M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS 447–455 CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS CIKLI^NI ODGOVOR KONSTRUKCIJ IZ STRI@NIH STEN, SPETIH S SKRITIMI SPONAMI, V ZGRADBAH Z MAJHNIM ŠTEVILOM NADSTROPIJ Min Gan 1,2 , Ming Kang 1*,2 , Chuan Long 1,2 , Liren Li 1,2 1 School of Civil Engineering, Chongqing University, No. 174 Shazheng street, Shapingba District, Chongqing 400045, China 2 Key Laboratory of New Technology for Construction of Cities in Mountain Area (Chongqing University), Ministry of Education, Chongqing 400030, China Prejem rokopisa – received: 2019-06-22; sprejem za objavo – accepted for publication: 2020-02-12 doi:10.17222/mit.2019.135 To study the cyclic response of high-strength-concrete, low-rise shear walls of concealed, cold-formed, thin-walled, sectional steel trusses, two scaled specimens with concealed-angle steel bracings with axial pressure ratios of 0.52 and 0.60 were designed to analyze the anti-seismic performance of low-rise shear walls under different axial pressure ratios. A scaled specimen with an axial pressure ratio of 0.52, but with a larger diagonal bracing area was designed to study the effects of increased area of diagonal bracing under a high axial pressure ratio on the anti-seismic performance. Low-cyclic repeated loading tests were conducted to determine the cracking loads, ultimate loads, and skeleton curves for each construct. The axial pressure ratio had significant effects on the cracking loads. Comparative analyses of the properties of the tested constructs, including the energy-dissipation capacity and the strength attenuation revealed 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 finding provides a basis for the optimized design of shear walls. Keywords: low-rise shear walls, cold-formed sectional steel, anti-seismic performance, concealed bracing Avtorji so v {tudiji raziskovali cikli~ni odgovor konstrukcije iz stri`nih sten, izdelanih iz visoko trdnega betona, vgrajenih v zgradbe z majhnim {tevilom nadstropij. Stri`ne stene so vertikalni konstrukcijski elementi v gradbeni{tvu, ki se upirajo bo~nim silam v ravninah sten s stri`enjem in upogibanjem. Avtorji so stene povezali z mre`o spon iz hladno deformiranega gradbenega jekla. V ta namen so za analizo oblikovali dva vzorca stri`nih sten, spetih s skritimi kotnimi jeklenimi sponami z aksialnim tlakom 0,52 in 0,60 ter analizirali njihove protipotresne lastnosti pri razli~nih razmerjih aksialnega tlaka. Prvi vzorec z razmerjem aksialnega tlaka 0,52 toda z ve~jim diagonalnim presekom spone, je bil oblikovan z namenom {tudiranja vpliva pove~anja preseka pri visokem aksialnem tla~nem razmerju na protipotresne lastnosti konstrukcije. Izvajali so malocikli~ni preizkus obremenjevanja konstrukcije, da bi dolo~ili pogoje (obremenitve) pri katerih pride do nastanka razpok. Dolo~ili so tudi kon~ne obremenitve pri katerih je pri{lo do poru{itev in skeletne krivulje vsake zgradbe posebej. Izbrano razmerje aksialnega tlaka je imelo pomemben vpliv na obremenitve pri katerih je pri{lo do pojava razpok. Primerjalna analiza lastnosti testiranih zgradb, vklju~no s kapaciteto energije disipacije in zmanj{evanja trdnosti je odkrila, da pove~evanje diagonalnega spenjanja pri visokem razmerju aksialnega tlaka lahko izbolj{a protipotresne lastnosti stri`nih sten v zgradbah z majhnim {tevilom nadstropij. Te ugotovitve so osnova za optimizacijo oblikovanja stri`nih sten. Klju~ne besede: stri`ne stene, zgradbe z malim {tevilom nadstropij, sestavljeni profili iz hladno deformiranega jekla, proti- potresne lastnosti, skrite spone 1 INTRODUCTION With the continued development of modern architec- tural technology and economy, innovative buildings are being designed and constructed. With increased height, there are increased requirements of high-rise building structures for a horizontal carrying capacity and a verti- cal bearing capacity. The intrinsic poor ductility of high-strength concrete limits its application in building structures with higher anti-seismic requirements. How- ever, the excellent tensile ductility of profile steel offsets the shortcomings of high-strength concrete. Concrete covering strengthens the rigidity of the profile steel but has a poor fire-resistance offset. As the wall thickness of the hot-rolled section steel is generally thicker than that of