Experimental study on the seismic performance of short shear walls comprising cold-formed steel and high-strength reinforced concrete with concealed bracing

1 Introduction

High-strength concrete is being applied extensively with the rapid construction of various types of high-rise buildings. However, its application in structures requiring strong seismic performance has been severely limited by its poor ductility. The composite structure of section steel and high-strength concrete combines the advantages of the two materials.

In high-rise and super-high-rise buildings, shear walls are used as the main components resisting lateral forces, and their performance directly affects the safety of the entire structure. Short shear wall made from high-strength concrete has poor ductility, which greatly restricts its application in buildings requiring strong seismic performance. It is therefore necessary to adopt other structural forms that improve the seismic performance of high-strength concrete short shear wall. Concealed bracing can effectively limit the development of diagonal cracks in a shear wall, which makes its internal diagonal cracks smaller and denser and thus significantly improves the energy dissipation and ductility of the wall. A built-in steel truss in the shear wall can effectively improve seismic performance but ideally requires more steel. A large amount of hot-rolled steel and concrete steel should be used in areas with high seismic incidence probability or in mid- and high-rise buildings with high seismic requirements. Using cold-formed steel instead of ordinary hot-rolled steel can not only ensure sufficient seismic performance, but also reduce steel consumption and cost, which is in line with the current theme of a low-carbon society and environmental protection. Therefore, it is of great significance to explore the seismic performance of cold-formed steel and high-strength concrete short shear walls with concealed bracing in practical engineering applications, which is the focus of this study.

Many experiments and studies have been conducted on reinforced concrete shear walls, including fire resistance performance [1], seismic performance (reinforced concrete walls [2,3], reinforced steel-plate shear walls [4]), the effect of alkali–silicic acid reaction [5], resistance to explosion [6], calculating formula [7,8], composite shear walls of polyvinyl acetate and steel–fiber concrete [9], cyclic loading failure experiments [10], and numerical simulation research [11]. Zhang et al. [12,13] proposed that the shear wall including concealed bracing not only showed much higher lateral force resistance, but also had desirable resilient performance under the large deformation; the seismic resistance capacity of the recycled concrete frame-shear wall structure can be greatly improved by setting up concealed bracings inside the walls. Li et al. [14] showed that both the reinforcement bracings and the channel steel bracings restrained the development of cracks and decreased the crack widths, which showed a more remarkable effect on alleviating the damage of the wall. Wang et al. [15] indicated that the damage of the proposed shear wall is significantly reduced, and the cracks of the wall pier are less. Li et al. [16] studied the effects of bracing type on the seismic behavior of braced precast concrete shear walls in terms of failure mode, hysteretic behavior, and energy dissipation capacity. Gan et al. [17] indicated that increasing the area of the diagonal bracing under a high axial pressure ratio can improve the anti-seismic performance of low-rise shear walls.

This study investigates the effect of cold-formed steel on the seismic performance of high-strength concrete short shear walls. In the process of exploring more effective forms of diagonal bracing, lattice-shaped diagonal bracings are used in a number of specimens to investigate whether they are able to improve the seismic performance of high-strength concrete shear walls combined with concrete. Four cold-formed steel and high-strength concrete short shear wall specimens, namely, one without a diagonal bracing, one angle-steel bracing, and two lattice bracing with different steel ratio, are designed for testing their low cyclic loading. The crack development and deformation in the specimens are observed and recorded. Hysteretic and skeleton curves of the specimens are then obtained, and the ductility, energy dissipation, strength attenuation, and load-bearing capacity changes of the specimens are analyzed. A formula for calculating the shear resistance in the oblique section of the cold-formed steel and high-strength concrete short shear wall with concealed bracing is established. Finally, experimental results are concluded, and future work is presented in this article.

