Experimental Investigation on the Axial Loading Performance of Grooving-Damaged Square Hollow Concrete-Filled Steel Tube Columns

Abstract

Under the influence of material defects, structural grooving, environmental corrosion, and other factors in engineering, concrete-filled steel tubes incur local defects on their external surfaces that affect their structural integrity and service life. This work conducts axial compression tests on 10 grooving-damaged square hollow concrete-filled steel tube (SHCFST) columns to investigate the effect of grooving damage on their axial compressive ultimate bearing capacity and the effect of steel tubes on concrete confinement. It explores the effects of three parameters, namely, the length of grooves, presence of slots in internal and external steel tubes, and orientation of grooves, on structural static performance. This study analyzes the loading, failure mechanisms, and axial compressive ultimate bearing capacity of grooving-damaged SHCFST columns. Results indicate that grooving weakens the steel tube’s confinement effect on the concrete core, reducing the axial compressive ultimate bearing capacity of specimens. On the basis of this experimental research, a method for calculating the axial compressive ultimate bearing capacity and axial compressive stiffness of grooving-damaged SHCFST columns is proposed. The calculation results closely align with experimental outcomes, providing valuable insights for related scientific research and engineering applications.

1. Introduction

Concrete-filled steel tube (CFST) columns, which consist of inner and outer steel tubes and concrete filler, are widely used in construction and bridge engineering because of their simple node construction, convenient connection, and good bending resistance. Solid CFST bridge piers often experience excessive axial load-bearing capacity as engineering structures increase in height and span increased distances. Square hollow concrete-filled steel tube (SHCFST) columns and other profiles have been introduced into engineering projects to conserve materials, reduce costs, and meet bending strength requirements [1,2,3,4].

In engineering, grooving and reinforcement bar insertion into steel tubes are necessary due to the needs of some applications, such as the connection of CFST beam-to-column joints. This process weakens the steel tube’s confinement effect on the concrete core at the joint and reduces cross-sectional stiffness, adversely affecting the mechanical performance of the CFST column at the joint [1,2,3]. He et al. [5] studied the influence of local geometric defects on the mechanical performance of CFST columns. They found that in CFST columns, the above damage weakens mechanical properties, destroys structural integrity, and affects service life. Research has been conducted on this topic domestically and internationally.

Research on CFST columns with grooving damage includes the work of Zhu et al. [6], who performed axial compression tests on grooving-damaged circular CFST short columns fabricated by using 12 different circumferential and longitudinal grooving methods. They analyzed the effects of various grooving methods on steel tube-confined concrete. Yu et al. [7,8] conducted experimental studies on the axial compressive performance of grooving-damaged circular CFST short columns. They investigated changes in the mechanical properties, such as stiffness, load-bearing capacity, and ductility, of specimens after grooving. Chang et al. [9] performed axial compression tests on 15 grooving-damaged circular CFST short columns. They analyzed the effect of groove length and direction, concrete strength, and steel content and proposed a formula for calculating the axial compressive bearing capacity of grooving-damaged circular CFST short columns. Liu et al. [10] conducted experimental studies on 14 hexagonal CFST short columns with different groove directions, positions, and sizes. They analyzed the strain development, failure mechanisms, and axial compressive ultimate bearing capacity of grooving-damaged hexagonal CFST short columns and established a method for calculating the ultimate strength of components. Guo et al. [11] performed experimental studies on 15 groups of square CFST columns with grooves, including central transverse, longitudinal, diagonal, and edge transverse and longitudinal grooves. They explored the effects of groove length, thickness, and direction on the axial compressive bearing capacity of square CFST columns and proposed a formula for calculating the axial compressive bearing capacity of grooving-damaged square CFST columns. Han et al. [12] investigated the mechanical performance of square CFST column components after corrosion and conducted a comparative analysis of the load-bearing capacity of square CFST column components by using different standards. Alzand et al. [13] revealed the flexural performance of square CFST beams stiffened with V-shaped grooves, and the bending moment capacity and the flexural stiffness results obtained from the experimental and numerical investigations were validated with predicted values from different existing standards. Wei et al. [14] investigated the behavior of rectangular concrete-filled fiber-reinforced polymer (FRP) and steel composite tube (CFCT) columns with stress-release grooves under axial compression and found stress-release grooves only play a positive role in specimens with relatively large values of the corner radii.

