1. Introduction
Infill walls are widely used in RC frames as interior partitions and exterior enclosures. The poor performance of infilled RC frames has been reported in the investigation of past earthquakes such as the Wenchuan earthquake [
1] and the Yushu earthquake [
2]. Under actual seismic action, infill walls interact with the surrounding frames and have a significant impact on the seismic performance of RC frame structures [
3]. The presence of Infill walls alter the dynamic characteristics of the structure [
4], including lateral stiffness, lateral load capacity, and natural vibration period. Over the past decades, a large amount of research on the effects of configuration [
5,
6,
7] and the openings [
5,
8,
9] of infilled walls on the seismic performance of RC frames has been conducted.
In fact, infill walls are usually arranged irregularly in a vertical direction due to different architectural functions. In a building, several floors may be divided into small rooms by infill walls, while other floors may be designed as larger spaces for parking, or conference rooms. The irregular vertical arrangement of infill walls results in nonuniform distributions of lateral stiffness. Therefore, it is important to study the effect of the vertical arrangement of infill walls on the seismic performance of RC frames. Chen et al. (2019) investigated the seismic response of vertical irregular RC frames under different sites and fortification intensities [
10]. Mondal and Tesfamariam (2013) studied the effects of the vertical irregularity and thickness of unreinforced infill walls on the robustness of RC-framed buildings [
11]. Gong et al. (2019) compared the seismic performance of pilotis with bare RC frame structures by shaking table tests [
12].
There are many kinds of blocks made of different materials. Different materials have different mechanical properties such as elastic modulus and compressive strength. It is necessary to study the effect of different masonry materials on the seismic performance of RC frames. Kakaletsis and Karayannis (2006) studied the influence of clay brick and ceramsite block infill walls on the seismic performance of RC frames [
13]. Pan et al. (2018) investigated the efficiency of various retrofitting schemes using carbon-fiber-reinforced polymers in improving the seismic performance of a masonry-infilled RC frames [
14]. Andreas (2009) studied the influence of unreinforced concrete infill walls on the seismic performance of RC frame structures [
15].
With the development of the finite element method, some refined fine finite element models were proposed for simulating the behavior of infilled frames. Unfortunately, they were not suitable for the analysis of multistorey and multibay structures because of the large amount of calculation [
16,
17,
18]. Therefore, some simplified analysis models [
9,
19,
20] were proposed. Polyakov (1956) proposed an equivalent diagonal bracing model in the analysis of the failure mechanism of the infill walls [
21]. The effective width of the equivalent brace is the key parameter, and it is difficult to determine. Holmes (1961) recommended that the effective width of the equivalent brace be one third of the diagonal of the infill wall [
3]. However, the single strut is insufficient to describe the interaction between RC frames and infill walls. Many researchers have proposed multiple strut models to simulate the wall–frame interaction effect [
22,
23,
24]. Fiore et al. (2012) simulated the complex behavior of infilled frames under lateral loads using two equivalent struts [
24]. EI-Dakhakhni (2003) presented an equivalent three struts based on the failure modes of masonry-infilled frames [
25]. Furtado et al. (2017) simulated the seismic behavior of infill walls with or without openings by using a five-strut model [
26].
In order to investigate the influence of the vertical arrangement and masonry material of infill walls on the seismic performance of RC frames, six RC frames with different arrangements of infill walls were designed. A five-strut simplified model was utilized to simulate the seismic contribution of the infill wall, and three-dimensional finite element models were built in OpenSees (v3.2.1, UC, Berkeley, Berkely, CA, USA). Then, the pushover analysis and time–history analysis were performed, and the base shear-top displacement relation, development of plastic hinges, and interstorey drift ratio are discussed to evaluate the seismic performance of the structure.
