Experimental and finite element analysis: Out-of-plane mechanical performance of infill walls with flexible connection

Abstract

Flexible connection between the wall and the frame can improve the seismic performance of the structure, thus minimizing the pushing effect of the infill wall on the main frame. Masonry infill walls with flexible connection have lower out-of-plane stability than walls with rigid connection. However, local collapses are more likely to occur. A comparative study was conducted on the load-carrying capacity, initial stiffness, deformation and ductility of different wall structures based on a monotonous static load test, and the out-of-plane mechanical properties exhibited by flexible masonry infill walls were investigated. A simplified separated finite element model of masonry infill wall was built in accordance with the test results, and then the monotone out-of-plane static loading was performed. The structural configuration, the wall-frame connection method, and the spacing between structural columns were found to have significant effects on the out-of-plane mechanical behavior of the infill wall. The wall structural configuration was found to have the strongest effect, followed by the wall-frame connection method. Under the conditions of flexible connection, when the wall structure was in the “grid-beam” form, and the spacing between structuring columns was 2.5–3.0 m, the frame-infill wall was found with the best restraint effect, so the optimal out-of-plane comprehensive mechanical performance would be achieved.

Keywords

masonry infill wall flexible connection out-of-plane mechanical performance test finite element analysis

Introduction

Masonry infill wall frames have been one of the most commonly used building structures. The infill wall refers to a non-structural element that may collapse locally or be thrown out completely during an earthquake, which are the most common causes of casualties and economic loss due to seismic damage (Godínez-Domínguez et al., 2021; Liu et al., 2021). Accordingly, the out-of-plane seismic performance of infill walls should be comprehensively and accurately investigated. A rigid connection scheme has been generally used between the infill walls and the main frame to ensure the stability of masonry infill walls, in which the infill walls and the main frame are embedded with mortar; the connection between them is improved by tie bars or cast-in-place bands. A complex interaction emerges between rigidly connected infill walls and the main frame under the action of a horizontal earthquake, thus resulting in additional internal force and premature damage to the infill walls. The flexible connection between walls and frames can be separated to reduce the pushing effect of infill walls on beams and columns. China’s certain technical specifications specify a flexible connection between the infill wall and the frame. When flexible connection is employed between the frame column and wall, a 20 mm gap should remain in which flexible materials (e.g., polyphenylene board or foaming agent) should be filled (Huan, 2013). However, creating a wall-frame flexible connection will change the out-of-plane stability of the infill walls (Chen, 2002).

The research on out-of-plane seismic behavior of frame infill walls mainly focuses on the following aspects. First, the effect of wall-frame connection modes and wall structural configurations on the out-of-plane seismic performance of infill walls has been investigated. In general, the damage degree of infill walls with flexible connection between the RC frame and the infill walls under the action of an earthquake is lower than that of a rigid connection between the RC frame and the infill walls. The degree of seismic damage to the wall decreases with the decrease in the tie bar spacing (Tang et al., 2019). A study by Cheng and Liu (2011) investigated the out-of-plane stability of four sets of two-layer infill walls in a frame through a shaking test, in which four different connection modes between the RC frame and the infill walls were adopted, including inclined bricks at the top of walls under rigid connection, disconnection, flexibility, as well as semi-flexibility. The test results suggested that the infill walls with flexible and semi-flexible connection modes exhibited the strongest integrity.

Second, the out-of-plane mechanism of infill walls has been explored in existing studies. Mc Dowell (1956) first proposed the theory of arched action of unreinforced masonry walls, in which cracks at both ends and the center of a wall are assumed to divide the wall into two interlinked but rotating parts (rigid bodies), thus leading to the formation of a three-hinge arch mechanism. In a study, Edri et al. (2019) examined the response of arch masonry walls based on out-of-plane static load and built a multi-degree-of-freedom analysis model in terms of their non-linear out-of-plane behavior. MD Domenico et al. (2018, 2019) studied the out-of-plane performance of walls with different thicknesses and boundary conditions by performing several tests on unreinforced masonry infill walls. The result suggested that horizontal arch failure was caused by the more fragile connection of three sides of masonry wall with a frame mortar; vertical arch failure arose from the only connection with the mortar on the upper and lower sides of the frame. The studies mentioned above have primarily investigated the bearing mechanism of arches under rigid connection conditions. There have been rare studies on the out-of-plane force mechanism under flexible connection conditions.