Studies on SHCFST columns include the research of Zhao et al. [15], who established theoretical models for the ultimate strength of SHCFST short columns and beams by conducting a series of axial compression and bending tests on SHCFST short columns. Ayough et al. [16] performed nonlinear analyses on double-layer CFST columns under axial compression. They suggested that increasing the wall thickness of the outer steel tube can reduce the sensitivity of the steel tube to local buckling, thereby improving the overall performance of columns. Muhammad Rizwan et al. [17] developed a calculation modeling method using fiber discretization technology. Their method considerably saved model development and computation time compared with finite element methods. They proposed an expression for designing rectangular hollow CFST columns and concluded that the yield stress of the steel had no remarkable effect on the ductility of columns. Ding et al. [18] create a three-dimensional solid model that considered the effects of various parameters on the ultimate bearing capacity and pressure stresses of concrete. They proposed practical calculation formulas for the load-bearing capacity of SHCFST short columns. Guo et al. [19] subjected 10 square CFST short columns with double inner steel tubes to axial compression tests. They systematically studied the influence of concrete strength, eccentricity, and section slenderness on the strength, deformation, and ductility of the specimens, presenting a simplified calculation formula for the ultimate strength of hollow CFST short columns.

As can be observed from the abovementioned works, research on the axial compression of grooving-damaged SHCFST columns is limited. By building on existing research, our research group (1) conducted axial compression tests on 10 grooving-damaged SHCFST columns; (2) analyzed the effect of three different grooving defect factors, including the length of grooves, presence of slots in internal and external steel tubes (number of grooves), and direction of grooves, on the mechanical performance of SHCFST columns; and (3) recommended a formula for the bearing capacity of grooving-damaged SHCFST columns by utilizing the bearing capacity formula for SHCFST columns proposed by Ding [18].

2. Experimental Study

2.1. Test Design

Ten grooving-damaged SHCFST columns with three varying parameters, namely, the length of grooves, presence of slots in internal and external steel tubes (number of grooves), and orientation of grooves, were designed for the experiments.

Table 1. Specimen size and material characteristics of CFST column.

	No.	Orientation	Location	L × b (mm)	D × t × H (mm)	fs (MPa)	fcu (MPa)	Nt (kN)	Nc (kN)
1	SHCFST-1	Vertical	Inside and outside	120 × 16	200 × 2.8 × 600	359	29.6	1500	1458.8
2	SHCFST-2	Vertical	Inside and outside	80 × 16	200 × 2.8 × 600	359	29.6	1570	1591.8
3	SHCFST-3	Vertical	Inside and outside	40 × 16	200 × 2.8 × 600	359	29.6	1590	1724.8
4	SHCFST-4	Vertical	Outside	120 × 16	200 × 2.8 × 600	359	29.6	1600	1665.0
5	SHCFST-5	Vertical	Outside	80 × 16	200 × 2.8 × 600	359	29.6	1723	1731.5
6	SHCFST-6	Vertical	Outside	40 × 16	200 × 2.8 × 600	359	29.6	1755	1798.0
7	SHCFST-7	Horizontal	Inside and outside	80 × 16	200 × 2.8 × 600	359	29.6	1774	1711.5
8	SHCFST-8	Horizontal	Inside and outside	40 × 16	200 × 2.8 × 600	359	29.6	1803	1778.0
9	SHCFST-9	Horizontal	Outside	80 × 16	200 × 2.8 × 600	359	29.6	1795	1791.3
10	SHCFST-10	Horizontal	Outside	40 × 16	200 × 2.8 × 600	359	29.6	1824	1824.6

shows the specimen size and material characteristics of a CFST column. All specimens had the same cross-sectional dimensions. Prior to testing, the material properties of the steel as well as the mechanical properties of cubic concrete specimens were determined in accordance with standard testing methods. The specimens are depicted in Figure 1, wherein l is the length of the steel tube groove, b is the width of the steel tube groove, D is the outer square side length, t is the steel material thickness, H is the specimen height, f_s denotes the yield strength of the steel material, f_cu indicates the compressive strength of cubic concrete specimens, and p is the specimen perimeter.

