numerical study on seismic behaviour of reinforced
TRANSCRIPT
https://aip.scitation.org/toc/apc/1977/1?expanded=1977
https://aip.scitation.org/doi/10.1063/1.5042918
Numerical study on seismic behaviour of reinforced concrete structures with steelbrace and infill wallIda Ayu Made Budiwati, Made Sukrawa, and Ida Wahyuni
Citation: AIP Conference Proceedings 1977, 040027 (2018); doi: 10.1063/1.5042997View online: https://doi.org/10.1063/1.5042997View Table of Contents: http://aip.scitation.org/toc/apc/1977/1Published by the American Institute of Physics
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Numerical Study on Seismic Behaviour of Reinforced Concrete Structures with Steel Brace and Infill Wall
Ida Ayu Made Budiwati1, a), Made Sukrawa1, b), and Ida Wahyuni2
1Lecturer at Department of Civil Engineering, Faculty of Engineering, Universitas Udayana, Bali, Indonesia 2Student at Department of Civil Engineering, Faculty of Engineering, Universitas Udayana, Bali, Indonesia
a)Corresponding author: [email protected]
Abstract. The behaviour of the structure of reinforced concrete (RC) due to seismic load was studied numerically where RC structures of 2, 3, and 4 storey were modelled to reveal their seismic behaviour. Nonlinear static pushover analysis was also carried out to determine its performance. The structures were strengthened using steel frames, steel braces or infill walls with concentric openings. RC open frame structure (MF) was firstly designed in accordance with Indonesian Code, SNI 2847:2013. The MF model was then strengthened, particularly in the middle span, either with steel frame on surrounding beams and columns (MFF) or reinforced with X, A, or V steel bracing (BFX, BFA, and BFV). The bracing model was further added with steel frames (FBFX, FBFA, and FBFV). The MF was also reinforced with infill walls with concentric opening of 20%, 40%, and 60% (IF20, IF40, and IF60). From the results of the analyses, it can be concluded that the addition of bracing with and without steel frames and infill walls with concentric opening can stiffen the structures. Steel framed brace model was the most rigid one. The level of performance of the reinforced MF model using steel bracing with and without steel frames increased from Immediate Occupancy (IO) to Operational Level (B), but the level decreased into Collapse Prevention (CP) for the model with infill walls. The performance of RC reinforced with steel frames remained at Immediate Occupancy (IO) level.
INTRODUCTION
Infilled frame is a structure consisting of columns and beams made of reinforced concrete or steel with an infill wall inside of it. Reinforced concrete (RC) frame is the most widely used structural system in high-rise buildings especially in areas with relatively inexpensive labour wages. Due to the rigid monolithic connection between beam and column, the structure system is rigid enough to hold both gravity and lateral loads in case there is an earthquake. In low to medium-rise building, RC frame is usually used as a system to hold vertical and lateral loads. Whereas in high-rise building, RC frame is more effectively used to carry vertical load, thus lateral load is retained by lateral load-bearing systems, such as shear wall, braced frame, infilled frame, and other systems that have a greater stiffness than reinforced concrete moment frame system.
The use of steel bracing enhances the strength and stiffness of the structural system, and does not have a significant effect on the weight of the structure [1]. Steel bracing types of X, A, and V are widely used for seismic retrofitting on RC frame due to the ease of their application in compared with the use of reinforced concrete bracing. In addition, the use of steel bracing among others, provides space that can be used for the placement of doors and windows thus to enable the light on the braced frame.
Research conducted on infilled frame also shows similar performance. The infilled frame with opening wall appears to have a relatively large stiffness in compared with the moment frames [2,3]. In this study, the two lateral load-bearing systems, namely steel braced frame and infilled frame with concentric openings were analysed and compared, and their performances in resisting lateral loads from earthquake were also studied. The comparison of lateral stiffness and performance of the structure between moment frames, moment frame with steel frame, frame
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with steel bracing X, A, and V with and without steel frame, and infilled frames with concentric openings of 20%, 40% and 60% were determined.
