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The axial compressive performance of novel L-shaped and T-shaped concrete-filled square steel tube (L/T-CFSST) column was assessed in this study. Ten L/T-CFSST columns were tested to failure under axial load. The experimental data were used to determine various failure modes, bearing capacities, and load-displacement curves. The test parameters included the section form, steel tube thickness, steel yield strength, and slenderness ratio. The axial compressive performance of the L/T-CFSST column proved favorable, and each square steel tube showed strong cooperative performance. The failure mode of the stub column specimen (

Steel and concrete composite structures are widely used in civil engineering projects as they make full use of the superior material properties of steel and concrete [

The steel column of traditional residential buildings is commonly a regular section. In this case, the column may protrude from the wall and affect the building’s function [

Cross-sections of special-shaped CFST columns: (a) SS-CFST column; (b) SS-CFST column with binding bars; (c) SC-CFST column with stiffener ribs; (d) SC-CFST column connected with steel plates; (e) multicell SC-CFST column.

Recent researchers have conducted experimental and FEA investigations to probe the effects of various parameters on the compressive performance and seismic behaviors of SS-CFST columns. Lei et al. [

However, there are some unresolved problems related to SS-CFST columns including the stress concentration at the internal and external corners, complex connection structure, and large amounts of welding and residual welding stress. These problems must be solved to satisfy the requirements of prefabricated steel residential buildings. In China, some prefabricated steel structure residential buildings adopt novel special-shaped concrete-filled steel square tube columns. The novel special-shaped column is composed of many manufactured square steel tubes connected via structural weld at the chamfer of the square steel tube in the factory and then combined into L-shape, T-shape, and crisscross-shape sections poured with concrete at the construction scene. The section form is shown in Figure

Novel special-shaped CFST column sections. (a) L shape. (b) T shape. (c) Crisscross shape.

In order to study the performance of the novel special-shaped CFST column under axial compression, ten L/T-CFSST column specimens were tested under axial load conditions with varying parameters including section form, steel tube thickness, steel yield strength, and slenderness ratio. The failure modes, bearing capacities, and load-displacement curves of the specimens were analyzed accordingly. Corresponding FEA models were established. Parametric studies were carried out based on the FEA models to establish the bearing capacity calculation formula for L/T-CFSST columns based on modification of the AIJ code. The calculation results were in close agreement with both FEA and experimental results, which suggests that the proposed model may provide a workable reference for practicing engineers and designers.

Ten L/T-CFSST columns were tested under axial compression to observe the effects of various parameters including the section form, steel tube thickness and strength, and slenderness ratio. Figure _{xo} and _{yo} are the slenderness ratio of the column around the _{0} and _{0} axis, respectively. _{s}, _{s}, _{c}, and _{c} are the elastic modulus and the moment of inertia of steel and concrete, respectively, _{s} and _{c} are the section area of steel tube and concrete, _{y} is the yield stress of steel, and _{ck} is the characteristic strength of concrete.

Cross-section of specimens.

Specimens in detail.

Specimen | Steel grade | _{x0} | _{y0} | _{uT} (kN) | _{uFEA} (kN) | _{uFEA}_{uT} | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