2 Test overview

2.1 Test design

The shear span ratio of short shear wall used in the test was λ = 1. The dimensions of the designed specimens are shown in Figure 1. In view of actual conditions, concrete with a strength grade of C60, cold-formed steel section, and HPB300 reinforcement was selected. The cold-formed steel section could not be welded, and the steel section frame was thus connected using self-tapping screws. The axial compression ratio in the test was 0.24. Four specimens with the same external dimensions, numbered as SRHCW-1–4, were studied. The wall section had a dimension of 800 mm × 120 mm, and the length of the marginal component was 0.2 times that of the wall section.

Figure 1

Specimen dimensions.

The steel section and reinforcement of marginal components in the four specimens were the same as those of the entire wall (Figure 2), with the change only in the form and steel ratio of diagonal bracing. Parameters of the four specimens are given in Table 1. SRHCW-1 had no diagonal bracing and was used only for comparison. SRHCW-2 had a diagonal bracing in the form of equilateral angle steel. SRHCW-3 had a diagonal bracing in the form of a latticed column, together with section steel having an area the same as that of the steel section on SRHCW-2. SRHCW-4 had diagonal bracing in the form of a latticed column, together with section steel having an area larger than that of the steel section of SRHCW-3. The mix proportion of the C60 concrete is given in Table 2.

Figure 2

Reinforcement details. (a) SRHCW-1, (b) SRHCW-2, (c) SRHCW-3, and (d) SRHCW-4.

Table 1

List of specimen parameters

Specimen parameters of cold-formed steel and reinforced high-strength concrete short shear walls with concealed bracing
Wall body	Specimen no.	SRHCW-1	SRHCW-2	SRHCW-3	SRHCW-4
	Wall thickness (mm)	120	120	120	120
	Wall width (mm)	800	800	800	800
	Net wall height	650	650	650	650
	Shear span ratio	1	1	1	1
	Size of marginal component (mm)	120 × 160	120 × 160	120 × 160	120 × 160
	Height of marginal component (mm)	800	800	800	800
	Horizontal distribution reinforcements of wall	6.5@100	6.5@100	6.5@100	6.5@100
	Reinforcement ratio of horizontally distributed reinforcements (%)	0.66	0.66	0.66	0.66
	Vertical distribution reinforcements	6.5@120	6.5@120	6.5@120	6.5@120
	Reinforcement ratio of vertically distributed reinforcements (%)	0.55	0.55	0.55	0.55
	Main bar of marginal component	6 × 8	6 × 8	6 × 8	6 × 8
	Section steel of marginal component	60 × 30 × 2.2	60 × 30 × 2.2	60 × 30 × 2.2	60 × 30 × 2.2
	Reinforcement ratio of main reinforcements in marginal component (%)	1.57	1.57	1.57	1.57
	Distribution ratio of section steel in marginal component (%)	2.06	2.06	2.06	2.06
	Stirrups of marginal components	6.5@100	6.5@100	6.5@100	6.5@100
	Volumetric reinforcement ratio in marginal component (%)	1.20	1.20	1.20	1.20
	Section of diagonal bracing	—	Equilateral angle steel ∟30 × 1.9	Lattice column (2 pcs, 60 × 1.0)	Lattice column (2 pcs, 80 × 1.4)
	Stay plate of diagonal bracing	—	—	30 × 1.0@100	50 × 1.0@100
	Steel ratio of diagonal bracing (%)	—	0.119	0.125	0.233
Loading beam	Length (mm)	900	900	900	900
	Width (mm)	300	300	300	300
	Height (mm)	300	300	300	300
	Main bar	4 × 20	4 × 20	4 × 20	4 × 20
	Stirrup	8@100	8@100	8@100	8@100
Bearing beam	Length (mm)	1,500	1,500	1,500	1,500
	Width (mm)	350	350	350	350
	Height (mm)	450	450	450	450
	Main bar	8 × 20	8 × 20	8 × 20	8 × 20
	Stirrup	8@100	8@100	8@100	8@100

Table 2

Mix proportion of the C60 concrete

Material composition	Sand	Pebble	Cement	Fly ash	Water	Water reducer
Mix proportion	0.90	1.85	1	0.08	0.26	0.01
kg·m−³	544	1,120	605	50	160	7.5

Table 3 gives the mechanical properties of the steel, whereas Table 4 gives the concrete strength and vertical loading of each specimen.