the cold-formed section steel, and its deformation ca- pacity is better than that of the cold-formed section steel, under the same conditions, the hysteretic curve of the hot-rolled section steel shear wall is fuller than that of the cold-formed section steel shear wall, whose energy dissipation and deformation performance are also better. The use of built-in steel trusses can considerably im- prove the anti-seismic performance of high-strength con- crete, but hot-rolled profile steel is expensive. The use of cold-formed thin-walled sectional steel instead of hot-rolled profile steel can substantially reduce the amount of steel required, thus decreasing the costs while ensuring sufficient anti-seismic performance. Given this benefit, studies of the anti-seismic performance of built-in, cold-formed, thin-walled, sectional steel, high-strength concrete can have economic significance. Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 447 UDK 69.01:69.032.2:699.841:669.14.018.291 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(4)447(2020) *Corresponding author's e-mail: kmingcqu@cqu.edu.cn (Ming Kang) Many studies have examined concrete shear wall composite structures. Buddika et al. studied the seismic response of a concrete shear wall prepared by the post- tensioning method from mixed prefabricated compo- nents. 1 Tong et al. designed an I-shaped steel-reinforce- ment bar concrete wallboard composite shear wall on a 1/3 scale and conducted a low-cyclic repeated test with- out the application of a vertical load. The result indicated that in the initial stage of loading, the stud connecting the concrete wallboard and the steel plate starts to yield, suggesting significant effects of this stud on the speci- men’s bearing capacity. 2 J. H. Mun et al. 3 discussed the shear resistance capability of gravity dams with con- struction joints. F. M. Ren et al. 4 examined the stress- strain model and anti-seismic performance of steel-fiber composite boundary shear walls and found that that these shear walls exhibit excellent toughness. W. Kassem et al. 5 proposed an analytical method to predict the shear strength and behavior of structural shear walls under the action of a single or cyclic load. A. Tuken et al. 6 dis- cussed an anti-seismic assessment for reinforcement-bar concrete shear walls. H. J. Lee et al. 7 studied the anti-seismic strength of shear walls in a flexible seismic isolation structure. A. K. Bhowmick 8 studied the anti- seismic performance of center-hole circular steel-plate shear walls. P. D. Moncarz et al. 9 introduced the theory and application of experimental model analyses in earth- quake engineering. To investigate anti-seismic measures for low-rise shear walls under a high axial pressure ratio, a low-rise shear wall component was designed with diagonal brac- ing of a high steel ratio. The effects of the ratio of the steel in the diagonal bracing on the bearing capacity of low-rise shear walls under a high axial pressure ratio were analyzed, with a comparison of the properties of failure mode, the hysteretic curve, the skeleton curve, en- ergy-dissipation capacity, and the strength attenuation. 2 TEST OVERVIEW 2.1 Test design 2.1.1. Specimen design According to the technical specification for the seismic tests of buildings (JGJ101-2015), 10 three high- M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS 448 Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 Table 1: Detailed parameters of specimens wall Specimen number SRHCW-2 SRHCW-3 SRHCW-4 wall thickness (mm) 120 120 120 wall width (mm) 800 800 800 clear height of wall 650 650 650 shear span ratio 1 1 1 edge component dimension (mm) 120×160 120×160 120×160 edge component height (mm) 800 800 800 horizontally distributed reinforcement bars in wall 6.5@100 6.5@100 6.5@100 ratio of reinforcement of the horizontally distributed reinforcement bars 0.66% 0.66% 0.66% vertically distributed reinforcement bars 6.5@120 6.5@120 6.5@120 ratio of reinforcement bars in the vertically distributed reinforcement bars 0.55% 0.55% 0.55% main reinforcement bar in the edge component 686 86 8 profile steel in edge components 60×30×2.2 60×30×2.2 60×30×2.2 ratio of reinforcement of main reinforcement bars in edge components 1.57% 1.57% 1.57% ratio of profile steel in edge components 2.06% 2.06% 2.06% stirrups of edge components 