSTRACT FROM AUTHOR]

An analytical solution is presented for the interaction of an I-core coil with a metal tube of finite length. Using the Truncated Region Eigenfunction Expansion (TREE) method, the impedance variation of the I-core coil is expressed in a closed form. Notably, the 1D finite element method (1D FEM) is adopted to calculate the complex eigenvalues and eigenfunctions of the steel tube, by which the overflow issue of complex eigenfunctions has been surmounted. And the 1D FEM also simplifies the calculation in the ferrite core region immensely. A performant algorithm is obtained due to the matrix formalization and the high speed of the sparse eigenvalue solver. The tubes made of aluminum alloy and stainless steel are investigated by the proposed method and tested in the experiment. The high reliability and efficiency of the proposed method are proved, and the results are also validated by the comparison with simulation and measurement data. [ABSTRACT FROM AUTHOR]

In this paper, the aim is to discuss the applicability of polyethylene terephthalate fiber-reinforced polymer (PET FRP) in DSTC structures. PET FRP can improve the strength and stability of structures by providing constraints for concrete. Herein, experimental and response surface analyses of the axial compressive properties of PET FRP–concrete–steel double-skin tubular stub columns (DSTCs) based on 26 DSTC specimens subjected to axial compression testing are presented. The height of each DSTC specimen is 600 mm, the outer diameter is between 305 and 315 mm, and the thickness of the added concrete is 43 mm. The main parameters are the number of layers on the PET FRP, the compressive strength of the concrete, and the thicknesses of the steel tubes. The experimental results show that the ultimate load and ultimate axial strain can be significantly increased by increasing the number of PET FRP layers, reaching 27.34% and 28.79%, respectively. When the compressive strength of the concrete increases from C30 to C40, the ultimate load and ultimate axial strain values of the DSTCs increase by 12.54% and 8.99%, respectively. In addition, as the thickness of the steel tube increases from 6 to 8 mm, the ultimate load and ultimate axial strain increase by 34.95% and 118.90%, respectively. These results indicate that the introduction of PET FRP significantly improves the overall performance of DSTCs. Increasing the number of PET FRP layers helps to limit the circumferential strain of DSTCs. P6-S8-C40 has the best ultimate load-bearing capacity and ultimate axial strain capacity, which reach 3356.18 kN and 0.1992, respectively. The main purpose of this paper is to study the influences of the PET FRP thickness (in different layers), steel tube thickness, and concrete strength on the properties, damage mode and damage process of DSTCs. [ABSTRACT FROM AUTHOR]

Summary: This study aims to develop an algorithmic approach to obtain optimum designs for tall buildings having composite columns and investigate the material cost advantages of these buildings over steel structures. The social spider optimization (SSO) algorithm, a new meta‐heuristic optimization method that has shown promising results in optimizing frame structures, was used to obtain the optimum designs. Concrete‐filled steel tube sections were chosen for composite columns. To define the optimization problem, we considered the material cost of the structure as the objective function, the size of columns (strength, deflection, drift, and geometric limitations) as the constraint functions, and ready steel sections as the design variables. We tested eight different frame structures of varying heights and irregularities to analyze how cost varied according to these parameters. Our results demonstrate that composite columns are a more cost‐effective option than steel structures, even for buildings that are not considered high rises. We found that the difference in cost between the two types of structures increases with building height and irregularity. Additionally, our optimization algorithm was unable to find feasible designs for steel structures taller than 180 m using ready steel profiles. [ABSTRACT FROM AUTHOR]

Summary: Precise estimation of the shear strength of concrete‐filled steel tubes (CFSTs) is a crucial requirement for the design of these members. The existing design codes and empirical equations are inconsistent in predicting the shear strength of these members. This paper provides a data‐driven approach for the shear strength estimation of circular CFSTs. For this purpose, the authors evaluated and compared the performance of nine machine learning (ML) methods, namely linear regression, decision tree (DT), k‐nearest neighbors (KNN), support vector regression (SVR), random forest (RF), bagging regression (BR), adaptive boosting (AdaBoost), gradient boosting regression tree (GBRT), and extreme gradient boosting (XGBoost) in estimating the shear strength of CFSTs on an experimental database compiled from the results of 230 shear tests on CFSTs in the literature. For each model, hyperparameter tuning was performed by conducting a grid search in combination with k‐fold cross‐validation (CV). Comparing the nine methods in terms of several performance measures showed that the XGBoost model was the most accurate in predicting the shear strength of CFSTs. This model also showed superior accuracy in predicting the shear strength of CFSTs when compared to the formulas provided in design codes and the existing empirical equations. The Shapley Additive exPlanations (SHAP) technique was also used to interpret the results of the XGBoost model. Using SHAP, the features with the greatest impact on the shear strength of CFSTs were found to be the cross‐sectional area of the steel tube, the axial load ratio, and the shear span ratio, in that order. [ABSTRACT FROM AUTHOR]

Extrusion tests were conducted on recycled coarse aggregate concrete with circular steel tubes (RCCST, 100% replacement of recycled coarse aggregate) to investigate the impact of freeze–thaw cycles on bond properties. RCCST samples were subjected to 0, 25, 50, 75, 100, 125 and 150 freeze–thaw cycles and their bond properties were then compared with those of natural coarse aggregate concrete with circular steel tubes (NCCST) after 0, 100 and 150 freeze–thaw cycles. The experimental results indicate that freeze–thaw cycling damages the internal structure of RCCST, causing a significant decrease in interfacial bond strength with an increase in the number of freeze–thaw cycles. Freeze–thaw RCCST exhibits lower peak load and higher peak slip compared to NCCST, with increases of 25.2%, 73.2%, and 70.3% after 0, 100, and 150 freeze–thaw cycles, respectively. Regression analysis was used to derive an exponential equation that describes the relationship between longitudinal strain and strain gauge position. Additionally, a segmented fitting method was employed to obtain an expression for the bond slip of the freeze–thaw RCCST with a circular steel pipe. [ABSTRACT FROM AUTHOR]

