This paper is based on the experimental study of ten slender steel tubular columns of circular and square sections filled with plain, fibre reinforced and partial replacement of coarse aggregate by rubber concrete. The specimens were tested under axial compression to investigate the effects of fibre reinforced and rubber concrete on the strength and behavior of slender composite columns. Hollow steel columns of similar specimen were also tested as reference columns. The test results were illustrated by load-deflection curves. Various characteristics such as strength, energy absorption capacity and failure mode are discussed. Interpretation of the experimental results indicates that the use of fibre reinforced and rubber concrete as infill material has a considerable effect on the strength and behavior of slender composite columns.
IntroductionIn the recent past and presently construction is generally carried out with construction materials such as concrete and steel rebar’s. Structural elements such as slabs, beams and columns are constructed with the above two materials, whereas the concrete takes compression and steel rebar’s takes tension. The main disadvantage while designing the elements is the cross sectional dimension. While loading over the elements is of heavy, the cross section considerably increases. This problem leads to the headroom effect in a building while a beam is designed. Likewise in column sections, the section becomes uneconomical which needs a huge amount of concrete. In order to avoid such situations, to carry the heavy load with the minimum dimensions the composite type of construction is introduced. In this type of construction steel rebars are replaced with the rolled steel joist (RSJ) sections such as channel sections, I sections and angle sections.
The technique of composite construction has assumed great importance due to some of its inherent advantages in comparison to the cast-in-situ construction in concrete. A major application of this technique in housing is the construction of composite beams, columns and slabs made of reinforced cement concrete and rolled steel sections. For stanchions, the I sections are mostly provided and it is cased with concrete so that the strength of section is considerably increased. It also includes the advantages such as prevention from corrosion and improved fire resistant. When fibres are introduced to such sections, it adds the advantage of distributing the crack evenly throughout the section. Steel-Concrete composite construction has emerged as one of the fast methods of construction in India. The inherent advantage of the steel-concrete composite beam lies in that the two principal elements-the–steel and the concrete are normally used in a manner so that full potential of both may be realized and the bestutilization of their respective properties can be made. This comparatively new method of construction quickly gained popularity in the Western World because of its applicability in bridges, multi–storied buildings, car parks, etc. with reduced construction time. There were valuable research studies, to support the design basis. It has been reported that saving in high yield strength steel can be up to 40% in composite construction.
In modern structural construction, concrete filled steel tubular (CFST) columns have become increasingly popular in structural applications like highrise buildings, bridges, and large industrial workshops and so forth They have better structural performance than that of bare steel or reinforced concrete column. They have proven to be economic, as well as providing for rapid construction and additional cost saving from the elimination of form work. Concrete prevents the local buckling of hollow steel sections, and significantly increases the strength and ductility of the section. In the past, the behavior of CFST columns has been the subject of many investigations, and a recent review of most of the studies is given by Shanmugham and Lakshmi , methods of load application , diameter-tothickness ratio and effects of local buckling  that influence the behavior of circular CFST columns have been investigated and reported by many researchers.
This previous research on slender circular composite columns shows that the strength of the column decreases as the slenderness ratio increases. Because of the slenderness effects, the slender columns did not exhibit the beneficial effects of composite behavior in terms of increased strength due to confinement. This was most likely caused by the increasing strain gradient associated with increasing flexure. Hence in the study presented in this paper, fibre reinforced concrete (FRC) was used as an infill material, as it has greater flexural strength and tensile strength than plain concrete. The purpose of this study was to examine the effects of fibre reinforced and rubber concrete on the strength and behavior of slender composite columns. The studies carried out by the authors in the last two years on the influence of FRC on the behavior of such columns have shown that the addition of steel fibres in the core concrete have an effect on the load bearing capacity of long columns. Furthermore, experiments have been conducted on slender columns with pinned end conditions, subjected to single curvature bending and the results are analyzed in detail in the present investigation. Many studies have been carried out to investigate the behavior of CFST columns subjected to concentric loadings.
However, most of the tested specimens are circur and square CFST columns. For axially loaded thin-walled steel tubes, local buckling of the steel tube does not occur if there is sufficient bond between the steel and concrete. CFST melambers provide excellent seismic resistance in two orthogonal directions as well as good damping characteristics.