MATERIALS AND METHODS
Retrofit is intended for strengthening the structure of a building or improving an existing structure, and also used in structural renovation. The addition of shear wall or steel brace is the most widely used retrofit method, since the methods are more effective and less costly than enlarging column and beam dimensions.
Based on research conducted by [4], there is a significant difference between moment frame (open frame) and the frame structure with reinforcement. Retrofitting of structures, such as by adding steel and wall bracing, can increase the strength and stiffness of the structures and reduce the size of the displacement, and decrease the ductility of the structure. Reinforcement with solid walls is the most rigid one. Structural design with retrofitting of steel bracing and infill wall are applied to hold most of the lateral loads in structures that will reduce the internal forces in beams and columns.
Bracing is a very effective and economical system used to withstand the lateral loads. Generally, the type of bracing used is concentric bracing because its implementation is easier and cheaper in compared with an eccentric bracing. Studies and researches related to the use of concentric steel bracing with and without frames as retrofitting in reinforced concrete frame have demonstrated that the structure with added steel bracing either with or without frame is able to increase the stiffness and strength of the structure itself [5-7].
The addition of wall panels on the structure of a reinforced concrete moment frame is widely accepted to significantly improve the stiffness and strength of surrounding frame structure [8-10]. The test results show that the infilled RC frames with openings are still much more rigid and stronger than the moment frame structure [2-3, 8, 10]. Infilled RC frames can be modelled with shell elements and strut diagonal in finite element analysis. Gap element is used to connect walls and RC frame when the walls are modelled using shell elements. The stiffness of the gap element is determined using equation develop by [11]. The width of the strut (Wds) for modelling infill wall as a diagonal strut can be calculated using equation (1) proposed by [12].
cdWds .tan4
(1)
05,109.220.1 2 rrc (2)
Wds is the width of the strut (mm), d is the diagonal length (mm), is the diagonal strut angle, tan is the ratio
of height and length of the wall (H⁄L), c is the coefficient of stiffness of wall, and r is the opening ratio. This strut width equation is applied only to infilled RC frames with concentric openings (r) of 10% to 60% and of 33° to 51. This equation is a modification of the equation previously proposed by [13]. The elastic modulus of the concrete material used is in accordance to SNI 2847: 2013 [13]. To calculate the elasticity modulus value of the wall material, a formula available at [14] is used.
In this study, the structural performance was analysed by using nonlinear static pushover analysis. Pushover is a static-nonlinear analysis method in which the structure is given gravitational load and monotonic lateral load pattern that continues to increase until the final condition is reached. The performance level of each RC frame model in SAP 2000 software application is shown with different colours. Operational condition (B) is shown in purple. Other performance levels namely Immediate Occupancy (IO), Life Safety (LS), Collapse Prevention (CP) to Collapse (C, D, E) are shown in dark blue, light blue, light green, yellow, orange, and red, respectively. According to FEMA [15,16], the buildings performance level during earthquake is in condition B means that the building can be used because it does not pose a risk to the life safety, IO means the building can be reoccupied because it does not suffer structural damage, LS means the building needs repairs before re-using it because of extensive damage, CP means the building may endanger the safety because of non-structural component failure even though the building is still standing, and C, D, E means the building has already collapsed.
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Research Method
In this research, RC frames of 2, 3, and 4 storey (MF model) were firstly designed to fulfill the requirement of SNI 2847:2013 as a special moment resisting frame. Based on MF model, other ten (10) models were developed namely MF model with steel frame surrounding beams and columns (MFF), MF model with steel braced of X, A, and V types (BFX, BFA, and BFV), MFF model with steel braced of X, A, and V types (FBFX, FBFA, and FBFV), and MF model with concentric infill wall openings of 20%, 40%, and 60% (IF20, IF40, and IF60). All models were applied in 2, 3, and 4 storey and named as shown in Table 1.