In this study, a novel special-shaped steel tube concrete column steel beam joint is proposed. Bolt connections are used to connect the pre-welded steel beam and the special-shaped steel tube concrete column. Based on the finite element software ABAQUS, we established finite element models of the column-beam joints, and conducted a series of finite element analyses to investigate the influence of various parameters on the seismic performance of the joint. These parameters include the shape and size of the flange hole, the shape and size of the web hole, and thickness of the spliced plate on the flange. The results show that the difference between the finite element analysis and experimental results for bearing capacity and displacement is within 10%. This indicates that finite element analysis can be effectively utilized to study the seismic performance of the joint. Compared to joints with non-spliced beam, the rotational deformation capacity of the joint is reduced when using bolts and spliced plates designed by the equal-strength method. The energy dissipation capacity and ductility deformation capacity of the joints with spliced beams are significantly improved by using elliptical bolt holes in the flange, and plastic hinges are formed in the spliced region. The elliptical holes in the web with varying sizes have little effect on the seismic performance of the joint. The thicknesses of the spliced plate ranging from 6 mm to 8 mm has little effect on the seismic performance of the joint, and a thin spliced plate will reduce the seismic performance of the joint. The study presented in this paper can provide a reference for promoting the engineering application of special-shaped steel tube concrete structures. [ABSTRACT FROM AUTHOR]

In the paper, the eccentric compression behavior of the truss-reinforced cross-shaped concrete-filled steel tubular (CCFST) column is investigated. A total of eighteen CCFST columns were tested under eccentric compression, and the key test variables included the reinforced truss node spacing (s = 140 mm and 200 mm), slenderness ratio (λ = 9.2, 16.6, and 23.1), and eccentricity ratio (η = 0, 0.08, and 0.15). The failure mode, deformation characteristic, stress distribution, strain distribution at the mid-span of the steel tube, and the eccentric compression bearing capacity were assessed. The results show that due to the addition of reinforced truss, the steel plates near the mid-span of eccentrically compressed CCFST columns experienced multi-wave buckling rather than single-wave buckling after the peak load was reduced to 85%, and the failure mode of concrete also changed from single-section to multi-section collapse failure. Comparisons were made with the unstiffened specimen. The ductility coefficient of the stiffened specimen with eccentricity ratios of 0.08–0.15 and node spacings of 140 mm~200 mm increased by 70~83%, approaching that of the multi-cell specimens with an increasing steel ratio of 1.8%. In addition, by comparing the test results with the calculation results of four domestic and international design codes, it was found that the Chinese codes CECS159-2018 and GB50936-2014, and the Eurocode 4 (2004) can be better employed to predict the compression bearing capacity of truss-reinforced CCFST columns. [ABSTRACT FROM AUTHOR]

This study proposes a novel reduced beam section concrete‐filled double steel tube (RBS CFDST) beam‐to‐column joint and investigates the effect of RBS length and the beam moment of inertia on the plastic hinge formation in such joints. Therefore, a set of nine RBS CFDST connections were fabricated and cast with self‐consolidating concrete in the laboratory. Then, parameters including failure pattern, buckling mode, plastic hinge location, joint maximum load‐bearing capacity, and column rotation were inspected. The findings reveal that when the RBS length is equal to that of the beam dimension, the entire plastic hinge length is formed within the RBS zone. As such, the plastic hinge occurs away from the column face and brittle failure is avoided, while the joint column rotation is significantly reduced. It was also concluded that the maximum load‐bearing capacity is the highest when the RBS length is at its lowest. [ABSTRACT FROM AUTHOR]

In this study, the authors conducted five low-cycle reciprocating loading tests on hexagonal ultrahigh-performance concrete (UHPC)-filled steel tubular (UHPCFST) and UHPC-filled double-skin steel tubular (UHPCFDST) columns. The main parameters investigated in this research program are the hollow ratio, axial load ratio, and column type. Based on the experimental results, the failure modes, lateral load-story drift ratio curves, lateral load-strain curves, ultimate lateral load and corresponding base moment, and ultimate story drift ratios are obtained and discussed. The results show that hexagonal UHPCFST and UHPCFDST satisfy the requirements of ductility in GB 50936-2014. Moreover, the authors developed a fiber beam model (FBM) to predict the hysteretic behavior and strength under combined compression and bending. Subsequently, a parametric study was conducted, based on the validated FBM, to investigate the effect of material strength, outer steel tube thickness, and hollow ratio on the strength of hexagonal columns under combined loadings. Comparisons of the predictions from T/CESS 7, AISC 360, and EC4 and numerical results suggest the necessity of a design method with higher accuracy. Finally, the authors proposed a calculation formula for hexagonal UHPCFST and UHPCFDST columns, which effectively predicts the strength of these two types of columns under combined compression and bending. [ABSTRACT FROM AUTHOR]

This article presents the properties of a Ti coating (angle of grain growth, thickness of the deposited layer, adhesion, and roughness of the deposited layer) deposited on the inner surface of a steel tube. The Ti coating was deposited using the EB PVD (electron beam physical vapor deposition) method. The areas in which the measurements were carried out are defined by the height of the measured area under the target (from 305 to 425 mm) and the radius of the tube (from 100 to 140 mm). The measurements showed that the angle of grain growth is in an interval from 40 to 61°. The results of the statistical analysis demonstrate that the height of the measured area and the tube radius significantly impact the angle of grain growth and adhesion. On the other hand, the impact of the height of the measured area and the tube radius on the roughness of the applied coating was not confirmed. [ABSTRACT FROM AUTHOR]

High-speed railway vehicle bogies are the main load-bearing components, which are welded together from steel plates and tubes. Butt joint and T-joint are the main types and the joint thickness is between 8–20mm. The stability of laser-arc hybrid welding (LAHW) process correlated strongly with keyhole behaviour. In this paper, the common quality defects in full penetration root pass by LAHW were analysed. Controlling the root gap is adopted to stabilize the keyhole, to improve the stability of weld quality. A novel type of butt joint with micro structure based on Y-shaped grooves has been designed, to meet both the requirement of controlling root gap and industrial large-scale batch production. Two new structure parameters, 2 mm of the remaining blunt edge thickness and 0.3 mm of the half width of root opening, were proved by the flat plates welding test. Finally, the joint with micro structure and the root pass welding parameters were transferred to the welding process of tube crossbeam components. The LAHW process verification was completed, and the test results showed the process meets the requirements of ISO 15614. [ABSTRACT FROM AUTHOR]

