Eccentric Compression Tests Research Articles

Effect of size on eccentric compression performance of steel tube and sandwiched concrete jacketed CFST columns

This study investigated the size effect of steel tube and sandwiched concrete jacketed CFST columns under eccentric compression. A series of eccentric compression tests were conducted on 3 geometrically similar un-strengthened CFST columns and 13 strengthened CFST columns with varying structural sizes (with a ratio of 1:1.5:2), different eccentricities, and different outer steel ratios. The eccentric performance of the specimens, including their failure behaviours, load-axial displacement curves, and longitudinal and circumferential strain distribution were explored. Additionally, the size effect on nominal stress-longitudinal displacement relationships, nominal axial strength, nominal ultimate deflection and ductility index were investigated. The test results confirmed the presence of a size effect in both un-strengthened and strengthened CFST columns, with nominal strengths closely following Bažant's proposed "size effect law (SEL)". Finally, considering the influence of size effect, this study proposed axial force-bending moment (N-M) correlation curves for predicting the bearing capacity of strengthened CFST stub columns based on EC4, AISC-360, AIJ and GB 50936. Among these codes, AIJ provided the most accurate predictions when compared to the test results. Introducing the size effect coefficients mitigated the tendency to overestimate the load-bearing capacity of large-size retrofitted columns.

Experimental investigation on the compressive behaviour of foam concrete light steel keel composite wall

ABSTRACT This study aims to explore a new load-bearing structure of foam concrete-light steel keel (FCLS) composite wall. Compared with the traditional beam-column frame structure, this structure has the characteristics of lightweight, thermal insulation, sound insulation, fire resistance, etc. and has better Carrying capacity. The compression performance of 10 FCLS composite walls was studied in depth by conducting axial and eccentric compression tests. The effect of the spacing of foam concrete, middle horizontal steel and self-tapping screws on the compressive bearing capacity of the middle column of FCLS composite wall under compressive load was investigated. The results showed that the bearing capacity of the composite wall filled with foam concrete increased by 70%-80%, while reducing the spacing of self-tapping screws could increase the bearing capacity by 5%-6%. In addition, the tie bar settings have little impact on the bearing capacity. Based on these influencing factors and through the analysis of test data, the calculation formula for the bearing capacity of the composite wall was obtained, and the effectiveness of the method was verified.

Bearing Capacity of Hybrid (Steel and GFRP) Reinforced Columns under Eccentric Loading: Theory and Experiment

In order to reveal the mechanical behavior of short concrete columns reinforced with hybrid steel and glass FRP bars, 10 specimens were designed for eccentric compression tests. The effect of eccentricity and load–displacement/strain of the specimens was studied. Test results indicate that the damage process and failure mode of these hybrid RC columns was similar to those in the conventional steel-reinforced concrete columns. The mode of failure for all specimens is characterized as large eccentricity compression failure, and the ultimate bearing capacity of the columns decreases with the increase in eccentricity. However, the impact of the varying axial stiffness ratio between GFRP and steel bars on the bearing capacity can be considered negligible. In addition, based on theoretical analysis, two boundary states for distinguishing failure mode and the formulae for calculating ultimate bearing capacity in different failure modes of eccentrically loaded hybrid RC columns are proposed. The computed results agree well with test results.

An improved CSM-based design method for H-section aluminium slender columns under eccentric compression

This study proposed an improved design method for predicting the load-carrying capacity of H-section aluminium alloy slender columns under eccentric compression based on CSM. Through eccentric compression tests and extensive numerical simulations, the study explored the influence of cross-sectional dimensions, member slenderness ratios, and initial geometric imperfections on the load-carrying capacity of columns. The proposed CSM-based design method utilized CSM limiting stress and CSM buckling reduction factors tailored to aluminium alloy columns, accounting for the specifics of their material behavior and geometric properties. The comparison and evaluation of the load-carrying capacities predicted from the proposed design method, European design code, and American design code, as well as with experimental and numerical results, were conducted. Meanwhile, the reliability analyses on these design method and codes were also performed, adopting the method provided by the AISC and EN 1990, Annex D. The comparison and reliability analyses results indicate that, compared to existing design codes, the proposed CSM-based design method could provide more accurate, consistent, and reliable load-carrying capacities of H-section aluminium alloy slender columns under eccentric compression.