Thin-walled steel tubes were formed by bending and welding thin plates. By contrast, the surfaces of square steel tubes were welded together from two L-shaped steel plates after crimping, and the inner and outer square steel tubes had two welds, as shown in Figure 1d. During production, it was ensured that the cross sections at both ends of the steel tube were smooth and without distortion. A 3 mm thick steel plate was welded to the bottom of the formed steel tube. The locations and sizes for groove preparation were positioned and marked on the steel tube, and a cutting machine was carefully used to create grooves on the steel tube corresponding to specifications. Tape was used to seal the grooved area when pouring concrete. The steel tube was then erected. Concrete was poured from the top and compacted with a vibrating rod until it was densely packed. The concrete at the ends was smoothed. The specimens were naturally cured.

2.2. Loading Scheme

Tests were conducted by using a 5000 kN hydrostatic machine for loading. The experimental setup for the SHCFST columns and field test are illustrated in Figure 2. Resistive strain gauges were installed along the midsection of the specimens, as shown in Figure 2a, to measure their vertical and horizontal deformations.

A displacement-controlled loading procedure was employed. During the elastic and plastic phase, each load level was approximately 1/15th of the ultimate load, and each load level was sustained for approximately half a minute. Loading was maintained at a slow, continuous pace, and corresponding data were synchronously collected. As the specimens approached their ultimate load-bearing capacity, they were slowly and continuously loaded until failure. Each test on the specimens lasted for approximately 10 min. In accordance with the characteristics of the specimen, the test was interrupted when the axial displacement reached 0.025 H.

2.3. Loading Phenomenon

SHCFST columns affixed with strain gauges were placed at the premarked center of the loading platform. A preliminary preload was applied to the SHCFST columns by using a hydrostatic machine to eliminate any gaps between the hydrostatic machine and upper surface of the SHCFST columns. Subsequently, the displacement dial indicator was zeroed in preparation for the start of the test. The SHCFST-1 specimen, which had vertical internal and external grooves 120 mm in length, were used to illustrate loading. Representative stages during loading are depicted in Figure 3.

When the load reached 200 kN, the axial displacement of the specimen was 0.20 mm, indicating the early stage of the elastic phase. At this point, no remarkable buckling or other deformations were observed on the surface of the specimen. Under a load of 1000 kN, the axial displacement of the specimen was 1.11 mm, signifying the initial stage of the elastic–plastic phase. Slight buckling was observed on the upper external steel tube of the specimen. At an axial displacement of 3.01 mm and a loading of 1500 kN, the specimen was on the verge of failure. Audible sounds of material compression and fracture could be heard at the test site. The degree of buckling on the outer steel tube of the upper section of the specimen continued to increase. At an axial displacement of 7.15 mm, the specimen entered the failure stage. The load reading on the hydrostatic machine had dropped from its peak of 1500 kN to 1098 kN by this point. The buckling of the upper external steel tube had drastically increased and was spreading downward. As the load decreased to 711 kN and the axial displacement of the specimen reached 15.84 mm, buckling was observed to have extended from the middle to the top four corners of the specimen. Loading was stopped to prevent the specimen from bursting.

As depicted in Figure 4, which illustrates the typical failure modes of the specimens, all specimens experienced buckling phenomenon at their top or bottom during the test.

After the completion of the experiments, the steel tube of the specimens was cut and stripped to reveal the condition of the concrete core, as shown in Figure 5. The following observations were obtained: (1) specimens with vertical grooves lacked apparent signs of concrete damage in their grooved section. However, they exhibited noticeable concrete damage near their grooved section and at their upper and lower parts, as depicted in Figure 5a,b. (2) Specimens with horizontal grooves showed minor concrete damage in their grooved section but considerable concrete damage in their upper part, as shown in Figure 5c,d. (3) The concrete core failure of the specimens with inner and outer steel tube grooves was more obvious than that of the specimens with only outer steel tube grooves.

