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This editorial highlights six papers published in the Proceedings of the Institution of Civil Engineers: Structures & Buildings. The topics covered include the bond properties of steel rebars in cracked concrete, flexural strengthening of reinforced concrete beams, repairing fire-damaged circular RC columns, seismic performance of a connection between concrete-filled steel columns and RC frames, optimal sensor placement for detecting rail damage, and the mechanical behavior of fasteners in laminated bamboo lumber. The papers present experimental and theoretical research, providing valuable insights into various aspects of civil engineering. [Extracted from the article]

The seismic performance of an innovative lightweight bridge with CFST composite truss girder and lattice pier was studied in this paper. Took Ganhaizi Bridge as prototype, a 1:8 scale specimen with two spans and three lattice high piers was designed for multi-shaking tables test. Adopted design seismic wave of prototype, dynamic characteristics, seismic performance and failure characteristics were analyzed. Results indicate that the frequency ratio and the displacement ratio between specimen and prototype are corresponding to similitude relationship. Under transverse or longitudinal excitation, acceleration in lattice zones significantly amplifies, and decreases the acceleration of the deck. It is unnecessary to consider the influence of bi-directional excitations. Displacement on the top of pier is less than the value limitation under design ground motions. The corresponding finite element simulations, using OpenSees, were carried out and the accuracy was verified. The finite element analysis results agree with experimental data. In addition, the plastic hinges were predicted under transverse and longitudinal excitation respectively, revealed that lightweight bridge with CFST composite truss girder and lattice pier has a favorable seismic performance. [ABSTRACT FROM AUTHOR]

Abstract: Core concrete subjected to confinement effect from steel tube is the main advantage of concrete filled steel tubular (CFST) column under compression. However, this beneficial composite action could be weakened even destroyed by debonding, which is almost unavoidable as a common phenomenon in CFST structures and has not been considered satisfactorily by current design codes. In this paper, the influence of debonding on circular CFST stub columns was investigated experimentally and numerically. The main experimental parameters were the debonding arc-length ratio, the confinement factor and the load mode. Meanwhile, the finite element models were implemented using the ABAQUS finite element software, of which the validity and accuracy were verified by test results. Utilising the finite element model, the parametric analysis was carried out to study the influence of the following parameters on the ultimate load capacity of debonding specimens: debonding arc-length ratio, debonding thickness and confinement factor. Based on the results of parametric analysis, a simplified calculation formula was proposed to calculate the ultimate load capacity of circular CFST stub columns with debonding. [Copyright &y& Elsevier]