Eccentric compressive performance and design of asymmetric steel columns with complex edges

Box-type houses often use asymmetric cold-formed thin-walled steel columns with complex edges to meet structural requirements. However, there is a need to clarify the mechanical properties of eccentric loading in this section of the steel column. This study conducted eccentric compression tests on 24 asymmetric cold-formed thin-walled steel columns with complex edges, clarifying the specimens' ultimate load-bearing capacity and buckling mode. The results showed that the resistance to pressure provided by the corner was higher than that provided by the two crimped edges. Subsequently, a finite element model was established, and its accuracy was verified by comparing it with experimental results. Finally, based on the formula for calculating the nominal axial strength in AISI, the direct strength method formula for the stable bearing capacity of asymmetric cold-formed thin-walled steel columns with complex edges under in-plane eccentric compression was proposed. This method takes into account the local-global interactive buckling and distortional-global interactive buckling. The comparison between the theoretical calculation results and numerical simulation results demonstrated that the design method proposed in this study is effective and conservative. It can provide a reference for future engineering applications.

Eccentric compression behavior of L-shaped column fabricated by thin-walled square steel tubes based on self-drilling screw connections

In this study, a new L-shaped column fabricated by thin-walled square steel tubes (LFTST columns) based on self-drilling screw connections is proposed. The LFTST columns consisted of square steel tubes, U-shaped parts, angle parts, gusset plates, and self-drilling screw connections. LFTST columns possess several advantages including easy transportation, rapid assembly, and eco-friendliness. Consequently, they are suitable for low-rise buildings, such as village-building, low-rise dormitories, and low-rise office buildings. However, the compression behavior of LFTST columns remains silent. Five full-scale LFTST column specimens were subjected to eccentric compression tests. The variables under consideration included eccentricity (with values of 0 mm, 40 mm, and 80 mm), thickness (2 mm and 4 mm), and the number of gusset plates (0, 1, and 3). The failure modes, bearing capacity, load–displacement response, and strain development of the LFTST specimens were obtained. Subsequently, finite element (FE) models of LFTST columns were established and used to analyze the eccentric compression behavior of LFTST columns. The FE modeling results agreed well with the experimental results. A detailed parameter analysis was conducted to evaluate the effects of various factors. These factors included the thickness of the plates (2 mm, 3 mm, and 4 mm), the width of the gusset plates (100 mm, 150 mm, and 200 mm), limb spacing values (0 mm, 150 mm, and 300 mm), and eccentricity (0 mm, 20 mm, 40 mm, 60 mm, and 80 mm). In addition, the calculation formula for estimating the ultimate bearing capacity of the LFTST columns was derived employing the double coefficient product method. The proposed formula was validated by experimental and FE results.

Eccentric compressive performances of circular steel-reinforced concrete-filled stainless steel tubular column

Steel-reinforced concrete filled stainless steel tube (SRCFSST) columns are predicted to offer excellent durability, load bearing capacity, ductility, fire resistance, impact resistance, and seismic performance. Therefore, eccentric compressive tests of 18 circular SRCFSST columns were carried out to investigate the effects of the slenderness ratio, load eccentricity, and embedment of carbon-profiled steel (CPS) on circular SRCFSST columns. The ultimate strength and ultimate deflection of the composite columns were improved significantly after the CPS was embedded. A finite element (FE) model was established and the FE analysis indicated that the CPS was proved to be an effective means of constraining concrete. Based on the FE parametric analysis, it was shown that the stability coefficient decreased with increasing concrete strength, proof strength of stainless steel, and CPS yield strength, but increased with increasing rotation radius-diameter ratio. The greater the concrete strength, the more similar the N∼M interaction curves of the SRCFSST columns and reinforced concrete columns. The higher the CPS yield strength, stainless-steel ratio, slenderness ratio, carbon steel ratio, or proof strength of stainless steel, the more similar the interaction curves of the SRCFSST columns and profiled steel columns. Finally, the simplified model was refined to forecast the carrying capability of a circular SRCFSST specimen under the eccentric compression.