This paper contributes to the further development of seismic resilient infrastructure by introducing a novel prototype bridge which integrates tubular steel piers with superelastic shape memory alloy (SMA). The proposed bridge pier is composed of a circular steel tube which is bonded to a superelastic SMA tube in regions of high stress. The pier is distinguished by its ability to minimize residual drifts following inelastic deformations induced by cyclic loading. Three‐dimensional continuum finite element (FE) models are utilized to examine its lateral behavior. Experimental data is used to demonstrate the effectiveness of the continuum FE procedure in replicating the cyclic response and capturing both global and localized behaviors. A novel composite material model is proposed to represent the degradation of strength and accumulation of irreversible strains in the cyclic response of superelastic SMAs. An iterative procedure for the calibration of this material is presented. Investigations, employing the calibrated FE procedure, focus on the quasi‐static cyclic response of steel piers with superelastic SMA in the plastic hinge zone, aiming to identify the optimal length of the SMA tube for achieving a self‐centering response with reduced residual deformation. The study is then further expanded to examine the seismic response of a bridge structure incorporating such piers. Development of the FE model for the prototype bridge includes the modelling of the piers using continuum elements, while the superstructure, bearing units, abutment walls, and backfill material are modelled using discrete elements. Nonlinear time history analyses are undertaken to investigate the effects of column wall thickness and materials used in the plastic hinge zones of the piers. Dynamic FE study results indicate that bridges employing such piers are capable of returning to their original position, provided the SMA tube is of adequate length. [ABSTRACT FROM AUTHOR]

The bonding strength and LBE corrosion resistance of the Fe15Cr11Al2Si, Fe15Cr11Al0.5Y, and Fe15Cr11Al2Si0.5Y coatings heat‐treated at 500–650°C for 500 h were investigated. The results showed that the as‐deposited Fe15Cr11Al0.5Y coating has the strongest bonding strength with the F/M steel cladding tube compared with the Fe15Cr11Al2Si and Fe15Cr11Al2Si0.5Y coatings. Heat treatment deteriorates the bonding performance of the coatings, and obvious enrichment of Cr and Al elements appeared. The consumed Al element inside the heat‐treated coatings promotes the formation of Fe3O4 on the surface of the coatings after the corrosion test. The Y element can inhibit the enrichment of elements and the formation of Fe3O4. The bonding strength of the heat‐treated coatings can be improved after the corrosion test. The underlying mechanism of the evolution of microstructure and properties of the coatings after heat treatment and corrosion test was discussed. [ABSTRACT FROM AUTHOR]

During the normal laying and operation of a three-core umbilical cable, AC current can easily lead to AC electrochemical corrosion on the outer surface of the steel tube. To explore the electrochemical corrosion mechanism and the factors affecting the three-core umbilical cable, this paper optimizes the internal induced potential calculation method for three-core umbilical cables. It analyzes the changes in the characteristics of the induced potential and explores the variations in the density of induced current under different conditions. The research results show that by optimizing the calculation method for the induction potential of the umbilical cable's steel pipe, for the electromagnetic significance of the smallest repeating unit, the induction potential on the steel pipe's surface exhibited a cyclic change. The peak part of the induction potential is most likely to experience electrochemical corrosion. Additionally, reducing the radius of the outer insulation aperture of the steel pipe and improving the conductivity of seawater will increase the density of the induced current in the insulation aperture, thereby increasing the risk of electrochemical corrosion. As the cable pitch and AC frequency increase, the current density in the steel pipe pores will also rise. [ABSTRACT FROM AUTHOR]

This study investigates the effect of concrete-steel debonding on the modal dynamic properties, i.e. frequency values and vibration mode shapes, and deals with the identification of interfacial debonding zones in CFST columns. In this context, debonding, defined as a decrease in concrete stiffness at the connection with the steel tube by a depth of 3 mm, was analyzed through the frequency analysis of the column. The findings indicate that debonding results in a discernible disparity between the primary and secondary mode shapes, as assessed by the Modal Assurance Criterion (MAC). Furthermore, there is a reduction in the frequency values across all modes. This paper proposes the irregularity detection indices of the Modal Curvature Index (MCI), Modal Curvature-Slope Index (MCSI) and Modal Curvature-Curvature Index (MCCI) using the concept of curvature to detect the debonding zones. These indices indicate the relative extreme values of irregularities in debonding zones for all modes, such that the MCSI and MCCI show an increase in the height of irregularities regardless of negative or positive signs in a debonding zone as the damage severity in that zone increases. [ABSTRACT FROM AUTHOR]

A multi-scale numerical model was used to study the mechanism of steel tube restraint effect on the concrete of concrete-filled steel tube (CFST) columns under compression. In the developed model, aggregates, mortar, and interfacial transition zones (ITZ) of concrete core were fabricated in meso-scale; however, steel tube was considered to be in macro-scale to investigate the interactions of all phases. Parametric analyses with different diameters and steel ratios were conducted to evaluate multi-scale model. The developed multi-scale numerical simulation could be employed to provide a better understanding on cracking processes of concrete and stress paths of steel tubes in CFST. [ABSTRACT FROM AUTHOR]

Wind turbine towers are growing taller to obtain larger thrust load and capture more wind energy at higher elevations. To address the excessive vibration problem of conventional steel tube (ST) towers, a concrete-filled double-skin tubular (CFDST) tower can be a great alternative owing to the advantages of excellent mechanical properties and cost efficiency, especially for tall and slender wind turbines. This paper developed a theoretical model of the CFDST-based wind turbine system by combining the Lagrangian method with the Galerkin method. Subsequently, the analytical solutions of both fundamental frequency and cross-wind displacement response were obtained. The wind tunnel tests of scaled CFDST and ST wind turbine systems, with variations in hollow ratio, tube length, and wind speed, were used to verify the proposed model. The predicted results from the analytical model showed reasonable agreement with the test results. Finally, the validated analytical solutions were employed to further analyze the influence of hollow ratio, length-to-diameter ratio, and tip-mass ratio. This study makes a contribution to the integration analysis of the CFDST-based wind turbine system in theory and experiment, and develops a simplified method to predict its dynamic behavior. [ABSTRACT FROM AUTHOR]