Primary concerns at present regarding the out-of-plane mechanical performance of infilled walls under flexible connections include flexible connection methods and boundary conditions. Different connections between the frame and the infill wall could change the crack formation and failure modes of the wall (Kahrizi and Tahamouliroudsari, 2020). Paton-Cole et al. (2012) performed the shaking table tests on a full-size steel frame-filled wall, and it was found that the infill wall exhibited typical geometric characteristics in different directions. The performance of the infill wall impacted by the out-of-plane seismic action can be improved by weakening the decorative surface, connecting the wall and frame with tension bars, and installing a bolt system. Jiang and Liu (2015) tested infill walls under low cycle reverse loads with different flexible connection types, and it was found that the material and connection of the infill wall had a significant effect on its contribution to the RC frame. The structural details of a wall-frame connection should be carefully considered for an effective seismic design.

In summary, compared with a rigid wall-frame connection, an effective flexible connection can improve the out-of-plane seismic performance of infill walls to a certain extent. Existing studies on the out-of-plane seismic performance of infill walls under flexible connection conditions have primarily investigated the flexible connection mode. While in fact, the strengthening of the structural configuration of the infill wall is also a vital factor for its out-of-plane seismic performance. Accordingly, the out-of-plane mechanical properties of infill walls with different structural configurations were analyzed in this study under the wall-frame flexible connection. Experimental and finite element analyses were conducted on the out-of-plane seismic performance of frame infill walls under the wall-frame flexible connection. Both total and local flexible connections were considered. The influence rules of wall-frame connection mode and wall structural configuration on the out-of-plane seismic performance of infill walls were determined to build an optimized infill wall structural scheme. In this study, the effect of the structural configuration of the infill wall on the seismic performance was preliminarily explored, which can provide theoretical and technical support for how the infill wall meets the out-of-plane seismic requirements under the wall-frame flexible connection mode.

Experimental program

Specimen design and fabrication

Three fully flexibly connected masonry-infilled RC frames were designed and fabricated for this experiment. A gap of 30 mm between the walls and the frames was reserved to meet the displacement angle of the weak layer under rare earthquakes. The gaps between the walls and the frames were infilled with polystyrene foam boards. The wall-frame connection adopted the flexible connection device as presented in Table1. As depicted in Figure 1, the connecting structure consisted of composite connectors embedded in frame columns or frame beams and U-shaped tie bars. Under flexible connection, the above connection structure could ensure the effective separation between the infill wall and the main frame, while ensuring the appropriate connection between them. It could solve the common problems in the construction of tie bar (e.g., the unstable planting of reinforcement and the inaccurate correspondence between the position of embedded reinforcement and the mortar joint of block), ensure the stability of force transmission path, and avoid the additional stress or stress concentration at the connection between the tie bar, the main frame and the infill wall. The embedded connectors consisted of connector I and connector II. Anchor bars were arranged at four corners on one side of the vertical plate of connector I, and the other side was connected with connector II. Connector II was provided with corresponding rectangular reinforcement holes which had a height of 30–35 mm and a width of 10–15 mm (Zhou, 2018, CN PAT). To compare with the flexible connection, a rigid connected masonry-infilled frame specimen was set (GF), of which the size was the same as that of the flexible connected frame. The tie bar was embedded in the frame column for 150 mm and extended into the wall for 500 mm. The mortar joint at the junction of the wall and column was full and dense. The vertical brick between the wall and the top beam was obliquely compacted and filled with mortar.

Table 1.

Basic information of specimen.

Specimen number	Wall-frame connection scheme	Structural mode of tie bar	Infill wall construction
RF	All flexible connections	Embedded connector and U-shaped tie bar	—
RF1		Embedded connector and U-shaped tie bar	Setting up two structured side columns and three horizontal tie beams
RF2		Embedded connector and U-shaped tie bar	Setting up two structural middle columns and three horizontal tie beams
GF	All rigid connections	Embedded tie bar into the frame column	—

Figure 1.

Connection between wall and frame and practical operation diagram.

A 300 mm × 150 mm × 120 mm autoclaved aerated concrete block was applied as the masonry material. The section sizes of structural columns and horizontal tie beams in the infill wall were all 50 mm × 150 mm. Concrete types of grade C20 and 2^–6 longitudinal reinforcement were employed to fabricate the structural columns and horizontal tie beams. Figure 2 presents the basic conditions of the specimens frame. Figure 3 presents the structural configuration of infill walls. The U-shaped tie rods from the frame beam and column extend into the infill wall with a length of 350 mm and 500 mm respectively. While The U-shaped pull rod of the specimens with horizontal tie beam or structural/side column passed through the connectors at both ends, which allowed it to penetrate the horizontal tie beam or structural/side column.

Figure 2.

Geometry and reinforcement of frames.

Figure 3.

Structural configuration of infill wall.

The mechanical properties of the materials applied are listed in Table 2.

Table 2.

Measured mechanical properties of materials.