Table 3

Mechanical properties of the steel

Type of steel		Yield strength (MPa)	Ultimate strength (MPa)	Elastic modulus (MPa)	Yield strain (με)
Thickness of section steel (mm)	1.0	307.0	395.7	194461.5	1,579
	1.4	329.5	369.5	193902.9	1,699
	1.9	389.8	409.8	206043.2	1,892
	2.2	384.2	414.1	212491.7	1,808
Diameter of bar (mm)	6.5	388.7	536.7	210666.5	1,845
	8.0	376.5	520.0	218761.7	1,721

Table 4

Concrete strength and vertical loading of specimens

Specimen No.	Axial compressive strength (MPa)	Designed value of compressive strength (MPa)	Axial tensile strength (MPa)	Designed axial compression ratio	Vertical load (kN)
SRHCW-1	48.7	22.6	2.3	0.24	432
SRHCW-2	50.1	23.2	2.3	0.24	444
SRHCW-3	53.0	24.4	2.4	0.24	469
SRHCW-4	59.5	27.3	2.5	0.24	524

2.2 Test device and loading system

2.2.1 Test device

Following Specification of Testing Methods for Earthquake Resistant Building (JGJ101-2015) [18], a horizontal low-cyclic-loading test was conducted under the action of fixed axial pressure (including vertical loads and horizontal loads). A horizontally sliding roller was installed between the vertical load and the reaction beam to ensure that the vertical jack could move along with the wall in the horizontal loading process, and the vertical load was kept unchanged during the test. The horizontal reaction force was balanced by the reaction wall because of high horizontal load. The force was provided by a hydraulic jack. The loading apparatus is shown in Figure 3.

Figure 3

Loading apparatus. 1 – 150T vertical hydraulic jack; 2 – 150T vertical sensor; 3 – sliding bearing; 4 – horizontal connecting device; 5 – 150T horizontal sensor; 6 – 150T horizontal actuator; 7 – specimen; 8 – ground anchor bolt; 9 – displacement meter; 10 – angle-steel frame; 11 – reaction wall; 12 – reaction beam; 13 – distribution beam; 14 – bearing; 15 – 30T jack.

2.2.2 Test loading system

Low-cyclic-loading method was adopted. The testing process can be divided into three stages.

1. Pre-loading stage: A vertical load of 150 kN was first applied to the top of the wall and the load then fell to zero. This loading was repeated twice. The vertical load was then increased to the predetermined value for testing (refer to Table 4 for details) and kept constant during the test. The horizontal repeated pre-loading was then applied with a cycle of 20 kN → 0 → −20 kN to determine whether the test instruments and equipment were operating normally.

2. Force-controlled loading stage: After the confirmation of normal operation, cracking load was identified by controlling the force. A positive horizontal load was first applied with the cycle of 20 kN → 40 kN → 60 kN and then increased in intervals of 10 kN until the specimen cracked. The positive cracking load P _cr was thus obtained, and the load was then released to zero. Similarly, the load was reversed until the specimen cracked to obtain the reverse cracking load P _cr, and the load was then released to zero.

3. Displacement-controlled loading stage: After the specimen cracked, a loading was applied with displacement control. The loading scheme is shown in Figure 4. Each stage of the displacement loading process is divided into two cycles. In the first cycle, loading and unloading were performed for three equal displacements; in the second cycle, loading and unloading were conducted once. The test continued loading until the specimen completely lost its vertical-load-bearing capacity or horizontal-load-bearing capacity. The layout of the strain gauges of the reinforcement and section steel in the wall is shown in Figure 5.

