Earthquake Response of Reinforced Concrete Frame with Open Ground Storey

Transcription

1 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Earthquake Response of Reinforced Concrete Frame with Open Ground Storey J. Prakashvel, C. UmaRani, K. Muthumani, N. Gopalakrishnan Abstract--- Open ground storey buildings have consistently shown poor performance during past earthquakes across the world. For example during 1999 Turkey, 1999 Taiwan and 2003 Algeria earthquakes, a significant number of them have collapsed. For instance, the city of Ahmedabad alone has about 25,000 five-storey buildings and about 1,500 eleven-storey buildings; majority of them have open ground storey. There are huge numbers of such buildings in urban areas of moderate to severe seismic zones of the country. The collapse of more than a hundred reinforced concrete frame buildings with in Ahmedabad (~225km away from epi-centre) during the 2001 Bhuj earthquake has emphasised that such buildings with open ground storey are extremely vulnerable under earthquake shaking. The presence of walls in upper storeys makes them much stiffer than the open ground storey. Still Multi storey reinforced concrete buildings are continuing to be built in India which has open ground storeys. These buildings are not designed as per the earthquake resistant design. It is imperative to know the behaviour of soft storey building to the seismic load for designing various retrofit strategies. Hence it is important to study and understand the response of such buildings and make such buildings earthquake resistant based on the study to prevent their collapse and to save the loss of life and property. Based on the above an attempt is made in this paper to assess the seismic performance of the soft storey reinforced concrete building by shake table test. Keywords--- Infill-Wall, Reinforced Concrete Frame, Shake Table Test, Soft Storey D I. INTRODUCTION EVASTATING earthquakes strike at a regular interval in various parts of India. About 60% of India s land area is reported to be under the threat of moderate to severe seismic hazard. Reinforced concrete frame buildings are built in India which has open ground storeys. Owing to the high cost of land and small sizes of plots, parking is often accommodated in the ground floor area of the building. Frame bays in the ground floor are not infilled with masonry walls, as it is done in the upper stories. These buildings are normally not designed as per the earthquake resistant design proposed in the BIS codes. J. Prakashvel, Senior Technical Officer, CSIR-SERC, E- mail:pra_vel@yahoo.com C. UmaRani, Associate professor, Anna University, E- mail:umarani@annauniv.edu K. Muthumani, Chief Scientist, CSIR-SERC, kmm@serc.res.in DOI: /BIJIEMS.1662 Past earthquakes have proved that these are highly vulnerable. There are huge numbers of such buildings in urban areas of moderate to severe seismic zones of the country. Hence it is important to study and understand the response of such buildings and make such buildings earthquake resistant based on the study to prevent their collapse and to save the loss of life and property. An attempt is made in this paper to overview the major issues associated with soft storey buildings. 1.1 Basic Features and the Behaviour of the Soft Storey Buildings Reinforced concrete frame buildings are becoming increasingly common in urban India. Many such buildings constructed in recent times have a special feature the ground storey is left open for the purpose of parking, i.e., columns in the ground floor do not have any partition walls (of either masonry or Reinforced concrete) between them. Such buildings are often called open ground storey buildings. The relative horizontal displacement in the ground storey is much larger than storeys above it. The total horizontal earthquake force it can carry in the ground storey is significantly smaller than storeys above it. The soft or weak storey may exist at any storey level other than ground storey level. Open ground storey buildings have consistently shown poor performance during past earthquakes across the world (for example during 1999 Turkey, 1999 Taiwan and 2003 Algeria earthquakes); a significant number of them have collapsed. A large number of buildings with open ground storey have been built in India in recent years. For instance, the city of Ahmedabad alone has about 25,000 five-storey buildings and about 1,500 elevenstorey buildings; majority of them have open ground storeys. Further, a huge number of similarly designed and constructed buildings exist in the various towns and cities situated in moderate to severe seismic zones (namely III, IV and V) of the country. The collapse of more than a hundred RC frame buildings with open ground storeys at Ahmedabad (~225km away from epi-centre) during the 2001 Bhuj earthquake has emphasised that such buildings are extremely vulnerable under earthquake shaking. A normal frame fails in a beam side sway mode and undergoes reduction in all the modal frequencies after the earthquake damage. Stiffness values of all the floors get reduced. However the stiffness reduction is more for the bottom storey. A soft storey frame fails in a column side sway mode and undergoes reduction mostly in its first mode frequency after the earthquake damage. Also, the stiffness of the bottom storey alone undergoes changes. 1.2 Definitions of Open Soft Storey Building After the collapses of RC buildings in 2001 Bhuj earthquake, the Indian Seismic Code IS: 1893 (Part 1) has included special design provisions related to soft storey