6.5@100 6.5@100 6.5@100 volume-stirrup ratio of edge components 1.20% 1.20% 1.20% cross section of diagonal bracing equilateral angle steel I 30×1.9 equilateral angle steel I 30×1.9 angle steel I 50×30×3 steel ratio of diagonal bracing 0.12% 0.12% 0.25% loading beam length (mm) 900 900 900 width (mm) 300 300 300 height (mm) 300 300 300 main reinforcement bars 4C20 4C20 4C20 stirrups 8@100 8@100 8@100 support- ing beam length (mm) 1500 1500 1500 width (mm) 350 350 350 height (mm) 450 450 450 main reinforcement bars 8C20 8C20 8C20 stirrups 8@100 8@100 8@100 Note: The reinforcement of SRHCW-1 is the same as that of SRHCW-2 and SRHCW-3 strength-concrete, low-rise shear walls (SRHCW) were designed with internal cold-formed sectional steel trusses of the same external dimension with a 120×800 mm wall cross-section with a 1/4 scale, as shown in Figure 1. The three components, SRHCW1-3, used concealed bent angle steel, provided with concealed bracings for angle steel with a cross-section of (30×30×1.9) mm (SRHCW1 was designed and investigated by refer- ence 11 ). SRHCW-4 contained angle steel with a cross- section of (50×30×3) mm. The detailed parameters of the specimens are shown in Table 1 and Figure 2. 2.1.2 Concrete preparation The tests used high-strength concrete with a strength grade of C60 and the mix proportion indicated in Table 2. Table 2: Mix proportion of C60 concrete Material components Stones Sand R42.5 cement Fly ash Water Polycar- boxylic acid water reducer mix proportion 1.85 0.90 1.00 0.08 0.26 0.01 kg/m³ 1119 546 602 50 165 7.5 2.1.3 Mechanical properties of the steel The reinforcement bars used in the test included 6.5 horizontally distributed reinforcement bars, vertically distributed reinforcement bars, 8 vertically and longi- tudinally load-bearing reinforcement bars in the edge components, diagonal bracing of the angle steel with 1.9 mm and 3 mm thicknesses (made of steel plates), and 2.2-mm-thick channel steel. All the reinforcement bars were hot-rolled, with the type being HRB 300. The diagonal bracing of the angle steel and the channel steel in edge components were cold-formed, thin-walled, sectional steel, the type of which is Q235. The mechani- cal properties of the steel are shown in Table 3. M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 449 Figure 1: Specimen dimensions Figure 2: Detailed drawing for reinforcement of specimens: a) detailed drawing showing the reinforcement of SRHCW-1, SRHCW-2, and SRHCW-3, b) detailed drawing showing the reinforcement of SRHCW-4 Table 3: Mechanical properties of steel and profile steel Type Elasticity modulus (Mpa) Yield strain (μ ) Yield strength (Mpa) Ultimate strength (Mpa) diameter of profile steel (mm) 1.9 206043.2 1892 389.8 409.8 2.2 212491.7 1808 384.2 414.1 3.0 204561.3 1786 365.5 425.5 diameter of reinforcement bar (mm) 6.5 221100.0 1620 358.3 512.3 8.0 237073.6 1529 362.5 520.4 2.1.4 Measured concrete strength and axial pressure on specimens The four specimens in the test were prepared from concrete with a strength grade of C60. In this experi- ment, the concrete is poured in four different batches, corresponding to SRHCW-1-4. When the cured concrete reaches the required strength, the experiment is carried out. The corresponding f cu,k is 50.1 Mpa, 59.3 Mpa, 60.1 Mpa and 59.2 Mpa. The standard value of the measured concrete strength f cu,k was consistent with the mean of the measured compressive strength of three 150mm standard cubes, as shown in Table 4.InTable 4, f c is the design value of the compressive strength of the concrete, f t is the design value of the tensile strength of the concrete, and N t is the loading axial force applied in the test. Table 4: Concrete strength and vertical load on specimens Specimen Number Design axial pressure ratio fcu,k (MPa) fc (MPa) Ft (MPa) Nt (KN) SRHCW-1 0.24 50.1 23.2 2.3 444 SRHCW-2 0.52 59.3 27.2 2.4 1140 SRHCW-3 0.60 60.1 27.5 2.4 1320 SRHCW-4 0.52 59.2 27.2 2.4 1140 Note: Design axial pressure ratio is 1.2 times the loading axial force divided by the product of the concrete section area and the design value of the concrete axial compressive strength 2.2 Test device and loading system 2.2.1 Test device According to the technical specification for seismic tests of buildings (JGJ101-2015) 9 , horizontal low-cyclic repeated quasi-static loading tests were conducted under the action of a stable and vertical axial force, with both vertical loading and horizontal loading. The vertical loading devices included a reaction beam, a vertical oil jack, and a jack pulley. The horizontal loading devices included a reaction wall, a horizontal actuator, and a horizontal connection device. The end of the horizontal actuator and the horizontal connection device were hinged. The vertical jack oil pump was manually con- trolled to ensure a stably applied vertical load. The horizontal actuator oil pump was manually controlled to achieve low-cyclic repeated horizontal loading. The loading devices are shown in Figure 3. 