Figure 4

Loading scheme with displacement control.

Figure 5

Layout of strain gauges. (a) Layout of strain gauges on vertical reinforcements, (b) layout of strain gauges on horizontal reinforcements, (c) layout of strain gauges on the marginal component, and (d) layout of strain gauges on the SRHCW-2 diagonal bracing.

3 Analysis of test results

3.1 Type and process of specimen failure

Low-cyclic-loading tests were respectively performed on four cold-formed steel and high-strength concrete short shear walls with the axial compression ratio remaining at 0.24. The type of specimen failure was determined according to the change in bearing capacity, the development of cracks, and the form of final failure, as shown in Figure 6.

Figure 6

Final failure diagrams of specimens. (a) Final failure diagram of SRHCW-1, (b) final failure diagram of SRHCW-2, (c) final failure diagram of SRHCW-3, and (d) final failure diagram of SRHCW-4.

The SRHCW-1 specimen did not have concealed bracing. The bearing capacity of the specimen began to fall from the horizontal bearing capacity to the final failure only after two cycles. The horizontal bearing capacity dropped sharply, and the failure indicated obvious brittleness. There were a number of wide oblique cracks in the specimen. At the time of failure, the concrete on the wall web was divided and broken by the oblique cracks and then bulged out, resulting in the loss of the vertical and horizontal bearing capacities. The final failure form of the specimen indicated that the specimen concrete was divided and broken by the diagonal cracks, which developed rapidly, and the central concrete largely bulged out and broke away, rapidly losing its horizontal bearing capacity. The failure type was thus oblique shear failure. When the horizontal displacement was 15 mm, the failure type of SRHCW-1 was shear failure, accompanied by obvious brittleness.

The concealed bracing of the SRHCW-2 specimen was in the form of equilateral angle steel. The bearing capacity of the specimen began to fall from the horizontal bearing capacity to the final failure after multiple cycles. The horizontal bearing capacity declined slowly, and the failure indicated relatively good ductility. Oblique cracks increasingly developed in the specimen, and horizontal cracks became wider in the later stage. At the time of failure, the cracks on the tensile side were very wide, whereas the oblique cracks had not widened appreciably. The final failure form of the specimen indicated that the concrete at the corners of the compression zone of the specimen was crushed by diagonal and vertical cracks, the compressed section steel and diagonal braces had severely yielded and bulged, the tensile horizontal cracks had severely widened and were stressed, and the longitudinal reinforcements had been pulled to breaking point. The horizontal displacement of the specimen was 22 mm when the specimen failed, and the failure type of SRHCW-2 was bending shear failure with good ductility.

The diagonal bracing of the SRHCW-3 specimen was a lattice column, the cross-sectional area of which was the same as that of the SRHCW-2. When the specimen failed, the horizontal displacement was 22 mm, and the failure process was similar to that of SRHCW-2. The failure type of SRHCW-3 was bending shear failure with good ductility.

The diagonal bracing of the SRHCW-4 specimen was a lattice column, with the cross-sectional and stay plate areas being larger than those of SRHCW-3. When the specimen failed, the horizontal displacement was 20 mm, and the failure process was similar to that of SRHCW-2. The failure type of SRHCW-4 was bending shear failure with good ductility.

The analysis of the failure modes and processes of the four specimens reveals that the use of the concealed bracing made of cold-formed steel section could change the failure mode of the high-strength concrete short shear wall and improve the ductility. The failure mode, failure process, and maximum displacement of the specimens for lattice diagonal bracing and angle-steel diagonal bracing were basically the same. When the failure mode of high-strength concrete short shear wall was changed to bending shear failure by setting up concealed bracing, and increasing the section area of diagonal bracing steel had little effect on the failure mode and maximum displacement.