TABLE 1. Name of the models Storey MF MFF Bracing (BF) Framed Brace (FBF) Infill Frame (IF)
X A V X A V 20% 40% 60% 2 MF2 MFF2 BFX2 BFX2 BFX2 FBFX2 FBFA2 FBFV2 IF220 IF240 IF260 3 MF3 MFF3 BFX3 BFX3 BFX3 FBFX3 FBFA3 FBFV3 IF320 IF340 IF340 4 MF4 MFF4 BFX4 BFX4 BFX4 FBFX4 FBFA4 FBFV4 IF420 IF460 IF460 All RC frames were modelled in 2D that consisted of three spans with length of 6000-mm and height of 3500-
mm as shown in Figure 1. The addition of bracing was applied in the middle span.
FIGURE 1. RC Structures of 2, 3, and 4 storey.
The RC frame was designed using concrete compressive strength (f'c), modulus of elasticity (Ec), steel yield
stress (fy), and steel ultimate stress (fu) of 25 MPa, 23500 MPa, 240 MPa, and 370 MPa, respectively. For steel bracing, material used was ST-37. The material properties used to model the practical column were 10 MPa and 14862 MPa for compressive strength value (f't) and modulus of elasticity (Et), respectively. For infill wall, the materials used were 1650 MPa and 9702.5 N/mm/mm as modulus elasticity (E'm) and stiffness of gap element, respectively. A panel showing all types of bracing involved in this study is shown in Fig. 2.
(a) Steel frame. (b) Steel brace.
(c) Steel framed brace.
(d) Infill wall.
FIGURE 2. A Panel with various types of bracing. All models were subjected to dead, live, and earthquake loads without including load factors. Dead loads from
the weight of the structure were calculated automatically in the SAP 2000 program [17], while the additional dead loads were calculated based on PPIUG 1983 [18]. Live load was determined based on the function of the building
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that is an office building listed in SNI 1727: 2013. Earthquake loads were applied using the Auto Lateral Load arrangement of IBC 2009 in accordance with SNI 1726: 2012 [19], seismic design category D. The buildings were located in Denpasar City with the condition of medium soil.
The RC frame 2, 3, and 4 storey models used the dimension of 250/400 mm for beams and tie beams, and IWF 200x200x8x12 mm for all steel brace and steel frame on each level. The column dimensions of each level can be seen in Table 2.
All the RC frame models were analysed using SAP 2000 program and the displacement results were compared. Beams, columns, steel frame, and steel brace were modelled using frame elements while the infill wall was modelled using shell element. The nonlinear static pushover analysis was then performed to obtain the structural performance, i.e., mechanisms for the occurrence of plastic hinge, initial yield, performance levels, and base shear forces as presented in the form of capacity curves.
TABLE 2. Columns dimension of the models StoreyLevel
Column (mm) Column (mm) Column (mm)
1 300 x 300 400 x 400 400 x 4002 300 x 300 350 x 350 400 x 4003 300 x 300 350 x 3504 300 x 300
RESULTS AND DISCUSSION
Displacement and Drift Ratio
Displacement results for all models are shown in Figure 3. From the figures, it can be observed that all structures were more rigid in compared to open frame structure (MF). RC frames with steel brace (BF and FBF) were the stiffest. Steel frame models were less stiff than Infilled frame structures. The increase in stiffness was 142% -143%, 182% -193%, 186% -194%, and 139% -179% for MFF, BF, FBF, and IF models, respectively. The drift ratio of all RC frame models have met the requirements of SNI 1726: 2012, which is not greater than 2%.
0
1
2
0 5 10 15
Stor
ey
Displacement (mm)
MF2 MFF2 BFV2BFA2 BFX2 FBFV2FBFA2 FBFX2 IF220IF240 IF260
(a) 2 Storey RC frame. (b) 3 Storey RC frame.
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0
1
2
3
4
0 10 20 30St
orey
Displacement (mm)
MF4 MFF4 BFV4BFA4 BFX4 FBFV4FBFA4 FBFX4 IF420IF440 IF460
(c) 4 Storey RC frame.