2.2.2 Loading system of test Low-cyclic repeated quasi-static loading was per- formed in the test. The loading process was as follows: (1) Preloading stage: A vertical load of 150 kN was first applied at the top of the wall and then unloaded to 0 KN. This was repeated twice. Next, the load was increased to the pre-set value of the component under the appropriate axial pressure ratio (Table 4). The horizontal repeated load was pre-loaded with the same pre-set value for a micro cycle of +20 kN 0 –20 kN, to verify the normal operation of the experimental apparatuses. (2) Loading stage of cracking load: After the micro- cycle was complete, the apparatus operated normally. The load was increased in the sequence 0 kN 100 kN 150 kN 200 kN 250 kN, and then with 20 kN as the gradient load until cracks occurred in the specimens. The positive load at the point of cracking was recorded, and then the horizontal load was unloaded to 0. Simi- larly, negative loading was performed to obtain negative cracks and determine the negative cracking load. Finally, the load was removed to zero. (3) Displacement control loading stage: After the cracks occurred, the loading was controlled by dis- placement. Each level included two cycles. The first cycle was loaded and unloaded by trisection and the M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS 450 Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 Figure 4: Loading system of the displacement control Figure 3: Loading device 1-150T vertical oil jack, 2-150T vertical sensor, 3-sliding support, 4-horizontal connection device, 5-150T horizontal sensor, 6-150T hor- izontal actuator, 7-specimen, 8-anchor bolt, 9-displacement meter, 10-angle steel shelf, 11-reaction wall, 12-reaction beam, 13-distribu- tion beam, 14-support, 15-30T jack second cycle was loaded and unloaded at a single time. Failure of the specimen and a test finished were when the sample completely lost its vertical bearing capacity and its horizontal force decreased to 70 % or less of the maximum horizontal force. See Figure 4 for the loading for each level in the displacement control. 3 ANALYSIS AND COMPARISON OF THE TEST RESULTS 3.1 Analysis of modes and types of component failure In the test, the shear span ratio of each of the four specimens was 1.0. The designed axial pressure ratio of SWHCW-1 was 0.24, and the axial pressure ratio for SRHCW-2 and SRHCW-4 was 0.52. The axial pressure ratio of SRHCW-3 was 0.60. The specimen failure mode was determined by the appearance of cracks in the com- ponents, attenuation in the bearing capacity, and final failure mode. The final failure patterns of the specimens are shown in Figures 5 and 6. The failure process and characteristics are as follows: (1) The concealed bracing for SRHCW-1 was sin- gle-leg 30×1.9 angle steel. In the low-cyclic repeated loading test, the specimen experienced eight complete cycles from loading to failure and failed at a displace- ment of = 22.0mm. The hysteretic curve was flat and the horizontal carrying capacity decreased slowly. When the component failed, the ultimate horizontal displace- ment was large, many oblique cracks developed in the wall, and the main oblique cracks were well developed. In the later stage, horizontal cracks occurred at the bot- tom of the wall. The main oblique cracks showed no sig- nificant signs of widening, and the horizontal cracks at the bottom of the wall became longer and wider when failure occurred. The compressed part in the corner of the specimen was crushed due to crack development and repeated loading and unloading. The compressed profile steel and diagonal bracing yielded and protruded under compression and the longitudinal tensile