SEISMIC PERFORMANCE OF RC BUILDINGS WITH SHEAR WALL
M. Danish1, Zaid M.2, M. Shariq3, A. Masood4& A. Baqi5
M. Tech. Student, Aligarh Muslim University, Aligarh, India, Email:
[email protected] 2 M. Tech. Student, Aligarh Muslim University, Aligarh, India, Email:
[email protected] 3 Asst. Professor, CES, University Polytechnic, AMU, Aligarh, India, Email:
[email protected] 4 Professor, Aligarh Muslim University, Aligarh, India, Email:
[email protected] 5 Professor, Aligarh Muslim University, Aligarh, India, Email:
[email protected] 1
ABSTRACT
An earthquake force is a very strange force and behaves quite differently than Gravity and Wind loads, striking the weakest spot in the whole three dimensional structure. kills, in fact ignorance in design and poor quality construction results in many weaknesses in the structure that cause serious damage to life and property. Masonry Infill are frequently used to fill the gap between the vertical and horizontal resisting elements of the building frames with the assumption that these infills will not take part in resisting any kind of load either axial or lateral. Hence, its significance in the analysis is generally neglected by the designer. In fact, infill wall and shear wall considerably enhance the rigidity and strength of the frame structure. Various researches suggest that the bare frame has comparatively lesser stiffness and strength than the infill frame and frame with shear wall, therefore their ignorance cause failure of many multistorey buildings when subjected to seismic loads. In the present study, the finite element analysis of RC frame models viz. a bare frame; a frame with shear wall considering infill; a bare frame with shear wall has been carried out and the number of storeys vary as G+3, G+5, G+7 and G+9. Linear analysis of all RC frame structures has been performed as per IS: 1893 (Part 1) - 2002 and IS: 456 - 2000. In this study only in-plane stiffness of masonry wall has been considered and infill panels modelled as equivalent diagonal strut elements. The behaviour of buildings subjected to Gravity and Seismic loads with the help of Response Spectrum Analysis using FEM based software and the effect on Time Period, Mass Participation factor, and Storey Drift has been observed. Strength and Rigidity of RC bare frame structures is found increasing after the inclusion of infill panels and shear wall. Keywords: Seismic analysis, Shear Wall, Equivalent diagonal strut, Response Spectrum, SRSS
INTRODUCTION RC buildings often have vertical plate-like RC walls called Shear Walls (or structural walls) in addition to slabs, beams and columns. These walls start at foundation level and continuous throughout the building height having thickness as low as 150 mm, or as high as 400 mm. Shear walls are provided along length and width of a building (Fig 1) like vertically-oriented wide beams that carry earthquake loads downward to the foundation. Properly designed buildings with shear walls have shown very good performance in past earthquakes however they require special detailing in high seismic risk zones. Shear wall is a popular choice in many earthquake-prone countries like Chile, New Zealand and USA because they are easy to construct and reinforcement detailing is relatively straight-forward and therefore can easily be implemented at site. Moreover these walls are efficient, both in terms of construction cost and in minimizing earthquake damage in structural and non-structural elements (like glass windows and building contents). But in order to get maximum advantage, it must be symmetrically located in plan to reduce ill-effects of twist in buildings (Fig 1). It should be placed symmetrically along one or both directions in plan and prove more effective when located along exterior perimeter of the building because such a layout increases resistance of the building to twisting. Due to the fact that shear wall is a very essential component providing sufficient lateral strength and preventing severe damage to life and property but inadequate detailing, poor quality material and substandard construction could lead to its failure (Fig.2) which may be fatal to both life and property. LITERATURE REVIEW Smith (1962, 1966) used an elastic theory to propose the effective width of the equivalent strut and concluded that this width should be a function of the stiffness of the infill with respect to that of bounding frame. By analogy to a beam on elastic foundation, he defined the dimensionless relative parameters to determine the degree of frame-infill interaction and thereby, the effective width of the strut. Singh (1995) found in his research that in the dynamic analysis of a complete building system, the inclusion of the effect of infill is essential for a realistic prediction of its behaviour and concluded further that there is very limited literature available on dynamic response of 3-D infilled reinforcement concrete frames. Bell and Davidson (2001) found that a review of international research and guidelines indicate that infill panels, where present in a regular arrangement, have a significant beneficial influence on the behaviour of RC buildings. These contrasts with New Zealand guidelines, which can give an impression that infill masonry panels have a detrimental influence on the behaviour of buildings due to soft storey effects. The reviewed sources indicate that due to stiffness, strength, and damping effects of infill panels, deformations are below that required for a soft storey mechanism. Das and Murty (2004) carried out non-linear pushover analysis on five RC frame buildings with brick masonry in-fills, designed for the