2. Design Information of Infilled RC Frames
A six-storey and three-bay RC frame structure was designed according to the Chinese seismic design codes [
27,
28]. Dead load is 5 kN/m
2 and live load is 2 kN/m
2. The fortification intensity is VII (the design acceleration is 0.1 g) and the site classification is II. The plan and elevation are shown in
Figure 1. The arrangement of beams in each storey is illustrated in
Figure 2, in which Bxn and Byn represent the beams in the two directions. The section of beams in x and y directions are 200 × 400 mm
2 and 250 × 500 mm
2, respectively. The concretes C30 and C35 are used for columns and beams, respectively. The reinforcements HRB 400 and HPB 300 are used for the longitudinal bar and stirrup, respectively. The design information of columns and beams is listed in
Table 1 and
Table 2, respectively. The stirrups in all the beams are A8@100/150. The mechanical properties of the concrete and steel bar are described in
Table 3.
Since infill walls are always arranged irregularly along the height of the building to accomplish different functions, six models with different infill wall arrangements were analyzed, as described in
Figure 3. Model M1 is the bare frame. Model M2 is the frame with infill walls on all the storeys. Model M3 is the frame without infill walls on the first two storeys. Model M4 is the frame without infill walls on the third and fourth storeys. Model M5 is the frame without infill walls on the top two storeys. Model M6 is the frame without infill walls in the middle bay.
In addition, four different masonry blocks were adopted as infill wall material to investigate the effect of material property on the seismic performance of the frame structure. The masonry blocks, named as I1, I2, I3, and I4, are fired common brick, hollow clay brick, concrete hollow block, and ceramsite concrete block, respectively. The mechanical properties of the four masonry materials are listed in
Table 4.
6. Conclusions
This manuscript investigates the seismic performance of multistorey RC frames, with special focus on the effect of vertical arrangement and masonry material of infill walls. A five-strut simplified model of infill wall is validated by the experiments. Then, through the static elastic–plastic analysis, the base shear, the interstorey drift ratio, and the plastic hinge distribution of frames with different vertical infill wall arrangements were studied. Finally, according to the nonlinear dynamic history analysis under three different earthquake ground motions, the base shear and interstorey drift ratio of structures were investigated. The following conclusions are drawn from this study:
- (1)
Pushover analysis is carried out on infilled RC frames, considering different vertical arrangements. Compared with the bare frame (M1), the regular vertical infill-arranged wall in the RC frames (M2 and M6) can improve the overall bearing capacity and stiffness of the structure greatly and decrease the interstorey drift ratio more than 57%. Additionally, the number of plastic hinges of M2 decreases 24% when the maximum interstorey drift ratio is 2%. For the frame with an absence of infill walls at the first two storeys (M3), the lateral capacity is 80% lower than that of M2. As the storeys with an absence of infill walls move up, the lateral capacity of the structure becomes increasingly larger and the interstorey drift ratio is decreased. The damage of the pilotis frame with weak ground storey is more serious than that of frames with an absence of infill wall at other storeys. The interstorey drift ratio of M5 is about 16% larger than that of M1. Therefore, this type of irregular vertical infill arrangement should be limited in the design process.
- (2)
Dynamic time–history analysis was then performed on RC frames with different infill wall materials. The vertical irregularly arranged infill walls lead to the discontinuity of stiffness. The analytical results show that the largest interstorey drift ratio occurs in storeys with an absence of infill walls. For the frames with vertically irregular arrangement, the base storey with an absence of infill walls does the greatest harm to the structures, the interstorey drift ratio of which is the largest (0.95%). The base shear of M2 is the largest among all structures, which is at least three times larger than that of M1. The absence of infill walls at the base storey decreases the base shear by 56–70% compared with M2.
- (3)
The base shear increases with the increase in the elastic modulus of the infill wall material. For the structures with vertical regularly arranged infill walls, the larger elastic modulus of masonry material can decrease the interstorey drift ratio of the structures. On the contrary, for the models with vertical irregularly arranged infill walls, the larger elastic modulus of masonry material can increase the interstorey drift ratio. Therefore, the disadvantages induced by the elastic modulus of masonry material, in the frames with vertical irregularly arranged infill, should be considered in the seismic design and assessment.