reinforcement bars of the concealed column were broken. The above analysis indicated that the failure mode of SRHCW-1 was flexural shear failure. (2) The reinforcement, edge profile steel, and con- cealed bracing of SRHCW-2 were the same as that of SRHCW-1, but with an increased axial pressure ratio of 0.52, falling into the category of medium and high axial pressure ratios. The specimen was subjected to five complete cycles in the low-cyclic repeated loading test. During positive loading at a displacement = 13.0mm, the specimen failed and the horizontal carrying capacity declined slowly. However, the horizontal displacement increased sharply and the vertical bearing capacity was lost rapidly when the specimen failed. The specimen ex- hibited significant brittleness during failure, and many cracks developed in the wall. The main oblique cracks and the horizontal tensile cracks at the bottom of the wall did not widen significantly, and no slip cracks appeared at the bottom of the wall. The concrete in the corner of the component was crushed under repeated loading and the longitudinal load-bearing reinforcement bars and profile steel yielded under compression. A large amount of concrete spalled in the periphery of the compressed area in the corner, causing the component to rapidly lose its vertical bearing capacity and horizontal carrying capacity. Based on the above analysis, the failure mode of SRHCW-2 was small eccentric compression failure. (3) The reinforcement of SRHCW-3, profile steel, and concealed bracing were the same as that of SRHCW-1 and SRHCW-2. However, the axial pressure ratio was increased to 0.60, falling into the category of a high axial pressure ratio. In the low-cyclic repeated load- ing test, the specimen only experienced two complete cycles before failing at a displacement = 8.0 mm. When the failure occurred, the flexural shear oblique cracks and main oblique cracks were well developed. Neither the main oblique crack nor the horizontal tensile cracks at the bottom of the wall had widened signi- M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 451 Figure 6: Final failure pattern of SRHCW-4 Figure 5: Final failure pattern of SRHCW-2 ficantly, and the compressed area in the corner was crushed first. Subsequently, the concrete failure area extended towards the middle of the wall under the re- peated action of the horizontal load. Finally, the concrete in the middle of the wall was crushed, causing rapid loss of the vertical bearing capacity and the horizontal carrying capacity with a poor deformation capacity. The above analysis indicated that the SRHCW-3 failure mode was small eccentric failure. (4) The concealed bracing of SRHCW-4 was single- leg 50×30×3 angle steel. The area of the diagonal brac- ing of SRHCW-4 was twice that of SRHCW-2 and both specimens had the same axial pressure ratio. In the low-cyclic repeated loading test, the specimen experi- enced eight complete cycles and failed at a displacement = 22.0 mm. Its horizontal carrying capacity decreased slowly. The specimen exhibited excellent ductility and energy dissipation capacity at the time of failure and the cracks were well developed. The horizontal cracks be- tween the wall bottom and the support were wide, and the oblique cracks showed no significant widening. With the increase in loading, the concrete in the two corners of the specimen was crushed successively. With the in- crease in displacement, the range of the compressed area increased. Finally, the concrete skin in the middle of the wall spalled on a large scale, with a loss of vertical bear- ing capacity and horizontal carrying capacity, marking the failure of the specimen. The above analysis indicated that the failure mode of SRHCW-4 was eccentric failure. 