Study on mechanical properties and bearing capacity calculation of eccentric compression of RC columns reinforced with prestressed lattice steel

The external steel structure and RC column under eccentric compression have a reduced ability to work together, and using prestressing technology can significantly improve their ability to work together. Therefore, the paper proposes a new prestressed column reinforcement technology by adopting a space profile steel structure instead of angle steel and combining prestressing technology with the bolted connection. By conducting eccentric compression test studies on 11 specimens with different reinforcement methods, prestressing and eccentricity, load-displacement curves, stress distribution laws, ultimate bearing capacity, damage modes and prestressing influence rules were obtained. The main conclusions are as follows: prestressing can significantly improve the ability of external reinforcement steel structure and core column to work together and inhibit the development of concrete cracks. Compared with the traditional angle steel reinforcement, the bearing capacity is increased by 17.5 %, the stiffness is increased by 26.6 %, and the ductility is increased by 12.3 %. However, the reinforcement effect will be reduced if the prestressing is too large. It is recommended that the prestressing should be applied according to the size of 0.09 fc of the unilateral area of the core column. According to the prestressed lattice steel constraint mechanism, a mechanical analysis model of the reinforced member was established. Finally, based on the cross-section equilibrium method, a calculation method of eccentric compressive ultimate bearing capacity was proposed, and the research results can provide a basis for the engineering application of the new reinforcement method.

Local–overall buckling behavior of corroded intermediate compression-bending H-section steel columns

The primary aim of this paper is to assess the impact of corrosion damage on intermediate H-section steel columns subjected to combined compression and bending about the strong axis. The study conducted a comprehensive investigation into the corrosion-induced transition in the buckling behavior of steel columns through experimental and numerical analyses. Six H-section Q355 steel intermediate columns were designed, with five undergoing an artificial spray-accelerated corrosion process. The three-dimensional scanning technology was employed to quantify the general corrosion and corrosion pits. Eccentric compression tests were carried out on intermediate H-section steel columns to investigate the effect of the different corrosion levels on the failure mode and load-displacement curves, as well as to determine local buckling load based on the load-strain curves. The test results showed that the corrosion characteristic parameters of each cross-section of the steel column exhibited irregularity and variation in all locations, reflecting the heterogeneity of corrosion. The density of corrosion pits exhibited a decreasing trend with increased corrosion duration. Corrosion remarkably reduced both the ultimate carrying capacity and local buckling capacity, with the most severely corroded specimen exhibiting an average cross-sectional area loss rate of 37 %, resulting in a 43 % and 51 % reduction in ultimate load and buckling load compared to the uncorroded specimen. Furthermore, the corrosion features substantially influenced the failure mode, resulting in a transition from overall buckling to local buckling or local-global interaction buckling. A numerical methodology was developed to incorporate the actual surface morphology of corrosion, and the reliability of the modeling method was validated through comparison with experimental results, further revealing the failure mechanism of corroded intermediate H-section steel columns under compression and bending about the strong axis.

Cross-section behaviour and capacity of cold-formed austenitic stainless steel flat-oval hollow sections under combined compression and bending