Blocks	Mortar	Concrete (structural columns and horizontal tie beams)	Longitudinal reinforcement (structural columns and horizontal tie beams)
Average compressive strength (MPa)	Average compressive strength (MPa)	Average cubic compressive strength f_cu,m (MPa)	Average yield strength f_ym (MPa)	Average limit strength f_um (MPa)	Average elastic modulus E_Sm (×10⁵) (MPa)
3.47	5.12	19.3	335.8	479.1	2.3

Test setup and instrumentation

The test setup is illustrated in Figure 4. The bottom beam of the frame was fixed to the ground using a ground anchor and pressure beam. The rigid support consisted of a reaction frame, and the steel beam formed the effective lateral fixed boundary of the frame, as presented in Figure 4(a). Four out-of-plane concentrated loads were applied using a hydraulic brake and steel distribution beam, as presented in Figure 4(b).

Figure 4.

Test setup and loading system and displacement gauge arrangement. (a)Side view of test device. (b) Front of test device. (c) Test loading system.

A load-controlled system of hierarchical load-controlled loading method was applied until the wall cracked. The load range was appropriately reduced close to the estimated cracking load. After the first observable crack was detected in the wall, a displacement (out-of-plane displacement of the midpoint of the wall) controlled system was applied. The control displacement of the respective stage was 0.5 mm. in accordance with Specification of seismic test of buildings of China, the test was completely performed once the load fell below 85% of the peak load. We consider that the infill wall was damaged under the above condition. Figure 4(c) plots the test loading system curve.

Test results and analysis

Failure process and failure modes of specimens

The final failure mode and crack distribution of each specimen are presented in Figure 5, respectively. The out-of-plane failure characteristics of rigid connection specimens were basically similar to those of flexible connection specimens under different structural configurations. A horizontal crack first occurred in the middle of the back of the wall. Then, the crack developed diagonally in a stepped manner to the four corners of the wall, thus eventually forming an X-type main crack that ultimately damaged the wall.

Figure 5.

Final failure mode and final crack distribution of specimens.

Compared with the rigid connection specimen GF, the damage of the flexible connection specimen RF without structural configuration was more obvious due to the small constraint effect of the main frame on the infill wall, whereas the damage of the specimens RF1 and RF2 with structural configurations was less than that of RF and GF. Thus, the use of horizontal tie beams and structural columns was found to effectively restrict the wall under the flexible connection.

The out-of-plane bearing capacity suddenly decreased in the RF specimen without any structural configuration when the specimen was damaged, ultimately resulting in damage characterized by brittle failure. In contrast, the bearing capacity of RF1 and RF2 specimens tended to decrease with the increase in the displacement, and the out-of-plane ductility was significantly improved. Compared with specimens RF1 and RF2, the out-of-plane displacement of the wall when the specimen RF was damaged was large, with the main cracks up to 10 mm wide, and the wall was severely damaged, thus indicating that certain structural configurations could be beneficial to the out-of-plane stability of flexibly connected infill walls.

Upon final failure, the crack development law of the specimen RF2 was more significant than that of other specimens, with final crack distribution more uniform. In addition, cracks in the specimen RF2 were narrower than those in the specimen RF1, which revealed that the mechanical performance of the “grid-beam” formed by the horizontal tie beam and structural center column was higher than that of the “sub-frame” formed by the horizontal tie beam and structural side column in terms of the out-of-plane working performance of the infill wall.

Test results and analysis

Table 3 lists the experimental measurements of specimens. Figures 6 and 7 show the load-displacement, the out-of-plane tangential stiffness degradation, and the out-of-plane deformation curves, respectively.

Table 3.

Measured load, displacement and displacement ductility of specimens in each stage.

Specimen number	Crack		Peak		Failure		$μ$
Specimen number	P_cr/kN	Δ_cr/mm	P_max/kN	Δ_max/mm	P_u/kN	Δ_u/mm	$μ$
RF	14.72	1.18	44.22	11.18	37.59	21.46	18.19
RF1	20.82	0.80	102.23	11.71	86.02	17.20	21.50
RF2	26.12	0.83	108.62	12.80	91.82	18.46	22.24
GF	32.34	1.04	146.76	10.51	121.58	13.92	13.38

* $P_{cr}$ , $P_{\max}$ , $P_{u}$ : Load of the specimens corresponding to the cracking point, peak point, and failure point, respectively; $Δ_{cr}$ , $Δ_{\max}$ , $Δ_{u}$ : Displacement of the specimens corresponding to the cracking point, peak point, and failure point, respectively; $K_{e}$ : Equivalent initial stiffness of the wall, $K_{e} = P_{cr} / Δ_{cr}$ ; $μ$ : Displacement ductility, $μ = Δ_{u} / Δ_{cr}$ .