3.2 Analysis of component skeleton curve Based on an analysis of the hysteretic curves of SRHCW-1-SRHCW4, it was concluded that the hyste- retic curves of the four specimens have the following characteristics: (1) In the initial stage, all specimens are in an elastic state and the unloading curve can return along the loading curve without any plastic deformation. In the stage from cracking to yielding of components, the area of the hysteretic curve enveloping is small. This is be- cause the plastic deformation is small with a low per- centage in the whole deformation; meanwhile, in this stage, the horizontal carrying capacity of each specimen increases with an increase in the displacement. After the specimen yields, with the increase in displacement, the horizontal carrying capacity of the specimen is declining gradually. The total deformation is dominated by plastic deformation. Thus, the hysteretic enveloping ring in- creases. The rigidness and horizontal carrying capacity of each specimen in the second cycle are lower than that in the first cycle. (2) As can be seen in the hysteretic curves for the SRHCW-1-SRHCW-3 components, when the axial pressure ratio increases from 0.24 to 0.6, SRHCW-1 has a hysteretic curve with the best plumpness; SRHCW-2 has a hysteretic curve with moderate plumpness; SRHCW-3 has a hysteretic curve with the poorest plumpness. The SRHCW-3 component fails although it shows no significant signs of plastic deformation or a rheostriction effect. The three components can be arranged in a descending order in terms of ultimate displacement and energy-dissipation capacity, SRHCW-1 > SRHCW-2 > SRHCW-3. (3) Based on a comparison between SRHCW-2 and SRHCW-4, it has been found that as the area of con- cealed bracing in SRHCW-4 is twice that in SRHCW-2, it has a far plumper hysteretic curve and better plastic deformation and energy dissipation capacity under a axial pressure ratio of 0.52 than that of SRHCW-2. The skeleton curve is obtained by smoothly con- necting all the maximum stresses beyond the last loading in the stress and strain curves in the same direction of M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS 452 Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 Figure 7: Skeleton curves of shear wall specimens: a) skeleton curves of SRHCW-1 and SRHCW3, b) skeleton curves of SRHCW-2 and 4 loading (pushing or pulling). The skeleton curve is a locus curve of the maximum horizontal load for each cycle loading and can directly reflect the loading and deformation characteristics of the specimens in different stages. The skeleton curves enable a clear comparison of different specimens for rigidness, strength, energy-dissi- pation capacity, and ductility. The skeleton curves of SRHCW-1 to SRHCW-4 were determined and are shown in Figure 7. An analysis of the above skeleton curves revealed the following characteristics: (1) The initial rigidness of all four components is in- dicated as a straight line before cracking, suggesting that each component is in an elastic state before cracking occurs. The rigidness of each component starts to decline as the specimen cracks. (2) As can be seen in Figure 7(a), the specimens can be arranged in a descending order by initial rigidness, SRHCW-3>SRHCW-2>SRHCW-1, suggesting increased initial rigidness with the increase in axial pressure. (3) As can be seen in Figure 7a, the specimens can be arranged in a descending order by the ultimate hori- zontal displacement, in the order SRHCW-1 > SRHCW-2 > SRHCW-3, suggesting that with the in- crease in axial pressure, there is a continuous decrease in the ultimate horizontal displacement of each component. In the later stage, SRHCW-3 exhibits the fastest decrease in rigidness after it reaches the horizontal carrying ca- pacity, SRHCW-2 shows a moderate decrease in rigid- ness, and SRHCW-1 shows the slowest decrease in rigid- ness. This suggests that the increase in axial pressure accelerates the decrease in component rigidity in the later stage and accelerates the component failure. The specimens can be arranged in order of decreasing ulti- mate horizontal carrying capacity, SRHCW-3 > SRHCW-2 > SRHCW-1, suggesting that increased axial pressure may be an effective strategy to increase the ulti- mate horizontal carrying capacity of the components. (4) As can be seen in Figure 7b, the initial rigidness of SRHCW-2 was slightly higher than that of SRHCW-4, suggesting that increasing the area of diagonal bracing has little effect on the initial rigidness. In terms of the horizontal ultimate bearing capacity and the ultimate dis- placement, SRHCW-4 was better than SRHCW-2, sug- gesting that an increased diagonal bracing area can slightly increase the ultimate horizontal bearing capacity and effectively control the decrease in component rigid- ness in the late stage to provide sufficient ductility of the components. In conclusion, increasing the axial pressure ratio can increase the ultimate horizontal bearing capacity and the initial rigidness of specimens. However, an increase in the axial pressure ratio decreases the horizontal ultimate displacement and accelerates the decline in rigidness in the late stage, thus decreasing the seismic resistance. Under a high axial pressure ratio, increasing the area of diagonal bracing can slightly increase the horizontal bearing capacity, substantially increase the horizontal ultimate displacement, and control the decline in com- ponent rigidness in the late stage. 