In the context of green and low-carbon materials development, the influence of replacing cement with circulating fluidised bed fly ash (CFBFA) on the expansion properties of paste under different curing conditions was first systematically investigated. Then, self-compacting concretes (SCCs) with recycled aggregate (RA) and CFBFA were prepared for filling steel tubes. The experimental results showed that, because of its unique physicochemical properties, the incorporation of CFBFA significantly increased the water requirement needed to achieve standard consistency of the paste and the pore structure was optimised by a reduction in the number of large pores due to the formation of more expansion components (ettringite, gypsum and portlandite are the main expansive products of CFBFA–cement paste). The influence of factors such as curing condition, age and CFBFA on the expansion effect was notable. With a 40% CFBFA content, the RA SCC in the steel tube showed an expansion of 41 μm/m, while the 28-day compressive strength reached 48.9 MPa. [ABSTRACT FROM AUTHOR]

In recent years, Concrete-Filled Double-Skin Tube (CFDST) columns have become a promising structural system, demonstrating improved strength, ductility, and sustainability when compared to conventional column designs. CFDST columns comprise two steel tubes placed concentrically infilled with concrete. Square and circular hollow steel sections are commonly used in CFDST. The higher strength-to-weight ratio is a primary factor in preferring CFDST columns over Concrete-Filled Steel Tube (CFST) columns. In this study, a circular CFDST column is modeled and validated using the Finite Element Analysis-based software ABAQUS. The response of the CFDST column under blast loading is explored. The influence of variables such as axial loading, outer steel tube thickness, hollowness ratio, angle of incidence, and scaled distance on blast resistance is determined through the parametric study. The study also explores the effectiveness of the introduction of steel reinforcement in the concrete infill of the CFDST column for enhancement of blast resistance. The results highlight that increasing the outer steel tube thickness and incorporating steel reinforcement lead to an enhanced blast resistance of structures. However, the study does not find definitive evidence indicating that the hollowness ratio affects the behavior of the CFDST column. [ABSTRACT FROM AUTHOR]

This paper delves into the impact of the damage accumulation effect, which leads to the degradation of material strength and stiffness, on the seismic resistance of transmission towers. Building upon the elastic–plastic finite element theory, a mixed hardening constitutive model is derived for circular steel tubes, standard elements in transmission towers, incorporating the damage accumulation effect. A user material subroutine, UMAT, is created within the LS–DYNA framework. The program's validity and reliability are established through axial constant–amplitude loading tests on single steel tubes. The subroutine is employed to conduct the incremental dynamic analysis (IDA) of an individual transmission tower and to contrast it with the structure utilizing the Plastic Kinematic material model, assessing the discrepancies in tower top displacements and segment damage indices (SDIs) at both macroscopic and microscopic scales. The results shows that the Plastic Kinematic model inflates the seismic performance of the transmission tower. When considering the damage accumulation effect in structural failure, the damage index of the members increases, leading to a reduction in both the structural strength and stiffness. The dynamic response in the plastic phase becomes more pronounced, and the onset of structural failure is accelerated. Consequently, structural analysis under seismic conditions should account for the damage accumulation process. Through the delineation of member and segment damage, the extent of damage to transmission tower segments can be quantitatively assessed. Subsequently, the ultimate load–bearing capacity and the most vulnerable location of the transmission tower can be ascertained. Finally, this paper provides a detailed analysis of the transmission tower collapse process under seismic action and summarizes the mechanism of collapse for the structure. [ABSTRACT FROM AUTHOR]

To address the issue of local buckling in thin-walled steel tube concrete columns, a form of helical stiffening ribs was proposed. Axial compression tests were conducted on five sections of square steel tube concrete column specimens. The research revealed that, compared to ordinary steel tube concrete columns, the axial compression bearing capacity and deformation capacity of steel tube concrete columns with helical rib constraints increased by 18.5% and 7.7%, respectively. The helical ribs effectively enhanced the buckling resistance of the thin-walled steel tube concrete components. The failure pattern of this new type of component was characterized by diagonal cracks in the encased concrete aligning with the direction of the helical ribs, and the buckling of the steel tube walls was concentrated between the helical stiffening ribs. Based on the experiments, an analysis of the buckling performance of thin-walled steel tubes with helical rib constraints was conducted, and this was incorporated into the bearing capacity calculation. The test, simulation, and theoretical calculations showed that the bearing capacity error of the composite columns for each specimen was within 10%. Ultimately, a formula for the critical buckling bearing capacity of the helical rib steel tubes was proposed. The research findings provide a foundation for the engineering application of this new type of component. [ABSTRACT FROM AUTHOR]

Concrete-filled built-up cold-formed steel (CFS) columns offer enhanced load-carrying capacity, improved strength-to-weight ratios, and delayed buckling through providing internal resistance and stiffness due to the concrete infill. Integrating sustainable alternatives like self-compacting geopolymer concrete (SCGC) with low carbon emissions is increasingly favoured for addressing environmental concerns in construction. This review aims to explore the current knowledge regarding CFS built-up composite columns and the performance of SCGC within them. While research on geopolymer concrete-filled steel tubes (GPCFSTs) under various loads has demonstrated high strength and ductility, investigations into built-up sections remain limited. The literature suggests that geopolymer concrete's superior compressive strength, fire resistance, and minimal shrinkage render it highly compatible with steel tubular columns, providing robust load-bearing capacity and gradual post-ultimate strength, attributed to the confinement effect of the outer steel tubes, thereby preventing brittle failure. Additionally, in built-up sections, connector penetration depth and spacing, particularly at the ends, enhances structural performance through composite action in CFS structures. Consequently, understanding the importance of using a sustainable and superior infill like SCGC, the cross-sectional efficiency of CFS sections, and optimal shear connections in built-up CFS columns is crucial. Moreover, there is a potential for developing environmentally sustainable built-up CFS composite columns using SCGC cured at ambient temperatures as infill. [ABSTRACT FROM AUTHOR]

Summary: This study aims to investigate the bonding performance of novel composite specially shaped concrete‐filled steel tubes (CFSTs). Seven novel composite L‐shaped CFST specimens were designed utilizing variations in steel tube wall thickness, steel tube length, and concrete strength as the primary parameters. Their failure modes, load–slip relationships, and longitudinal strain distribution patterns were examined through push‐out tests. Additionally, finite element models of the members were established using nonlinear spring elements based on the experimental data and subjected to numerical analysis. The research findings indicate that the ultimate bond strength of the composite L‐shaped CFSTs is positively correlated with steel tube wall thickness, steel tube length, and concrete strength. The strain distributions on the concave and convex faces of the L‐shaped steel tubes are identical. The results obtained from the finite element analysis closely match the experimental findings. [ABSTRACT FROM AUTHOR]