FIGURE 3. The displacements for all models.
Nonlinear Static Pushover Analysis
Results from the pushover analysis can be seen in Tables 3-5. Initial yield point, performance point, and ultimate point for each model are given in the table. In overall, the RC frame model with bracing can improve the ability of the structure to resist the base shear forces that occur at the performance point and the ultimate point.
For steel frames models (MFF), the base shear forces at the performance point and ultimate point increased by 1.0-1.25 times in compared to the MF model. For steel brace models (MBF), the performance point increased in the range of 2.82-3.77 times, similar to the results of steel frames brace models (FBF) which were of 2.54-3.59 times. Meanwhile, for the infill models (MIF), the values were in the range of 1.93-2.52 times.
At ultimate point, the increase of base shear forces was higher in compared to those at performance point. For steel brace models (MBF), the ultimate point increased in the range of 4.37-10.39 times in compared to the MF models. For the steel frames brace models (FBF), the increase values were of 4.70-10.72 times. However, for the infill models (MIF), the values were only 2.66-7.06 times.
The analysis results revealed that the increases in either at performance point or ultimate point were demonstrated by steel frames models, infill wall models, steel brace models, and the highest by steel frames bracemodels. At performance point steel brace, type A obtained the highest while at the ultimate point, type X showed the highest. For infill wall, it showed that the wider, the opening the smaller the values.
From Tables 3-5, it can be seen that performance level of 2-4 storey RC frame (MF) is shown as an Immediate Occupancy (IO). By strengthening it using steel frame (MFF), the performance was remained IO. But once the RC frame was braced using either steel brace or steel frames brace, their performances became B (Operational Level), better than MFF. However, RF frames with infill wall (IF) did not show good performance. The performance of IF model decreased into Collapse Prevention (CP).
TABLE 3. Pushover Analysis Results of 2-storey Models Model of
RC FrameInitial Yielding point Performance Point Ultimate point Performance
levelVy (kN) Δy (mm) Vf (kN) Δf (mm) Vu (kN) Δu (mm)MF2 195 22 260 71 273 115 IOMFF2 106 3 325 59 329 60 IOBFV2 781 7 727 7 2006 24 BBFA2 1453 10 740 5 1615 15 BBFX2 1357 9 735 5 2837 28 BFBFV2 338 2 661 5 2079 22 BFBFA2 298 2 687 4 1745 15 BFBFX2 457 2 689 4 2927 27 BIF220 492 6 655 10 1927 90 CPIF240 316 6 557 16 1310 92 CPIF260 202 6 545 28 901 95 CP
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TABLE 4. Pushover Analysis Results of 3-storey Models
Model of RC Frame
Initial Yielding point Performance Point Ultimate point Performance levelVy (kN) Δy (mm) Vf (kN) Δf (mm) Vu
(kN)Δu (mm)
MF3 268 30 368 84 391 170 IOMFF3 156 7 395 72 404 91 IOBFV3 825 10 1045 13 2085 32 BBFA3 1236 12 1116 11 1753 21 BBFX3 1737 16 1104 10 3027 36 BFBFV3 444 4 980 11 2212 30 BFBFA3 387 3 1081 9 1837 20 BFBFX3 570 4 1003 8 3044 36 BIF320 570 9 880 20 1827 109 CPIF340 392 7 723 29 1309 124 CPIF360 275 10 710 47 1089 151 CP
TABLE 5. Pushover Analysis Results of 4-storey Models Model of
RC FrameInitial Yielding point Performance Point Ultimate point Performance
levelVy (kN) Δy (mm) Vf (kN) Δf (mm) Vu (kN) Δu (mm)MF4 295 43 389 101 413 194 IOMFF4 194 12 485 86 496 108 IOBFV4 1413 25 1433 26 2181 47 BBFA4 1518 21 1470 20 1806 28 BBFX4 1830 24 1450 19 3020 49 BFBFV4 758 11 1340 21 2326 45 BFBFA4 891 10 1398 16 2044 30 BFBFX4 945 11 1391 16 3115 42 BIF420 543 12 1105 40 2101 165 CPIF440 373 12 975 61 1492 167 CPIF460 260 12 787 76 1099 186 CP
Pushover Curves