3.2 Hysteretic curve

A hysteretic curve describes the load–deformation curve of specimens under repeated loads, which comprehensively reflects changes in the bearing capacity, displacement, stiffness degradation, deformation characteristics, and energy dissipation of the specimens in the process of repeated loading and is often used to determine the comprehensive performance of the specimen under the action of repeated low cyclic loading. Four findings were taken from the hysteretic curves, as presented in Figure 7.

Figure 7

Hysteretic curves of specimens. (a) SRHCW-1 P–Δ hysteretic curve, (b) SRHCW-2 P–Δ hysteretic curve, (c) SRHCW-3 P–Δ hysteretic curve, and (d) SRHCW-4 P–Δ hysteretic curve.

In the initial stage of loading, the specimen was in a linear elastic stage and the elastic hysteretic curves of the four specimens basically had the same shape with a sharp point.

The horizontal bearing capacity increased as a function of the horizontal displacement load. The hysteretic curves of the four specimens started to tilt toward the displacement axis. In the absence of horizontal load, the displacement was not fully recovered, and the stiffness decreased in the second cycle, indicating that the plastic deformation of the specimen began, which was attributed to developing cracks and reduced stiffness.

As the horizontal displacement increased, the hysteretic curves of the four specimens continuously tilted toward the displacement axis, the stiffness continuously decreases, the bearing capacity began to decrease, followed by an obvious pinch effect, and the area enclosed by the hysteretic curve of each cycle increased.

When the horizontal displacement of SRHCW-1 reached 15 mm, the horizontal bearing capacity of the specimen drops sharply and is lost finally. The component failed shortly after the decrease in the horizontal bearing capacity. The failure exhibited obvious brittleness. The hysteretic curves of RHCW-2, SRHCW-3, and SRHCW-4 showed similar trends, enveloping areas and pinching effects. Compared with SRHWC-1, the number of cycles of the hysteretic curves of the other three specimens relatively increased, with fuller hysteretic curve.

The analysis of the hysteretic curves of the four specimens reveals that the concealed bracing made from cold-formed steel section in the high-strength concrete short shear wall increased the effective number of hysteretic cycles, which restricted the development of diagonal cracks, reduced pinching effect, and made hysteretic curve fuller. The lattice diagonal bracing and angle-steel diagonal bracing had almost the same effect on the shape and change trend of hysteretic curves of high-strength concrete short shear walls.

3.3 Skeleton curve

A skeleton curve is the force–displacement curve of a specimen under monotonic loading. The curve provides an intuitive understanding of the sizes and change trends of the bearing capacity, deformation capacity, stiffness, and ductility of the specimen in the loading process. Skeleton curves of the four specimens are shown in Figure 8. The following conclusions are obtained from the comparison of the skeleton curves.

Figure 8

Skeleton curves of specimens and their comparison. (a) SRHCW-1 skeleton curve, (b) SRHCW-2 skeleton curve, (c) SRHCW-3 skeleton curve, (d) SRHCW-4 skeleton curve, and (e) comparison of skeleton curves of specimens.

The skeleton curve of a specimen increased linearly in the pre-cracking stage (when the displacement was 0–0.5 mm), indicating that the specimen was in the elastic stage. The initial slopes for the four specimens were similar, indicating their similar initial elastic stiffness.

The change trend of the bearing capacity shows that the bearing capacity of SRHCW-1 suddenly decreased and the horizontal bearing capacity was lost when the displacement reached 15 mm. The change trends of the horizontal bearing capacity of the other three specimens were similar, with the horizontal bearing capacity gradually decreasing with an increase in displacement. SRHCW-1 exhibited obvious brittleness in the case of sharp failure. The horizontal bearing capacity for SRHCW-2, SRHCW-3, and SRHCW-4 gently dropped, which implies that the high-strength concrete short shear wall without a diagonal bracing failed suddenly, and the horizontal bearing capacity could be controlled by a cold-formed steel diagonal bracing arranged in the specimen so that it decreased incrementally with the increase of displacement.