Figure 6.

Load-displacement curves.

Figure 7.

Stiffness degradation curves.

According to Specification of seismic test of buildings, the secant stiffness can represent the stiffness of the specimen. The secant stiffness was calculated by equation (1). Combined with the provisions of the test code and the actual loading mode of this test, the out of plane secant stiffness kJ of each infill wall specimen was calculated by equation (2). The out-of-plane stiffness degradation curve of each specimen is plotted in Figure 7.

K_{i} = \frac{| + F_{i} | + | - F_{i} |}{| + X_{i} | + | - X_{i} |}

(1)

*+F_i,- F_i: the i-th forward and reverse peak point load value;

+X_i,- X_i: the displacement value of the i-th forward and reverse peak points.

K_{j} = \frac{F_{j}}{Δ_{j}}

(2)

*F_j,Δ_j:the j-th load and displacement value respectively.

As depicted in Table 3 and Figures 6 and 7, the out-of-plane bearing capacity of wall-frame rigid connection was higher than that of flexible connection, but the ductility was low. While in the same case of flexible connection, the structural configuration significantly improved the out-of-plane ultimate bearing capacity of the wall specimens. The out-of-plane bearing capacity of the specimen RF2, in which a “grid-beam” stress system was formed using horizontal tie beams and structural middle column, was the largest among them. The structural measures also improved the out-of-plane ductility of infill wall with flexible connection, thus contributing to the structural energy consumption and the plastic deformation requirements under rare earthquakes. According to the data in the Table 3, the displacement ductility of specimen RF2 was higher than that of specimen RF1.

As depicted in Figure 7, the initial out-of-plane stiffness of wall-frame rigid connection was the largest, thus indicating that frame infill wall exhibited better stiffness than flexible connection under the rigid connection. With the reciprocating loading, the stiffness of each specimen tended to decrease, and the stiffness of rigid connecting wall GF tended to be consistent with that of RF without structural configuration, whereas it was generally lower than RF1 and RF2. Under flexible connection, the initial out-of-plane stiffness of specimens RF1 and RF2 with structural configuration was significantly higher than that of specimens RF without structural measures, thus implying that structural measures significantly increased the out-of-plane stiffness of the infill wall, among which the out-of-plane stiffness of specimen RF2 with a “grid-beam” stress system was the highest.

Compared with RF1, the wall deformation curve of RF2 was relatively uniform at different times, and the area with the largest out of plane displacement of the wall was closer to the middle of the wall, thus revealing that the overall stress distribution of RF2 wall was relatively uniform. It was therefore confirmed that the addition of structural columns on the basis of horizontal tie beams could make the two work together, thus eliminating the inequality of the constraints of the main frame on the infill wall, balancing the stress characteristics of the wall, making the deformation of the wall relatively stable and consistent, and improving its integrity.

In brief, the overall stress distribution of the specimen RF2 in which a “grid-beam” stress system was formed using horizontal tie beams and structural middle columns was relatively uniform. The horizontal tie beams and structural middle columns were distributed and cooperated with each other, which could eliminate the inequality of the constraints of the main frame on the infill wall, while balancing the stress characteristics of the wall, thus making the wall deformation relatively stable and consistent and improving its integrity and stiffness.

Establishment and verification of finite element model

Model establishment

The separated reinforced concrete frame models were built in ABAQUS. The reinforcement unit was embedded into the concrete using the embed technology to build the simplified separated masonry infill wall models (Kong et al., 2012; Zhou et al., 2019). The simplified separated model was that the mortar was divided and evenly distributed in the surrounding blocks, the blocks and mortar formed the homogeneous blocks. A viscous joint contact interface was formed between the homogeneous blocks, i.e., the actual mortar position. The tangential force on the contact surface of the model connected the friction force and the normal stress at the interface through the friction coefficient. In accordance with Code for Design of Masonry Structures in China, the friction coefficient of masonry structure sliding along masonry or concrete in a certain dry environment was taken as 0.5. The reinforcement was established using an ideal elastic-plastic model and the T3D2 truss unit, while the masonry and concrete were set using the ABAQUS concrete plastic damage model and the C3D8R unit. In this case, the constitutive relation of masonry was double-folded (equation (3) and Figure 8(a))(Guo, 1980) according to the test results and with reference to the literature (Huang, 2011; Liu, 2005). The disadvantage was that the downward section of load-displacement curve could not be described. The extrudable foam model was employed for the flexible filling material. Figure 8(b) plots the yield stress-plastic strain curve of foam (Zhuang, 2009).