Flat-oval hollow section is composed of two semi-circular elements and two flat elements, with the semi-circular elements (exposed to wave and wind) offering a low level of hydrodynamic and aerodynamic drag and the flat elements facilitating connections with other members perpendicular to the wave and wind directions. This paper presents an experimental and numerical study of the cross-section behaviour and resistance of cold-formed austenitic stainless steel flat-oval hollow section stub columns under combined compression and bending. An experimental programme, including initial local geometric imperfection measurements and ten major-axis and minor-axis eccentric compression tests, was firstly carried out, with the test setups, procedures and results fully reported. Following the experimental programme, a numerical modelling programme was performed, with finite element models developed and validated against the eccentric compression test results and then used to conduct parametric studies to generate additional numerical data. Based on the test and numerical results, the relevant codified design interaction curves for cold-formed austenitic stainless steel rectangular hollow sections were evaluated for the applicability to their flat-oval hollow section counterparts. It was found from the evaluation results that the codified design interaction curves led to inaccurate resistance predictions, mainly due to the conservative end points (i.e. cross-section resistances under pure compression and pure bending) and inefficient shapes. Finally, a new design interaction curve was proposed through the use of more accurate end points and more efficient shape and offered more accurate and consistent resistance predictions for cold-formed austenitic stainless steel flat-oval hollow sections under combined loading than the codified design interaction curves.

Global buckling and resistances of austenitic stainless steel semi-oval hollow section beam-columns

As advanced manufacturing technologies become increasingly mature in civil engineering, novel tubular sections, characterised by appealing architectural appearance and prominent structural efficiency, can be fabricated, with an example being semi-oval hollow section. The present paper reports an experimental and numerical investigation into the global buckling behaviour and resistances of austenitic stainless steel semi-oval hollow section beam-columns under combined compression and bending. An experimental programme was firstly performed on ten austenitic stainless steel semi-oval hollow section beam-column specimens and included initial geometric imperfection measurements and eccentric compression tests. The obtained test failure loads, load–mid-height lateral deflection curves and failure modes were fully reported. Following the experimental programme, a numerical modelling programme was conducted, including validation studies to validate developed finite element models against the test results and parametric studies to generate additional numerical data. Given that the existing European code and American specification do not cover the design of stainless steel semi-oval hollow section components, the applicability of the codified design interaction curves for austenitic stainless steel circular hollow section beam-columns to their semi-oval hollow section counterparts was evaluated based on the obtained test and numerical data. The European code was found to result in relatively accurate and consistent resistance predictions for austenitic stainless steel semi-oval hollow section beam-columns with Class 1 and 2 cross-sections, but the resistance predictions for those with Class 3 cross-sections were conservative and scattered. The American specification was shown to offer overall accurate and consistent resistance predictions, but some resistance predictions were marginally unsafe.

Experimental and numerical investigations of austenitic stainless steel semi-oval hollow sections under combined compression and bending

Semi-oval hollow section is an innovative cross-section profile, including one semi-circular flange, one flat flange and two flat webs. While the semi-circular flange (exposed to fluid or wind) offers a low level of hydrodynamic or aerodynamic drag, the flat elements facilitate connections with other members. This paper presents experimental and numerical investigations into the local buckling behaviour and resistances of austenitic stainless steel semi-oval hollow sections under combined compression and bending. A testing programme was firstly conducted and included initial local geometric imperfection measurements and ten eccentric compression tests. In conjunction with the testing programme, a numerical modelling programme was performed, where finite element models were developed to validate against the test results and conduct parametric studies for expanding the test data pool over a wider range of cross-section dimensions and loading combinations. The obtained test and numerical data were used to evaluate the applicability of the relevant design interaction curves for austenitic stainless steel rectangular hollow sections, as prescribed in the European code and American specification, to austenitic stainless steel semi-oval hollow sections. On the basis of the evaluation results, the codified design interaction curves were found to provide conservative resistance predictions. Finally, an improved design interaction curve, with more accurate end points and suitable shape, was developed and led to more accurate and consistent resistance predictions for austenitic stainless steel semi-oval hollow sections under combined compression and bending.