In the later stage, the horizontal bearing capacity of SRHCW-1 was lost instantly, while that of the other three specimens decreased slowly. When the displacement reached about 28 mm, the vertical and horizontal bearing capacity of the specimen remained at about 50% of the maximum bearing capacity, respectively, and the specimen completely collapsed.

The analysis of the skeleton curves of the four specimens indicates that cold-formed steel concealed bracing had a minor effect on the initial elastic stiffness of high-strength concrete short shear walls; the use of cold-formed steel concealed bracing led to a gradual decline in the horizontal bearing capacity of high-strength concrete short shear wall and a significant increase in the ultimate displacement; and the use of cold-formed steel concealed bracing in a high-strength concrete short shear wall guaranteed the integrity, bearing capacity, and deformation capacity of the wall under large deformation.

3.5 Analysis of energy dissipation

The energy dissipation capacity is a measure of the ability of a component or structure to consume energy during an earthquake through plastic deformation and thus is an important index reflecting the seismic performance of a component or structure. In practical engineering, the energy dissipation coefficient E and the equivalent viscous damping coefficient h _e are usually adopted to represent the energy dissipation capacity of a structure or a component. E and h _e can be calculated from the area surrounded by the load–displacement hysteretic curve in each cycle during loading.

The size of component, the strength of concrete, the strength and area of reinforcements and section steel, the development and extension of cracks, and the bonding slip between reinforcement and concrete are the important factors affecting energy dissipation capacity. The relative energy dissipation of specimens (i.e., the energy dissipation divided by the compressive strength of concrete) was compared to eliminate the effect of concrete strength on the energy dissipation capacity of specimens.

The energy dissipation, energy-consumption coefficient, and their comparison for the four specimens are presented in Table 6. Figure 9 presents a transverse comparison of the relative energy dissipation of the four specimens.

Table 6

Energy dissipation capacity of the specimens

Specimen no.	Displacement (mm)	Relative energy dissipation	E	h e (%)	Relative cumulative energy dissipation	Relative value of energy dissipation
SRHCW-1	3	56	0.96	15.33	740	1.00
	5	88	1.07	17.03
	8	153	1.20	19.10
	10	181	1.12	17.83
	13	263	1.36	21.65
SRHCW-2	3	66	1.07	17.05	1,360	1.84
	5	90	1.11	17.71
	8	154	1.13	18.05
	10	191	1.22	19.39
	13	250	1.33	21.18
	15	278	1.32	21.01
	18	330	1.42	22.58
SRHCW-3	3	39	0.74	11.70	1,376	1.86
	5	90	1.05	16.63
	8	151	1.16	18.52
	10	174	1.16	18.51
	13	265	1.35	21.41
	15	303	1.41	22.51
	18	353	1.44	22.88
SRHCW-4	3	43	0.83	13.20	1,267	1.71
	5	88	1.07	16.98
	8	155	1.23	19.60
	10	191	1.27	20.14
	13	239	1.39	22.15
	15	273	1.45	23.06
	18	279	1.36	21.60

Figure 9

Comparison of the relative cumulative energy dissipation of specimens.

The larger the loading displacement, the larger the area around the hysteresis curve, and the larger the energy dissipation of the specimen. However, the energy dissipation tended to stabilize owing to the decrease in the bearing capacity of the specimen with the increase in the displacement.

The energy dissipation for the four specimens was basically the same under the same displacement.

The total energy dissipation capacity was the weakest for SRHCW-1 but stronger for SRHCW-2, 3, and 4 because the effective displacement was 13 mm and the overall deformation capacity was low when SRHCW-1 failed, whereas the cumulative displacement was 18 mm and the cumulative energy dissipation for the other three specimens became stronger. The aforementioned results indicate that the deformation capacity could be improved using cold-formed steel concealed bracing in a high-strength concrete short shear wall so as to greatly improve the accumulative energy dissipation.

