Research Article Volume 6 Issue No. 7

Transcription

1 ISSN XXXX XXXX 216 IJESC Research Article Volume 6 Issue No. 7 Seismic Performance of Multi Storey RC Frame Buildings with Soft Storey from Pushover Analysis Basavaraju Y K 1, Dr. B S Jayashankar Babu 2 M.Tech Student 1, Professor 2 Civil Department P.E.S. College of Engineering, Mandya, Karnataka, India Abstract: Earthquakes are one of the most devastating natural calamities where both life and economic losses occur. Mos t of the losses and tragedies are due to structural collapse or damage; hence we need to design the structure to withstand these earthquakes. In multi storeyed buildings, damages are due to location of irregularities where large concentration of stresses leads to failure. Soft storey collapse is one such reason for failure of framed structures during earthquake. In soft storey buildings, stiffness of the la teral load resisting system at that storey is quite less compared to that of other storey. Pushover analysis is a nonlinear static approach for the seismic analysis of structures subjected to permanent vertical load and gradually increasing lateral load at very large strains up to failure. In this paper, the behaviour of multi-storeyed building will be studied for various position of soft storey along the height. For this purpose, ETABS software has been used and typical two dimensional RC frames are considered. Storey displacement, storey drift, base shear, roof displacement experienced, and status of performance point are the parameters used to quantify the performance of RC frames. It is inferred that soft storey structures are most vulnerable to earthquake. They possess lower lateral load carrying capacity and experience increased roof displacement. Key Words: Soft storey, pushover analysis, performance point, stiffness ratio, ductility demand, storey drift. I. INTRODUCTION Earthquakes are the most destructive and life threatening phenomenon of all the times. Earthquakes are caused due to the large release of strain energy during a brittle rupture of rock. The effect of an earthquake event is mainly due to the factors such as the load path distribution, effect of source and local site effect. Earthquake causes the ground to vibrate and in turn structures supported on them are subjected to ground motion and oscillations. The various factors which contribute to the structural damages occur during an earthquake event are vertical and plan irregularities, strength and stiffness irregularity, mass irregularity, torsional irregularity etc. Construction of soft storey in multi storeyed buildings is a common practice in India from past few years. This is an unavoidable feature and generally adopted for parking or reception lobbies in the first stories. Also soft storeys at different levels of buildings are constructed for offices or for any other purpose such as communication hall, banks etc. Provision for commercial and parking areas with higher floor heights and less infill walls reduce the stiffness of the lateral load resisting system at that storey and progressive collapse becomes unavoidable during earthquake for such buildings. The main focus of the present work is to carry out nonlinear static analysis to evaluate the capacity and performance of RC framed structures under seismic loading especially those structures which are having an uneven stiffness distribution in elevation. For this purpose, RC frames are initially analyzed and designed using ETABS v9.7.1 finite element software, under the combination of gravity loading and seismic loading for different seismic zones. ETABS performs the initial design depending on the earthquake zone and the type of structure. Further, it carries out pushover analysis providing the desired information. Further, the performance under non-linear incremental loading is studied based on the capacity curves generated upto the maximum displacement. II. SOFT STOREY BEHAVIOUR As per IS 1893 (Part 1): 22, irregular buildings are mainly classified into two categories, viz., plan and vertical irregularities. Vertical irregularities may be categorized as soft storey, mass irregularity, vertical geometric irregularity, in plane discontinuity and weak storey. Although soft storey and weak storey structure may cause similar structural damages during an earthquake event. The building code distinguishes between soft and weak storeys. Soft storeys are less stiff, or more flexible, than the storey above; weak storeys have less strength. A soft or weak storey at any height creates a problem, but since the cumulative loads are greatest towards the base of the building, a discontinuity between the first and second floor tends to result in the most serious condition. During an earthquake, if abnormal inter storey drifts between adjacent storeys occur the lateral forces cannot be well distributed along the height of the strucutre. This situation causes the lateral forces to concentrate on the storey(or storeys) having large displacement. In addition, if the local ductility demands are not met in the design of such a structure for that storey and the inter-storey drifts are not limited, a local failure mechanism or even worse, a story failure mechanism, which may lead to the collapse of the system, may be formed due to the high level of loaddeformation (P-Δ) effects. Figure 1 shows the collapse mechanis m of such structure with a soft storey under earthquake loads. As per Indian standard IS 1893 (Part 1) :22 International Journal of Engineering Science and Computing, July

