An improper configuration of masonry infill walls in RC frame may lead to short column effect on the columns, which is harmful to the seismic behavior of the structure. In this study, a bare frame and two single-story, single-bay RC frames, partially infilled with masonry, were tested under cyclic loading. The failure mechanism and seismic performance of these partially infilled RC frames (with an infill height of 600 mm) with different types of connections were analysed. Based on the experiment, nonlinear finite element simulation and analysis were conducted to study the effects of the infill walls and connections. The results show that both mechanical performance and failure mode are affected by the infill height, the type of connection between the frame and the infill, and the ratio of shear bearing capacity of the frame column to that of the infill. For the masonry-infilled frame with rigid connection, the higher the infill wall is, the lower the shear bearing capacity ratio will be. Thus, the effect of the lateral constraint of the infill wall on the column increases, and the shear span ratio of the free segment of the column decreases, resulting in the short column effect. Based on the analysis results, a value of 2.0 is suggested for the critical shear bearing capacity ratio of the frame column to the infill wall. If the shear bearing capacity ratio is less than 2.0 and the shear span ratio of the column free segment is not more than 2.0, the short column effect will occur. For the infilled frame with flexible connection, both the lateral constraint from the wall to the column and the wall-frame interaction decrease; this reduces or prevents the short column effect. The conclusion can present guidance for the design and construction of masonry-infilled RC frame structure.
A large number of earthquake investigations have revealed that nonstructural members, especially the masonry infill walls, may have a large influence on the seismic behavior of the main structure. In some cases, the effect may be positive, but in other cases the masonry infill walls may cause more serious damage to the framed structure. In the recent decades, extensive studies [
For ease of construction and to ensure the stability of the infill wall, tie bars and cement mortar are embedded between the wall and frame to form a rigid connection. With a rigid connection, the mechanism of force transfer changes because of the presence of wall-frame interaction [
In this study, a bare frame and two single-story, single-bay RC frames partially infilled with masonry were tested under reversed cyclic loading. The failure mechanism and the seismic performance of these partially infilled RC frames with different types of connections were analysed; the details are presented in the next section. Based on the results of the experiment, nonlinear finite element analysis was performed to investigate the effect of infill wall on the mechanical performance of the frame structures; the analysis was performed with several parameters. The details are covered in Section
Three single-story, single-bay masonry-infilled RC frames were tested under low reversed cyclic loading. The masonry infill wall was built with fly-ash thermal insulation consisting of a hollow block of grade MU3.5 and masonry mortar of type M5. Concrete types of grades C30 and C20 was used for fabrication of the RC frames and core columns, respectively. The stirrups and longitudinal bars of the RC frames were made of HPB300 and HRB335, respectively. The axial compression ratio of the frame columns was 0.25. For the specimen with flexible connection, a gap of 30 mm between the walls and the frames was reserved to meet the displacement angle of the weak layer in the case of frequent earthquakes and rare earthquakes, and the gap was infilled with 32 mm thick polystyrene foam boards between the walls and the frames. The details of the specimens are presented in Table
Description of specimens.
Specimen number | Connection between the wall and frame | Connection between the wall and frame column | Height of infill wall | Core column |
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PF | Bare frame | — | None | |
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GWF | Rigid connection | Tie bars into the column and wall; the longitudinal reinforcement of core column stretches into the sill. | Partially infilled | 2 |
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RWF | Flexible connection | Tie bars into the column and wall; tie bars and steel mesh reinforcement were connected through the wall; foam polystyrene filled seam reserve; the longitudinal reinforcement of core column stretches into the sill. | Partially infilled | 2 |
Geometry and reinforcements of specimens.
Specimens GWF and RWF.
Specimen GWF. Note:
Specimen RWF. Note:
The test setup is shown in Figure
Test setup.
Test setup
Schematic of test setup. Note: ① vertical loading device; ② frame; ③ jack; ④ Low-friction sliding plate; ⑤ steel beam; ⑥ steel crossbeam; ⑦ bolt; ⑧ pressure beam
Loading history.
Strain gauges were placed on the stirrups and longitudinal bars of the RC frames at the critical sections, as shown in Figure
Strain gauge layout in the bars of beams and columns.