The analysis of the energy dissipation capacity of the four specimens indicates that the use of cold-formed steel concealed bracing greatly improved the total energy dissipation capacity of high-strength concrete short shear wall compared with ordinary high-strength concrete short shear wall, and the increase in the total energy dissipation capacity was mainly attributed to the higher total number of hysteretic cycles. Additionally, compared with the use of an angle-steel diagonal bracing, the use of lattice diagonal bracing did not significantly improve the energy dissipation capacity of high-strength concrete short shear wall because the lattice diagonal bracing steel had no obvious restraint effect on high-strength concrete.

3.6 Analysis of the strain skeleton curves of the reinforcement and diagonal bracing

By connecting the strain values of bar and section steel under each horizontal maximum displacement, the strain skeleton curve was obtained, and its influence on the strain variation of specimen was analyzed. Strain skeleton curves for hg-10 and xc-3 are shown in Figures 10 and 11, respectively.

Figure 10

Comparison of strain skeleton curves of horizontally distributed reinforcements hg-10.

Figure 11

Comparison of strain skeleton curves of diagonal bracing section steel xc-3.

The strain of horizontal reinforcements for SRHCW-1 was higher than those for the other three specimens under the same displacement, which indicates that cold-formed steel diagonal bracing in a high-strength concrete short shear wall greatly reduced the stress of horizontal reinforcements, delayed the yield of horizontal reinforcements, and limited the development of shear cracks in the shear wall.

The main mechanism of the diagonal bracing was to share the tensile stress of horizontally distributed reinforcements in the middle position of the wall. The diagonal section steel may not yield in the middle position of the wall when the cross-sectional area of the diagonal steel reached a certain value, and its benefit to the horizontal reinforcements tended to be slight.

The advantage of the diagonal bracing section steel to the horizontal reinforcements in the wall was mainly due to share tensile stress, rather than the superposition effect, so that the maximum horizontal bearing capacity of the high-strength concrete short shear wall was not obviously improved.

3.7 Analysis of the variation law of diagonal bracing hysteresis curves

According to the measured data, select the strain values at the representative positions (xc-1, xc-3) of the diagonal braces of the SRHCW-2, 3, and 4 specimens, and draw the strain hysteresis curves of the horizontal reinforcement of the three specimens, as shown in Figure 12.

Figure 12

Diagonal braces xc-1 and xc-3 hysteresis curves. (a) SRHCW-2 xc-1 hysteretic curve, (b) SRHCW-3 xc-1 hysteretic curve, (c) SRHCW-4 xc-1 hysteretic curve, (d) SRHCW-2 xc-3 hysteretic curve, (e) SRHCW-3 xc-3 hysteretic curve, (f) SRHCW-4 xc-3 hysteretic curve.

It can be seen in Figure 12 that the diagonal braces of the three specimens are mainly compressive strain near the foot of the wall and yields under compression when the displacement reaches about 15 mm, and tensile strain rarely occurs, indicating that the diagonal braces at this position are mainly compressed under low cycle repeated action. The tension strain of diagonal braces near the wall belly is basically dominant, and the diagonal braces are not yielded in tension, and the compression strain is very small. Since the diagonal braces are tensioned and do not yield, the development of shear cracks in the specimen is greatly constrained.

4 Comparison of calculated and measured bearing capacities

The bearing capacity is an important parameter of structural components. However, there is no clearly established formula for the bearing capacity of cold-formed steel section and high-strength concrete short shear walls with concealed bracing at present. It is therefore important to establish a formula for calculating bearing capacity and confirm its accuracy. The formula used for calculating the shear capacity for shear sections of common shear walls should be in line with current specifications.