3.3 Analysis of energy-dissipation capacity of the com- ponents The energy-dissipation capacity of a structure deter- mines its anti-seismic performance level in earthquakes. The structural energy dissipation is mainly reflected by the plastic deformation of components in seismic res- ponses, as indicated by the hysteretic curve. In this study, the equivalent viscous damping coefficient h e and the energy-dissipation coefficient E were used to quantify the energy dissipation capacity of specimens. According to the technical specification for seismic tests of build- ings (JGJ101-2015) 9 , E and h e can be computed in each level of the cycle depending on the area of the hysteresis loop corresponding to the hysteretic curve. Figure 8 presents the hysteretic curve for a certain cycle obtained by low-cyclic repeated loading. E and h e are computed according to the following formula, which refers to the indicated parameters in Figure 8: E S SS = + (ABCD) (0EB) (0FB) h E e = 2π (1) Table 5 lists the energy-dissipation coefficients for SRHCW-1 to SRHCW4. As can be seen in Table 5: (1) With increased displacement, the energy gene- rated in each cycle increased gradually, but the increase of the energy dissipation slows when the displacement reaches the peak. (2) The specimens can be arranged in descending or- der of total energy dissipation, SRHCW-1 > SRHCW-2 > SRHCW-3. The results suggest that with the increase in axial pressure ratio, the energy-dissipation capacity of the components decreases. The energy-dissipation ca- pacity of the components under a high axial pressure ratio is only 18% of that of components under a low axial pressure ratio. M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 453 Figure 8: Diagram illustrating the computation of the equivalent vis- cous damping coefficient (3) For the energy-dissipation capacity, the specimens can be arranged in descending order as SRHCW-4 > SRHCW-1 > SRHCW-2. The data suggest that increas- ing the area of diagonal bracing can substantially in- crease the energy-dissipation capacity of components under the same axial pressure ratio. The horizontal carrying capacity of SRHCW-4 was higher than that of SRHCW-1, consistent with the slightly better energy- dissipation capacity of SRHCW-4 compared to that of SRHCW-1 in the same cycle. 3.4 Analysis of the component-strength attenuation Strength attenuation, as defined here, means that the internal damage of the component that occurs in the first cycle of the low-cyclic repeated loading test causes a decrease in the horizontal carrying capacity in the second cycle. The ratio of the horizontal carrying capacity in the first cycle to the horizontal carrying capacity in the second cycle was used to reflect the strength attenuation. The amplitude of the strength attenuation reflects the capacity of the component to resist deformation. Based on the data in Table 6, it can be concluded that: (1) Component strength attenuation generally de- creases with the increase in cycle and the strength attenuation coefficient is minimum in the last cycle (approximating failure). (2) In the cycle with a displacement of 3 mm, the specimens can be arranged in order of increased strength-attenuation coefficient, SRHCW-1 < SRHCW-2 < SRHCW-3. For displacements larger than 3 mm, the specimens can be arranged in order of decreased strength-attenuation coefficient, SRHCW-1 > SRHCW-2 > SRHCW-3, suggesting that a higher axial pressure ratio of the component leads to a smaller internal loss and a larger strength-attenuation coefficient under a small displacement. However, with the increase in the displacement, the strength-attenuation coefficient de- creases, suggesting increased component-failure velo- city. (3) At the same displacement, SRHCW-4 has a higher strength-attenuation coefficient than SRHCW-2, indicating that increasing the area of the diagonal bracing can reduce the internal loss of the component and improve the capacity to resist the horizontal repeated load, thus increasing the strength-attenuation coefficient. 