Analysis of RC Frame Buildings Having Open Ground Storey

Transcription

1 Journal of Engineering Technology (ISSN ) Volume 8, Issue, Jan. 9, PP Analysis of RC Frame Buildings Having Open Ground Storey Md Tasleem,, Dr K. Narayan, Dr S. Choudhury Department of Civil Engineering, Institute of Engineering and Technology, Sitapur Road, Lucknow, U.P., India 6 Department of Civil Engineering, NIT Silchar, India Abstract: In practice, buildings with Open Ground Storey (OGS) are most common types of vertically irregular buildings. Such a building may fail due to low stiffness strength of ground storey. Such failures are due to faulty design of not considering the effect of infill in the building model. RC frame buildings of various heights have been designed as per IS code and the building's performance has been evaluated through Non-linear Time History Analysis (NLTHA) and Pushover Analysis (POA). Initially buildings have been designed without considering infill strut elements in the computer model and, later on the infill strut elements were incorporated while designing. Five different spectrum compatible ground motions (SCGMs) have been considered for NLTHA. This study evaluates the performance of ground soft storey buildings. It was observed that the buildings performed well when the open ground storey was designed for.5 shears and moments. However, when ground storey columns were designed for. times shears and moments, the and 6-storey performed well as there was no formation of soft storey. However, soft storey formation was observed in buildings having 8 and -storey. Keywords: Infill strut element, Push over Analysis, Non-linear Time History Analysis, Open Ground Storey, Inter-storey Drift Ratio. Introduction Open ground storey buildings are commonly being used for parking and open entrance etc. These buildings are very advantageous because they provide extra free space for parking. In these buildings due to use of infill walls in upper storeys and absence of infill walls in ground storey, the ground storey column may fail during earthquakes because of lowering of stiffness strength in the ground storey as compared to upper storeys. During an earthquake it has been seen that the columns of OGS are more vulnerable. The major types of failures in ground storey columns may be, failure of lateral ties or core concrete crushing or it may be buckling of longitudinal reinforcing bars etc. In these buildings the upper storeys with infill moves as a single block almost together, and in the ground storey only most of the horizontal displacement occurs locally. 56

2 Figure. Typical soft storey mechanism of OGS building. There is a necessity that the columns of ground storey must be sufficiently strengthened and must have adequate ductility. In designing the masonry walls are supposed to act as non-structural elements but in actual practice they have an important role in performance of structures. Masonry walls are generally used as infill partition walls. Due to combined effect of beam-column and infill walls additional strength and stiffness is achieved. Under lateral loading the infill walls behave as a strut at compression corners, whereas at tension corners there will be a gap. There is no role of infill at tension corners, but in reverse of the load, tension corner goes under compression and compression corner in tension, and during these loadings infill walls act like strut elements. Figure. Equivalent strut model. Codal provisions for designing As per IS 89(part ): 6, which gives guidelines for designing earthquake resistant structures, the lateral force or base shear (V B ) along any principal direction will be determine by the following expression: 57

3 Where, = Design acceleration spectrum value in horizontal direction. = Seismic weight of the building. The design horizontal seismic coefficient A h for a structure shall be determined by the following expression: Where, Z = Zone factor I = Importance factor, R = Response reduction factor, = Average response acceleration coefficient. The approximate value of fundamental natural period of vibration ( ), in seconds, for moment resisting frame building with infill is given by Where, h = Building height, in m. d = Dimension, along the considered direction at the level of plinth of the building in m. Modelling aspects Buildings of different height groups have been considered and the performance of these buildings have been analysed by SAP software. M5 concrete and Fe5 reinforced bars have been used in all the buildings. For analysis, a regular plan with different storeys has been considered. Zone factor has been taken as zone V and soil site has been considered as medium. Capacity design has been done for achieving the strong column weak beam concept. The weight of slab has been assigned at the beams by trapezoidal rule and weight of infill walls have been assigned as uniformly distributed loads at the beams. Two types of buildings have been analysed (a) buildings without infill effect and (b) buildings with infill effect. The infill effect has been considered by replacement of the infill with infill strut elements, and thickness of the equivalent diagonal strut element will be considered as the thickness of the infill wall itself. The width of strut element recommended as per FEMA56 as, Where, and, = Height of Column, in. = Height of infill walls, in. = Modulus of elasticity of material of RC frame, ksi = Modulus of elasticity of material of infill wall, ksi = Column s Moment of inertia, in. = Infill wall s length, in. = Infill wall s diagonal length, in. = Thickness of equivalent strut which is equal to infill wall s thickness, in. Θ= Angle, The tangent of which is the infill height to length aspect ratio, radians. 58

