Effect of soft storey in a structure present in higher seismic zone areas

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1 Effect of soft storey in a structure present in higher seismic zone areas by Neelima Patnala, Pradeep Kumar Ramancharla in Urban Safety of Mega Cities in Asia 2014 (USMCA 2014) Myanmar, Burma Report No: IIIT/TR/2014/-1 Centre for Earthquake Engineering International Institute of Information Technology Hyderabad , INDIA November 2014

2 Effect of soft storey in a structure present in higher seismic zone areas Neelima V. S. PATNALA 1 and Pradeep K. RAMANCHARLA 2 1 MS by Research Scholar, Earthquake Engineering Research Centre, International Institute of Information Technology, Hyderabad, India patnala.neelima@research.iiit.ac.in 2 Professor, Earthquake Engineering Research Centre, International Institute of Information Technology, Hyderabad, India ABSTRACT The use of unreinforced brick masonry as infill material in reinforced concrete frames is inevitable even in higher seismic zone areas. With increasing demands for architectural features in buildings, the provision of soft storey is a common practice in many multi storeyed structures throughout the world, for parking. The standard code of practice in many countries suggests higher design forces for the columns present in the soft storey. In order to understand the affect of increase in design seismic forces on the columns in a structure present in higher seismic zone areas, a study is carried out by taking pushover analysis as a tool for obtaining capacity of the structure. A comparative study between three types of arrangements; type I: structure with infill walls in all floors, type II: structure with open ground storey, type III: structure with open ground storey and columns designed for increased forces. It is observed that there is an increase in maximum load carrying capacity for the type III structure as compared to type II structure with no considerable change in behaviour of the two types of structures. It can be concluded that the increase in design forces of the columns at open ground storey may not lead to the safety as of structure type I. Keywords: Unreinforced brick masonry (URM), multi storeyed structures, higher seismic zone areas, soft storey, pushover analysis 1. INTRODUCTION Multi storeyed buildings gained huge popularity with the reliable applicability of reinforced concrete frames infilled with masonry walls. Out of all the kinds of masonry units used for constructing infilled walls, brick masonry is one of the oldest materials used. Due to easy construction and low cost, brick masonry is used as infill material even till date in many parts of the world. Though the presence of infill is inevitable, it is considered as a non structural element according the standard codes of practice in many countries. This leads to improper analysis of the seismic behaviour of a building as a whole. Because of increase in demand to aesthetic appearance of a building, the civil engineering details of a building have lost their importance. Open ground storey is one such detail which is mostly prevalent in the present day multi-storeyed buildings throughout the world. The presence of open ground storey in a building leads to soft

3 November 2014, Yangon, Myanmar storey effect. Though standard codes in some countries like India, Europe, Japan, etc., provide special clauses for the modeling the soft storey. The seismic behavior of the building with soft storey is not completely understood. Hence there is a need to verify the provisions given in different standard codes for the safety of the building with soft storey. Significant research has been carried out in understanding the effect of soft storey on seismic performance of brick infilled reinforced concrete frames. In a similar study, Murty et al. (2000), conducted experiments on twelve single bay single storey RC frames of full scale and scaled models subjected to reverse cyclic displacement controlled loading. The main conclusions of this work were that the average initial stiffness, strength and ductility of infilled frames is 4.3 times, 70% and 4 times more than the bare frame, respectively. In order to improve the performance of soft storey buildings, Dande et al. (2013) conducted an analytical study on 12 storey symmetric building with strut modeling for infill walls. A comparative study is carried out by performing linear response spectrum analysis between the two types of retrofitting techniques; 1) by providing stiffer columns at ground floor and 2) the other by providing infill walls at the corners. It is observed that the provision of stiffer columns in the ground floor may not reduce the displacement and force demands at the open ground storey but can be reduced to some extent if infill walls are provided at the ground floor. This paper aims at understanding the seismic behaviour of building with soft storey and tries to verify the clauses given in Indian standard code for the safety of building. A case study is carried out by performing pushover analysis on a commercial building. The comparison is made between the structure with and without soft storey and interpretations are derived. 2. NUMERICAL MODELING OF SOFT STOREY The architectural plan and the grid line diagram of the structure are shown in Figure 1. The structure is assumed to be present in zone IV area with high expected seismic hazard. The geometric details of the structure considered are given in the Table 1. The structure with soft storey is designed for gravity loads and seismic loads, according to the factors applicable for zone IV area as given in IS and IS New Technologies for Urban Safety of Mega Cities in Asia

4 Figure 1: Architectural plan and grid line marking of building Table 1: Details of considered building Geometric details 1. Purpose Commercial 2. Total plot area 24 x 22.9 m 3. Bays 4 x 4 in x & y dir 4. Floor Height 3.6 m Loading (Seismic) 1. Seismic Zone IV 2. Zone factor Response reduction factor 3 (Non ductile) 4. Importance factor 1 5. Base Shear 1488 kn 6. Time period in x direction 0.33 sec 3. PUSHOVER ANALYSIS To carry out this study, four different cases are considered namely bare frame structure, structure with soft storey, structure without soft storey and structure with soft storey designed according to Indian standard code. As a first step, the pushover analysis is performed on bare frame structure and the pushover curve is shown in the Figure 2 Pushover analysis is performed by modeling the structure with and without soft storey using Applied Element Method (AEM) (Bishnu, 2004). The pushover curve obtained from AEM analysis is shown in Figure 3 and Figure 4. The propagation of crack from Effect of soft storey in a structure present in higher seismic zone areas