2.4. Analysis of Test Results

Figure 6 illustrates the load–displacement curves of the specimens. The static test on the steel tube concrete columns can be divided into three stages: elastic, elastic–plastic, and failure stages. (1) Elastic stage: During the initial loading stage (approximately 60% of the ultimate load), the axial deformation of the specimens exhibited nearly linear growth as the load increased. The appearance of the specimens remained unchanged. (2) Elastic–plastic stage: After surpassing the yield load, the load–deflection curve exhibited nonlinear growth. The linear growth in axial displacement decelerated as the load increased, showing a gradual upward trend. The appearance of the specimens still did not show any remarkable change. (3) Failure stage: After reaching the ultimate load, the buckling of the specimen was aggravated with the increase in axial deformation.

3. Effects of Three Parameters on the Axial Compression Performance of SHCFST Columns

3.1. Influence of Notch Length

Figure 7 depicts the influence of notch length on the load-bearing performance of SHCFST columns. The groove lengths tested were 40, 80, and 120 mm. As the groove length increases, the steel tube’s constraining effect weakens, leading to the low ultimate load-bearing capacity of the specimens.

For specimens with vertical grooves, the load-bearing capacity of the specimen with inner and outer steel tube groove lengths of 40 mm is 1.3% and 6.0% greater than that of the specimens with groove lengths of 80 and 120 mm, respectively. The load-bearing capacity of the specimen with an external groove length of 40 mm is 1.9% and 9.7% greater than that of the specimens with groove lengths of 80 and 120 mm, respectively.

For specimens with horizontal grooves, the load-bearing capacity of the specimen with inner and outer steel tube groove lengths of 40 mm is 1.6% greater than that of the specimen with an 80 mm groove length, and the specimen with an external groove length of 40 mm has a load-bearing capacity that is 4.0% greater than that of the specimen with an 80 mm groove length.

3.2. Influence of Notch Location

Figure 8 illustrates the effect of the presence of internal and external grooves on the load-bearing performance of SHCFST columns with grooves in their internal central and external central parts. When the inner and outer steel tubes are slotted, the ultimate bearing capacity of the SHCFST column is smaller than that when only the outer steel tube is slotted.

For specimens with vertical grooves, the load-bearing capacity of specimens with external grooves is 8.9% greater than that of specimens with inner and outer steel grooves. For specimens with horizontal grooves, the load-bearing capacity of specimens with external grooves is approximately 1.2% greater than that of specimens with inner and outer steel grooves.

3.3. Influence of Notch Direction

Figure 9 illustrates the effect of groove direction on the load-bearing performance of SHCFST columns, with groove directions being vertical and horizontal. When internal and external grooves are present, the load-bearing capacity of specimens with horizontal grooves is 13.0% greater than that of specimens with vertical grooves. When only external grooves are present, the load-bearing capacity of specimens with horizontal grooves is 11.2% greater than that of specimens with vertical grooves.

4. Calculation of the Ultimate Bearing Capacity and Axial Compression Stiffness of Notched SHCFST Columns

4.1. Bearing Strength

The following formula proposed in the literature [18] is adopted for SHCFST column specimens without grooves:

$$N_{\mathrm{c}}=f_{\mathrm{c}}{A}_{\mathrm{c}}+{\mathit{Kf}}_{\mathrm{s}\mathrm{o}}{A}_{\mathrm{s}\mathrm{o}}+f_{\mathrm{s}\mathrm{i}}{A}_{\mathrm{s}\mathrm{i}},$$

(1)

where N_c is the calculated capacity of columns; A_c is the section area of concrete; f_c is the compressive strength of concrete, $f_{\mathrm{c}}=0.4f_{\mathrm{c}\mathrm{u}}^{7/6}$; and K is the constraint coefficient of the concrete-filled steel tube. Ding et al. [18] reported that for SHCFST column specimens with hollow ratios between 0 and 0.77, $K=1.2+0.15X_{\mathrm{o}}-0.3\sqrt{X_{\mathrm{o}}}$, where X_o is the hollow ratio. The steel test specimen has square internal and external cross sections; 100 and 200 mm inner and outer edges, respectively; and calculated K = 1.08. f_so and A_so are the yield strength and area of the outer steel tube, respectively, and f_si and A_si are the yield strength and area of the inner steel tube, respectively.