A three-roll rotary piercing process (TRPP) with severe plastic deformation was conducted to fabricate stainless steel seamless tubes with high mechanical properties. To address production issues such as front stuck (FS) where the front end of the tube blank cannot be accessed, back stuck (BS) where the back end of the tube blank cannot be penetrated and steel-heaping, the process parameters were optimized through simulation and testing in this study. Additionally, a novel structure for the three-roll piercing machine was designed and developed. The changes in roll feed angle and plug advance were systematically analyzed for defects in the tubular billet. Numerical simulations of the TRPP were performed using commercially available Finite Element Method (FEM) software (Simufact Forming) to analyze the stress, strain, temperature, friction stress and contact pressure distributions in the pierced tubes. Experimental analysis was conducted on seamless tubes with different diameter defects, leading to the discovery of optimal rules. Based on the experimental and numerical investigations, it was found that optimizing plug advance and roll feed angle can enhance the forming quality of the tubes. [ABSTRACT FROM AUTHOR]

The aim of the research was to select a material from which a washer can be made, so that it can be connected to an E355 steel tube by Rotary Friction Welding (RFW). It was decided to choose the steel grade X6CrMo17-1. The numerical model of the RFW process was built using the finite element method (FEM) using the ADINA System software. The numerical model takes into account the friction coefficient with variable values depending on the temperature. Numerical simulations of the process made it possible to determine the temperature fields in the weld cross-section. For the assumed process parameters: rotational speed of 14,000 rpm, friction time of 1.5 s and friction force of 600 N, the peak temperature occurred in the middle of the friction surface at the end of the friction phase and amounted to 1050 °C. The results of the temperature analysis are one of the most important parameters for the implementation of subsequent calculations, such as the calculation of structural changes, hardness, residual stresses and deformations. [ABSTRACT FROM AUTHOR]

Hybrid fiber-reinforced polymer (FRP)–concrete–steel hybrid double-skin tubular columns (DSTCs) consist of an outer FRP tube, an inner steel tube, and a layer of concrete filled between the two tubes. Previous studies have demonstrated their structural performance compared with conventional concrete columns. However, their durability performance, particularly in aggressive environments, is not well understood. Therefore, this study aims to investigate the durability of hybrid DSTCs subjected to a wet–dry cyclic environment for up to 2 years. The time-dependent behaviors, including axial load–strain (axial and hoop) curves, ultimate load, and ultimate axial and hoop strains, are tested and discussed with regard to the aging time. The test results indicate that the ultimate load of hybrid DSTCs with a 6-mm glass fiber–reinforced polymer (GFRP) tube continuously increased by 15.1% after 2 years of exposure primarily owing to the increase in concrete strength. By contrast, the ultimate load of hybrid DSTCs with a 3-mm GFRP tube increased by 15.2% after 1 year of exposure and increased by 11.6% after 2 years of exposure. The results indicate that the reductions can be attributed to the degradation of the GFRP tubes, especially for a thinner GFRP tube. In addition, a design-oriented stress–strain model for concrete in DSTCs is verified against the test results. [ABSTRACT FROM AUTHOR]

One of the main barriers to the acceptance and adoption of fiber-reinforced polymers (FRPs) in structural engineering is the brittle failure mode and limited ductility caused by the elastic-brittle behavior of FRP materials. Introducing a compression-yield (CY) system is a promising way to improve the ductility of the structures. This study adopted perforated steel cylinders as CY systems. Six perforated steel cylinders with different parameters were tested, and the applicability of the existing constitutive model on the perforated steel cylinders was verified. A design approach for perforated steel members on the structures was proposed. Based on the proposed design approach, ultralightweight cement composite (ULCC)-filled carbon fiber-reinforced polymer (CFRP)-wrapped steel tube arches with perforated steel cylinders were designed and experimentally investigated. The ULCC-filled CFRP-wrapped steel tube arches with CY systems achieved high ductility while maintaining favorable bearing capacity. A model was then proposed to evaluate the damage degree of the structures based on the longitudinal strain in the CY system. The damage development of the structures can be detected based on the measurement of the CY system, which enables early warning to be provided before structural failure. [ABSTRACT FROM AUTHOR]

The hybrid fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular column (DSTC) is an innovative composite member consisting of an outer FRP tube and an inner steel tube, with the space between them filled with concrete. The incorporation of ultrahigh-performance concrete (UHPC) in the DSTC yields numerous advantages over the DSTC with normal-strength concrete, particularly in terms of enhanced impact resistance. While the lateral impact behavior of UHPC DSTC has been investigated, its residual performance after lateral impact remains unexplored. To comprehend the postimpact behavior of UHPC DSTC, a series of specimens underwent lateral impact, followed by static axial compression. Additionally, intact specimens were subjected to static axial compression for comparative analysis. Subsequently, refined numerical models were developed and validated using the obtained test data. Detailed parametric studies were carried out to investigate the influences of key variables on the postimpact behavior of UHPC DSTCs. Finally, based on the ratio of residual midheight deflection to the column height, a prediction formula was proposed for the rapid damage evaluation of impacted UHPC DSTCs, and threshold values for different damage levels were suggested based on a defined damage index. [ABSTRACT FROM AUTHOR]