2 ,soft storey is one in which the stiffness of the lateral load resisting system is less than 7 percent of that in the storey above or less than 8 percent of the average lateral stiffness of the above three storeys. The soft storey can be considered in many ways say by columns with lesser flexural rigidity or by higher floor height at that storey and also by not considering the infill at that storey. performance point. The capacity curve is transformed into capacity spectrum by ETABS as per ATC4 and demand or response spectrum is also determined for the structure depending upon the seismic zone, soil conditions and required building performance level. The seismic performance of a building can be evaluated in terms of pushover curve, performance point, displacement, ductility, plastic hinge formation etc. Pushover curve with performance levels and ranges are as shown in Figure 3. FIGURE 1 SOFT FIRST STOREY FAILURE MECHANISM III. PUS HOVER ANALYS IS Push over analysis is a static non-linear procedure in which the magnitude of the lateral load is incrementally increased maintaining a predefined distribution pattern along the height of the building. W ith this type of loading, failure modes and weak links of the buildings are found. Material non-linearity effects are modeled and then the structure is pushed until a collapse takes place which helps in assessing the status of plastic hinges formed in the structure. At each step, the base shear and the roof displacement can be plotted to generate the pushover curve is as shown in Figure 2. FIGURE 2 BUILDING MODEL AND SIMPLE PUSHOVER CURVE Generally, there are two types of pushover analysis namely, force control analysis and displacement control analysis. In force control analysis, the structure is subjected to incremental lateral force and the displacements corresponding to these forces are calculated. In displacement control analysis the structure is subjected to incremental increase in displacement and response force or base shear is evaluated. The target displacement or target force is intended to represent the maximum displacement or maximum force likely to be experienced by the structure during the design earthquake. Response of structure beyond maximum strength can be determined only by displacement-controlled pushover analysis. Hence, in the present study, displacement-controlled pushover method is used for analysis of RC frames. A structural analysis software package ETABS version has been used for the purpose. A pushover analysis consists of two components namely, capacity curve and demand spectrum. The point of intersection of capacity and demand spectrum at 5% damping is known as Here, FIGURE 3 CAPACITY AND DEMAND SPECTRUM WITH PERFORMANCE LEVELS IO Immediate Occupancy LS Life Safety CP Collapse Prevention IV. MODELLING AND ANALYS IS In the present study, 5 bay- 1 storey 2D RC frame with typical floor height being 3.1m and bay width of 5m is modelled. Soft storey is considered at different levels along the height of frame say Ground (G), G+3, G+6 and G+9 levels respectively. Soft storey is considered by varying the stiffness ratios as 1.,.69,.46,.32and.23 respectively. Stiffness ratio (SR) can be defined as the ratio of height of column section of soft storey to that of other storeys. Here the seismic zone IV and type of soil II are considered in the analysis. The modelling is carried out in ETABS v9.7.1 software and the loads are applied. The models are analyzed for different combinations of gravity and lateral loads. Equivalent static force method of analysis is carried out as per IS-1893 (Part I): 22. Then they are designed according to the Indian Standard code IS-456: 2 in ETABS v After the design is carried out, default plastic hinge properties available in ETABS as per ATC-4 are assigned to the frame elements and then the models are subjected to pushover analysis. The target displacement for pushover analysis is taken as 4% of the total height of the frame. The results obtained from pushover analysis are compared with those from the equivalent static method. Base shear carried, roof displacement experienced, status of the performance point and the number and status of plastic hinges formed in the structure are the parameters used to judge the performance of the models. The building parameters considered for the analysis are presented in Table I. International Journal of Engineering Science and Computing, July