2 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December buildings. The Indian seismic code IS:1893 (Part 1) 2002 Criteria for Earthquake Resistant Design of Structures, part-1- General provisions and buildings, BIS, [7] gives the following technical definition for soft open storey buildings in Table 5 Definition of irregular buildings Vertical irregularities (Clause 7.1) i) (a) Stiffness Irregularity (Soft Storey): A soft storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the average lateral stiffness of the three storeys above. (b) Stiffness Irregularity (Extreme Soft Storey) A extreme soft storey is one in which the lateral stiffness is less than 60 percent of that in the storey above or less than 70 percent of the average stiffness of the three storeys above. A weak storey is one in which the storey lateral strength is less than 80 percent of that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the storey shear in the considered direction. As per commentary by National Information Centre for Earthquake Engineering at IITK, Kanpur, soft storey is one in which the lateral stiffness is less than 60% percent of that in the storey above or less than 70% of the average lateral stiffness of the three storeys above. As per commentary by National Information Centre for Earthquake Engineering, weak storey is one in which the storey lateral strength is less than 70% percent of that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the storey shear in the considered direction. The IS Code suggests that the forces in the columns, beams and shear walls (if any) under the action of seismic loads specified in the code, may be obtained by considering the bare frame building (Without any infill). However, beams and columns in the open ground storey are required to be designed for 2.5 times the forces obtained from this bare frame analysis. If it is not feasible to increase the capacity of the columns in soft/weak storey, shear walls should be provided, preferably on the periphery of the building. Care should be taken to ensure symmetric arrangement of the shear walls to avoid the torsional effects. The shear walls should be designed for 1.5 times the seismic demand for the storey as per calculations while the columns are designed for 100% of seismic demand. II. REVIEW OF RESEARCH WORKS ON SOFT STOREY BUILDINGS Handbook on seismic retrofit of buildings, CPWD, IBC, and IITM [1] illustrates very good methods on retrofit strategies. In this handbook, they have given local and global retrofitting strategy for improving the strength and other attributes of resistance of a building or a member to seismic forces. Global retrofit strategy improves the performance of the entire building under lateral loads. Local retrofit develops the seismic resistance of a member. Arslan and Korkmaz [2] investigated the failure modes during the earthquake at Turkey. In Turkey, generally, building stock is formed from reinforced concrete structures and during last earthquakes, a large number of these buildings in the epicenter regions were collapsed leading to widespread destruction and loss of life. In this paper, the performance of reinforced concrete buildings during recent earthquakes in Turkey is discussed. The failure modes consist of foundation failures, soft stories, strong beams and weak columns, lack of column confinement, poor detailing practice and non-structural damages. Observations from the earthquake damages are discussed. The Reconnaissance Report of January 26, (2001) Bhuj India Earthquake [3]. It is found that large number of open ground storey buildings in Ahmedabad, Bhuj, Gandhidham, and other towns suffered severe damage or dramatic collapse. Out of the 130 buildings that collapsed in Ahmedabad, most were of open ground story configuration. Among those that did not collapse, the damage was confined mostly to the open ground storey columns. The RC frames with masonry infill formed a relatively stiff and strong lateral load resisting system in the upper stories, in contrast to the columns with few or no infill walls in the ground storey. As a result, almost the entire lateral deformation is concentrated in the ground storey columns, and the upper stories moved laterally as a rigid block. Moreover, unlike the upper storey columns, the ground storey columns in such buildings could not share the lateral shears with the infill walls. Since these columns were neither designed for lateral forces