Glass fiber-reinforced polymer (GFRP) anchor bolts are a new type of high-performance nonmetallic anchor with significantly higher tensile strength, a lighter weight, better corrosion resistance, and a lower cost than steel bars. Therefore, exploring the durability and bonding performance of GFRP anchor systems is of great importance for the structural design of protective engineering, especially in coastal environments. However, insufficient research has been conducted on the durability of GFRP resin bolts in seawater conditions, with no universal standard on the pullout testing of GFRP bolts. To study the durability and bonding performance of GFRP resin bolts, durability experiments were conducted in this work using artificial seawater, and the pullout tests were conducted using a large-scale concrete platform with different compressive strengths (21.2, 40.8, and 61.3 MPa). The results of the durability experiments indicated that the strength variations of the GFRP rods and epoxy resin materials in artificial seawater environments were less than 5%. Subsequently, indoor pullout tests using steel tubes filled with epoxy resin were conducted, and the test results indicated a critical anchor length value. Pullout tests of the GFRP resin bolts embedded in large-scale concrete blocks were also conducted with different strengths. According to the test results, all GFRP resin bolts embedded in the three concrete blocks with different compressive strengths exhibited rod fracture failure. The failure mode was not controlled via the compressive strength of the concrete blocks due to the high bonding strength between the resin and the rod, as well as between the resin and the concrete. Therefore, this GFRP resin anchor system could fully utilize the tensile strength of GFRP rods. This research offers significant practical value in verifying the safety and reliability of GFRP resin bolts in corrosive marine service environments, and it contributes to the application and development of GFRP materials in the engineering field, serving as a valuable reference for the structural design and further study of GFRP bolts. [ABSTRACT FROM AUTHOR]

A novel technique for evaluating roller expansion joint quality of ASTM A192 carbon steel tubes to SA 299 Gr.A tube sheet by correlation with its magnetic coercive force (MCF) has been presented. Roller expansion joint strength in respect of different percentages of apparent wall reduction (%WR) and temperature rise up to 600 °C was studied. The MCF, hardness values, pull-out strength, and microstructure of roller expanded joints were compared and analyzed concerning various %WR. It is observed that MCF and hardness values increase uniformly up to 12%WR with an increase in pull-out strength. A correlation between hardness and MCF was found. However, the development of severe plastic deformation at the inner tube surface above 12%WR gives a sharp rise in MCF values with decreased pull-out strength, which indicates over-expansion. Recovery and recrystallization of expansion joints during repair of neighboring tube reduce hardness, MCF, and pull-out strength with little change in microstructure. The MCF value after rolling has been critical to obtaining maximum strength. The study evaluated criteria for expansion joint quality using MCF within the recommended expansion range of 6-12%WR. It is inferred that MCF can effectively monitor expansion joint strength during initial rolling and after repair. [ABSTRACT FROM AUTHOR]

A newly developed microseismic (MS) monitoring system was employed in the Tianshan-Shengli tunnel to detect MS activities and then predict and provide early warning of rockburst disasters. The system not only has the advantages of accuracy of artificial analysis but also real-time analysis and warnings. The positioning accuracy for MS events is approximately 5-10 m. A new sensor installation scheme was proposed to achieve fast sensor installation and recovery, taking advantage of semicircular steel tubes and hose clamps. In addition, the rockburst risk level prediction criteria adopted multiple evaluation indexes such as MS event energy and moment magnitude and number, and it revealed that the evolution of maximum energy has a good positive correlation with that of maximum moment magnitude through analyzing the monitored MS events. It also showed that the rockburst generally occurred 2 days after the rock mass was exposed by the tunnel boring machine (TBM) tail shield and belonged to the delayed rockburst category, according to the field statistical results. The preliminary application cases indicated that the rockburst prediction and early warning based on MS monitoring agree with the site survey results. Therefore, the new MS monitoring system is a reliable tool for predicting and providing early warnings of rockburst disasters. [ABSTRACT FROM AUTHOR]

The present study was conducted to investigate the root cause failure analysis of 316L stainless steel air tube inlets at the weld joint in a High-Speed Power turbine (HSPT). Fractography analysis revealed ratchet marks and striations on the fracture surfaces, indictive of vibration-induced fatigue in the heat affected zone of the tube-to-flange weld joint, resulting from the rotation of the HSPT at high speed, identified as the primary cause of failure. Additionally, radiography and visual inspection unveiled a sharp interface between the weld metal and parent metal within the inner wall, acting as a stress riser. X-ray diffraction analysis demonstrated that the parent metal predominantly consisted of austenite (γ), while the weld metal comprised a mix of austenite (γ, ≈ 90%) and delta ferrite (δ, ≈ %10). Hardness readings in the weld zone ranged from 260–302 HV, compared to ≈ 140 HV in the parent metal. Microstructural examination showed inclusions and impurities rich in manganese, and silicon, likely introduced during manufacturing and/or welding processes. Conclusively, the study attributes failure to the use of low-quality materials and consumables, poor welding practices, and insufficient vibration condition monitoring. [ABSTRACT FROM AUTHOR]

To address the structural concerns of a 12.0 m-span landing assembled single-tube frame (LASF) for Chinese solar greenhouses subjected to snow loads, the internal forces and deformations of LASF and its reinforced counterpart (RLASF) were numerically simulated to determine the ultimate bearing capacities (Lu) and the failure loads (Lf). During the simulations, steel tubes were modeled as beam188 elements and cables as link180 elements. The frame constraints and the connections were assumed to be fixed supports and rigid, respectively. The loads were determined according to the Chinese standard (GB51183-2016). Simulations revealed that the LASF and RLASF primarily withstand bending moments and are prone to strength failures under snow loads. Both exhibited lower Lu and Lf under non-uniform snow loads than under uniform snow loads. The results also indicated that crop loads could deteriorate the structural safety of the LASF and RLASF. Lu and Lf were found to be proportional to the section modulus of the tubes. The effects of wind loads and initial geometry imperfections on Lf of the LASF and RLASF can be neglected. Furthermore, the RLASF exhibited higher Lf compared to the LASF. Steel usage of the RLASF could be further reduced by replacing circular tubes with rectangular tubes, making the RLASF a feasible option for constructing Chinese solar greenhouses. [ABSTRACT FROM AUTHOR]

This article mainly investigates the axial compressive behavior of GFRP-repaired dented steel tubes through experimental and numerical methods. First, the experimental tests are implemented to introduce dented damage, perform repair processing, and research the final compression response. Subsequently, numerical simulation is conducted to characterize the dented deformation and the axial compression behavior of GFRP-repaired dented steel tubes. Finally, the parametric studies of GFRP-repaired steel tubes are further conducted numerically. The results demonstrated that GFRP can evidently polish up the mechanical behavior of dented steel tube, delay local buckling, and finally improve the ultimate compression strength of steel tubes. [ABSTRACT FROM AUTHOR]