Test SpecimensA total of ten full-scale column specimens of Circular (designated C) and Square (designated S) sections were tested in this study. The column specimens were classified into five different groups. Each group consisted of two specimens filled with plain concrete (designated P), FRC (designated F) and partial replacement of coarse aggregate by rubber 25% and 50% (designated R25 and R50 respectively). The rest of the column specimens were tested as hollow sections for comparison (designated H). All column specimens were slender with same lengths and of similar cross sectional dimensions, given in Table 1. As shown in Table 1, the slenderness ratio (L/D) in this study is 12. All the specimens were fabricated from circular and square hollow steel tube and filled with four types of concrete. The average values of yield strength and ultimate tensile strength for the steel tube were found to be 260 and 320 MPa respectively. The modulus of elasticity was calculated to be 2.0 x105 MPa. In the present experimental work, the parameters of the test specimens are the strength of concrete and steel, cross-sectional aspect ratio and volumetric steel-to-concrete ratio (á = As/Ac). All the selected parameters are within the ranges of practical limits. In order to prevent the steel hollow column section from local buckling, both AISC and ACI required the widthto- thickness (B/t) ratio of the steel hollow section not greater than the following limit: B/t < (3Es/fy) = 48.03
During preparation of the test specimens, concrete was cast in layers and light tamping of the steel tube using wooden hammer was performed for better compaction. The specimens were cured for 28 days in a humiditycontrolled room.
Test Setup and ProcedureAll of the tests were carried out in an Electronic Universal Testing Machine with a capacity of 1000 kN. The columns were hinged at both ends and axial compressive load applied. A small pre-load of about 5 kN was applied to hold the specimen upright. Dial gauges were used to measure the lateral deflections of the columns at midheight. The load was applied in small increments of 20 kN. At each load increment, the deflection measurements were recorded. All specimens were loaded to failure.
Test Results and Discussions
In comparison with the reference column, the ultimate strength of the composite column was approximately 45–100% higher. The strength of each column is plotted and shown in Figure 2. It also shows that FRC filled specimens gain an increase in ultimate strength compared with plain concrete filled specimens. Stiffness in the present context is defined as the ability to resist lateral deformation, and stiffness values are calculated as the ratio of the load increment to the corresponding increase in the measured displacement at the midheight of the columns.
Also, it shows quite clearly that FRC filled and rubber concrete columns behaved in a stiff manner over the entireload range and has higher stiffness than plain concrete filled columns. Ductility and energy absorption are extremely important criteria for structures that are located in seismic areas. Ductility is defined as the ability to possess non-linear deformation under loading. The ductility factor of the column is herein defined as the ratio of the deflection at ultimate or peak load to the deflection at the load corresponding to yielding of the extreme fibre in the compression region. The energy absorption is the work done by the external load up to the failure of the column specimen. The area of the loaddeflection diagram is representative of the energy absorption under monotonic loading. Table 1 shows the experimentally observed values of energy absorption of the columns. The results show that the energy absorption was the highest in the columns filled with fibre reinforced and rubber concrete (R25). The mean energy absorption of fibre reinforced and rubber concrete filled columns is equivalent to double that of hollow columns. The comparison of the energy absorption capacity of all types of columns is shown in Figure 3.
This clearly shows considerable improvement in the energy absorption capacity of columns if fibre reinforced and rubber concrete is used as the in-fill material. All specimens filled with concrete failed at the mid-height due to concrete crushing and steel yielding in the compression zone. The concrete filled steel tubular columns did not show any signs of local buckling of the shell and the columns were able to sustain more loads before failure due to an overall buckling. Figure 4 shows the column after test at an advanced stage of loading. Figure 5 illustrates the typical failure mode of columns filled with concrete. Local buckling of the steel tube could only be observed for the empty reference column on the compressive side at the mid-height of the column.
- The use of FRC and rubber concrete has resulted in considerable improvement in the structural behavior of slender composite columns subjected to axial loading.
- The ductility is found to be almost equal for both plain and FRC filled steel tubular columns and high for rubber concrete.
- The use of fibre reinforced and rubber concrete in the steel tube results in an enhanced energy absorption capacity of the composite columns.
- The use of fibre reinforced and rubber concrete as a filling material increases the load bearing capacity to a much greater extent compared with that of unfilled columns and reduces the lateral deflection.
- It is observed that the energy absorption of the composite column specimen with square cross-section is small compared to that with a circular cross section in the case of large axial compressions Figure 3.
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