4 COMPARISON OF MEASURED VALUES AND COMPUTED VALUES OF A COMPONENT’S BEARING CAPACITY The formula in Section 8.1.4 of technical specifi- cation for a steel-reinforced concrete composite structure M. GAN et al.: CYCLIC RESPONSE OF SCALED LOW-RISE SHEAR WALLS WITH CONCEALED BRACINGS 454 Materiali in tehnologije / Materials and technology 54 (2020) 4, 447–455 Table 5: Calculated energy-dissipation capacity values of specimens Specimen Number Cycle (mm) Energy dissipation (kN·mm) E H e (%) Cumulative energy dissipation Relative value SRHCW-1 3 1531.2 1.07 17.1 31552.0 1 5 2088.0 1.11 17.7 8 3572.8 1.13 18.1 10 4431.2 1.22 19.4 13 5800.0 1.33 21.2 15 6449.6 1.32 21.2 18 7656.0 1.42 22.6 SRHCW-2 3 1570.0 0.72 11.4 15236.0 0.48 5 2829.0 0.78 12.4 8 5219.0 1.10 17.5 10 5690.0 0.89 14.1 SRHCW-3 3 1627.0 0.71 11.2 5673.0 0.18 5 4136.0 0.88 14.0 SRHCW-4 3 1590.0 0.77 12.3 38443.0 1.22 5 2814.0 0.82 13.1 8 4664.0 0.80 12.7 10 5750.0 1.04 16.6 13 6754.0 1.12 17.8 15 7423.0 1.16 18.5 18 9448.0 1.18 18.8 Table 6: Strength-attenuation coefficients of specimens Displacement (mm) SRHCW-1 SRHCW-2 SRHCW-3 SRHCW-4 -18 0.89 0.91 -15 0.98 0.92 -13 0.98 0.93 -10 0.95 0.88 0.92 -8 0.99 0.96 0.95 -5 0.94 0.95 0.74 0.95 -3 0.93 0.96 0.98 0.97 3 0.91 0.95 0.97 0.98 5 0.94 0.93 0.93 0.97 8 0.92 0.94 0.95 10 0.97 0.92 0.95 13 0.99 0.95 15 0.94 0.94 18 0.93 0.81 (JGJ138-2016) 12 was used to compute the shear bearing capacity of the cold-formed, sectional steel, shear walls with concealed bracings. The computational results and measured values are shown in Table 7. As can be seen in Table 7, when the component has a low axial pressure ratio, the computed value shows good agreement with the measured value, with no significant difference. How- ever, with an increase in the axial pressure ratio, the error between the computed value and the measured value increases, and all the computed values are smaller than the measured values. A higher axial pressure would lead to poorer ductility and anti-seismic performance of the components. Thus, the computation formula does not adequately consider the role of the axial pressure on the increase in the horizontal carrying capacity of the com- ponent. As a result, the formula further decreases the horizontal carrying capacity under a high axial pressure ratio, increases the safety margin for the horizontal carrying capacity of the low-rise shear wall, and properly overcomes the insufficient ductility of the low-rise, shear wall under a high axial pressure ratio. 5 CONCLUSIONS By analyzing the failure mode, skeleton curve, energy-dissipation capacity, and strength attenuation of four scaled specimens, the results allow the following conclusions: (1) Under a high axial pressure ratio, the component is mainly compressed. The component rapidly loses its vertical bearing capacity in a short period of time from concrete spalling to significant brittle failure. A higher axial pressure ratio leads to earlier and more abrupt failure. Under a high axial pressure ratio, increasing the area of the diagonal bracing can delay component failure but does not change the failure mode. (2) The tests have shown that the ratio of steel in the diagonal bracings of the high-strength concrete low-rise shear walls is an important factor influencing anti-seis- mic performance. Under a medium or high axial pressure ratio, increasing the area of the diagonal bracing two- fold neither changes the failure mode of the components nor substantially increases the horizontal carrying ca- pacity, but improves the energy-dissipation capacity of components by 150%. (3) A comparison of the measured values of the shear bearing capacity of the cold-formed, sectional steel, shear wall with concealed bracings and the computed values derived according to the formula in the code reveals a large safety margin for the effects of diagonal bracings on the horizontal carrying capacity under a high axial pressure ratio. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 51778087). 6 REFERENCES 1 H. A. D. S. Buddika, A. C. 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