ε = [0.93 (\frac{σ}{f_{m}}) + 0.25 {(\frac{σ}{f_{m}})}^{4}] (\frac{1550}{\sqrt{f_{1}}} + \frac{450}{\sqrt{f_{2}}}) \times \sqrt{f_{m}} \times 10^{- 6}

(3)

Figure 8.

Stress-strain curve of masonry and foam (a) masonry. (b) Foam.

* fm denotes the compressive strength of masonry; f1 represents the compressive strength of the block; f2 is the compressive strength of mortar. (unit:MPa)

Model verification

The wall RF without structural configuration and the wall RF2 with the best integrity and stiffness in the test were selected for model verification. The structural configuration of the infill wall, the material parameters, and the loading regime of the model were the same as those of the test.

Load-displacement curve

Figure 9 presents the simulation and test results for the ascending section of the load-displacement curve of the respective wall. The finite element simulation curves were well consistent with the test results at the initial stress stage. At later stress stages, the finite element simulation curves showed a certain degree of lift and extension as compared with the test curves. There were some deviations in the detailed data of the whole process, which was primarily explained below. First, there were many factors for the masonry of the test wall and the test process, thus having little influence at the initial stage of the experiment. With the increase in the wall stress, the effect of various factors tended to emerge, while the conditions of the finite element analysis were basically ideal except for the effect of wall parameters. Moreover, in the test process, there was a certain time interval in the loading process at all levels, which was adopted to record the test data and the test phenomena, while the simulation process was continuously conducted without interruption.

Figure 9.

Comparison of load-displacement curves.

Distribution of wall damage

Figure 10 presents the distribution of simulated cracks and ultimate tensile damage of the respective wall, basically consistent with the test phenomena. The simplified separated masonry modeling method could accurately indicate the rising section development trend and out-of-plane load-displacement relationship of the filling wall besides its cracking and failure modes.

Figure 10.

Comparison of simulated damage distributions and experimental failure.

The above analysis suggested that the simplified separated modeling method selected in this study could better indicate the overall trend of the rising section of the out of plane load displacement relationship of the infill wall, while accurately reflecting the cracking and failure mode of the wall. Accordingly, it is reasonable and feasible to determine the modeling method.

Influence of wall-frame connection and wall structural configuration on out-of-plane mechanical behavior of infill walls

Four infill walls with locally flexible connections were modeled and analyzed to further investigate the effects of different wall-frame connection modes and structural column or horizontal tie beam settings under the same connection mode on the out-of-plane mechanical behavior of infill walls (Table 4).

Table 4.

Model settings.

Number	Model number	Wall-frame connection scheme	Wall-column connection	Wall-beam connection	Structural configuration of infill wall
1	RF3	Locally flexible connection	Rigid connection	Flexible connection	Only two structural central columns
2	RF3-1		Rigid connection	Flexible connection	Only two horizontal tie beams
3	RF4		Flexible connection	Rigid connection	Only two horizontal tie beams
4	RF4-1		Flexible connection	Rigid connection	Only two structural central columns

Load-displacement curve

Figure 11 plots the out-of-plane load-displacement curves of the respective wall. As depicted in Figure 11(a) and (b), the out-of-plane initial stiffness and ultimate bearing capacity of the only structural column walls were higher than those of the only horizontal tie beams regardless of the wall-frame connection scheme. As depicted in Figure 11(c) and (d), the out-of-plane initial stiffness and ultimate bearing capacity of the wall were nearly the same under the different wall-column (beam) connection methods, thus revealing that the influencing factor for infill wall structural configuration was more important than that of the wall-frame connection method. As depicted in Figure 11(c) and (d), the out-of-plane displacement of the wall with the wall-column flexible connection was generally more significant than that with a wall-beam flexible connection upon reaching the ultimate load. As a result, the out-of-plane deformation capacity of the infill wall with the wall-column flexible connection was better than that with the wall-beam connection.

Figure 11.

Load displacement curve. (a) Wall-beam flexible connection (b) Wall-column flexible connection. (c) Wall only with structural columns (d) Wall only with horizontal tie beam.

Analysis of wall tensile damage

Figure 12 illustrates the distribution of tensile damage of the respective wall. Under the same wall-column (beam) connection method, the damage area of the structural column in the wall was significantly larger than that of the horizontal tie beam, thus revealing that the structural column played a more significant role in the wall’s function. Under the same structural configuration, the wall exhibited similar mechanical properties under the wall-beam flexible connection and the wall-column flexible connection. Moreover, the tensile damage distribution under the limit state of the wall was nearly the same. The wall damage distribution of the wall-column flexible connection was wider than that of the wall-beam flexible connection when the structural configuration remained unchanged, thus revealing that the wall-column flexible connection was more advantageous in terms of the mechanical properties of the wall.