Compression performance of FRP bar-reinforced seawater sea-sand concrete encased FRP-wrapped steel tube columns

This paper developed a new type of composite column made of fibre-reinforced polymer (FRP) bar-reinforced seawater sea-sand concrete (SWSSC)-encased FRP-wrapped steel tubes, which is suitable for maritime engineering. In this composite form, the inner steel tube was isolated to the chloride ions by wrapping with an FRP sheet, and FRP bars were employed to reinforce the exterior SWSSC. To avoid/delay the occurrence of internal steel tube buckling, two measures were implemented, that is, filling with ordinary concrete, and bundling and welding using four hollow small-sized steel tubes. The compression performance was explored by axial compression and eccentric compression tests, and three experimental variables were set, including the steel tube thickness (2.5 mm and 5 mm), number of steel tubes (0, 1 and 4), and eccentricity (20 mm, 40 mm and 60 mm). The test results showed that increasing the thickness of the steel tube could improve the bearing capacity and lateral deformation of specimens (e.g., from 32.66 MPa to 36.99 MPa), whereas the number of steel tubes had a minor positive effect on the specimens. Meanwhile, the bearing capacity of the specimens decreased with increasing eccentricity of the eccentric compression test from 41.51 MPa to 36.99 MPa. Finally, a calculation method was proposed to predict the peak eccentric strength of specimens, which can provide a reference for the design of this kind of composite column.

Performance of eccentrically loaded ultra-high-strength concrete-filled Q770 steel tubular columns with varying slenderness ratios: Experimental and numerical investigations

This study conducts experimental and numerical investigations into the response of ultra-high-strength (UHS) concrete-filled ultra-high-strength steel tubular (UHSCFST) columns under eccentric loading, varying the slenderness ratio, to develop effective design guidelines for engineering applications. A series of eccentric compression tests were conducted on twelve UHSCFST columns with varying slenderness ratios to identify failure mechanisms and load-bearing characteristics. In the experimental program, critical influencing factors, including load eccentricity ratio, slenderness ratio, and UHS steel strength, were considered. Finite element models of eccentrically loaded UHSCFST columns were developed using ABAQUS and validated through a comparison of tested and simulated results to analyze load-sharing ratios and stress distribution for each column component. The findings indicated that UHSCFST column failure modes were characterized by compression-controlled and flexure-dominated damage under the combined effects of slenderness ratio and load eccentricity ratio. UHS concrete played a dominant role in bearing the eccentric load, and a judicious selection of UHS steel yield strength can improve column capacity and resistance. Additionally, these validated models were employed to conduct parametric studies that evaluated the effects of design and load parameters on the eccentric load capacity of UHSCFST columns. Design equations were developed based on tested and simulated results to predict the ultimate load capacity of UHSCFST column with varying slenderness ratios under eccentric loading conditions.

Structural behaviour and resistances of high strength concrete-filled stainless steel tube (HCFSST) beam-columns

This paper reports experimental and numerical investigations into the structural behaviour and resistances of high strength concrete-filled stainless steel tube (HCFSST) beam-columns under combined compression and bending. An experimental investigation was conducted on sixteen HCFSST beam-columns specimens fabricated from stainless steel tubes with two cross-section sizes and high strength concretes with four material grades, and included concrete cylinder tests, tensile coupon tests, initial global geometric imperfection measurements and eccentric compression tests. The test setups and procedures were fully reported and the experimental observations were discussed and analysed in detail. It was found that the plane cross-section assumption was valid for eccentrically loaded HCFSST sections and the lateral deflection distribution patterns for HCFSST beam-columns were approximately half-sine wave shapes. The test results were used in a subsequent numerical investigation for the validation of finite element models, which were then employed to conduct parametric studies to generate additional numerical data over a wide range of cross-section dimensions, member lengths and loading combinations. Based on the experimental and numerical results, the relevant design rules, as set out in the European code, American specification and Australian/New Zealand standard, were evaluated for their applicability to HCFSST beam-columns. The evaluation results revealed that the European code provided accurate but scattered failure load predictions when used for HCFSST beam-columns, while the American specification and Australian/New Zealand standard resulted in conservative and scattered failure load predictions.