Fig. 1 RC Shear Walls built at corner in a building frame (left), Shear Walls must be Symmetric in Plan layout (right)
Fig 2 Shear wall ripped off due to earthquake same seismic hazard as per Euro-code, Nepal Building Code and Indian Code and the equivalent braced frame method given in literature. Infills are found to increase the strength and stiffness of the structure, and reduce the drift capacity and structural damage and also reduce the overall structure ductility, but increase the overall strength. Building designed by the equivalent braced frame method show better performance. Amato et al (2008) discussed the mechanical behaviour of single storey-single bay infilled frames and generalized analytical procedures available in the literature for the identification of a pin-jointed strut equivalent to the infill to take the influence of vertical loads into account. Detailed numerical investigation on infilled meshes has proved that in the presence of vertical loads it is possible that a strong correlation between the dimension of the equivalent diagonal strut model and a single parameter, which depends on the characteristics of the system. Baran and Sevil (2010) have found through various analytical and experimental studies that hollow brick infills could not only increase both strength and stiffness of RC frames but also adequately be modelled by diagonal compression struts. Asteris et al (2011) conducted quasi-static experiments on frames with masonry infill panels with openings that reveal important insights regarding the global as well as the local response of the tested infill frames. In particular, the experimental
results indicate that the failure modes of the infilled frames classified into distinct modes. Such a classification of the failure modes (crack patterns) enhances considerably the understanding of the earthquake resistant behaviour of infilled frames and leads to improved comprehension of their modelling, analysis and design. Mohan and Prabha (2011) concluded that Equivalent Static Method can be used effectively for symmetric buildings up to 25m height. For higher and unsymmetrical buildings, response spectrum method shall be used. However for important structures, time history analysis shall be performed as it predicts the structural response more accurately in comparison with other two methods since it incorporates P-linearity, which is true in real structures. Danish et al (2013) has found that structures with shear walls perform better than both infilled and bare frames under seismic loads. Therefore, we can conclude that the presence of infill and shear wall influence the behaviour of moment resisting frame and the characteristic configuration of the infill panels can alter the predominant mode of structural action particularly when the frame is subjected to lateral loads. METHODOLOGY In the present study, the finite element analysis of RC frame models viz. a bare frame; with shear wall considering infill; a bare frame with shear wall has been carried out and the number of storeys varied from G+3 to G+9. Seismic analysis of all RC frame structures performed as per IS: 1893 (Part 1) - 2002 and IS: 456 2000 using response spectrum method (SRSS). Masonry walls are modelled as equivalent diagonal struts while considering only in-plane stiffness. Shear walls provided symmetrically at the corners of the building (Fig. 3) and start at ground level continuous throughout the top floor and considered to be of uniform thickness of 250 mm. The plan (Fig. 4) of the RC frame structure is 14.4 m wide and 24.4 m long measured along the central line of the columns fixed at ground level and storey heights are taken to be 3.35 m each with solid RCC slab of 110 mm thick carrying dead and live loads. Section details for beams, columns and thickness of infills are given in Table 1. Infills without openings are considered in the analysis and its width of equivalent diagonal strut is calculated (Table 2) using where,
.
Em= Elastic Modulus of masonry wall Ef = Elastic Modulus of masonry of frame material t = Thickness of the infill wall h = Height of the infill wall L = Length of the infill wall Ic = Moment of Inertia of the column of the frame Ib = Moment of Inertia of the beam of the frame
(h/L) w = Width of the Equivalent Strut -1
Fig 3 Shear wall located symmetrically at Fig. 4 Plan of a building also showing all corners of building presence of infills (highlighted as red) Table 1 Section details (*along z-direction; **along x-direction) MEMBER SIZE (mm) Beams (Transverse*) 500 × 300 Roof Beams (Longitudinal**) 300 × 300 Corridor Beams(Longitudinal) 300 × 300 External Beams (Longitudinal) 350 × 300 Columns 600 × 500 External Walls 250 Internal Walls 150 Slab 110 Table 2 Width of equivalent diagonal strut for both external and internal masonry infill H (m) L (m) w (m) h l 3.35 6.2 1.51 2.7 1.55 3.35 6.2 1.71 3.07 1.76 3.35 6.0 1.51 2.67 1.54 3.35 6.0 1.71 3.03 1.74 3.35 2.2 1.47 2.04 1.28 3.35 2.2 1.44 1.66 1.10 3.35 3.05 1.64 1.68 1.20 1.675 3.05 1.27 2.40 1.00 Table 3 Nomenclature of building Type of frame Bare Frame Frame with shear walls at corners Frame with infills and concrete shear wall at corners
Nomenclature A B C
RESULTS AND DISSCUSSION We have considered two cases firstly, the effect of shear wall and infill on bare frame; secondly, the effect of increasing height of building and then made a comparison of dynamic characteristics i.e. time period, modal mass participation factor (%) and storey drift. 1.