The axial compressive behaviours of coal gangue concrete-filled steel tube (GCFST) columns after chloride salt corrosion were investigated numerically. Numerical modelling was conducted through the static analysis method by finite element (FE) analysis. The failure mechanism, residual strength, and axial load–displacement curves were validated against tests of the coal gangue aggregate concrete-filled steel tube (GCFST) columns at room and natural aggregate concrete-filled steel tube (NCFST) columns after salt corrosion circumstance. According to the analysis on the stress distribution of the steel tube, the stress value of the steel tube decreased as the corrosion rate increased at the same characteristic point. A parametric analysis was carried out to determine the effect of crucial variation on residual strength. It indicated that material strength, the steel ratio, and the corrosion rate made a profound impact on the residual strength from the FE. The residual strength of the columns exposed to chloride salt was in negative correlation with the corrosion rate. The impact on the residual strength of the column was little, obvious by the replacement rate of the coal gangue. A simplified design formula for predicting the ultimate strength of GCFST columns after chloride salt corrosion exposure was proposed. [ABSTRACT FROM AUTHOR]

Six hybrid steel and PVA fiber cementitious composite (S&PFCC)‐encased concrete‐filled steel tube (CFST) columns were designed and fabricated, and the effects of axial compression ratio, core concrete strength grade and steel tube thickness on the seismic performance of composite columns were studied. The test results show that the increase of core concrete strength can obviously improve the bearing capacity and energy dissipation capacity of composite column and can restrain the strain growth rate of steel tube after yielding. Low axial compression ratio and thicker steel tube thickness can improve the overall seismic performance of composite columns. Three calculation methods are modified, and the modified An method can predict the lateral bearing capacity of composite column well. Finally, based on the OpenSees finite element software, the seismic performance of the CFST column wrapped in S&PFCC is simulated to verify the test, and the numerical study shows that the seismic performance of the composite column can be optimized by increasing the steel fiber content, while PVA fiber has a weak effect in the elastic stage, and can effectively improve the ductility of the composite column in the plastic stage. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tubular (CFST) columns have been widely used in multi-story and high-rise frame structures. During the service period, they may suffer vehicle impact due to traffic accidents or terrorist attacks. This paper numerically evaluates the performance of CFST columns under vehicle impact and investigates the effects of carbon FRP (CFRP) wrapping arrangements on performance improvement of the columns. Before that, a numerical model was developed to simulate the responses of CFST columns without and with FRP wrapping under vehicle impact and post-impact axial compression, and then calibrated by reported tests. Evaluation results show that the performance of CSFT columns under vehicle impact is divided into five levels, i.e., no repair required, rapid repair required, minor repair needed, major repair needed, and replacement needed. The performance level decreases with the increase in the vehicle weight or speed and increases with the increase in the column diameter or steel tube thickness. The column height has little effects on the performance level. A higher axial load ratio, e.g., 0.5, might reduce the performance level. Besides, a CFST column tends to fail in flexure mode when hit by F800 medium truck, while it may fail in flexure & shear mode when hit by C2500 pickup truck. Investigation results indicate that FRP wrapping with each layer orientation of 90° (i.e., in the longitudinal direction) and 0° (i.e., in the hoop direction) present the best performance improvement for a CFST column possibly undergoing flexure & shear and flexure failure, respectively. The increase of the number of FRP layers effectively improves the performance levels of CFST columns but the excessive demand may be not economical. It is not necessary to employ an FRP wrapping range of 100% for improving the vehicular impact performance level of a CFST column to the expected one. [ABSTRACT FROM AUTHOR]

In the engineering structures using circular steel tube (CST), the eccentric loading of CST members often occurs due to installation error and structural form. Existing research mainly focuses on the bearing resistance of circular steel pipes under the condition of eccentricity at one end. In practical application, both ends of CST members also may be eccentric. And with the increase in eccentricity, material yielding may occur prior to flexural buckling. So far, it's lack of reports about the mechanical behavior of CST members subjected to eccentric loading at both ends and the boundary between material yielding and flexural buckling. This paper presents experimental, numerical and theoretical studies on bearing resistance of thin-walled circular steel tube with slenderness ratio of 30, 40 and 50, subjected to eccentric loading at one end and both ends, respectively. The study reveals a significant discrepancy in the prediction of bearing resistance for circular tubes subjected to eccentric loading at both ends according to existing design codes. Considering the synthesis of bending moment and deflection caused by eccentric loading at both ends, the calculation method of the bearing resistance based on flexural buckling of CST is established, which enhances the prediction accuracy of test verification. In addition, a theoretical boundary between the two failure modes appearing in CST members under eccentric loading at one and both end(s) - flexural buckling and reaching the material yielding strength - was established as dominated by the slenderness ratio and loading eccentricity. [ABSTRACT FROM AUTHOR]

At high temperatures, coatings provide a protective scale development on surfaces to maintain long-term stability. In the current study, ASTM-SA210-Grade A1 (GrA1) and ASTM-SA213-T-11 (T11) boiler tube steels were coated with NiCrAl alloy with high-velocity oxy-fuel (HVOF) to prevent oxidation and hot corrosion. For hot corrosion and oxidation, 50 cycles at 900°C were taken into account. Additionally, tests of hot-corrosion behavior were conducted in an atmosphere containing molten salt (Na2SO4-60%V2O5), while tests of oxidation behavior were conducted in static air. The kinetics of oxidation were calculated using the thermogravimetric method. Using XRD, EPMA, and SEM/EDAX methods, the produced oxide scales were characterized. The oxidation rate of NiCrAl-coated steels was found to be lower than that of uncoated steels. The coated steels subjected to oxidation in air exhibit slow scale growth kinetics and oxides of α-Al2O3 and Cr2O3 on the outermost surface, while accelerated oxidation caused by the molten salt exhibits metastable Al2O3. Along the nickel-rich splat boundary, Cr and Al were formed a preferential oxidation, which prevents other oxygen from entering the coating via pores and voids, resulting in steady-state oxidation. [ABSTRACT FROM AUTHOR]