The discussion of the above experimental results reveals that the length, number, and direction of grooves have a certain effect on the ultimate load-bearing capacity of composite columns. Therefore, the following equation for calculating the ultimate load-bearing capacity of grooved SHCFST columns is proposed:

$$N_{\mathrm{c}}=f_{\mathrm{c}}{A}_{\mathrm{c}}+K_1({\mathit{Kf}}_{\mathrm{s}\mathrm{o}}{A}_{\mathrm{s}\mathrm{o}}+f_{\mathrm{s}\mathrm{i}}{A}_{\mathrm{s}\mathrm{i}})$$

(2)

In the formula, the mathematical expression of K₁ is

$$K_1=1.0-(0.8{\beta }_1+1.6{\beta }_2)$$

(3)

Here, ${\beta }_1$ and ${\beta }_2$ are the parameters related to groove length and width, number, and direction, whereas S represents the perimeter of the steel material’s cross section.

The following definitions are provided for specimens with inner and outer grooves:

$${\beta }_1=\{\begin{array}{c}b/H\\ b/S\end{array}\begin{array}{l}\mathrm{Transverse}\mathrm{slotting}\\ \mathrm{Vertical}\mathrm{slotting}\end{array},$$

(4)

$${\beta }_2=\{\begin{array}{c}l/S\\ l/H\end{array}\begin{array}{l}\mathrm{Transverse}\mathrm{slotting}\\ \mathrm{Vertical}\mathrm{slotting}\end{array}.$$

(5)

The following definitions are provided for specimens with outer grooves:

$${\beta }_1=\{\begin{array}{c}2b/H\\ 2b/S\end{array}\begin{array}{l}\mathrm{Transverse}\mathrm{slotting}\\ \mathrm{Vertical}\mathrm{slotting}\end{array},$$

(6)

$${\beta }_2=\{\begin{array}{c}2l/S\\ 2l/H\end{array}\begin{array}{l}\mathrm{Transverse}\mathrm{slotting}\\ \mathrm{Vertical}\mathrm{slotting}\end{array}.$$

(7)

The average N_c/N_t ratio is 0.992, and the dispersion coefficient is 0.034. N_t is the tested capacity of the columns. Figure 10 shows that the results calculated by using the formula are in good agreement with the test results.

4.2. Confinement Effect

In summary, the grooving damage of steel tube weakens the confinement effect on the concrete core, making the mechanical properties of the damaged CFST column worse than those of undamaged CFST column. Therefore, the evaluation of the confinement effect is a crucial issue. The column strength of a steel tube concrete section is the sum of contributions from the steel tube, concrete, and interaction between the steel tube and concrete. Therefore, it can be expressed as:

N_c = A_sf_s + A_cf_c + F_s.

(8)

F_s is the contribution of the interaction between the steel tube and concrete, F_s = N_c − A_sf_s − A_cf_c.