A graph resulted from pushover analysis was created in simple form to ease the explanation of the base shear force. Figure 4 shows that the stiffness of all models initially increases in compared to bare frame model (MF). A small increase is shown from the model with added steel frame (MFF) in compared to the original brace frame (MF). For the 2-storey RC frame, the increases were 3%, 4%, and 8% for FBFX2, FBFV2 and FBFA2 models in compared to the BFX2, BFV2, and BFA2 model, respectively. For the 3-storey RC frame model, the increases were 1%, 6%, and 5% in FBFX3, FBFV3 and FBFA3 models in compared to BFX3, BFV3 and BFA3 model,respectively. Meanwhile, for the 4-storey RC frame models, the increases were 3%, 7%, and 13% on FBFX4, FBFV4, and FBFA4 model, respectively, in compared to BFX4, BFV4, and BFA4 model.
(a) The 2-Storey RC frame Models. (b) The 3-Storey RC frame Models.
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(c) The 4-Storey RC frame Models.
FIGURE 4. The pushover curves for all models.
Figure 4 also shows that the MFF2, BF2, FBF2, and IF2 model can increase ultimate shear force by 17%, 83% -90%, 84% -91%, and 70-86%, in compared to the MF2. MFF3, BF3, FBF3, and IF3 model that can improve ultimate shear forces by 3%, 78% -87%, 79%-87%, and 64%-79%, in compared to the MF3. MFF4, BF4, FBF4, and IF4 model that can improve ultimate shear forces by 17%, 77% -86%, 80% -87% and 62% -80% in compared to the MF4 model.
For models with infill wall, the base shear force increased by 80%, 72%, and 62% for IF420, IF440, and IF460 model, respectively, in compared to MF4 Model. For the 3-storey models, the increases were 79%, 70%, and 64% while for the 2-storey, they were 86%, 79%, and 70%.
Based on the pushover curves, it can be recognized that the lowest performance was shown by open frame models (MF). By strengthening the structures using infill wall, steel frames, steel brace or steel frames brace, the performance level can increase. The lowest increase is shown by steel frames models (FF) and the highest is shown by the steel frames brace (FBF), and in between are shown by the infill wall (IF) and steel brace (BF).
Compared with [4], the performance results of all models are similar, except that FBFA is lower than IF20. Research conducted by [4] found that steel frames brace models withstand greater base shear force than the infilled RC frame models.
CONCLUSIONS
Based on the results from the numerical analyses, the following conclusions can be drawn. RC frame structures with reinforcement using framed and ordinary steel braces X, A, and V type and infilledframes (IF) with concentric openings of 20%, 40%, and 60% show more rigid structure, produce smallerdrift and withstand larger base shear force.The stiffness of RC frame strengthened with steel frames (MFF) increases by 50% in compared with openframe (MF).The stiffness of RC frame strengthened with steel brace (BF) or steel frames brace (FBF) increases by 90%in compared with open frame (MF).The stiffness of RC frame strengthened with infill wall (IF) increased by 75% in compared with open frame(MF). However, insignificant increase is shown for higher opening ratio.Base shear force for all models increases in compared with open frame models (MF). The increase by 10times is shown by FBFX model, which obtained the highest value.Pushover analysis shows that the performance level of steel brace models improves from ImmediateOccupancy (IO) to Operational level, while it decreases into Collapse Prevention (CP) for infill models.Models with steel frames (MFF) are still at IO level.
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ACKNOWLEDGMENTS
The authors would like to thank Engineering Faculty of Universitas Udayana for the funding support of the research project grant in 2017.
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