nor detailed for ductile behaviour, many of them sustained brittle shear failure or flexural failure resulting from large moment and axial load. Once the ground floor columns failed, the gravity load-carrying capacity of the building was partially/completely lost resulting in partial/complete collapse of many buildings. Infill alter the behaviour of buildings from one of predominantly frame action to one of predominantly shear action and also carry the lateral seismic force as compression axial loads along their diagonals. Dyavanal and Gudadappanavar [4] evaluated three, six, and nine storey soft open storey buildings designed for 1.0, 1.5, 2.0 and 2.5 times the seismic demand and investigated. It is found that same design forces may well be used for three storeys, soft open ground buildings as calculated for bare frame to perform within collapse prevention levels. For the six to nine storey, soft open ground buildings, the design forces calculated for bare frame may be increased to 1.5 times to the ground storey columns to perform within collapse prevention levels. Hejazil et al [5] investigated the effect of soft storey on structural response of high rise buildings. The lower level containing the concrete columns behaved as a soft storey in that the columns were unable to provide adequate shear resistance during the earthquake. Usually the most economical way of retrofitting such a building is by adding proper bracing to soft stories. In this paper occurrence of soft storey at the lower level of high rise buildings subjected to earthquake has been studied. Also it has been investigated on adding of bracing in various arrangements to the structure in order to reduce soft story effect on seismic response of building. From these few models, bracing is one of the method that can be used to resist earthquake compare to moment resisting frame. It is because beside increase the strength in the member, it also increases the overall stiffness in the building. Kaushik et al [6] investigated various strengthening schemes for their effectiveness in improving the performance of the buildings based on nonlinear analyses of typical RC frames. A rational method has been developed for calculation of required increase in strength of open first storey columns instead of designing for very high forces as recommended in some of the national codes. In this paper, the strengthening schemes such as providing additional columns, diagonal bracings and lateral

3 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December buttress in the open ground storey are studied. Codal methods are found to increase only lateral strength of such frames whereas, some of the alternate schemes studied were found to improve both lateral strength and ductility for improved seismic performance. Jaswant Arlekar et al [8] evaluated the seismic response of RC frame building with soft first storeys. This paper highlights the importance of explicitly recognizing the presence of the open first storey in the analysis of the building. The error involved in modeling such buildings as complete bare frames, neglecting the presence of infills in the upper storeys, is brought out through the study of an example building with different analytical models. This paper argues for immediate measures to prevent the indiscriminate use of soft first storeys in buildings, which are designed without regard to the increased displacement, ductility and force demands in the first storey columns. Alternate measures, involving stiffness balance of the open first storey and the storey above, are proposed to reduce the irregularity introduced by the open first storey. Kadid and Boumrkik [9] conducted the pushover analysis to evaluate the performance of framed building under future expected earthquakes. The Boumerdes 2003 earthquake which has devastated a large part of the north of Algeria has raised questions about the adequacy of framed structures to resist strong motions, since many buildings suffered great damage or collapsed. Three framed buildings with 5, 8 and 12 storeys respectively were analyzed. PMM hinges for columns and M3 hinges for beam as described in FEMA-356 has been adopted in this study. Beams and columns are modeled as nonlinear frame elements with lumped plasticity at the start and the end of each element. From this study results, it is understood that well designed frames will perform well under seismic loads. The author gives the following conclusions. The pushover analysis is a relatively simple way to explore the non-linear behaviour of the buildings. The behaviour of properly detailed reinforced concrete frame building is adequate as indicated by the intersection of the demand and capacity curves and the distribution of hinges in the beams and the columns. Most of the hinges developed in the beams and few in the columns but with limited damage. The causes of failure of reinforced concrete during the Boumerdes earthquake may be attributed to the quality of the materials used and also to the