For grooved specimens, only part of the steel tube can withstand the axial load due to the existence of notches. As can be seen from Equation (8), the column strength of grooved SHCFST columns is

$$N_{\mathrm{c}}=A_{\mathrm{s}\mathrm{e}}{f}_{\mathrm{s}}+A_{\mathrm{c}}{f}_{\mathrm{c}}+F_{\mathrm{s}}.$$

(9)

For specimens with internal and external vertical grooves, the effective section area $A_{\mathrm{s}\mathrm{e}}$ of the steel tube is defined as

$$A_{\mathrm{s}\mathrm{e}}=(1-2l/H)\times S.$$

(10)

For specimens with external vertical grooves, the effective section area $A_{\mathrm{s}\mathrm{e}}$ of the steel tube is defined as

$$A_{\mathrm{s}\mathrm{e}}=(1-l/H)\times S.$$

(11)

For specimens with internal and external transverse grooves, the effective section area $A_{\mathrm{s}\mathrm{e}}$ of the steel tube is defined as

$$A_{\mathrm{s}\mathrm{e}}=(1-2l/p)\times S.$$

(12)

For specimens with external transverse grooves, the effective section area $A_{\mathrm{s}\mathrm{e}}$ of the steel tube is defined as

$$A_{\mathrm{s}\mathrm{e}}=(1-l/p)\times S.$$

(13)

Hence, the confinement factor defined in Equation (14) was applied to estimate the confinement effect of a grooved steel tube on the concrete core:

$$\mathsf{\lambda }=F_{\mathrm{s}}/N_{\mathrm{c}}.$$

(14)

The confinement coefficients of all tested notched specimens are shown in

Table 2. List of constraint coefficients of specimens.

No.	${\mathit{A}}_{\mathbf{se}}$ (mm2)	${\mathit{A}}_{\mathbf{s}\mathbf{e}}{\mathit{f}}_{\mathbf{s}}+{\mathit{A}}_{\mathbf{c}}{\mathit{f}}_{\mathbf{c}}$ (kN)	${\mathit{N}}_{\mathbf{c}}$ (kN)	${\mathit{F}}_{\mathbf{s}}$ (kN)	λ
SHCFST-1	2016.0	1348.5	1458.8	110.3	0.076
SHCFST-2	2464.0	1509.3	1591.8	82.5	0.052
SHCFST-3	2912.0	1670.1	1724.8	54.7	0.032
SHCFST-4	2688.0	1589.7	1665.0	75.3	0.045
SHCFST-5	2912.0	1670.1	1731.5	61.4	0.035
SHCFST-6	3136.0	1750.5	1798.0	47.5	0.026
SHCFST-7	2464.0	1509.3	1711.5	202.2	0.118
SHCFST-8	2912.0	1670.1	1778.0	107.9	0.061
SHCFST-9	3136.0	1750.5	1791.3	40.8	0.023
SHCFST-10	3248.0	1790.8	1824.6	33.8	0.019

. The confinement factor increases as the groove length of the steel tube increases.

For specimens with vertical grooves, the confinement factor of the specimen with the internal and external steel tube groove length of 120 mm is 139% and 45.9% greater than that of the specimens with 40 and 80 mm groove lengths, respectively. The load-bearing capacity of the specimen with an external groove length of 120 mm is 71.3% greater than that of the specimen with an 80 mm groove length and 27.6% greater than that of the specimen with an 80 mm groove length.

For specimens with horizontal grooves, the load-bearing capacity of the specimen with internal and external steel tube groove lengths of 80 mm is 94.7% greater than that of the specimen with a 40 mm groove length, and the specimen with an external groove length of 80 mm has a load-bearing capacity that is 22.6% greater than that with a 40 mm groove length.

That is, the damage caused by the grooving of the steel tube has a considerable effect on structural bearing capacity and exhibits a nonlinear relationship. In engineering applications, grooving should receive careful consideration.

4.3. Axial Compression Stiffness

In this study, the ultimate load corresponding to 50% of the specimen’s limit load is taken and associated with the secant stiffness at the axial compression stiffness of the specimen. (EA)_c is defined as

(EA)_c = E_sA_se + E_cA_c.

(15)

Figure 10 shows the axial compression stiffness of the specimens. (EA)_t is the measured axial compressive stiffness of the specimens. The average (EA)_t/(EA)_c is 0.952, and the dispersion coefficient is 0.132. The results obtained with Equation (15) are in good agreement with the experimental results.

Table 3. List of axial compression stiffness of specimens.