3 TABLE I BUILDING PARAMETERS CONSIDERED IN THIS WORK PARAMETERS TYPE/VALUE Structure Type S.M.R.F Nu mber of stories Ground + 9 Typical storey height 3.1m Type of building for use Public building Bay width 5m Importance factor I 1. Response reduction factor 5. R Material properties Grade of concrete M25 Young s modulus of 25 x1 6 kn/m 2 concrete, E c Grade of steel Fe415 Poisson s Ratio of.2 reinforced concrete Member properties Thickness of slab.15m Beam size.3 x.5m Column size.5 x.5m Wall thickness.23m Load Intensities Live load on floor 3kN/m 2 Live load on roof 1.5kN/m 2 Floor finishes 1kN/m 2 FIGURE 4(B) G+3 STOREY SOFT FIGURE 4(A) GROUND STOREY SOFT FIGURE 4(C) G+6 STOREY SOFT International Journal of Engineering Science and Computing, July

4 FIGURE 5(B) DISPLACEMENT VS FOR SR= FIGURE 4(D) G+9 STOREY SOFT V. RES ULTS AND DIS USSION An attempt is made to find the vulnerability location of soft storey by considering the soft storey at the different levels (i.e., Ground (G), G+3, G+6 and G+9 levels) along the height of the frame. A. Variation of Storey lateral displacement FIGURE 5(C) DISPLACEMENT VS FOR SR= FIGURE 5(A) DISPLACEMENT VS FOR SR= FIGURE 5(C) DISPLACEMENT VS FOR SR=.23 International Journal of Engineering Science and Computing, July

5 BASE SHEAR(KN) BASE SHEAR(KN) BASE SHEAR(KN) BASE SHEAR(KN) Variations of displacement with soft storey at different floors for different stiffness ratio are shown in Figure 5, It can be seen that frame having SR=.23 is showing the maximum displacement and frame having SR=.69 has minimum displacement. The displacement is maximum in the floor where soft storey occurs. Soft storey structures having SR=.23 are more vulnerable to seismic excitation compared to others B. Variation of Storey drift STOREY DRIFT FIGURE 6 VS STOREY DRIFT FOR SR=.69 Variations of drift with soft storey at different floors for different stiffness ratio are shown in Figure 6. It can be seen that frame having SR=.23 is showing the maximum drift and frame having SR=.69 has minimum drift. The drift is maximum in the floor where soft storey occurs. For SR=.23 there is a higher storey drift at the levels where soft storey is located. This shows that soft storeys attract lateral forces and because of their lower stiffness, they undergo large deflections under lateral loads and frame are more vulnerable to seismic excitation compared to others. C. Variation of Pushover curve Without soft storey FIGURE 7(B) PUSHOVER CURVES FOR SR= FIGURE 7(C) PUSHOVER CURVES FOR SR= FIGURE 7(A) PUSHOVER CURVES FOR SR= FIGURE 7(D) PUSHOVER CURVES FOR SR=.23 The resulting pushover curves are shown in Figures 7(A), 7(B), 7(C) and 7(D) are obtained for different stiffness ratios and in all the cases, soft storeys is considered at different locations like Ground (G), G+3, G+6 and G+9 storey levels respectively. The curves show the similar features. They are initially linear but start to deviate from linearity consequently as the beams and the columns undergo inelastic actions. When the buildings are pushed well into the inelastic range, the curves become linear again but with a smaller slope. The curves show a decrease in the lateral load carrying capacity of the buildings International Journal of Engineering Science and Computing, July

6 DUCTILITY RATIO well before the target displacement is reached, indicating the need for seismic retrofitting STIFFNESS RATIO G+3 storey soft G+6 storey soft G+9 storey soft FIGURE 8 DUCTILITY RATIO FOR VARYING STIFFNESS RATIO AND SOFT STOREY AT DIFFERENT LEVELS Figure 8 shows the variation of ductility ratio with different stiffness ratios and the soft storey is placed at different levels. Ductility demand of structure reduces with decrease in stiffness ratio and also when soft storey is placed at ground level. Storey level TABLE II BASE SHEAR FOR STIFFNESS RATIO.69 Pushover Elasic Ratio Base Base (V po /Ve) shear shear V po (kn) Ve(kN) Collapse dis placement (m) G G G G TABLE III BASE SHEAR FOR STIFFNESS RATIO.23 Storey level Pushover Base shear V po (kn) Elasic Base shear Ve(kN) Ratio (V po /Ve) Collapse dis placement (m) G G G G SR D. Performance point Variation TABLE IV COORDINATES OF PERFORMANCE POINT Soft Storey R Location S a S d V B (kn) (m) GROUND G G G GROUND G G G GROUND G G G GROUND G G G FIGURE 9 PERFORMANCE POINTS FOR SR=.69 The elastic base shear for all the 2D frame models is obtained from the equivalent static method as per IS-1893(Part I): 22 and is compared with the base shear obtained from pushover analysis and the results are presented in Table II and Table III respectively. Base shear increases when soft storey is located at higher level, also base shear obtained from pushover analysis is much more than the base shear obtained from the equivalent static analysis. From Table II and Table III, it can be seen that soft storey at ground level having a least collapse displacement compare to soft storey located at other storeys. It means that model having a soft storey at ground level will fail earlier than the other models. Thus, it can be said that it is the most vulnerable case during earthquake. FIGURE 1 PERFORMANCE POINTS FOR SR=.46 International Journal of Engineering Science and Computing, July