4 Acceleration (g) Acceleration (g) Earthquake Ground Motions Five Spectrum compatible ground motions (SCGM) have been considered with the name assigned as GM-, GM-, GM-, GM-, and GM-5 as detailed in Table Table. Details of Spectrum Compatible Ground Motions (SCGM) Sl. Name Background Earthquake(Eq.) PGA (g) Duration No. (Sec) GM- Northridge EQ /7/9, :, Canoga Park.6.99 GM- Victoria, Mexico 6/9/8, :8, Chihuahua GM- Morgan Hill //8, :, Gilroy Array GM- Erzikan //9 7:9, Erzikan Eastwest Comp GM-5 Dinar //95, 5:57, Dinar There is a real earthquake in background of each corresponding SCGMs whose characteristics like phase and duration have been used in generation of the SCGMs. The SCGMs have been generated using software developed by Kumar (). The generated ground motions represent MCE level of earthquakes and have been presented in Figure a, b, c, d and e. These ground motions have been used for nonlinear time history analysis of the considered building. GM Time (T) in sec. Figure a. SCGM after Northridge Earthquake (99) GM Time (T) in sec. Figure b. SCGM after Victoria, Mexico Earthquake (98) 59

5 Acceleration (g) Acceleratin (g) Acceleration (g) GM Time (T) in sec. Figure c. SCGM after Morgan hill Earthquake (98) GM Time (T) in sec. Figure d. SCGM after Erzikan Earthquake (99) GM Time (T) in sec. Figure e. SCGM after Dinar earthquake (995) 5 Behaviour of buildings without infill effect These buildings have been designed without considering infill strut elements but infill weight has been considered. The infill effect in these buildings has been considered through periodic formula and base shear correction as per codal provisions. The non-linear analysis (static pushover analysis and time history analysis) have been done to find out the performance of the buildings. The fundamental natural time periods of the all considered buildings as per code and as per analysis have been shown in Table 5

6 Base Shear (kn) Name of Buildings Table. Fundamental periods of designed buildings Code specified time period ( in sec Period from dynamic analysis (T) in sec Long direction Short direction Long direction Short direction A A A A To avoid the over estimation of design lateral force, base shear correction factor has been applied to each building. It has been observed that the fundamental time period for all the buildings in dynamic analysis are greater than the code specified time periods. -storey in short direction storey in long direction storey in short direction..... Figure. Push-over curves.... Push-over curves for buildings in short and long direction have been shown in figure. The graphs are plotted between roof displacement and base shear for mode proportion and for uniform load condition. The performance point for the design basis earthquake and maximum considered earthquake have been shown in figure. The performance levels, namely IO, LS, CP have been shown on curves plotted between base shear and roof displacement. 8-storey in long direction MODE PROPORTINAL 5

7 -storey in short direction GM GM GM GM -storey in long direction GM GM GM GM Storey in Short direction GM GM GM GM Storey in long direction GM GM GM GM Figure 5. The inter storey drift ratio (IDR) for all s have been shown in the figure 5 and it was observed that the Inter storey drift ratio for all buildings is different, whereas it was also observed that the IDR for the different ground motions at the different storeys are different. It has been observed that if the IDR of any storey is similar for two or more ground motions, it is not necessary that it will be the same or will be in the same ratio for another storey also with same ground motion. 6 Performance of open ground storey buildings Performance of soft storey buildings having soft storey at ground level, designed as per IS code have been studied. For analysis, buildings having a regular plan with different storeys say -storey, 6- storey, 8-storey and -storey have been considered. All buildings are open at ground floor and all above levels are with infill strut elements. Initially, a factor of.5 is used for designing the Open Ground Storey, thereafter a lesser value of.5 and. has been taken and all the buildings have been designed and results have been obtained. After analysis it has been found that the code specified time is totally different from fundamental time periods recorded after dynamic analysis and which have been shown in Table 5

8 Table. Fundamental periods of designed OGS buildings Name of Buildings Code specified time period ( in sec Period from dynamic analysis (T) in sec Long direction Short Long direction Short direction direction B B B B storey in short direction storey in long direction storey in short direction storey in long direction Figure 6 Pushover curves for OGS building In figure 6 it has been observed that the curve of base shear versus roof displacement is totally different. In figure the plot is for bare frame analysis, whereas in figure 5 the plot is for the buildings which were analysed after applying the multiplication factors. The performances of the buildings were totally different. 5