No.	${\mathit{E}}_{\mathbf{s}}$ (Pa)	${\mathit{E}}_{\mathbf{c}}$ (Pa)	${\mathit{A}}_{\mathbf{se}}$ (mm2)	${\mathit{A}}_{\mathbf{c}}\left({\mathbf{mm}}^2\right)$	(EA)c (N)	(EA)t (N)	(EA)c/(EA)t
SHCFST-1	2.06 × 1011	3.35 × 1010	2016.0	30,000	1.420 × 109	1.11 × 109	0.821
SHCFST-2	2.06 × 1011	3.35 × 1010	2464.0	30,000	1.513 × 109	1.52 × 109	1.006
SHCFST-3	2.06 × 1011	3.35 × 1010	2912.0	30,000	1.605 × 109	1.89 × 109	1.179
SHCFST-4	2.06 × 1011	3.35 × 1010	2688.0	30,000	1.559 × 109	1.23 × 109	0.817
SHCFST-5	2.06 × 1011	3.35 × 1010	2912.0	30,000	1.605 × 109	1.59 × 109	0.991
SHCFST-6	2.06 × 1011	3.35 × 1010	3136.0	30,000	1.651 × 109	1.75 × 109	1.060
SHCFST-7	2.06 × 1011	3.35 × 1010	2464.0	30,000	1.513 × 109	1.25 × 109	0.826
SHCFST-8	2.06 × 1011	3.35 × 1010	2912.0	30,000	1.605 × 109	1.60 × 109	0.991
SHCFST-9	2.06 × 1011	3.35 × 1010	3136.0	30,000	1.651 × 109	1.21 × 109	0.815
SHCFST-10	2.06 × 1011	3.35 × 1010	3248.0	30,000	1.674 × 109	1.67 × 109	0.996

shows the List of axial compression stiffness of specimens.

5. Conclusions

This work conducted axial compression tests on 10 grooving-damaged SHCFST column specimens. It investigated the influence of parameters, such as the length of grooves, presence of grooves in internal and external steel tubes (number of grooves), and direction of grooves, on the load-bearing performance of steel tube concrete columns. This study examined structural failure modes, load–displacement curves, and other performance aspects. Additionally, it proposed an equation for calculating the ultimate load-bearing capacity and axial compression stiffness of grooving-damaged SHCFST columns. Its main conclusions are as follows:

(1)

The test results indicate that at the early stage of the elastic phase, no remarkable buckling or other deformations are present on the surface of the specimen. Different types of CFST columns exhibit the same failure modes. That is, all specimens experience buckling phenomenon at their top or bottom. After the completion of the experiments, the steel tube of the specimens was cut and stripped, which revealed the core concrete had different degrees of damage. The concrete core failure of the specimens with inner and outer steel tube grooves was more obvious than that of the specimens with only outer steel tube grooves;

(2)

The ultimate load-bearing capacity of grooved CFST columns is lower than that of intact CFST columns because grooved steel tubes cannot provide sufficient constraint to the concrete core within the column. Parameter analysis reveals that groove length and width, position, and direction have a certain effect on the strength of concrete columns in grooved steel tubes. It can be observed that as the groove length increases, the steel tube’s constraining effect weakens, leading to the low ultimate load-bearing capacity of the specimens. When the inner and outer steel tubes are slotted, the ultimate bearing capacity of the SHCFST column is smaller than that when only the outer steel tube is slotted. The load-bearing capacity of specimens with horizontal grooves is greater than that of specimens with vertical grooves;

(3)

The ultimate load-bearing capacity of undamaged SHCFST columns was predicted by using existing formulas. An empirical equation based on the formula proposed by Ding et al. [18] was established to predict the ultimate load-bearing capacity of grooved SHCFST columns. The calculated values align well with the experimental results. This work also proposes the confinement effect. Experiments indicate that grooving has a considerable influence on structural load-bearing capacity and has a nonlinear relationship;

(4)

The influence of damage caused by grooving in steel tubes is considered, and a formula for calculating the axial compression stiffness of the specimens is proposed. The calculation results are in good agreement with the experimental results.