Strain gauge layout in the longitudinal bars
Strain gauge layout in the stirrups
The final failure modes of the specimens are shown in Figure
Strain gauge layout in the bars of beams and columns.
Specimen PF
Specimen GWF
Specimen RWF
(2) Specimen GFW: Because of the stiffness effect of the infill wall on the column, at the drift ratio of
(3) Specimen RFW: As the gaps were infilled with polystyrene foam boards, the stiffness effect and the constraint effect of the infill wall on the column decreased, and hence the longitudinal bars yielded at the ends of the beam and the column at the drift ratios of
To sum up, for the specimen with rigid connection, the wall-frame interaction may give rise to additional shear on the frame column, which affects the failure modes and makes the structure vulnerable to damage under seismic action. For the specimen with flexible connection, the additional shear and short column effect can be reduced or even avoided. In this experiment, the infill wall which was rigidly connected to the RC frame did not have a significant effect on the behavior of the frame column because of the low strength of the infill wall. Otherwise, the short column effect may occur in the frame columns.
Figures
Hysteresis curves of specimens.
Specimen PF
Specimen GWF
Specimen RWF
Relationship between cumulative energy dissipation and displacement.
The same features can be seen in all the specimens until the specimens yielded. Before the crack load is reached, the relationship between the force and displacement was linear, and the energy dissipation was low. Subsequently, the hysteresis loop became spindle-shaped, indicating that the energy dissipation capacity has increased.
After yielding, however, the behaviors of the specimens were slightly different from each other. For specimens PF and GFW, the hysteresis loop became bow-shaped initially, and the energy dissipation increased; when the maximum load was reached, pinching effect was significant, and the bearing capacity as well as the energy dissipation capacity decreased. It may be noted that the area bounded by the hysteresis loop of specimen GFW was larger than that of specimen PF, implying higher energy dissipation at the same drift ratio. Finally, the hysteresis loop of specimen PF had a reversed S-shape, while that of specimen GFW had a shape lying between bow-shape and reversed S-shape. It can be inferred that the energy dissipation capacity of the structure improved because of the infill wall. For specimen RFW, the pinching effect was not significant initially owing to the flexible connection. Hence, the hysteresis loop was basically bow-shaped, and the area bounded by the hysteresis loop was large. On reaching the maximum load, the pinching effect could be observed, and the loop had a shape lying between spindle-shape and bow-shape. Compared with specimen GFW, the energy dissipation capacity of specimen RFW had improved.
Normally, a frame partially infilled with masonry can have a high energy dissipation capacity; however, with a rigid connection, which may cause an additional shear or short column effect, shear failure of the frame column can occur. Therefore, the flexible connection was an effective solution, as the specimen with flexible connection dissipated more energy than that with rigid connection.
Figure
Experimental results.
Specimens | | | | | | | | | | |
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PF | - | - | 166.3 | 9.59 | 17.34 | 334.7 | 44.37 | 284.5 | 55.28 | 5.76 |
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GWF | 188 | 4.64 | 263.3 | 9.10 | 28.93 | 375.9 | 43.21 | 319.5 | 51.58 | 5.67 |
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RWF | 241.8 | 7.77 | 260.0 | 9.65 | 26.94 | 360.7 | 44.90 | 306.6 | 55.97 | 5.80 |
Skeleton curves of specimens.
From the details in Figure
(2) Compared with the maximum load for specimen PF, the maximum loads of specimens GWF and RWF are higher by 12.3% and 7.8%, respectively. Thus, the addition of infill wall contributes to higher bearing capacity of the structure.
(3) Compared with the stiffness of specimen PF, the stiffness values of specimens GWF and RWF are higher by 66.8% and 55.4%, respectively. Thus, the addition of infill wall contributes to higher stiffness of the structure.
(4) The displacement ductility values of the partially infilled frame (
The strain in the longitudinal bars during the test is depicted in Figure
Strain in the longitudinal bars of the left column.
Specimen PF
Specimen GWF
Specimen RWF
(2) For specimen GWF with rigid connection, the strain in the longitudinal bar at half the height of the column increased sharply owing to the stiffness effect and constraining effect resulting from the infill wall and the rigid connection. The internal force of the column was distributed unevenly and discontinuously, leading to a change in the position of the critical section of the column. If the shearing capacity of the infill wall is greater than that of the frame, the column may fail in shear.