fact that most of buildings constructed in Algeria are of strong beam and weak column type. The results obtained in terms of demand, capacity and plastic hinges gave an insight into the real behaviour of structures. C.V.R. Murthy, [10] reviewed major issues associated with open ground storey building in this paper. These open ground storey buildings are highly vulnerable in shear generated during strong earthquakes. It is relatively flexible in the ground storey. The total horizontal displacement of the open ground storey is much larger than the storeys above. Also the open ground storey is relatively weak. The total horizontal earthquake force that can be carried by the open ground storey is significantly less than the storeys above. This paper also enumerates the reasons for the poor behaviour of open ground storey buildings in India. Santhi, M.H; et al [11] studied the two single-bay, three-storey space frames, one with brick masonry infill in the second and third floors representing a soft-storey frame and the other without infill which were designed and their 1:3 scale models were constructed according to non-seismic detailing and the similitude law. The models were excited with an intensity of earthquake motion as specified in the form of response spectrum in Indian seismic code IS using a shake table. The seismic responses of the soft-storey frame such as fundamental frequency, mode shape, base shear and stiffness were compared with that of the bare frame. It was observed that the presence of open ground floor in the soft-storey infilled frame reduced the natural frequency by 30%. The shear demand in the soft-storey frame was found to be more than two and a half times greater than that in the bare frame. From the mode shape it was found that, the bare frame vibrated in the flexure mode whereas the softstorey frame vibrated in the shear mode. The frames were tested to failure and the damaged soft-storey frame was retrofitted with concrete jacketing and, subjected to same earthquake motions as the original frames. Pushover analysis was carried out using the software package SAP 2000 to validate the test results. The performance point was obtained for all the frames under study, therefore the frames were found to be adequate for gravity loads and moderate earthquakes. III. 3.1 RC Frame Details EXPERIMENTAL INVESTIGATIONS Half scale model of two-bays in X-direction, single bay in Y-direction and three storey reinforced concrete frame with a total height of 4.8m has been constructed for the shake table experiment. Each storey height is 1.6 m. Plan and elevation of the reinforced concrete frame model is shown in figures 1 and 2. Beams and columns are of size 150 X 150 mm. Open space between the columns at the ground floor level is left open. Second and third floor is filled with the brick in-fills. In order to understand the importance of soft storey effect, a half-scale model of reinforced concrete building is tested in a 4 m X 4 m shake table. The reinforced concrete frame is open ground storey frame (OGS frame) having in-fills at higher floors and kept open at the ground floor. The slab thickness is 100 mm and base is a raft foundation, which is used for connecting the model to shake table. The longitudinal beam and transverse beam reinforcement consists of four numbers of 16 mm diameter and four numbers of 10 mm diameter bars respectively. Columns are reinforced with four numbers of 12mm diameter bars. Lateral ties in the columns and beams are 6 mm diameter two legged stirrups at a spacing of 150 mm c/c. Material used are M 25 grade concrete and Fe 415 steel. An additional mass of 1200 kg is added on each floor to represent equivalent live load. 3.2 Instruments Used High speed data acquisition units with necessary sensor/amplifier/recording devices are used to measure the strain, displacement, and acceleration. 1. Shake table 2. Accelerometer 3. LVDTs (Linear Variable Displacement Transducer) 4. NCDTs (Non Contact Displacement Transducer) 5. Strain gauges

4 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Shake Table 4m X 4m shake table is used to test the reinforced concrete frame model. Required earthquake time history is given to the shake table in the form of displacement values through the multi directional actuators in the shake table setup. Specification of the shake table Size of the table Maximum pay load : 4m X 4m : 30 tons Frequency of operation : 0.1Hz 50Hz Acceleration (Maximum): 1.0g (H&V) Velocity (Maximum) : 0.8 m/sec Wave form 3.3 Test Procedure : Sine wave, Sine sweep, Random Model reinforced concrete frame is placed on the shake table at the required position. Base raft slab of the frame is connected to the shake table with high strength bolts to make it as a rigid base. A stationary reference steel frame (I section) is fixed near the reinforced concrete model and away from shake table. LVDTs and NCDTs are connected to the steel frame at each floor levels to measure the linear displacement of the frame for every