Eccentric Compressive Behavior of Round-Ended Rectangular Concrete-Filled Steel Tubes with Different Central Angles

The application of round-ended rectangular concrete-filled steel tubes (RRCFSTs) in high-rise buildings or bridge structures is increasing, improving structural performance and meeting aesthetic requirements. Researching this novel steel–concrete composite helps to fully utilize the properties of the materials. In this study, 15 specimens were tested for analysis of the behaviors of RRCFSTs with different central angles under eccentric compression. Influences of central angles of round ends (θ), aspect ratios of rectangular parts (κ), steel strength and the eccentric ratio on failure modes, material utilization, confinement effect and eccentric bearing capacity are studied. Besides, the mechanism of confinement effects of steel tubes with different θ values was evaluated with the finite element method (FEM). The results show that local bulking usually occurs at the compression zone. When θ gradually changes from 0° to 180°, the local bulking position of straight steel plate changes from mid-length to both ends of the columns. Additionally, the interfacial stress between steel tube and concrete at round ends rises, but that at the corner, it decreases continuously, which results in an improved overall confinement effect and increased material utilization. In contrast, a larger κ leads to lower material efficiency because of the reduced overall confinement effect. The increases in both θ and κ enlarge the cross-sectional area and the eccentric ultimate bearing capacity, whereas θ has a better influence on the ductility than κ. A feasible simplified calculating approach for the eccentric ultimate bearing capacity of RRCFSTs is presented and validated.

Short columns composed of concrete-filled steel tubes in a fire situation – Numerical model and the “air-gap” effect

The increase in temperature reduces the strength of steel and concrete, in such a way that it is essential to verify concrete-filled steel tube columns in fire situations. Numerical simulations, with lower costs than laboratory tests, have great importance in checking resistance and defining simplified methods for design practice. However, peculiarities of the thermal and mechanical behavior of heated confined concrete and the air-gap effect (a phenomenon inherent to concrete-filled steel columns) must still be better understood. Therefore, this study presents the development of a numerical model performed in the ABAQUS software (Dassault Systemes SIMULIA Corp., 2014) for the thermomechanical analysis of short columns composed of circular and square concrete-filled steel tubes considering the air-gap effect. The air-gap phenomenon is presented and analyzed according to possibilities of implementation to the numerical model and, finally, the proposed numerical model is validated with experimental results presented in the literature. According to the study results, the numerical model can be used to define and adjust simplified methods for verification of composite columns in fire situation. The importance of considering the air-gap effect in numerical modeling was confirmed, taking into account that disregarding its effect may result in overestimated responses of the steel tube resistance in fire situations. Moreover, it was suggested thermomechanical joint analysis and the use of the explicit solver as a strategy to minimize processing time.

Novel Fuzzy-Based Optimization Approaches for the Prediction of Ultimate Axial Load of Circular Concrete-Filled Steel Tubes

An accurate estimation of the axial compression capacity of the concrete-filled steel tubular (CFST) column is crucial for ensuring the safety of structures containing them and preventing related failures. In this article, two novel hybrid fuzzy systems (FS) were used to create a new framework for estimating the axial compression capacity of circular CCFST columns. In the hybrid models, differential evolution (DE) and firefly algorithm (FFA) techniques are employed in order to obtain the optimal membership functions of the base FS model. To train the models with the new hybrid techniques, i.e., FS-DE and FS-FFA, a substantial library of 410 experimental tests was compiled from openly available literature sources. The new model’s robustness and accuracy was assessed using a variety of statistical criteria both for model development and for model validation. The novel FS-FFA and FS-DE models were able to improve the prediction capacity of the base model by 9.68% and 6.58%, respectively. Furthermore, the proposed models exhibited considerably improved performance compared to existing design code methodologies. These models can be utilized for solving similar problems in structural engineering and concrete technology with an enhanced level of accuracy.

Investigation on the Performance of Partial Penetration Welds in Multicell Concrete Filled Steel Tubes

This paper presents an experimental and analytical investigation on the performance of partial penetration welds used to adjoin steel plates in irregular shaped multicell concrete filled steel tubes. The experimental program of this study is designed based on an actual implementation of such members as mega columns in a super high rise building in China. A total of six specimens are designed with different plate arrangements for the purpose of testing the performance of the partial penetration welds at different locations of the specimen. The designed specimens are tested under different load procedures and directions; this is achieved by placing them in vertical and slantwise manners between two loading plates which impose monotonic and cyclic actions. The failure conditions of each of the tested specimens are presented and discussed in detail and are based on the conclusions drawn from the experimental observations; the partial penetration weld at the corner of the tested specimens is found to be the most vulnerable. To facilitate large scale analysis, a finite element model constructed by the finite element analysis program ABAQUS is verified against experimental results. The evaluation of the stress at the partial penetration welded corner is carried out following an empirical procedure, which is adopted due to the complexity of the problem domain. The adopted procedure consists of two steps: the first one is to initially evaluate the stress based on an existing method in the literature, and the second one is to fit the results of the initial evaluation with the finite element model results based on parametric and regression analysis. After performing regression analysis, a formula to predict the weld stress is concluded, and the results of the proposed equation are found to be satisfactory when compared with the finite element model results.