Figure 12.

Tensile damage distribution of walls.

Influence of structural column spacing

The out-of-plane mechanical performance of the infill wall was found to be more effective when the structural columns were in a dispersed layout and had fully flexible connections. Based on the “grid-beam” stress system (e.g., RF2 model), five finite element models were built by changing the spacing S of structural columns to investigate the effect of structural column spacing on the out-of-plane mechanical behavior of the wall. The structural columns tended to move toward both sides of the wall based on the central axis of the wall. The influence rule of structural column spacing on out-of-plane mechanical behavior of infill walls was determined through the comparison of the various load-displacement curve rising sections, wall failure modes and tensile damage distributions. The height-width ratio and height-thickness ratio of the wall model were set to 0.6 and 16, respectively, in accordance with the practical engineering application. Figure 13 plots the test modeling model size.

Figure 13.

Model diagram.

Load-displacement curve

Table 5 lists the out-of-plane ultimate bearing capacity and out-of-plane initial stiffness results of the respective wall. Figure 14 illustrates the load-displacement relationship of the respective wall.

Table 5.

Out-of-plane ultimate bearing capacity and initial stiffness of walls.

Model number	RF2-1	RF2-2	RF2-3	RF2-4	RF2-5
Structural column spacing S/m	0	0.66	1.33	2.66	3.94
Ratio of spacing to thickness λ	0	4.40	8.87	17.73	26.27
Initial stiffness K/kN/mm	7.03	8.58	18.42	6.38	4.62
Ultimate bearing capacity F/kN	72.21	80.22	91.23	58.74	42.01

Figure 14.

Load-displacement curve.

As depicted in Table 5 and Figure 14, the out-of-plane ultimate bearing capacity of the wall increased by 11.1% when the structural column spacing S was 0.66 m; it increased by 26.3% when the structural column spacing S was 1.33 m as compared with the RF2-1 specimen (structural column spacing S = 0). When the structural column spacing S was 2.66 m and 3.94 m, the out-of-plane ultimate bearing capacity of the wall decreased by 18.7% and 41.8%, respectively. In comparison with the RF2-1 specimen (S = 0), the out-of-plane initial stiffness of the wall increased by 22.1% when S was 0.66 m, and it increased by 162.0% when S was 1.33 m. The initial stiffness decreased by 9.3% and 34.3% when S was 2.66 m and 3.94 m, respectively. In the above calculation model, the out-of-plane ultimate bearing capacity and initial stiffness of the wall were optimal when the structural column spacing was 1.33 m.

Under the flexible connection, the main frame had a not significant effect on the wall. Besides the connection effect of mortar between blocks, the out-of-plane bearing capacity of the wall was primarily provided by the restraint effect of structural configuration on the masonry. During the simulation, the structural columns with different spacings served as the structural configuration of infill walls. The middle masonry of the wall with structural columns could form an arch effect in the horizontal direction between the structural columns. The above arch effect was found to transmit the out-of-plane load on the wall to the structural columns on both sides along the horizontal direction, limit the out-of-plane deformation of the wall, make the overall stress distribution of the wall more uniform and increase the plane bearing capacity. The action effect of the above arch effect was dependent on the ratio of unbraced height to thickness (Komaraneni et al., 2011), in the horizontal direction, it is expressed as the ratio of spacing S to wall thickness (Denoted by λ). When λ increased from 0 to 8.87, the arch effect was increasingly significant. When λ was higher than 8.87, the arch effect tended to decrease, the restraint effect of structural columns on the middle masonry decreased, and the out-of-plane bearing capacity decreased. When the λ was 17.73, the arch effect disappeared due to the large span in the middle, the middle masonry almost lost the restraint provided by the structural column. Thus, the out of plane force received by the wall could not be transmitted evenly, which resulted in the imbalance of stress and strain of the wall, and the failure type of the wall changed from strength failure to instability failure.

Failure mode of wall and out-of-plane deformation

Table 6 lists the out-of-plane displacement under the ultimate load of the respective wall. Figure 15 illustrates the deformation and out-of-plane displacement distribution of the respective wall. The development and distribution of cracks in the respective wall were found to be still similar under different structural column spacings. As the structural column spacing increased from zero, the cracking degree of the wall decreased first and then increased; when S = 1.33 m, i.e. λ = 8.87, the cracking degree was slight except where the wall was in contact with the structural column.

Table 6.

Out-of-plane displacement under ultimate load of the respective wall.