7 FIGURE 11 PERFORMANCE POINTS FOR SR=.32 varying the location of soft storey and stiffness ratio. The results obtained herein lead to the following conclusions. The seismic performance of structures is very sensitive to stiffness ratio. The lower the stiffness ratio of soft storey more vulnerable the structures will be to the earthquake excitations. Ductility capacity of the structure reduces when the structure becomes irregular in stiffness and also when the soft storey is placed at ground level. The performance point shifts towards the rightwards when stiffness ratio decreases indicating more vulnerability of the structure. In soft storey models, the storey drift and storey displacement is higher at the levels where stiffness ratio is least (SR=.23) or at higher floor height in the building. This is due to the fact that soft storeys attract larger lateral forces and because of their low stiffness then they undergo large deflections under lateral loads. The base shear carrying capacity of the structure reduces with the decrease in stiffness ratio. The base shear carrying capacity of the structure increases as the soft storey is placed at higher levels and is least when the soft storey is at the ground. The soft storey structures are most vulnerable to seismic excitation compared to regular structures. VII. REFERENCE [1] Ramya R. Pai, et al, Seis mic Performance of Soft Storey RC Frames at Different Storey Levels from pushover Analysis IOSR Journal of Mechanical and Civil Engineering, e-issn: , p-issn: X, PP FIGURE 12 PERFORMANCE POINTS FOR SR=.23 Figure 9, Figure 1, Figure 11 and Figure 12 shows the performance point for varying stiffness ratios.69,.46,.32 and.23 respectively and also for soft storey at different levels. The point of intersection of capacity and demand spectrums is known as performance point.the performance point shifts towards the elastic stage, when the soft storey is placed at higher levels. Also, capacity versus demand curves indicates that the performance point gets shifted towards lower spectral displacement and higher acceleration for soft storey located at higher levels. Hence, it can be said that soft storey located at lower levels of building cannot withstand higher level of ground accelerations and need to be retrofitted to perform better during seismic excitations. Hence the structure is most vulnerable when soft storey is at ground level. VI. CONCLUS IONS In the present work, the effect of stiffness irregularities on seismic performance of 2D RC frames has been carried out. Equivalent static method and pushover analysis has been performed to assess the seismic performance of the frames with [2] Sunil N, B Shiva Kumaraswamy and S K Prasad, Seis mic Performance Assessment of Soft Storey Structures Using Pushover Analysis, National Conference on Recent Advancements in Infrastructural Development (RAID 215) th March 215, JSS Academy of Technical Education, Bengaluru. [3] CSI (213), CSi Analysis Reference Manual, Computers and Structures, Inc., Berkeley, California. [4] Rahiman G. Khan and M. R. Vyawahare (213), Push Over Analysis of Tall Building with Soft Storey at Different Levels, International Journal of Engineering Research and Applications (IJERA), Vol. 3, Issue 4, pp [5] Sujay Deshpande, (29), Seismic Performance Study of R.C. Frames W ith Stiffness Irregularities Using Pushover Analysis, M.Tech Thesis, Department of Civil Engineering, SJCE, Mysuru. [6] Applied Technology Council, ATC-4, (1996), Seismic Evaluation and Retrofit of Concrete Buildings, Vol. 1, Applied Technology Council, Redwood City, California. [7] IS: (part 1), Indian Standard Criteria for Earthquake Resistant Design of Structures, fifth revision, Bureau of Indian Standards, New Delhi. International Journal of Engineering Science and Computing, July