The compressive behavior of steel tube reinforced concrete (STRC) columns under eccentric loads was investigated experimentally and numerically in this paper. Primary parameters included the eccentricity ratio, the concrete-filled steel tube (CFST) area ratio, and the longitudinal rebar ratio. This paper analyzed the impacts of these parameters as well as the failure modes, the full-range compression process, and the strain development. Similar to the reinforced concrete (RC) column, the full-range compression process of STRC column under eccentric loads can be divided into three stages, i.e., elastic stage, elastic-plastic stage (cracking stage), and plastic stage (descent stage). The experimental findings showed that, on the near-load side of the STRC column, the outer concrete crushed, and the longitudinal rebars buckled in compression. Conversely, on the far-load side, the concrete exhibited cracking in tension, and the longitudinal rebars exhibited tension. Finite element (FE) analysis results demonstrated that, similar to a RC column, three distinct failure mechanisms can be identified for STRC columns under eccentric loads, i.e., compression-controlled, balanced, and tension-controlled failures. During the compression process, analyses were conducted on the contact stress between the steel tube and concrete, as well as on the load distribution between the inner CFST and the outer RC. The ultimate load of the STRC column decreased with an increase in the eccentricity ratio, while it increased with larger longitudinal rebar ratio and CFST area ratio. Based on the experimental and FE analysis results, a simplified calculation method was developed to determine the eccentricity reduction coefficient for STRC columns. • The eccentric test result of steel tube reinforced concrete (STRC) column with low CFST area ratio was presented. • The effect of eccentricity ratio, CFST area ratio and rebar ratio on the compressive behavior of STRC column were analyzed. • The failure mechanisms, load distribution and contact stress was analyzed by refined finite element analysis. • The simplified calculation method of eccentricity reduction coefficient was proposed for the ultimate load of STRC column. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tubular (CFST) structures have been widely used in high-rise buildings and long-span bridges for the high strength and ductility due to the confinement effect of the concrete core provided by the steel tube. This beneficial composite action could be damaged by circumferential debonding gap (CDG), which is nearly unavoidable in real CFST structures. Reliable formulations to predict the influence of CDG on the mechanical performance of compressed circular CFST columns are needed. In this paper, the experimental parameters and results of compressed circular CFST columns with CDG are summarized to discuss the deficiencies of the parameters chosen in the tests. Detailed finite element simulation procedures for compressed circular CFST columns with CDG, based on ABAQUS, are then introduced, verifying their accuracy by test results. Based on the validated finite element models, a database of compressed circular CFST columns with and without CDG, including strength, ductility and stiffness of specimens is established; it is obtained by covering suitable parameter ranges for CDG and CFST structures, in accordance with real applications. Successively, new formulations able to predict the ultimate load capacity, the reduction factor, the ductility and the stiffness of specimens are derived by the Evolutionary Polynomial Regression (EPR) methodology, demonstrating good accuracy and low complexity. • FEMs to predict the mechanical performance of CFST columns with CDG are validated. • A database including strength, ductility and stiffness of CFST columns is established. • Formulas to predict ductility and stiffness of CFST columns are missing in literature. • New formulas for the ductility and stiffness of CFST columns are proposed by EPR. • The proposed models are consistent with the physical interpretation of the phenomena. [ABSTRACT FROM AUTHOR]

Concrete Filled Steel Tubular (CFST) structures are increasingly used not only for columns in tall buildings but also in the arch trusses of many arch bridges and in the truss girders of buildings and bridge decks. Therefore, the chords of Circular Hollow Section (CHS) truss arches and girders are increasingly filled with concrete, effectively making them CFST structures. In addition to the strength and stiffness of the CFST members, the failure mode of the CFST joint connecting them to the tubular member is also affected by the concrete filling. In this study, truss girders with different web arrangements were tested, and their behaviour investigated. The girders were not slender because they were designed to attain the peak limiting state for joint failure rather than chord failure due to bending moments. Moreover, two other types of girders were tested: one without concrete-filled chords (CHS girder) and another with only the upper chord filled with concrete; thus allowing an investigation of how a concrete-filled chord affects joint failure mode. The geometry of the CHS girder joints was such that only chord face failure and punching shear failure could occur. The former required an inward deformation that was prevented by the concrete filling in a CFST girder with similar geometry. Finally, the study considers extending the Eurocode 3 and the AWS D1.1 code formulae, originally proposed for CHS joints, to calculate the resistance of CFST joints. [ABSTRACT FROM AUTHOR]

To improve the seismic performance of tall-pier bridges, an innovative tall pier with a composite box section, composed of four-limb concrete-filled steel tube (CFST) columns and low-yield-strength (LYS) steel plates, was proposed based on the principle of capacity design and the replaceable fuse concept. Both axial and eccentric compression tests were conducted on a series of specimens to study the compressive performance of the composite tall pier. Parametric analysis was performed to investigate the effects of load eccentricity, slenderness ratio, thickness, and yield strength of the LYS steel plates on the compressive capacity using validated finite element (FE) models. The main results indicate that the composite tall pier fails with global bending deformation and elastoplastic instability under axial or eccentric compression. The compressive capacity and initial stiffness of the composite tall pier decrease significantly with an increase in eccentricity or slenderness ratio, and the influences of eccentricity and slenderness ratio are essentially independent. Additionally, the increase in the thickness of the steel plates enhanced the flexural stiffness and compressive capacity, and reduced the uneven loading distribution for the pier. Under axial compression, the pier can be simplified as a CFST lattice column connected by steel beams and LYS steel plates. Finally, practical formulas for the compressive capacity are proposed considering the connection and restraint effects of the LYS steel plates on the CFST columns, which can be used to predict the compressive capacity of the novel composite tall pier. • A seismic resilient composite tall pier with replaceable components is proposed. • Axial compressive experiments of the composite tall pier are conducted. • Compressive performance of the composite tall pier is analyzed. • Analytical models for calculating the resistance of the pier are proposed. [ABSTRACT FROM AUTHOR]