L-1 | Q235 | 600 | 200 | 3 | 4 | 9 | 12 | 0.18 | 1.4 | 2620 | 2363 | 0.90 |

L-2 | Q355 | 600 | 200 | 3 | 4 | 9 | 12 | 0.18 | 2.1 | 3288 | 2999 | 0.91 |

L-3 | Q235 | 600 | 200 | 3 | 6 | 9 | 12 | 0.29 | 2.3 | 3055 | 2909 | 0.95 |

L-4 | Q235 | 900 | 200 | 4.5 | 4 | 13 | 18 | 0.18 | 1.4 | 2403 | 2211 | 0.92 |

L-5 | Q235 | 1200 | 200 | 6 | 4 | 17 | 24 | 0.18 | 1.4 | 2239 | 2151 | 0.96 |

T-1 | Q235 | 900 | 300 | 3 | 4 | 11 | 16 | 0.18 | 1.4 | 3140 | 3159 | 1.00 |

T-2 | Q355 | 900 | 300 | 3 | 4 | 11 | 16 | 0.18 | 2.0 | 3808 | 3978 | 1.04 |

T-3 | Q235 | 900 | 300 | 3 | 5 | 11 | 16 | 0.23 | 1.8 | 3817 | 3734 | 0.98 |

T-4 | Q235 | 1200 | 300 | 4 | 5 | 15 | 21 | 0.23 | 1.8 | 3580 | 3521 | 0.98 |

T-5 | Q235 | 1500 | 300 | 5 | 5 | 19 | 26 | 0.23 | 1.8 | 3350 | 3297 | 0.98 |

The test specimens consist of three parts: the L/T-CFSST column, a top cover plate, and a bottom cover plate. When processing L/T-CFSST columns, the steel tube was first cut down according to the design length and positioned according to the section form. The steel tube chamfer was welded, and then the steel cover plate at the bottom of the specimen was welded. Self-compacting concrete was poured into the square steel tube and cured. The upper part of the specimen was grinded, and the top cover plate was welded to ensure that the steel tube and core concrete would bear force simultaneously throughout the test. Air vent holes were reserved in the steel tubes to ensure fully compacted concrete.

Standard tensile tests on coupons were conducted to measure the material properties of the steel tubes. The measured average yield strength (_{y}), ultimate strength (_{u}) are listed in Table _{s}) is 206 GPa. The core concrete of L/T-CFSST column is poured by commercial concrete, and the mix proportions of the concrete are as follows: cement: 280 kg/m^{3}; river sand: 700 kg/m^{3}; water: 156 kg/m^{3}; fly ash: 100 kg/m^{3}; coarse aggregate: 1150 kg/m^{3}; and additional high range water reducer (HRWR): 6.3 kg/m^{3}. The average measured compressive cube strengths (_{cu}) of the concrete at the time of testing were 38.44 MPa, and the values of elastic modulus (_{c}) for the concrete were 30 GPa.

Mechanical properties.

Material | _{y} (MPa) | _{u} (MPa) | |
---|---|---|---|

Steel Q235 | 4 | 320.5 | 480.6 |

Steel Q235 | 5 | 390.0 | 488.3 |

Steel Q355 | 4 | 456.7 | 546.7 |

The test was carried out on a 5000 kN computer-controlled servo-hydraulic universal testing machine, and the experimental devices are depicted in Figure _{0} axis, and the load line is applied on the fixed part through the knife-edge to realize the axially loading.

Loading devices.

The force-controlled mode was adopted at the beginning of the loading process. The subsequent loading speed was 100 kN/min. Once the load-vertical displacement curve changed from a straight line to a curve, the device was switched into displacement-controlled loading mode and the loading speed was controlled at 0.5 mm/min. The axial load-vertical displacement curve was directly obtained from universal testing machine. Linear variable displacement transducers (LVDTs) were arranged at three divided points of the specimen to monitor the lateral displacement. Strain gauges were attached to the midheight cross-section of each surface to measure longitudinal and horizontal strains. Surfaces and strain gauges number and instrumentation layout are shown in Figure

Surfaces, strain gauge number, and instrumentation layout.

According to the height of L/T-CFST column, the test phenomena can be divided into two categories. For the stub column (_{0} axis upon failure of steel tube after local buckling. It is worth noting that the universal testing machine leaked oil during the T-2 specimen test, so the test data of T-2 was incomplete. The final failure characteristics of stub columns are shown in Figure

Failure characteristics of stub columns: (a) L-1; (b) L-2; (c) L-3; (d) T-1; (e) T-2; (e) T-3.

There was no obvious phenomenon observable in the medium-long column from the beginning of loading to 80% of the ultimate load. As loading continued, the specimen began to show slight bending deformation. The lateral deformation grew more obvious as the loading increased until the steel tube of the L-CFSST column presented local buckling. Upon reaching the ultimate bearing capacity, the bearing capacity dropped off as lateral deformation continued to increase until overall bending occurred along the _{0} axis and the specimen was significantly damaged. The final failure characteristics of the medium-long column are shown in Figure

Failure characteristics of medium-long columns: (a) L-4; (b) L-5; (c) T-4; (d) T-5.