5 November 2014, Yangon, Myanmar starting stage to the end of the analysis is shown with figures at critical locations. The location of the crack is indicated by white colored lines in the brick masonry wall and with red colored circles in reinforced concrete elements. The red colored pointers are used to indicate the location of the crack and the scope of crack propagation. For clear understanding, the sequence of cracking for the two analyses is shown in the Figure 3 and Figure 4. The provision suggested by Indian standard code of practice for the safety of the structure with soft storey should be verified. For understanding the issue of increasing the moments and shear forces in columns, the same structure is designed for 2.5 times moments and shear forces. The increase in moments is done only in the ground floor where the structure is having open storey and the global pushover curve and the sequence of cracking in the analysis are shown in Figure 5 and Figure 8, respectively. Figure 2: Global pushover curve for bare frame New Technologies for Urban Safety of Mega Cities in Asia

6 Figure 3: Global pushover curve for structure with soft storey Figure 4: Global pushover curve for structure without soft storey Effect of soft storey in a structure present in higher seismic zone areas

7 November 2014, Yangon, Myanmar Figure 5: Global pushover curve for structure with soft storey designed for 2.5 times moments Figure 6: Sequence of cracking for structure with soft storey New Technologies for Urban Safety of Mega Cities in Asia

8 Figure 7: Sequence of cracking in structure without soft storey Figure 8: Sequence of cracking for structure with soft storey designed for 2.5 times moments Effect of soft storey in a structure present in higher seismic zone areas

9 November 2014, Yangon, Myanmar The pushover curve for bare frame structure shown in Figure 2 clearly indicates that the load carrying capacity of the soft storey is almost same as that of the bare frame. From this it can be concluded that a structure with soft storey reduces the lateral load carrying capacity to that of a bare frame without infill walls. This is serious loss because the strength and stiffness of the infill walls present in the top storeys are not at all responsible for the behaviour. With the presence of soft storey, the beneficial influence of the infill walls is completely neglected. This also causes brittle failure and easy collapse of the complete structure. The striking observation that can be made from the curves in Figure 3 and Figure 4 is that the lateral force carrying capacity of the structure with soft storey is much less when compared to the structure without soft storey. As the walls are not present in the ground floor, the transfer of load from top of the structure (i.e., the point of application of load) to the bottom is not continuous and is breaking at the ground floor beams. Due to this, the load applied at the top of the storey is transferred only through the beam column joints at the ground floor to the ground floor columns. Hence, it can be seen that the ground floor columns which acts as a soft storey, deflects more when compared to the other floors. Proving this discussion, the infill walls in the structure without soft storey, distributed the load uniformly depending on their relative stiffness. Due to this, the crack propagated uniformly in the infill walls from top to bottom of the structure. The diagonal cracks, perpendicular to the line of application of load, in the tension zone, formed in this structure show the bending behaviour of the structure is predominant. But the cracks are visible only at the ground floor columns in the structure with soft storey effect. In addition to this, there are no cracks observed in the top floor infill walls. This indicates that the load is not transferred to any of the elements in the top storey. At the second critical point, the cracks are concentrated at the first floor wall where the sudden change in stiffness of the structure is taking place. Later at the final stage, the infill wall at the first floor got separated from the RC frame. This is caused due to the concentrated damage at the first floor. Apart from this, another point to be observed is that, in the structure with soft storey, the ground floor columns have not deflected equally. The first column is completely straight whereas the other columns are almost reaching the collapse state. When the lateral force is acting on the structure towards positive X direction, the columns to the extreme right side are subjected to compressive forces and the columns to the extreme left side are subjected to tension forces. In addition to this, the columns in the ground floor are subjected to high lateral forces when compared to the other floors. At first floor, the tensile forces induced due to the bending behaviour of the structure are dominating the compressive forces applied at that floor. This causes columns on the left side to act rigidly to the compressive forces acting on it. On the other hand, the columns on the right side deform depending on the individual relative stiffness. This causes the difference in deformation patterns of the columns in the ground floor. The jaggerdness of the pushover curve for open ground storey structure is caused because of the spring failure which is the inherent property of the methodology used for the analysis. When the spring reaches the limit specified, an equal force on the opposite side is applied to make the net force zero. Because of irregular failure of springs in the case of soft storey structure, the curve is obtained in such a pattern. New Technologies for Urban Safety of Mega Cities in Asia