earthquake given to the shake table. Strain gauges pasted on to the steel reinforcement are connected to the multi channel data acquisition system. Two accelerometers are fixed at each floor level to measure the acceleration values. The earthquake time history compatible to response spectrum given in IS-1893, for soft soil spectrum for zone V is modified with the frequency scaling of square root of two by similitude law for the half scale model of reinforced concrete frame which is termed as. The earthquake time history in the form of displacement is given to the shake table actuators which are present in the lower part of shake table using control system software The magnitude of the earthquake time history is increased from 10% to 120 % and the response of the reinforced concrete frame model for every increment of 10% from -10% to -120% is observed and measured. These actuators are moved from its original position to achieve its input excitation. All the displacements, acceleration values are measured along the direction of table excitation. Dynamic strain values from the strain gauges of the columns, acceleration values from all accelerometers and displacement values from all the LVDT and NCDT s are received, digitized and recorded. High speed data acquisition units with necessary sensor/amplifier/recording devices are used to measure the strain, displacement, and acceleration. Instrumentation details are shown in figures 3 and 4 IV. RESULTS AND DISCUSSION During the shake table experiment, various earthquakes were imparted to the model reinforced concrete frame. Response of the reinforced concrete frame model for various earthquakes were analysed and behaviour of the structure is studied. The maximum peak ground acceleration value is 7.99 m/s 2 during the -120% earthquake. The magnitude is increased from 10 % to 120% and response is measured for every earthquake time history. SFT is the earthquake generated from IS-1893, soft soil spectrum. Modified SFT () is the modified spectrum compatible time history of IS-1893 soft soil spectrum with the frequency scaling of square root of two by similitude law for the half scale model of the reinforced concrete frame. The reinforced concrete frame model tested is the half scale model. Hence () the modified spectrum compatible time history of IS1893 soft soil spectrum is used for the test. Figure-5 shows ground acceleration and acceleration values measured at three floors of the reinforced concrete frame during -50 % earthquake. The maximum acceleration at the third storey level is 4.396m/sec 2. Due to the stiffness of the brick infill in the upper storeys, acceleration of all the three storeys is almost same as seen in the figure 5. Base shear during -50% earthquake is given in figure 6. The maximum value of the base shear is KN during -50%. It is observed that magnification is almost same for all the three storeys. However third storey has got higher values of magnification. Drift variation of the figure.10, shows the clear picture of soft storey behaviour of having higher inter-story drift at ground floor level. The maximum drift value is 17mm in the ground storey during -50% Figure 1: Side Elevation

5 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Figure 3: Strain Gauge Details Figure 4: Instrumentation Details Figure 2: Plan and Elevation

6 Base Shear (kn) Mean Acceleration (m/sec^2) Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December PGA Third floor Second floor First floor Time (Secs) Figure 5: Ground Acceleration and Mean Acceleration values Measured at Three Floors during -50 % Earthquake Time (Secs) Figure 6: Base Shear during -50% Earthquake

7 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Figure 7: Strain Variations in the Second Storey Columns during -120 % Earthquake Figure 8: Strain Variations in the Ground Storey Columns during -120 % Earthquake Note: Strain Gauges GC12 and GC21 were Failed due to Very High Strain hence not Shown in the above Figure

8 Magnification Factor (Acceleration) m/sec^2 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Table 1: Response of the Model RC Frame for Modified Spectrum (Soft soil-is1893) Description of Test 10% 20% 30% 40% 50% 70% 80% 90% 100% 110% 120% PGA Base Shear kn Normalised Base Shear % III Floor II Floor I Floor The RC frame tested is the half scale model. Hence () the modified spectrum compatible time history of IS1893 soft soil spectrum is used for the test. The magnitude is increased from 10 % to 120% and response is measured for every earthquake time history Description of Test Unit Table 2: Displacement of the Model RC Frame for Modified Spectrum (Soft soil-is1893) 10% 20% 30% 40% 50% 70% 80% 90% 100% 110% 120% First floor (mm) Second floor (mm) Third floor (mm)