Eccentric load-carrying capacity calculation of L-Shaped Columns with recycled aggregate concrete-filled tubes

A composite L-shaped column made of square steel tubes filled with concrete is well-suited for residential steel structures because it effectively prevents the beams and columns from protruding from the walls, thus maximizing the usable space within a room. Recycled aggregate concrete (RAC), a sustainable material, can efficiently recycle construction waste and reduce energy consumption. However, compared to natural concrete, RAC has lower elastic modulus and compressive strength. When RAC is filled into steel tubes, it effectively restrains the RAC and allows it to support the internal buckling of the steel tubes. This study investigates L-shaped columns of recycled aggregate concrete-filled steel tubes (L-RACFST) through eccentric compression tests and numerical validation. By applying the superposition principle, the study evaluates the eccentric compression strength calculation method of L-RACFST columns and plots the N-M relationship curve. Comparing the results of the tests and numerical studies, it is evident that the derived formula is reliable for predicting the eccentric compression bearing capacity of L-RACFST columns. Furthermore, the N-M relationship curve drawn from the formula provides a more accurate prediction of safety. This paper delves into the stress mechanisms of L-RACFST columns under eccentric compression loads. The calculation formula can be used to determine the bearing capacity of L-RACFST columns under eccentric compression loads, and the N-M relationship curve can be employed to predict the safety performance of L-RACFST columns under such loads.

Study on Eccentric Compression Mechanical Characteristics of Basalt Fiber-Reinforced Recycled Aggregate Concrete-Filled Circular Steel Tubular Column

During this study, eight basalt fiber-reinforced recycled aggregate concrete-filled circular steel-tubular (C-BFRRACFST) column specimens were subjected to eccentric compression tests with different replacement ratios of recycled coarse aggregate (RCA), basalt fiber (BF) contents, length-diameter (L/D), and eccentricity. The whole process of stress as well as failure mode of the specimens were observed, and a load–displacement curve as well as a load–strain curve for the specimens were measured. The impacts of various parameters upon the bearing capacity, peak displacement, and ductility coefficient of the specimens were analyzed. Subsequently, a 3D finite element model of the C-BFRRACFST column was established, and the whole process of stress was simulated. Based on the finite element simulation results, the N/Nu-M/Mu correlation strength curve of the C-BFRRACFST columns was verified. The exploration demonstrated that under eccentric load, the C-BFRRACFST column eventually underwent destruction of the overall instability. The load–axial displacement curve was characterized as three stress stages: elastic, elastic–plastic, declining, as well as declining stages. The strain of the mid-span section for the specimens follows the plane section assumption, and the lateral deflection basically follows the sine waveform curve. The ultimate bearing capacity of the specimens exhibited little change as the replacement ratio of RCA improved, while the ductility progressively reduced. Furthermore, the ultimate bearing capacity of the specimens failed to be obviously changed as the BF content enhanced, while the ductility progressively rose. Increasing the L/D gradually reduced the specimen’s ultimate bearing capacity alongside its ductility. The corrected N/Nu-M/Mu curve agreed well with the findings of finite element simulation.

Experimental and numerical investigation of corroded cold-formed steel channel section columns subjected to in-plane compression and bending

This paper investigates the effect of corrosion on bearing capacity of corroded cold-formed steel (CFS) channel section columns subjected to in-plane compression and bending. Herein, 14 CFS columns with different column lengths and corrosion degrees were investigated. The mass loss rate, surface morphology, and initial geometric imperfections of corroded CFS columns were measured. The eccentric compression test of corroded CFS columns that were accelerated corrosion by the outdoor periodic spray test were carried out. The relationships among the failure mode, load–strain curves and load–axial displacement curves, and mass loss rate were analyzed, and then the deterioration laws of bearing performance of corroded CFS channel section columns were discussed. The non-linear finite element model for corroded CFS columns considering surface morphology was then developed by using Geomagic Studio and ABAQUS. The results indicated that the local and overall initial geometric imperfections of corroded specimens were greater than those of non-corroded specimens. With increasing mass loss rate, the distortional buckling deformation of corroded CFS columns occurred in advance and the failure locations were diversified. Corrosion exhibited a greater impact on the buckling critical load than on the ultimate load and post-buckling strength. The failure positions of corroded CFS channel section columns subjected to in-plane compression and bending were found to be mainly related to the areas with severe corrosion of the flange.