Model number	RF2-1	RF2-2	RF2-3	RF2-4	RF2-5
Structural column spacing S/m	0	0.66	1.33	2.66	3.94
Ratio of spacing to thickness λ	0	4.40	8.87	17.73	26.27
Midpoint displacement of wall Δ/mm	57.15	78.63	85.42	98.06	43.89
Wall top displacement Δ/mm	35.64	46.72	59.17	127.65	92.36

Figure 15.

Deformation and displacement distribution of walls.

Under flexible connection conditions, the connection stiffness of the top of the infill wall was found to be significantly lower than that of the bottom of the wall. The upper part of the wall moved more significantly, which was promoted with the increase in the structural column spacing. When the ratio λ of spacing S to thickness was 4.40 and 8.87, the displacement of the midpoint of the wall was larger than that of the top, thus revealing that the structural columns inhibited the deformation of the wall to a certain extent. When the λ was 17.73 and 26.27, the displacement of the top of the wall was larger than that of the midpoint of the wall. The out-of-plane displacement difference between the top of wall and the midpoint of wall increased with S. For instance, when S = 2.66 m, λ = 17.73, the top displacement was 130.2% of the midpoint displacement of wall; when S = 3.94 m, λ = 26.27, the top displacement was 210% of the midpoint displacement of wall, i.e., the upper wall showed a larger tendency of outward displacement.

As depicted in Table 6 and Figure 15, when the spacing of structural columns was within a certain limit (1.33 m), the wall primarily underwent biaxial bending, and the stress was more uniform than that under other conditions, so the out-of-plane displacement of the wall had the symmetrical distribution. In addition, due to the different connection conditions between the top and bottom of the wall, there were significant differences in the stiffness distribution along the height direction of the wall. The stress of the masonry in the middle of the wall was similar to the cantilever structure, so the overall displacement of the wall was significantly inclined to the top of the wall, and the upper wall showed a significant outward dump trend. The larger the spacing of structural columns, the more serious the phenomenon would be.

Distribution of wall damage

As depicted in Figure 16, the walls with 0.66 m and 1.33 m structural column spacing exhibited similar mechanical characteristics. The distribution range of wall tensile damage covered almost the entire wall, though the damage at the intersection of the structural column and horizontal tie beam was more significant. Beyond S = 2.66 m, i.e. λ = 17.73, the distribution of tensile damage at the bottom of the wall was inhibited significantly. This phenomenon was intensified by larger structural column spacing. When S = 3.94 m, i.e. λ = 26.27, no relatively serious damage occurred at the bottom of the wall, and the damage at the intersection of the structural column and horizontal tie beam was not significant.

Figure 16.

Tensile damage distribution of walls.

In brief, when the ratio of spacing S to thickness was lower than 8.87, there was a significant interaction between structural columns and horizontal tie beams, thus leading to the even distribution of the arch effect in the width direction and height direction of the wall and further exerting a good restraining effect. With the increase in the spacing of structural columns, the above interaction tended to weaken. At this time, the structural columns and horizontal tie beams played a restraining role separately rather than mutually, and the restraining effect on the masonry in the middle was reduced, thus resulting in the deterioration of the integrity of the wall.

Conclusion

In this study, infill walls with different structural configurations and wall-frame flexible connections were analyzed through the out-of-plane mechanical performance testing and the finite element analysis. The conclusions are summarized as follows.

1) The structural configuration of the wall could significantly improve the out-of-plane mechanical performance of masonry infill walls with the flexible connection. A structural scheme based on a “grid-beam” stress system allowed horizontal tie beams and the structural column to cooperate with each other, thus leading to the distribution of cracks in the wall uniform, creating favorable crack development and increasing the out-of-plane bearing capacity, initial stiffness and ductility of the wall.

2) Wall-frame connection schemes and wall structural configurations were found as the primary factors for the out-of-plane mechanical performance of masonry infill walls. The effect of the latter was stronger than that of the former. Under the same wall structural configuration, a locally flexibly connected frame infill wall with wall-column flexible connection had better out-of-plane deformation performance than one with a wall-beam flexible connection. Under the same wall-frame connection scheme, the out-of-plane initial stiffness and out-of-plane bearing capacity of the filled wall with only structural columns were found to be higher than those of one with only a horizontal tie beam.

3) The ratio of structural columns spacing to wall thickness was confirmed as a vital factor for the out-of-plane stability of flexible connected masonry infill walls. The appropriate spacing of structural columns could effectively restrict the wall and avoid out-of-plane instability. Accordingly, in practical engineering, the maximum spacing of structural columns should be limited according to the wall thickness. As revealed by the model calculation results, the out-of-plane seismic performance of the wall was the best when the ratio of spacing to thickness was 8.87. Due to the factors (e.g., specimen scale and practical engineering application), for the wall-frame fully flexible connection autoclaved aerated concrete masonry infill wall, respectively, it was suggested that the ratio of structural columns spacing to wall thickness should be strictly controlled within 25.00.