9 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December Table 3: Drift Values of the Model RC Frame for Modified Spectrum (Soft soil-is1893) Description of Test Unit 10% 20% 30% 40% 50% 70% 80% 90% 100% 110% 120% Third floor (mm) Second floor (mm) First floor (mm) Figure 9: Displacement of the Model RC Frame for (Soft soil-is1893) Figure 10: Drift of the Model RC Frame for Modified Spectrum Modified Spectrum (Soft Soil-IS1893) Magnitude of strain variation in the ground storey column is higher than the second storey column during - 50%.The maximum magnitude of strain in the ground storey column is nearly 1500 micro strain whereas it is only 50 micro strains in the second storey column during -50%. Similar to the -50% earthquake, acceleration values of all the three storeys is almost same during -120% earthquake. This may be due to the presence of the brick infill in the upper storeys. The maximum value of the base shear is KN during -120%. During -120%, it is observed that magnification is almost same for all the three storeys. However third storey has got higher values of magnification. Drift variation during -120% depicts the behaviour of soft storey building wherein the inter-storey drift at ground floor level is very high compared to other storeys. Magnitude of strain variation in the ground storey column is higher than the second storey column during -120% earthquake as shown in figure 7 and figure 8.As far as - 120% earthquake is concerned the maximum magnitude of strain in the ground storey column is nearly 10,000 micro strain where as it is only 700 micro strain in the second storey column. Figure 9 and Figure 10 gives the floor wise graphical representation of the displacement and drift of the reinforced concrete model frame for the given earthquake (Modified spectrum for Soft soil-is1893). Soft storey behaviour is clearly depicted in the figures 9 and 10 wherein the drift value is very high for the ground storey compared to the second and third storey. Figure 9 and 10 indicates the severe deformation

10 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December demands in case of a building with a soft storey. Excessive deformations in the ground storey alone are not desirable since the columns in the ground storey become stressed well beyond the level anticipated in the design (source: Murty). Table 1 to 3 gives the response of the Model RC frame for Modified spectrum (Soft soil-is1893).the maximum peak ground acceleration value is 7.99 m/s 2 during -120% earthquake and the corresponding base shear is kn. Maximum base shear of kn has occurred during -110% earthquake. -120% earthquake consists of highest magnitude of peak ground acceleration (PGA) value of 7.99 m/s 2 and the maximum response acceleration of the above earthquake is m/s 2 at the third floor level. Hence the magnification for this earthquake is Amplification of the reinforced concrete frame model is less due to interaction of masonry infill with the bounding reinforced concrete frame in the in-elastic range which is the soft storey behaviour. Table 2 and Table 3 gives the displacement and drift values of the model reinforced concrete frame. As the structure has yielded beyond -100% earthquakes, both the values of the displacement and drift has been reduced for the earthquake -110% and 120%. Model reinforced concrete frame has undergone a maximum displacement of mm at third floor level during -100% earthquake whose PGA value is 6.81 m/s 2. But in the -120% earthquake with PGA 7.99 m/s 2 the maximum displacement is reduced to mm at the third floor level. This shows that the frame has yielded leading to the stiffness degradation. As the model reinforced concrete frame is a soft storey building, the ground storey drift is much higher than the second and third storey drift. Soft storey behaviour can be clearly demonstrated by comparing the ground storey drift values with that of second and third storey drift mm drift is the maximum drift experienced by the ground storey of reinforced concrete frame during -100% earthquake. Brick infill in the reinforced concrete frame model contributes to the stiffness value in addition to the column stiffness. Ground storey stiffness is very less (less than 30 %) compared to second storey stiffness for all the earthquakes. Due to damage in the reinforced concrete model frame, after the 110% earthquake, the stiffness percentage of ground storey to second storey is as low as 3.45%. In general the displacement profile and storey drift clearly depicts the soft storey behaviour. -120% earthquake consists of highest magnitude of peak ground acceleration (PGA) value of 7.99 m/sq sec and the maximum response acceleration of the above earthquake is m/sq sec at the third floor level. As the model RC frame is a soft storey building, the ground storey drift is much higher than the second and third storey drift. Soft storey behaviour can be clearly demonstrated by comparing the ground storey drift values with that of second and third storey drift. 