9 -Storey in Short direction GM GM GM GM -Storey in Long direction GM GM GM GM Storey in Short direction GM GM GM GM Storey in long direction GM GM GM GM Figure 7 IDR for OGS building Figure 7 shows the inter storey drift ratio for different ground motions after applying the codal provisions and it can be observed that the IDR for the all s are within limit of., whereas in figure the IDR is not within the code specified limit. 7. Results and conclusions In this paper dynamic analysis of different buildings has been done and the following results were obtained: () The performance of the buildings without infill strut elements has been observed and it was found that there was no soft storey formation in these buildings when infill effect was considered only through period formulae and base shear correction. () It has been observed that when the ground storey of OGS buildings was designed for.5 times storey shears and moments, it performed well and did not form soft storey. () It has also been observed that the building performed well when the ground storey was designed for.5 times storey shears and moments. () When the ground storey columns of OGS buildings were designed for. times storey shears and moments the following points have been observed i. The buildings having and 6-storey performed well and there was no formation of soft storey. ii. Soft storey formation has been observed in buildings having 8 and -storey and performance of such buildings was poor. On the basis of the above results we can say that the there should be codal provisions for designing the soft storey. The multiplication factor for shears and moments could be different for the different 5

10 types of plan irregularity and may change with the height of structure. It has also been observed that the code specified time period is totally different from recorded time period after dynamic analysis of buildings. References [] Mehmet Inel and Hayri B Ozmen(8), Effect of Infill Walls on Soft Story Behavior in Mid- Rise RC Buildings, The th World Conference on Earthquake Engineering October -7, Beijing, China. [] Jaswant N. Arlekar, Sudhir K. Jain, C.V.R. Murthy, Seismic Response of RC Frame Building With Soft First Storeys, Department Of Civil Engineering, Indian Institute Of Technology, Kanpur. [] Dr. Mizan DOGAN, Dr. Nevzat KIRAÇ, Dr. Hasan GÖNEN (), Soft-Storey Behaviour in An Earthquake And Samples of Izmit-Duzce, Department of Civil Engineering, Osmangazi University Eskisehir- TURKEY. [] Goutam Mondal and Sudhir K. Jain, M.EERI Lateral Stiffness of Masonry Infilled Reinforced Concrete (RC) Frames with Central Opening. [5] J. Prakashvel, C. UmaRani, K. Muthumani, N. Gopalakrishnan () Earthquake Response of Reinforced Concrete Frame with Open Ground Storey [6] Choudhury S. (8) Performance Based Seismic Design of Hospital Buildings Ph.D. thesis, Department of Earthquake Engineering, IIT Roorkee. [7] IS 56:, Indian Standard for Plain and Reinforced Concrete Code of Practice, Bureau of Indian Standards, New Delhi.. [8] IS 89 (Part ): 6, Indian Standard Criteria for Earthquake Resistant Design of Structures, Bureau of Indian Standards, New Delhi. [9] Holmes, M. (96), Steel frames with brick and concrete infilling. Proceedings of Institution of Civil Engineers [] FEMA 56 (), Prestandard and Commentary for the Seismic Rehabilitation of Buildings. American Society of Civil Engineers. USA.. [] EC 8 (), Design of Structures for Earthquake Resistance, Part-: General Rules, Seismic Actions and Rules for Buildings. European Committee for Standardization (CEN), Brussels.. [] PEER. Pacific earthquake engineering research centre: PEER Strong Motion Daaebase, University of California. [] IS 9: 99, Ductile Detailing of Reinforced Concrete Structures Subjected to seimic forces Code of Practice. [] Noor, Amin, Bhuiyan, Chowdhury and Kakoli (eds) (), Effect of Soft Storey on Multi- Storied Reinforced Concrete Building Frame th Annual Paper Meet and st Civil Engineering Congress, December -,, Dhaka, Bangladesh. [5] A. K. Chopra, D. P. Clough, R. W. Clough (97) Earthquake Resistance Buildings With A Soft First Storey Earthquake Engineering and Structural Dynamics, Vol., 7-55 (97). [6] M. J. N. Priestley, University of California, San Diego Performance Based Seismic Design (WCEE ). [7] J. D. Pettinga and M. J. N. Priestley, Dynamic Behaviour of Reinforced Concrete Frames Designed With Direct Displacement-Based Design [8] Scarlat, A. (997) Design of soft stories - A simplified energy approach. Earthquake Spectra