10 The stiffness of the structure with and without open ground storey is varying largely and the stiffness for open ground storey structure is less when compared to the structure without open ground storey. This is because of number of effective structural elements present for resisting the lateral forces acting on the structure. Finally, the ductility of the structure which is responsible for the deformation carrying capacity of the structure is more in the case of structure without soft storey and the ductility of the soft storey structure is relatively less. On the other hand, there are no visible sudden drop in the curve for soft storey structure which can be interpreted to be more ductile when compared to the structure with all walls. The pushover curve for structure, designed according to IS 1893, is almost similar to the pushover curve obtained for the structure with soft storey without increase in the design forces. Both the structure show similar behaviour in terms of the propagation of crack and the distribution of load throughout the structure. As the discontinuous load path is not treated in this structure also, the distribution of load is not uniform throughout the structure. Due to this, there are no cracks formed in the top storeys of the structure. This underestimates the strength of the infill walls present in the top storeys. In this structure, cracks are more concentrated in the first storey near the beam column joint where the sudden change in stiffness is taking place. But the only difference that is observed in the structures with and without increase in design moments is that the crack which is started at the first storey is not propagated to the top storey. They are concentrated more in the first storey for the structure without increase in design moments. But in the structure with increased design moments, the cracks initiated at the first storey propagated towards the top storeys. Hence the first bay which is directly subjected to the loads effects more than the other floors. With this observation, it can be concluded that the load transfer is taking place from top to bottom of the structure on one side in the soft storey structure with increased moments. The stiffness and strength of the two structures are quantitatively shown in Table 2. From the table it can be seen clearly that the structure with soft storey has very less strength and stiffness when compared to the structure without soft storey. This is true in the case of soft storey structure designed for 2.5 times the moments and shear forces. Table 2: Comparison of parameters Initial Stiffness (kn/m) Max Base Shear (kn) Structure without soft storey Structure with soft storey Structure with soft storey (2.5times) x x x CONCLUSIONS This paper aims at understanding the seismic behaviour of some special cases in Moment resisting frames with URM infilled frames. For fulfilling this objective, a RC building is considered to be present in seismic zone IV. The capacity of the structure was obtained and proper reasons were derived by observing the parameters involved in estimating the capacity and also the crack initiation and propagation patterns. Effect of soft storey in a structure present in higher seismic zone areas

11 November 2014, Yangon, Myanmar It was observed that the structure with soft storey losses greater initial stiffness and maximum strength when compared to the structure without soft storey. It was also observed that the load path from the point of application of load was not distributed properly in the structure with soft storey. Due to the improper distribution of lateral load, the structure with soft storey had very less capacity when compared to the structure without soft storey. From the structure designed according to clause given in IS 1893, it was observed that the structure had increased its maximum load carrying capacity with increase in design moments. The crack pattern had not changes much in both the cases. The assumption that the soft storey structure with increased design moments behaves as the structure with enclosed full walls is directly under question with this observation. There is no comparison between the soft storey with increased moments and the structure without soft storey. The maximum load carrying capacity and the initial stiffness is hugely varying in both the cases. REFERENCES Bureau of Indian Standards, Indian Standard code of practice for plain and reinforced concrete for general building construction, IS 456, New Delhi, India. Bureau of Indian Standards, Indian Standard Criteria for Earthquake Resistant Design of Structures. Part 1 General Provisions and buildings, IS , Part 1, New Delhi, India. Crisafulli, F. R., Seismic behaviour of reinforced concrete structures with masonry infills, PhD Thesis, University of Canterbury, New Zealand. Dande, P. S., and Kodag, P. B., Influence of soft storey in RC frame building in earthquake resistant design, International Journal of Engineering Research and Applications 3, Elouli, T., Effect of infill masonry panels on seismic response of frame buildings, The fifth Agdal Rabat Morocco, University of Mohammed. Kaushik, H. B., Rai, D. C., and Jain, S. K., A rational approach to analytical modeling of masonry infill in reinforced concrete frame buildings, The 14 th World Conference on Earthquake Engineering, October 12-17, 2008, Beijing, China. Lourenco, P. B., and Rots, J. G Multi-surface interface model for analysis of masonry structures, Journal of Engineering Mechanics 123. Murty, C. V. R., and Sudhir, K. J., Beneficial influence of masonry infill walls on seismic performance of RC frame buildings, The 12 th World Conference on Earthquake Engineering, January 30 February 4, 2000, Auckland, New Zealand. Pandey, B. H., and Meguro, K., Simulation of brick masonry wall behaviour under in plane lateral loading using applied element method, The 13 th World Conference on Earthquake Engineering, August 1-6, 2004, Vancouver, B.C., Canada. Pujol, S., Climent, A. B., Rodrigueand, M. E., and Smith-Pardo, J. P., Masonry infill walls: An effective alternative for seismic strengthening of low rise reinforced concrete building structures, The 14 th World Conference on Earthquake Engineering, October 12-17, 2008, Beijing, China. Samoila, D., Masonry infill panels analytical modeling and seismic behaviour, IOSR Journal of Engineering (IOSRJEN) 3, New Technologies for Urban Safety of Mega Cities in Asia