4.1 Summary Inter storey drift is high at the first storey level for the tested reinforced concrete frame model which is due to the lack of stiffness in the ground storey. Drift values for the upper storeys are comparatively low because of the relative increase in stiffness. The shear force acting at the ground storey level is higher than the other floors. Due to the reduced stiffness and increased shear force, ground storey column deformed to a large extent during the earthquakes. Damages are predominantly spread in the ground storey of the tested reinforced concrete frame model which is due to the high lateral shear force at the ground storey level and also due to the inadequate lateral load capacity of the reinforced concrete frame model. Sliding cracks are formed in the infill walls. V. CONCLUSIONS An open ground storey reinforced concrete frame model was tested in the shake table for the earthquake time history compatible to response spectrum given in IS-1893, for soft soil spectrum for zone V, modified with the frequency scaling of square root of two by similitude law for the half scale model of reinforced concrete frame. Based on the above experimental studies the following conclusions are drawn. 1) In the shake table experiment, it was found that the inter-storey drift is higher in the ground storey (28.88 mm) compared to other storey drifts (6.34mm & 3.43mm) in the upper floors. This clearly demonstrates the open ground storey frame effect. 2) It is found that the magnitude of strain variation in the ground storey column is in the range of 10,000 micro strain and it is only 700 microns in the second storey column during shake table test. During all the shake table tests, strain values are higher at ground storey columns and it is comparatively very low for other storey columns 3) During the shake table test, predominant failure hinges are formed in the open ground storey columns. 4) With reference to the behaviour of the infill wall, it is observed in the shake table test that the sliding cracks are formed in the in-filled walls instead of X cracks. It is found that this is due to the reason that the maximum base shear(11.65tons) value is higher than the strength of the infill (3.4tons) 5) In general for all the earthquakes given to the model frame, magnification is less due to the stiffness provided by the infill walls in the upper storeys. 6) It is found from the shake table experiment that the ratio of ground storey stiffness to second storey stiffness is 35% at the initial stage before testing. After testing the stiffness reduced to 3.32% leading to the exaggeration of the soft storey effect. 7) The force capacity and the maximum displacement obtained from the shake table experiment is tons and 37.42mm respectively. In view of the above, the effect of the open ground storey frame is clearly demonstrated from the drift profiles, displacement profiles, strain gauge values, stiffness ratio and damage pattern of the reinforced concrete model frame

11 Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December ACKNOWLEDGEMENT This paper is published with the approval of the Director, CSIR-SERC, Chennai, and his encouragement is gratefully acknowledged. REFERENCES [1] Amarnath Chakrabarti Devdas Menon and K. Amlan Sengupta, Handbook on seismic retrofit of buildings, CPWD, IBC, IITM, ISBN-13, Narosa Book Distributors Private Limited, New Delhi , [2] M.H. Arslan, and H.H. Korkmaz, What is to be learned from damage and failure of reinforced concrete structures during recent earthquakes in Turkey? Journal of Engineering Failure Analysis, Vol.14, No.1 Pp. 1-22, [3] Bhuj, India, Earthquake of January 26, Reconnaissance report- EERI. [4] S.S. Dyavanal, and B.M. Gudadappanavar, Performance Based Evaluation of Seismic Code Provisions for Soft Storey Buildings International Journal of Earth Sciences and Engineering, Vol.03, No.4, Pp , [5] F. Hejazi1, S. Jilani, J. Noorzaei, C. Y. Chieng, M. S. Jaafar, Effect of Soft Story on Structural Response of High Rise Buildings, IOP Conf. Series: Materials Science and Engineering, Vol.17, Pp.1-13, [6] H.B. Kaushik, D.C. Rai, S.K. Jain, Effectiveness of some strengthening options for masonry-infilled RC Frames with open first storey, Journal of Structural Engineering, ASCE, Vol.135, No.8, Pp , [7] IS: 1893(Part 1) Indian Standard Code of Practice for Criteria for Design of Earthquake Resistant Structures, Bureau of Indian Standards, New Delhi, [8] N. Jaswant Arlekar, K. Sudhir Jain, and C.V.R. Murty, Seismic Response of RC Frame Buildings with Soft First Storeys, Proceedings of the CBRI Golden Jubilee Conference on Natural Hazards in Urban Habitat, New Delhi, Pp.13-24, [9] A. Kadid, and Boumrkik A Pushover analysis of reinforced concrete frame structures, Asian Journal of civil engineering (Building and Housing) Vol.9, No.1, Pp.75-83, [10] C.V.R. Murthy, Open Ground Storey RC Frame Buildings with 230mm Columns unsafe during Earthquakes published in the National Seminar on Seismic detailing of R.C.C structures proceedings, Pp.1-30, [11] M.H. Santhi, G.M.S. Knight, and K. Muthumani, Evaluation of Seismic Performance of Gravity Load Designed Reinforced Concrete Frames, Journal of Performance of Constructed Facilities, ASCE, Vol.19, No.4, Pp , 2005.