The axial compression process of the L/T-CFSST column can be divided into three stages as per the test phenomena observed here: the elastic stage, the elastic-plastic stage, and the failure stage. There is no obvious phenomenon in the elastic stage, the steel tube and concrete bear the load separately, and the combined action of the steel tube and concrete has not yet occurred. In the elastic-plastic stage, the steel tube restrains the concrete once the horizontal deformation of concrete exceeds the horizontal deformation of the steel tube. At this time, the concrete is in a state of triaxial compression and the steel tube can be regarded as the plane stress state of axial compression and circumferential tension. As the load further increases, the steel tube yields under compression and local buckling or bending deformation can be observed. In the failure stage, the concrete is crushed and the bearing capacity of the specimen is markedly reduced. The strength failure caused by local buckling of the steel tube characterizes the failure mode of stub columns. For medium-long columns (

Figure

Axial load (P)-vertical displacement (∆) curves: (a) L-shaped column; (b) T-shaped column.

According to Table

The axial load (

Axial load (

To further analyze the influence of individual parameters on the axial bearing capacity of the L/T-CFSST column, a three-dimensional (3D) FEA model was established in ABAQUS software. In the FEA model, the cover plate was simulated by a rigid body, the square steel tube was simulated by a 4-node shell element with a reduced integral (S4R), and the concrete was simulated by 8-node brick elements with reduced integration (C3D8R).

The boundary conditions of each FEA model were the same as those in the test. The coupling point was specified at the centroid of the cover plate. The coupling point of the top cover plate was constrained in the

The stress-strain relationship of steel tube adopts the simplified bilinear model. The yield stress and elastic Young’s modulus obtained from tensile coupon test were employed in the analysis. The elastic modulus of the strengthened section in this case was 0.01_{s} and Poisson’s ratio was taken as 0.3.

To describe the compressive performance of the core concrete, an equivalent stress (_{0}, _{0}, and _{0} and _{0} are parameters related to the passive confinement provided by the steel tube to the concrete, _{c} is the elastic modulus of concrete (

A damaged plastic model was used to simulate the concrete where in the key parameters include the dilation angle (_{b0}/_{c0}), ratio of the second stress invariant on the tensile meridian to that on the compressive meridian (

Table _{uT} from the test and the peak load _{uFEA} from the FEM. The average value of _{uFEA}/_{uT} is 0.96, and the two sets of results are similar. A comparison between the failure characteristics of L-1 specimen obtained by finite element analysis versus the test is shown in Figure

Comparison of L-3 specimen failure mode between FEM and test.

Comparison of axial load-maximum strain curves between FEM and test: (a) L-1; (b) L-2; (c) L-3; (d) L-4; (e) L-5; (f) T-1; (g) T-3; (h) T-4; (i) T-5.

A series of parametric analyses were carried out to investigate the behaviors of L/T-CFSST column subjected axial load. The effects of steel thickness, steel strength, concrete strength, and slenderness ratio on the structural performance of axially loaded columns in terms of the load-displacement curves were assessed for each specimen. The numbering rules of specimen in the parameter analysis are shown in Figure

Specimen naming rule.

Figure

Load-vertical displacement curves of steel tubes with different thickness: (a) L-shaped; (b) T-shaped.

The effects of concrete strength on the load (

Load-vertical displacement curves of concrete with different strengths: (a) L-shaped; (b) T-shaped.

The load (

Load-vertical displacement curves of the steel tubes with different strengths: (a) L-shaped; (b) T-shaped.

According to the experimental phenomena and other data provided in Table

Load-vertical displacement curves of L/T-CFSST columns with different slenderness ratios: (a) L-shaped; (b) T-shaped.