(3) For specimen RWF with flexible connection, the strain in the longitudinal bar had almost a linear variation in the column height direction, owing to the reduction in both the stiffness effect and restraining effect of the infill wall. Consequently, the critical section of the column was at the bottom.
It can be concluded that in the case of rigid connection between the wall and frame, improper infill wall configuration may cause additional shear or even short column effect on the frame column, which is harmful to the seismic performance of the structure. The strength and height of the masonry infill as well as the type of connection between the wall and frame will affect the structural behavior and failure mode in most cases.
Using ABAQUS finite element software, three-dimensional models were established and nonlinear finite element analysis was performed on the masonry-infilled frames under monotonic horizontal load. The finite element model of specimen RWF is shown in Figure
Strength parameters of the materials.
Type | Masonry | Longitudinal reinforcement of frame | Column stirrup | Beam stirrup and core column reinforcement | Concrete used in frame | Concrete poured in core column |
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Compression strength (MPa) | 1.02 | 483.8 | 365.6 | 381.2 | 20.5 | 22.3 |
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Tensile strength (MPa) | 0.021 | 483.8 | 365.6 | 381.2 | 2.26 | 2.37 |
Elastic parameters of the materials.
Type | Masonry | Reinforcement | Concrete used in frame | Concrete poured in core column | Polystyrene foam board |
---|---|---|---|---|---|
Elastic modulus (MPa) | 443 | 2×105 | 3×104 | 1.48×104 | 6.15 |
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Poisson ratio | 0.315 | 0.3 | 0.2 | 0.25 | 0.5 |
Finite element model of specimen RWF.
The bilinear ideal elastic-plastic model was adopted for the reinforcements. The plastic damage model of ABAQUS and the constitutive relation recommended by the Chinese Code for Design of Concrete Structures (GB 50010-2010) were used for the concrete. For the masonry wall, the compression constitutive relation was adopted as given in the literature [
Yield stress versus plastic strain curve for polystyrene foam board.
The comparison of the experimental and analytical results is shown in Figure
Load-displacement skeleton curves.
The comparison of plastic damage in compression obtained from the models and the failure phenomenon observed in the experiment is shown in Figure
Plastic damage in compression from modeling and failure phenomenon from experiment.
Specimen PF
Specimen GWF
Specimen RWF
For the partially infilled RC frame with rigid connection, the failure mode of the frame column is affected by the degree of lateral constraint from the infill to frame column. If the strength of the infill wall is relatively high, then the lateral constraint on the column is strong and the free segment of the column decreases, causing short column effect. Hence, the shear bearing capacity ratio of the frame column to the infill wall is an important parameter that affects the failure mode of the column [
This ratio can be expressed by the following equation:
where
Three nonlinear finite element models of specimens with rigid connection were established. The masonry height is 600 mm in all the three models, and the compression strengths of the masonry are 2MPa, 2.5MPa, and 3MPa, respectively. Figure
Calculated load-displacement curves.
Plastic strain in RC frames.
Masonry strength
Masonry strength
Masonry strength
Plastic strain in infill walls.
Masonry strength
Masonry strength
Masonry strength
The following observations can be made from Figures
(2) For the structure with masonry compression strength of 2MPa, a relatively larger plastic strain appears on the column end and the beam end, and bending failure occurs. For the structure with masonry compression strength of 2.5MPa and 3MPa, the maximum value of plastic strain appears at the centre of the free segment of the right column, and shear failure occurs in the column. This may be attributed mainly to the increase in the lateral constraint of the infill wall to the column with the increasing in masonry compression strength. When the masonry compression strength increases to a certain extent, the clamping effect of the wall to the frame column increases, and the shear span ratio of the column free segment reduces, leading to an increase in stiffness and decrease in energy dissipation capacity. In the end, the stirrups yield ahead of the longitudinal reinforcements owing to insufficient shear capacity and short column effect.
(3) The reason for the reduction in the structure ductility is that the short column effect causes brittle shear failure associated with the low deformation capacity of the structure in the case of a strong infill wall surrounded by a weak frame.