Time Period In first case, the inclusion of shear wall in bare frame reduces time period of vibration to almost 33 % in G+3, 35% in G+5, 37% in G+7 and 40% in G+9. Further, when masonry infills become part of the frame then combine effect of both shear wall and infills comes into picture, thus time period reduced to minimum (see frame C in Table 4). In second case, increment in height causes increment in time period for each frame configuration (i.e. frames A, B, C and D). So we have found that the inclusion of infills and shear wall make a building frame more rigid but the increment in height makes it more vulnerable to vibration.
Table 4 Time periods obtained from dynamic analysis Time Time Period (sec) Frame Period Frame Building Building (sec) type type Mode 1 Mode 2 Mode 3 Mode 1 Mode 2 Mode 3 A 0.388 0.319 0.12 A 0.605 0.496 0.191 G+3 B 0.129 0.077 0.055 G+5 B 0.211 0.129 0.074 C 0.12 0.106 0.063 C 0.162 0.093 0.60 A 0.829 0.679 0.264 A 1.061 0.864 0.340 G+7 B 0.310 0.193 0.095 G+9 B 0.420 0.266 0.120 C 0.231 0.125 0.078 C 0.312 0.160 0.097 2.
3.
Modal Mass Participation factor Firstly, for different frame configurations (i.e. frames A, B, C and D), the mass participation factors for the 1st mode gets reduced (Table 5). Similar trend is followed by increasing the number of storeys. Storey Drift [Plot of height v/s storey drift (Fig. 5) for each frame configurations given in Table 3] best represent the behaviour of structure under seismic loads. In first case, maximum storey drift reduced to about 30% when shear walls are introduced in a bare frame and there is further reduction in storey drift to almost 18%, when the effect of both infill and shear wall is considered. Thus bare frame is more vulnerable to lateral movement under seismic event compared to a frame with infills and shear wall.
Building G+3 G+7
Frame type A B C A B C
Table 5 Modal Mass Participation factor Mass Participation factor Mass Participation factor Frame (%) (%) Building type Mode 1 Mode 2 Mode 3 Mode 1 Mode 2 Mode 3 A 81.96 0.121 11.51 80.50 0.14 11.02 G+5 77.18 0.00 12.55 B 74.26 0.00 15.56 C 0.16 81.72 0.00 79.33 0.06 12.34 79.70 0.15 10.95 A 79.15 0.14 11.03 72.28 0.00 17.06 G+9 B 71.11 0.00 17.51 76.66 0.22 14.48 C 74.30 0.40 16.14
Fig. 5 Height v/s Storey Drift for frames G+3 to G+9 In second case however, increasing number of storeys result in increasing lateral movement causing more storey drift and therefore we have found that high rise buildings with no shear wall are vulnerable to collapse under seismic loads and dangerous to both life and property.
CONCLUSIONS The present study leads to the following conclusions: Infill and shear walls considerably enhance the rigidity and strength of the frame structure therefore, neglecting them in analysis & design of structure will lead to failure due to stiffness irregularity. Symmetry in position of shear wall in plan is a key factor to obtain desirable performance of shear wall structure. Increment in number of storeys make the building frame more vulnerable and therefore shear wall becomes a necessity in high rise buildings to save damage due to earthquake. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Smith B S (1966), The Composite Behaviour of Infilled Frames In Proceedings of a Symposium on Tall Buildings with Particular Reference to Shear Wall Structures, University of Southampton, Department of Civil Engineering. Oxford Pergamon Press. Smith B S (1962), Lateral stiffness of infilled frames , Journal of Structural division, ASCE, 88 (ST6), 183-199. Smith B S (1966), Behaviour of square infilled frames , Journal of Structural division, ASCE, 92 (ST1), 381-403. Singh Excit Technology. Bell D K and Davidson nfill . Das D and Murty k Masonry Infills in Seismic Design of RC Frame Buildings: Part 2Amato G, Cavaleri L, Fossetti M, and Papia Load on the Engineering. Baran M. and Sevil T. (2011), Analytical and experimental studies on infilled RC frames , International Journal of the Physical Sciences, 5(13), 1981-1998. Asteris P G, Kakaletsis D J, Chrysostomou C.Z. and Smyrou E.E. (2011), Failure Modes of Infilled Frames , Electronic Journal of Structural Engineering 11(1). Mohan R and Prabha C (2011), Dynamic Analysis of RCC Buildings with Shear Wall , International Journal of Earth Sciences and Engineering, ISSN 0974-5904, 04(06), 659-662. Danish M., Shoeb, Shariq M. and Masood A. (2013) Seismic Performance of Masonry Infills - Innovation in Concrete construction, 2169-2191. IS 1893 (Part 1) (2002), Indian Standard: Criteria for Earthquake Resistance Design of Structures , New Delhi. IS 456 (2000), Indian Standard: Plain and Reinforced Concrete- Code of Practice , New Delhi.
