Circular Concrete-filled Tubular Columns Research Articles

Effect of bolted shear connectors on the axial load-bending moment interaction capacity of CFT columns

In Concrete-Filled Tubular (CFT) columns under the influence of compressive loading, an omnilateral confinement pressure is exerted upon the concrete core which leads to increase in the strength, stiffness, and ductility of the column. This paper investigates the influence of bolted shear connectors on the behavior of circular and square CFT columns. Also, a comparison is made between the interactional axial load-bending moment (P–M) curves obtained from the numerical analyses and the P–M relationships put forth by different design standards. For this purpose, 18 circular and 9 square CFT columns, categorized into six groups, are subjected to eccentric compressive loading. The investigated parameters include concrete compressive strength, diameter of the steel tube, load eccentricity, and the presence of bolted shear connector. In most cases, the presence of bolted shear connectors in the column causes strength and ductility to increase, leading to an improvement in the axial load-bending moment (P–M) interaction. The employment of bolted shear connectors at 12.5 cm to 25 cm intervals results in a minimum of 5% and a maximum of 45% increase in the strength of the column.

Numerical analysis on circular concrete-filled tubular columns subjected to fire

Purpose Nowadays, circular concrete-filled tubular (CFT) columns are largely used in construction because of structural and architectural advantages such as high load bearing capacity and aesthetic appearance. The behavior of CFT columns at ambient and high temperatures is good; however, there are problems related to their behavior in fire when inserted in a real building structure, as for example, the influence of the restraining to thermal elongation that have to be addressed in order to improve their design. This study aims to present the results of a numerical study on the behavior of CFT columns with restrained thermal elongation in case of fire. Design/methodology/approach The parameters tested in the numerical simulations included column slenderness, load level, surrounding structure stiffness and steel reinforcement ratio. A sequentially coupled thermal stress analysis was carried out. The numerical model was validated with results from a large series of fire resistance tests carried out at Coimbra University, in Portugal. From these, simple equations to evaluate CFT column critical times were derived. Findings The results were also compared with the ones obtained from the current EN 1994-1-2:2005 simplified calculation and tabulated data methods. For the analyzed cases, it was verified that, while the simplified calculation method led to safe results on the evaluation of the fire resistance of CFT columns with restrained thermal elongation, the tabulated data method led, in certain cases, to unsafe results. This research showed also lower critical times than those from literature on similar type of columns. Originality/value The influence of the stiffness of the surrounding structure on the behavior of CFT columns subjected to fire was not yet clear in the major part of the studies already carried out. So, this paper has the originality to consider this parameter in the numerical simulations of this type of columns.

Resistance of circular concrete-filled tubular sections to combined axial compression and bending

This paper describes strength formulae and simplified strength interaction curves for the direct strength method (DSM) for circular concrete-filled tubular (CFT) sections. A simple axial strength formula and a flexural strength formula for circular CFT sections are proposed to account for the post-local buckling strength of the circular steel skin and the increase in concrete compressive strength caused by confinement of the steel skin. The squash load predicted by the proposed strength formula is compared with test results in the literature, and those predicted by AISC specifications and Eurocode4. A simplified strength interaction curve for circular CFT members under combined axial compression and bending is proposed and compared with test results. The comparison confirms that the proposed axial and flexural strength formulae and simplified strength interaction curves can be used to conservatively predict the resistance of circular CFT columns to an axial load, and combined axial compression and bending.

Circular Concrete-Filled Tubular Columns: State of the Art Oriented to the Vulnerability Assessment

The vulnerability of framed structures has been analyzed until recently from two different perspectives: Structural and socio-economical. For the sake of assessing the former, indexes and objective measurements have been proposed in the literature. These indexes include relatively accurate assessments of the strength, ductility, energy absorption, fire, blast response and resilience of the elements in order to define a higher-level structural magnitude. Similar approaches are performed with the latter when it comes to assessing damage, economical aspects, social and other important factors. On the other hand, concrete-filled tubes (CFT) have proven structurally efficient due to their relatively high strength-toweight ratio. Considerably complete state-of-the-art reviews are available for these members when it comes to analyzing their strength and overall or local buckling in static and/or dynamic responses. Reviews concerning important issues related to the structural vulnerability of those members are, however, scarce. In this paper, a state-of-the art dealing with the behavior of concrete-filled tubes is presented. The novelty of such approach is to present research concerning CFT but, in this case, from a structural vulnerability perspective (not socioeconomical), that is to say, summarizing references concerning seismic response, fire resistance, impact response and other main characteristics that are further used when defining the aforementioned indexes. Relevant numerical, experimental and theoretical studies presented in recent years are pinpointed as well as potential research trends.