The circular hollow steel-reinforced concrete (HSRC) column consists of an inner circular hollow steel tube and outer circular hollow reinforced concrete (RC). This design provides several advantages, including being lightweight, having a wide sectional profile, and having a high flexural stiffness. This paper aims to investigate the behavior of the circular HSRC columns under axial compression through testing and finite element (FE) modeling. An FE model was established to simulate the circular HSRC columns under axial compression, which was validated against the test data. Additionally, the load distribution and the interface stress between the outer hollow RC and inner steel tube were analyzed. Subsequently, a systematic parametric analysis was conducted on the diameter (d) and thickness (t) of the steel tube; slenderness ratio (λ); strength of concrete (fcu); yield strength of steel tube (fsy), longitudinal rebar (fly), and stirrup (fgy); as well as the stirrup spacing (s). The critical influencing factors of the circular HSRC columns under axial compression were identified. fcu, λ, d, fly, and fsy dramatically influence the bearing capacity, and the stiffness is notably affected by λ and fcu. Finally, three simplified design methods were summarized and evaluated for calculating the bearing capacity of the circular HSRC columns under axial compression. [ABSTRACT FROM AUTHOR]

In the internal support system of foundation pit engineering, concrete support has been widely used in foundation pit engineering due to its advantages of high bearing capacity, unrestricted layout, cast-in-place end nodes and good integrity. However, the disadvantages of concrete support in design and construction are also very obvious, with problems such as large design section, long construction time, heavy pollution, high cost and serious waste. Therefore, in view of the advantages and disadvantages of concrete support in foundation pit engineering and combined with the characteristics of deep foundation pit engineering of metro open cut station, the prefabricated composite concrete filled steel tube internal support is designed in this paper. From the perspective of improving the stiffness and strength of end joints and achieving fixed connection at the end, a fixed end composed of long bolts and mold bag concrete is proposed, which is suitable for the prefabricated composite steel tube concrete internal support of metro foundation pit. The design scheme, structural characteristics and working principle of the fixed end are explained and compared with the cast-in-place concrete support node, which shows that the fixed end plays the role of fixed support in the support system, which is composed of special components such as embedded steel plate, hollow flange, bag concrete and long bolt. In order to establish the theoretical basis for the design and check of the fixed end, the analytical method of the compression and bending load-bearing capacity of the fixed end is obtained by analyzing the mechanical mechanism, failure mode and evolution law of the fixed end. In order to verify the feasibility of the theoretical analysis method of the bearing capacity of the fixed end, the theoretical and numerical results are verified and analyzed by numerical simulation, and the main factors affecting the bearing capacity of the fixed end are parameterized. [ABSTRACT FROM AUTHOR]

The use of carbon fiber–reinforced polymer (CFRP) grid-reinforced engineered cementitious composite (CFGRE) systems to strengthen concrete-filled steel tubes (CFSTs) offers potential to improve the structural response as well as the resistance to high temperatures and corrosion. This study consists of an experimental investigation on the behavior of eccentrically loaded square CFSTs strengthened with CFGRE systems. Nineteen specimens were tested using a compression-testing machine with loads, deformations, and recorded test observations. The investigated parameters included the number of CFRP grid layers, width-to-thickness ratio of a steel tube, concrete strength grade, and eccentric ratio. The strengthened columns exhibited ductile failure. Due to stress concentrations, the CFGRE systems ruptured at the corners on the compression side. After strengthening, the yielding and ultimate loads of the CFST columns increased from 21.8% to 53.5% and 21.1% to 34.9%, respectively. Although an increase in the number of CFRP grid layers did not significantly affect the bearing capacities of the square specimens, it significantly enhanced the ductility of the columns. An increase in the eccentric ratio weakened the confining effect. An average decrease of 9.7%, 27.6%, and 41.4% was observed in the ultimate loads of the strengthened columns when the eccentric ratio increased from 0 to 0.1, 0.25, and 0.4, respectively. A five-point N–M interaction model was developed based on the principles of force equilibrium and deformation compatibility to predict the eccentric bearing capacities of strengthened columns. The prediction results exhibited high accuracy. [ABSTRACT FROM AUTHOR]

Hybrid double-skin tubular columns (DSTCs), which consist of an outer fiber-reinforced polymer (FRP) tube, an inner steel tube, and a concrete annular layer between the two tubes, possess many advantages, such as high load resistance and large deformation capacity and excellent durability. The use of ultrahigh-performance concrete (UHPC) or engineered cementitious composites (ECCs) instead of normal strength concrete (NSC) in the annular layer, given the advantages such as the ultrahigh strength of UHPC and strain hardening and high toughness feature of ECC, endows the DSTCs with additional high performance, such as an enhanced load-carrying capacity (with UHPC) and an improved cracking resistance (with ECC). The behavior of such DSTC specimens with UHPC or ECC that are subjected to eccentric loading was investigated for the first time, to the best of the authors' knowledge, and the results are reported in this paper. The DSTC specimens with NSC and ECC under eccentric compression exhibited similar great ductility with a bilinear shape for the axial load–deformation curves. In addition, the initial cracking of the FRP tube at the tension side of DSTC specimens with ECC or NSC occurred later than that of DSTC specimens with UHPC, which might imply the better durability performance of DSTC specimens with ECC or NSC during the serviceability state. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the response and local buckling of locally sharp-notched C2700 brass circular tubes (LSN C2700 brass circular tubes) under cyclic bending loads. The study considers four different notch orientations (0°, 30°, 60°, and 90°) and five distinct notch depths (0.2, 0.4, 0.6, 0.8, and 1.0 mm). The results reveal that notch orientation and depth exert minimal impact on the moment–curvature relationship, leading to the formation of stable loops. The ovalization–curvature graphs demonstrate a trend of symmetry, serration, and growth with an increasing number of bending cycles. Additionally, larger notch orientations or smaller notch depths result in reduced ovalization. Furthermore, the double logarithmic coordinates of controlled curvature–number of cycles necessary to induce local buckling reveal five non-parallel lines representing different notch depths when the notch orientation is fixed. Finally, by adopting the formulas for smooth tubes and for locally sharp-notched 304 stainless steel circular tubes (LSN SS304 circular tubes), this study adjusts the related material parameters accordingly. These modifications effectively describe the controlled curvature–number of cycles necessary to induce local buckling for LSN C2700 brass circular tubes with different notch orientations and depths under cyclic bending, demonstrating reasonable agreement with the experimental results. [ABSTRACT FROM AUTHOR]