Experimental investigations on the compression capacity of rectangular steel tubular columns strengthened with intermittently welded angle steel

Many steel structure buildings in China were built from the 1970s to the 1980s, and they have been in service for decades. However, due to factors such as changes in their functionality, prolonged disrepair, and erosion of the surrounding environment. Studying the strengthening of many existing steel structures will become an important measure during the “reinforcement period.” Welding angle steel on the surface of steel columns for reinforcement is a commonly used method in practical engineering. However, the traditional continuous welding of angle steel to reinforce rectangular steel tube columns results in a relatively large welding workload on the construction site, extending the construction period and increasing the engineering cost, which cannot guarantee high welding quality. To reduce the workload of on-site welding and the cost of construction reinforcement, intermittent welding is proposed. This article designed 9 test specimens for axial and eccentric compression tests to obtain the failure modes, ultimate bearing capacity, and stress distribution of the cross-section of steel tube columns reinforced with different intermittent welding angle steels. The experimental results indicate that intermittent welding can replace continuous welding for the reinforcement of steel tube columns, significantly reducing on-site welding workload and fully utilizing the role of angle steel reinforcement. If the length of the intermittent welding is too large, local buckling will occur, thereby weakening the reinforcement effect of the angle steel. The existence of batten plates can improve the ultimate bearing capacity of intermittent welded angle steel tube columns. Comparative analysis was conducted using a finite element model, which revealed the force transfer mechanism and ultimate bearing capacity of intermittent welding angle steel reinforced steel tube columns. The optimal weld spacing and weld length for intermittent welding of angle steel to reinforce rectangular steel tube columns provide a theoretical reference for the design and application of practical reinforcement engineering.

Mechanical behavior of the sustainable square spiral stirrup-confined recycled aggregate concrete filled steel tubular columns under compression

Owing to the inherited structural defects of recycled aggregate concrete (RAC), such as deficiencies in durability and mechanical properties, its further applications in modernized civil engineering practices are significantly limited. To this end, this study aims to achieve the sustainable application of RAC and improve its mechanical properties by infilling it into the steel tube to form the recycled aggregate concrete filled steel tubular columns (RA-CFST columns). Numerous studies have indicated that embedding steel spiral stirrups inside the steel tube is helpful in improving the ultimate strength (Nu) and ductility of square CFST columns. Thus, this paper designed several square RA-CFST columns confined by high-strength steel (HSS) spiral stirrups (HSS-RA-CFST columns) and their concentric and eccentric compression behavior were experimentally studied. The test specimens were divided into two different parts, including (i) axial compression and (ii) eccentric compression test specimens. For the former, 16 columns were designed, including 4 traditional non-confined RA-CFST columns, 4 normal-strength steel (NSS) spiral-confined RA-CFST columns (NSS-RA-CFST columns), and 8 HSS-RA-CFST columns. For the latter, 8 specimens were prepared, including 2 non-confined and 6 HSS-RA-CFST columns. Effects of the critical test parameters, such as the steel tube thickness, steel spiral pitch, replacement ratio of recycled aggregate (RA) to natural aggregate (NA), and eccentricity ratio on the compression behavior of the specimens were examined. Subsequently, the failure modes, load-deflection responses, strain distributions, ductility, and the composite confinement mechanism of the columns were comprehensively studied. Finally, this paper also suggested the improved design models in predicting the Nu of the spiral-confined CFST columns (i.e., NSS-RA-CFST and HSS-RA-CFST columns), and the effectiveness and accuracy of the recommended design models were validated by using the experimental results acquired from this study and those studies collected from the literature. This study could serve as the experimental and theoretical references for the mechanical properties and determination of the Nu for the spiral confined-RA-CFST columns under both the concentric and eccentric compression.