(4) For the infill wall, the diagonal strut mechanism is evident, and the plastic deformation is concentrated mainly on the infill diagonal from the loading corner to the bottom of the left column.
The values of the shear bearing capacity ratio of the frame column to the infill wall for the three models are listed in Table
Shear bearing capacity ratio of the frame column to the infill wall (
Masonry strength | | | | Short column failure |
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2 | 225 | 111.7 | 2.01 | No |
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2.5 | 225 | 139.7 | 1.61 | Yes |
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3 | 225 | 167.6 | 1.34 | Yes |
Investigations were conducted for infill heights
Load-displacement curves.
Masonry strength of 2MPa
Masonry strength of 3MPa
Plastic strain in bare frame.
Plastic strain in frames partially infilled with masonry (
Plastic strain in infill walls (
Plastic strain in frames partially infilled with masonry (
Plastic strain in infill walls (
From the results, the following conclusions can be drawn.
(2) When the masonry strength is lower (e.g., 2MPa), irrespective of the filling rate, there is no increase in the ductility of the structure. When the masonry strength is higher (e.g., 3MPa), the ductility of the structure decreases with the increasing of filling rate, and the decline branch of the load-displacement curve is steeper (except for a filling rate of 1).
(3) When the filling rate is 0, a large plastic strain is observed at the ends of the column and beam. When the filling rate is 1, the diagonal strut of the infill wall is evident, and a large plastic strain is observed mainly on the column and beam ends. When the filling rate is 0.25, the infill wall does not affect the frame significantly, and the distribution of plastic strain is similar to that in the case of filling rates of 0 and 1. The higher the infill height is, the more easily the shear failure occurs because of the short column effect. When the filling rates are 0.5 and 0.75, the shear span ratios of the free segment of the column were 1.3 and 0.8, respectively, which lie in the short column range (
(4) The infill height is an important factor that affects the failure mode of the structure. In real structures, a value greater than 2.0 is recommended for the shear span ratio of the free segment of the column.
Two connection modes between the wall and frame, i.e., flexible connection and rigid connection, were investigated using finite element models. The compression strength of the masonry was 3MPa, and the heights of the infill wall were 600 mm and 900 mm. The load-displacement curves are shown in Figure
Load-displacement curves of models with different connections.
Plastic strain in RC frame with different connection modes.
Infill height of 600 mm with rigid connection
Infill height of 600 mm with flexible connection
Infill height of 900 mm with rigid connection
Infill height of 900 mm with flexible connection
Plastic strain in infill wall with different connection modes.
Infill height of 600 mm with rigid connection
Infill height of 600 mm with flexible connection
Infill height of 900 mm with rigid connection
Infill height of 900 mm with flexible connection
Stresses in reinforcement members in different connection modes.
Infill height of 600 mm with rigid connection
Infill height of 600 mm with flexible connection
Infill height of 900 mm with rigid connection
Infill height of 900 mm with flexible connection
From Figures
(2) The displacement ductility of the specimen with flexible connection is greater than that of the corresponding specimen with rigid connection; this is favourable for energy dissipation and plastic deformation under a severe earthquake.
(3) In case of the infilled RC frame with flexible connection, because of the decreasing of the wall-frame interaction and the reduction of lateral constraint from the wall to the column, the short column effect which occurs easily in case of rigid connection is prevented. Therefore, the flexible structure has better seismic resistance, and the using of flexible connection can also reduce the damage to the infill wall which is beneficial to the working performance of the structure as a whole.
Experimental tests and nonlinear finite element simulation were conducted to study the effects of the infill walls and wall-frame connections. The following conclusions were drawn: The infill wall can increase the strength, stiffness, and ductility of the frame structure. However, in the case of a partially infilled frame with rigid connection, when the masonry strength and the infill height are increased, the ductility of the frame structure may decrease owing to short column effect. The infill height The mechanical performance and failure modes of the frame column are affected by the connection mode between the wall and frame. With flexible connection, the interaction between the wall and frame is insignificant, and the lateral constraint from the wall to the frame is weakened, which reduces or eliminates the short column effect that occurs in case of rigid connection, resulting in a better seismic performance.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The research described in this paper was financially supported by the National Natural Science Foundation of China under Grant nos. 51678389 and 51408400.