Through the experiments and the finite element analysis, the effects of structural columns and horizontal tie beams on the out-of-plane failure mode, bearing capacity, stiffness and displacement of flexible connected autoclaved aerated concrete masonry infill walls were investigated, and the influence law of structural configuration on the out-of-plane performance of flexible connected masonry infill walls was preliminarily determined. The above preliminary conclusions can provide a reference for future research and expand more structural configurations on this basis.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research described in this paper was financially supported by the National Natural Science Foundation of China under Grant Nos. 51678389.

Data availability

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

ORCID iDs

Xiao-jie Zhou

Pei-qi Chen

References

Chen

(2002) Experimental study on YTONG aerated concrete rectangular and irregular roof slabs Seismic behavior of YTONG lightweight infill wall-reinforced concrete frame connection. M.S. Thesis. Tongji University, China.

Cheng

Liu

(2011) Shaking test research on connection modes between block infill wall and frame beam. Journal of Harbin Institute of Technology (New Series) 18(5): 123–129.

Domenico

Ricci

Verderame

(2018) Experimental assessment of the influence of boundary conditions on the out-of-plane response of unreinforced masonry infill walls. Journal of Earthquake Engineering 24(1): 1–39.

Domenico

Ricci

Verderame

(2019) Experimental assessment of the out-of-plane strength of URM infill walls with different slenderness and boundary conditions. Bulletin of Earthquake Engineering 17(7): 3959–3993.

Edri

Yankelevsky

Rabinovitch

(2019) Out-of-plane response of arching masonry walls to static loads. Engineering Structures 201: 109801.

Godínez-Domínguez

Tena-Colunga

Pérez-Rocha

(2021) The September 7, 2017 Tehuantepec, Mexico, earthquake: damage assessment in masonry structures for housing. International Journal of Disaster Risk Reduction 56(4): 102123.

Guo

(1980) Calculation and experimental basis of aerated concrete members. Report, Collection of scientific research reports of anti-seismic and anti-riot Engineering Research Office of Tsinghua University (Part II).

Huan

(2013) Research on the connection modes and numerical simu lation methods of RC frame and infill wall. Advanced Materials Research 671–674: 549–554.

Huang

(2011) Study on seismic behavior and elastic-plastic analysis method for seismic responses of Rc frame infilled with New Masonry. M.S. Thesis. Huaqiao University, China.

10.

Jiang

Liu

Mao

(2015) Full-scale experimental study on masonry infilled RC moment-resisting frames under cyclic loads. Engineering Structures 91(may 15): 70–84.

11.

Kahrizi

TahamouliRoudsari

(2020) Experimental study on properties of masonry infill walls connected to steel frames with different connection details. Structural Durability & Health Monitoring 14(2): 165–185 07816.

12.

Komaraneni

Rai

Singhal

(2011) Seismic behavior of framed masonry panels with prior damage when subjected to out-of-plane loading. Earthquake Spectra 27(4): 1077–1103.

13.

Kong

Zhai

(2012) Study on in-plane seismic performance of solid masonry-infilled RC frames. China Civil Engineering Journal(S2): 137–141.

14.

Liu

(2005) Experiment Study on the Mechanical Properties of the Autoclaved Aerated Concrete Bearing Masonry. M.S. Thesis. Tianjin University, China.

15.

Liu

Fang

Zhao

(2021) Reflection on earthquake damage of buildings in 2015 Nepal earthquake and seismic measures for post-earthquake reconstruction. Structures 30: 647–658.

16.

Mc Dowell (1956) Sevin E.Arching action theory of masonry walls. Journal of the Structural Division (ASCE) 82(ST2): 915/1–915/18.

17.

Paton-Cole

Gad

Clifton

, et al. (2012) Out-of-plane performance of a brick veneer steel-framed house subjected to seismic loads. Construction and Building Materials 28(1): 779–790.

18.

Tang

Chen

(2019) Seismic performance of RC frames with EPSC latticed concrete infill walls. Engineering Structures 197(15): 109437.

19.

Zhou

(2018) A flexible connection structure between infilled wall and main frame and its construction method. CN PAT, CN108222283A.

20.

Zhou

Kou

Cui

(2019) Seismic performance test and nonlinear finite element analysis of hollow block walls. Journal of Basic Science and Engineering 27(02): 341–358.

21.

Zhuang

(2009) Finite Element Analysis and Application Based on ABAQUS. China: Tsinghua University Press.