Concrete-encased CFST (CE-CFST) member refers to a hybrid structure consisting of outer reinforced concrete and inner CFST components. This paper studies the compressive behavior of CE-CFST stub columns with initial stress on steel tube. Nine compression tests on CE-CFST stub columns were completed. The influences of initial stress degree and eccentricity on the failure mode, load-displacement behavior and ultimate strength were investigated. The crushed concretes with locally buckled longitudinal rebars were observed at the middle height of specimens. The ultimate strength of CE-CFST stub column slightly decreases as initial stress degree increasing. The maximum decreasing range on the ultimate compressive strength was 9.3% as the initial stress degree increasing in range of 0–0.5. A finite element model of CE-CFST stub column was developed and verified against test results. The parameters study and full-range compression process analysis were performed using the verified finite element model. The strength contribution provided by the inner CFST and outer RC components and interactions between concretes and steel tube were discussed. The influence mechanism of initial stress on the compressive behavior of CE-CFST stud column was revealed by the test and finite element results. The constraint effect on the core concrete provided by steel tube decreased due to the presence of initial stress, which reduced the strength contribution of the CFST component to the CE-CFST column. A simplified method was proposed to predict the influence coefficient of initial stress on the ultimate strength of the CE-CFST column based on the test and finite element results. • Compression tests of CE-CFST stud column with initial stress on steel tube were completed. • Finite element analysis of CE-CFST stud column with initial stress on steel tube was realized. • The influence mechanism of initial stress on compressive behavior of CE-CFST column was revealed. • The influence coefficient of initial stress on ultimate strength of CE-CFST column was proposed. [ABSTRACT FROM AUTHOR]

The failure modes of K-joints of Concrete Filled Steel Tubular (CFST) truss structures were investigated through laboratory tests. CFST K-joint specimens consisted of a tubular chord filled with concrete and two braces inclined at the same angle with respect to the chord. Their size was chosen of the same scale as that of joints of real tubular structures currently used in bridges. The results were compared with those obtained by testing K-joint specimens of Circular Hollow Section (CHS) truss structures with the same size. Contrary to CHS K-joints, where a chord face failure mode occurred, in CFST K-joints no failure mode with inward deformation of the chord section can occur. Hence, punching shear failure was shown to be the typical failure mode of CFST K-joints, if no brace failure mode occurred before. Resistance of CFST K-joints to punching shear was then compared with the resistance values calculated through the Eurocode 3 and the AWS code formulae on CHS K-joints. The values provided by the former formula were quite close to (although always lower than) the actual resistance to punching shear of CFST K-joints. Finally, the favourable effect on joint resistance due to additional studs welded to the interior surface of the chord tube of CFST K-joint was also investigated. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tubular (CFST) columns are increasingly used around the world because they offer two significant advantages. The first one is the composite action of the steel tube and infilled concrete, which enhances the strength and ductility of the columns. The second one is the use of the steel tube as a permanent formwork for concrete casting, which allows saving construction time and costs. Although considerable research and several experimental tests have been carried out on CFST columns, there is not a probabilistic model for the evaluation of their axial capacity yet. Additionally, the effects of the axial load eccentricity and debonding have received little attention so far. Within this framework, the present paper proposes a physics-based probabilistic model to predict the ultimate axial capacity of CFST columns, which is developed as the sum of a deterministic part and a probabilistic correction term, together with two additional corrective models to describe its reduction due to load eccentricity and debonding, which are developed coherently with the axial capacity model as probabilistic correction terms. The accuracy of the proposed models is compared with that of existing capacity equations already in use within technical standards and available literature. Additionally, this study provides uncertainty factors to allow using the proposed capacity model for design applications. Thanks to their very good accuracy and compact form, the proposed models are suitable to be included within technical standards. On the other hand, differently from common deterministic models, they can also be updated as new experimental data are available. A case study with the derivation of fragility curves for a CFST bridge column shows a possible application and the advantages of the proposed model. • Probabilistic model for the axial capacity of Concrete Filled Steel Tubes (CFST). • Capacity reduction factors for load eccentricity and debonding. • Bayesian model calibration using all available data from the literature. • Comparison between formulations from literature and technical standards. • Calibration of uncertainty factors corresponding to different reliability levels. [ABSTRACT FROM AUTHOR]