According to Code for Design of Concrete Structures (GB 50010-2019) [19], the formula for calculating the shear capacity of a reinforced concrete shear wall under eccentric compression is expressed as:

where V w is the designed sectional shear capacity; λ is the shear–span ratio of the section, which is calculated by M / ( V h 0 ) . Its value is 1.5 when λ is less than 1.5, and its value is 2.2 when λ is greater than 2.2; and M is the bending moment corresponding to the shear force V w . When the distance between the section and the bottom of the wall is less than h 0 / 2 , the value of λ should be the bending moment and shear force from the h 0 / 2 bottom of the wall. f t is the designed concrete tensile strength. b and h 0 are, respectively, the width and height of the section. N is the designed axial pressure in the shear wall; N = 0.2 f c b h 0 was adopted when N > 0.2 f c b h 0 ; A is the cross-sectional area of the shear wall; A w is the cross-sectional area of the T-shaped or I-shaped wall web, and A = A w is adopted for the cross-sectional area of the rectangular shear wall; f y v is the designed tensile strength of horizontally distributed reinforcements in the shear wall; A s h is the cross-sectional area of horizontally distributed reinforcements in the shear wall; and s v is the spacing of horizontally distributed reinforcements in the shear wall.

According to Technical Specification for Steel Reinforced Concrete Composite Structures (JGJ138-2016) [20], the formula for calculating the shear capacity of a reinforced concrete shear wall with section steel at both ends under eccentric compression is expressed as:

where f c is the designed compressive strength of concrete; f a is the designed tensile strength of the medium-steel concealed column in the shear wall; A a is the cross-sectional area of the medium-steel concealed column in the shear wall; and the physical meanings of the other symbols are the same as those in formula (2). The calculated and measured values are compared in Table 7. Two findings are taken from the comparison; f y z is the designed tensile strength of cold-formed section steel for concealed support of shear wall; A s z is the sectional area of cold-formed steel for concealed support of shear wall; and α s is the angle between the concealed support profile steel and the horizontal direction of the shear wall.

The shear capacity of the shear wall calculated using formula (2) was lower than the measured value as the effect of section steel in the marginal component and diagonal bracing was not considered in formula (2). The wall safety could be guaranteed if the formula was adopted to calculate the shear capacity for the diagonal section of the cold-formed steel and high-strength reinforced concrete shear wall with concealed bracing, but the calculation was conservative.

The shear capacity of the shear wall calculated using formula (3) was lower than the measured value. The wall safety could be guaranteed if formula (3) was adopted to calculate the shear capacity of the cold-formed steel section and high-strength reinforced concrete wall with the concealed bracing, with the measured value agreeing well with the calculated one.

5 Conclusions

Four specimens of cold-formed steel and high-strength reinforced concrete short shear walls with different concealed bracing were tested and analyzed. The following conclusions are drawn:

1. The use of cold-formed steel concealed bracing in the high-strength concrete short shear wall can effectively change the failure mode of the wall into bending shear failure with good ductility, which indicates that the seismic performance of high-strength concrete short shear wall can be enhanced by using cold-formed steel. When the failure mode of high-strength concrete short shear wall was changed to bending shear failure by setting up concealed bracing, increasing the section area of diagonal bracing steel had little effect on the failure mode and maximum displacement.

2. The concealed bracing made from cold-formed steel section in the high-strength concrete short shear wall increased the effective number of hysteretic cycles, which restricted the development of diagonal cracks, reduced pinching effect, and made hysteretic curve fuller.

3. The use of cold-formed steel concealed bracing greatly improved the total energy dissipation capacity of high-strength concrete short shear wall compared with ordinary high-strength concrete short shear wall, and the increase in the total energy dissipation capacity was mainly attributed to the higher total number of hysteretic cycles.

4. The main mechanism of the diagonal bracing was to share the tensile stress of horizontally distributed reinforcements in the middle position of the wall, rather than the superposition effect, so that the maximum horizontal bearing capacity of the high-strength concrete short shear wall was not obviously improved.

5. The shear capacity of the cold-formed steel shear wall with concealed bracing calculated using the formula given in Technical Specification for Steel Reinforced Concrete Composite Structures (JGJ138-2016) is more consistent with the measured one.