The main specifications for CFST column bearing capacity calculation subjected axially load include BS 5400 [

The calculation for the ultimate bearing capacity of CFST specified in BS5400 (2005) is based on the superposition theory. The contribution of steel and concrete is appropriately reduced. The equation for the ultimate axial compressive bearing capacity of the square CFST column is_{u} is the ultimate axial compressive bearing capacity of the square CFST, _{s} and _{y} are the cross-sectional area and yield strength of the steel tube, and _{c} and _{cu} are the cross-sectional area and the cube compressive strength of concrete, respectively.

As specified in ANSI/AISC 360-05, the rectangular CFST column calculations are as follows:_{u} is the nominal compressive bearing capacity, _{cr} is the elastic critical buckling bearing capacity, _{0} is the nominal axial compressive strength without consideration of slenderness ratio, _{s}, _{y}, and _{s} are the cross-sectional area, yield strength, and elastic modulus of the steel tube, _{c}, _{c} are the cross-sectional area, the cylinder compressive strength, and the elastic modulus of concrete, respectively, _{s} and _{c} are the moment of inertia of the steel section and the concrete section about the elastic neutral axis, _{1} equals 0.85, _{2} is the coefficient for calculation of effective bending stiffness, and

Under EC 4 (2005), the strength of axially loaded square CFST columns is determined by summing the strength values of the steel tube and concrete. The axial load-carrying capacity of a square CFST column is calculated under this standard as follows:_{s} and _{c} are the material partial factors of steel and concrete, respectively, _{s} = 1.1 and _{c} = 1.5.

The ultimate axial bearing capacity of a square CFST column is calculated under AIJ (1997) as follows:

In Chinese CECS159 code, the ultimate axial bearing capacity of the square CFST column is the sum of the strength values of the steel tube and concrete:_{c} are the yield strength of steel and compressive strength of concrete, respectively.

The Chinese code GJB 4142-2000 adopts the unified strength theory, under which a CFST composed of two different materials (steel and concrete) is regarded as a new material. The CFST under axial compression is expressed as follows:

The axial bearing capacity of 72 L/T-CFSST stub columns with different section sizes was calculated to validate the above formula (_{u,c}) by comparison against the FEA result (_{u,FEA}) as shown in Figure

Comparison of FEA results against calculations from extant design codes.

As expected, most of the calculation results were partial to safety. The calculation results using the AIJ specification were close to the FEA values with a small discrete type. The AIJ code and effects of the confinement factor on the bearing capacity were utilized to establish a novel formula for the ultimate bearing capacity of L/T-CFSST columns after regression analysis of FEA results and test values:_{u} is the ultimate bearing capacity of L/T-CFSST stub columns, _{c} is the concrete cross-sectional area, _{s} is the steel tube cross-sectional area, _{y} is the yield strength of the steel tube, _{u} is the ultimate strength of the steel tube,

The influence of slenderness ratio is not incorporated into the formula presented above. The stability coefficient _{cr} is the axial stability bearing capacity of the medium-long column and _{u} is the axial strength bearing capacity of the stub column. Here, 48 L/T-shaped CF ST columns with different slenderness ratios were established and analyzed to determine the functional relationship between the stability coefficient

Above all, the bearing capacity of L/T-CFSST columns can be calculated as follows:

The failure modes of L/T-CFSST stub columns (

No weld tearing or separation of the steel tube occurred in the tests conducted here. The L/T-CFSST column shows strong mechanical properties and cooperative performance. The axial compressive process of L/T-CFSST columns can be divided into an elastic stage, elastic-plastic stage, and failure stage. The bearing capacity and ductility of the test specimens appeared to increase with steel tube thickness and strength increase and decrease as slenderness ratio increases.

Calculation formulas for the axial compressive strength and stability bearing capacity of L/T-CFSST columns were established and validated according to numerical and regression analyses based on AIJ code.

The data used to support the findings of this study are included within the article.

The authors declare that they have no conflicts of interest.

This paper was supported by the Scientific Research and Development Project for Wall Material Innovation and Building Energy-Saving of Shandong Province of China (Lucaijianzhi (2014) 139).