• A simplified FEM approach for CFST K-joints with and without steel studs is proposed. • The modeling key parameters for a reliable FEM procedure of CFST K-joints are discussed. • Cohesive-interaction properties for simulating the shear studs are investigated. • The stiffness and strength capacity of CFST K-joints is assessed by FEM strategy. Concrete Filled Steel Tubular (CFST) K-joints employed for truss structures gained high interest in the last years due to their widespread use in engineering practice. The overall performances of these joints can be efficiently improved by using steel studs welded in the inner surface of the steel chord filled with the concrete, avoiding punching shear failure, and improving the overall strength and ductility. However, a reliable prediction of the structural behavior of the joints is outmost of importance for the assessment of the capacity of new and existing structures, and there are no standardized design methods nowadays. In this paper, the structural performances of CFST K-joints with and without steel studs are investigated by Finite Element Modelling (FEM) approach with the aim to provide a predictive tool for the design. A comprehensive discussion of the key parameters that govern the FEM procedure as well as the calibration of the FE models was provided to give the basis for a reliable modelling of CFST K-joints with and without studs for the predictions of the load–displacement/strain response and the strength, considering the main failure mechanisms. In this context, in addition to the detailed FEM of CFST K-joints with steel studs, a simplified equivalent FEM approach is proposed to reduce computational effort keeping the same accuracy. [ABSTRACT FROM AUTHOR]

Concrete filled steel tubular (CFST) structures have become a viable alternative to reinforced concrete or steel structures due to several advantages. One of the most important is the confinement effect of the concrete core provided by the steel tube. However, this beneficial composite action will be probably weakened by debonding, so reliable formulations to predict the reduced ultimate resistance are needed. In this paper, a review of existing specifications and experimental tests carried out on compressed circular CFST columns with and without debonding, is given. Accordingly, more circular CFST long specimens with debonding should be necessary to understand the combined influence of slenderness ratio, load eccentricity ratio and confinement factor on the reduction coefficient (K D) of ultimate load capacities (N u). The combined influence of arc-length ratio and thickness of circumferential debonding gap on K D should be further studied by experimental tests. Moreover, the existing formulae for K D and N u show a low accuracy in predicting the test results and should be improved. To this aim, an evolutionary polynomial regression (EPR) MOGA-based methodology was performed to obtain more accurate formulations for N u and K D of circular CFST columns with debonding. The formulae extracted from the Pareto front of non-dominated solutions, demonstrate good accuracy, higher than the ones in literature. The proposed models are consistent with the physical interpretation of the studied phenomenon according to which N u and K D decrease as debonding parameters increase and can be used to calculate the real resistance of CFST structures. • Experimental tests on compressed circular CFST columns with debonding are summarized. • The accuracies of existing models to predict the reduction coefficient K D of N u are low. • Formulaeto predict the ultimate load capacities N u with debonding are missing. • New formulaefor K D and N u are proposed by EPR. • The proposed models have good accuracies and physical meaning. [ABSTRACT FROM AUTHOR]