Test and analysis on the seismic performance of a steel truss-to-circular CFT column sub-assembly

This paper presents an experimental and theoretical investigation on connection between steel truss and circular concrete-filled tubular (CFT) columns used for modern low-rise railway station platforms. Two full-scale cyclic tests were performed on connection sub-assembly and results showed that relative strength between the steel truss and column zone had a major effect on the subassembly's seismic performance and failure patterns. Results also indicated that a better deformability and energy dissipation capacity can be obtained if the column zone is slightly weaker than the steel truss. In addition, a simplified theoretical model was proposed to predict the experimental load–deformation histories and failure patterns. Satisfactory comparisons were observed.

Local Buckling Restraining Behavior of Thin-Walled Circular CFT Columns under Seismic Loads

AbstractThe local buckling restraining behavior of thin-walled circular concrete-filled tubular (CFT) columns is examined under seismic loads by conducting a bidirectional cyclic loading and a bidirectional shaking table test. According to the responses of the deformations and strains measured in the bidirectional cyclic loading test, the axial compressive force acting on the buckled part of a steel tube is reduced with the increase in its local buckling deformation because most of the compressive axial force is transferred from the steel tube to the in-filled concrete. The reduction in the compressive axial force in the steel tube slows the progress of the buckling deformation. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force acts repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the l...

Seismic Performance of Moment End-Plate Connections to CFT Column under Cyclic Loading

An experimental program under cyclic load is performed on two half-scale interior moment end-plate connections to concrete filled tubular (CFT) columns. Flat and curved stiffened extended end-plates were welded to the steel beams in the shop, and bolted on the site to the square and circular CFT column tubes respectively, using steel rods passing through the column. The experimental results demonstrated that both circular and rectangular end-plate connections showed similar performance in a ductile manner and the stiffener elements were effective to form the plastic hinges away from the welding zone, also the proposed curved end-plate connection with rods passing through the column in “X” shape was effective. The test specimens showed a plastic rotations capacity of 0.054 radian.

The Ultimate Strength of the Concrete-Filled Tubular (CFT) Section Columns

This paper describes design strength of concrete-filled tubular (CFT) section columns accounting for local buckling of steel skin. The local buckling has a negative effect on the ultimate compression strength based on the yield stress of steel skin and nominal compressive strength of in-filled concrete. A squash load formula for CFT stub columns is proposed to account for the post-local-buckling strength of steel skin. A compressive strength formula for filled-in concrete accounting for the confining effect of steel skin and strength ratio between filled-in concrete and steel skin is also proposed. The squash loads predicted by the proposed strength formula for the direct strength method were compared with the AISC (2010) and Eurocode4 (2004). The comparison showed that the squash load formula proposed can predict conservatively the squash load of circular and rectangular CFT columns with local buckling

Simple calculation model for evaluating the fire resistance of unreinforced concrete filled tubular columns

This paper presents a method for calculating the fire resistance of unreinforced axially loaded concrete filled circular hollow section columns, based on the guidelines of Eurocode 4 Part 1.2 for the fire design of composite columns. The different design approaches which are currently available worldwide are reviewed in this paper, focusing in particular on the European code, which is discussed and analysed in depth. Several of the aspects which are nowadays missing or thought to be arguable by researchers in the Eurocode 4 method are studied and on the basis of parametric studies a new proposal is developed, so that designers can easily apply the code in the future. The parametric studies investigate the main factors affecting the fire resistance of unreinforced concrete filled tubular (CFT) columns, such as the outer diameter, the thickness of the steel tube wall, the relative slenderness of the column at room temperature and the fire exposure time, widely covering the range of values which can be found in practice. From the results of these parametric studies, equations for obtaining the equivalent temperatures of steel and concrete are developed, and appropriate flexural stiffness reduction coefficients are integrated in the simple calculation model of Eurocode 4. The method presented here is valid for centrally loaded circular CFT columns filled with unreinforced normal strength concrete, and makes possible the calculation of slender columns, extending the current limitations in buckling length of the European code.

Prediction of the squash loads of concrete-filled tubular section columns with local buckling

This paper describes a series of compression tests of circular hollow steel (CHS) sections and circular concrete-filled tubular (CFT) sections. The diameter to thickness ratios of the test specimens ranged 45–140. This range was set to investigate the effect of local buckling in the circular steel skin on the ultimate strength of CFT columns. The confining effect against filled-in concrete was also studied. A squash load formula for CFT stub columns is proposed to account for the post-local-buckling strength of steel skin. The ultimate strength of steel skin was predicted by a kind of Direct Strength Method (DSM). The proposed DSM does not require the computation of the effective area of the steel skin, but uses the gross area of the steel skin and the design strength formula based on various test results. The compressive strength formula of the filled-in concrete accounting for the strength ratio of steel skin to filled-in concrete is also proposed to consider the increase in compressive strength of the filled-in concrete, due to the confining effect of the steel skin. The design strengths of CFT columns were compared with the test results for verification.