SEISMIC ANALYSIS OF SIX STOREYED RC FRAMED BUILDING WITH BRACING SYSTEMS

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1 SEISMIC ANALYSIS OF SIX STOREYED RC FRAMED BUILDING WITH BRACING SYSTEMS ABSTRACT Megha Kalra Assistant Professor, The North Cap University, Gurgaon(India) Multistoreyed buildings are most affected by earthquake forces in seismic prone areas. The major concern in the design of the multi-storey buildings is the structure to have enough lateral stability to resist lateral forces and to control the lateral drift of the building. The use of steel bracing systems in reinforced concrete frames is a viable solution for resisting lateral forces. Steel bracing is economical, easy to erect, occupies less space and has flexibility in design for meeting the required strength and stiffness. In the present study six storeyed building is analyzed with four different types of bracings and each bracing has been provided at three different locations. The types of bracing studied are X-brace, V-brace, inverted V-brace and K-brace. In the first location, bracings are provided in the exterior frame at corners. In the second location, bracings are again provided in the exterior frame, but in the middle bays. Finally, in the third location, bracings are provided in the middle bays in exterior and interior frames both. In all twelve different cases of braced RC frames are analyzed and compared with unbraced RC frame, using STAAD PRO-2007 with Response Spectrum method. It has been seen that X bracing shows the best performance. Index Terms: RC Frame, Steel Bracing, Fundamental Time Period, Base Shear, Lateral Displacement, Storey Drift, Axial Force I. INTRODUCTION The aftermath of an earthquake manifests great devastation due to unpredicted seismic motion striking extensive damage to innumerable buildings of varying degree, i.e. either full or partial. This damage to structures in turn causes irreparable loss of life with a large number of casualties. Therefore, most buildings are designed with lateral-force-resisting systems to resist the effects of earthquake forces. In many cases laterally braced systems make a building stiffer against horizontal forces, and thus minimize the amount of relative lateral movement and consequently the damage. It can be concluded that both structural and non-structural damages are observed during earthquake ground motions are primarily produced by lateral displacements. Therefore, in order to increase the seismic strength of framed structures, steel bracing or shear walls are often used [4]. However, considering the ease of construction and the relatively low cost, steel bracing appears to be a better alternative. It was therefore considered necessary to carry the present study on Structural Behavior of RC building with different laterally braced systems to assess the performance of the bracing system. 297 P a g e

2 1.1 Types of bracings Bracing systems are classified depending on whether the braces create perfect triangulation. Based on this they are most commonly used under two categories, Concentric bracing system and eccentric bracing system Concentric Bracing The Bracing is concentric when the center lines of the bracing members intersect. Concentric bracings increase the lateral stiffness of the frame, thus increasing the natural frequency and also usually decreasing the lateral drift [5]. However, increase in the stiffness may attract a larger inertia force due to earthquake. Further, while the bracings reduce the bending moments and shear forces in columns, they increase the axial compression in the columns to which they are connected. Different types of concentric bracing are: V brace- Bracing where a pair of braces, located both above beam, terminates at a single point within the clear beam span. Inverted V brace- is that form of chevron bracing that intersects a beam from below. X brace- Bracing where a pair of diagonal braces crosses near mid-length of the bracing members. K brace- Bracing where a pair of braces located on one side of a column terminates at a single point within the clear column height Eccentric Bracing In an eccentrically braced frame bracing members connect to separate points on the beam/girder. The beam/girder segment or link between the bracing members absorbs energy from seismic activity through plastic deformation [6]. Eccentric Bracings reduce the lateral stiffness of the system and improve the energy dissipation capacity. Due to eccentric connection of the braces to beams, the lateral stiffness of the system depends upon the flexural stiffness of the beams and columns, thus reducing the lateral stiffness of the frame. The vertical component of the bracing forces due to earthquake causes lateral concentrated load on the beams at the point of connection of the eccentric bracings. Fig.1: Typical arrangements of eccentric bracings II. PARAMETERS OF THE BUILDING The building considered had plan dimensions of 20m x 12m at each floor level and was considered to be in Indian seismic zone V. The frames were assumed to be firmly fixed at the bottom and the soil structure interaction was neglected. The building was analyzed with three different types of bracings viz X brace, V brace, inverted V brace and K brace at three different location L1, L2 and L3(Refer Figs. 1-3). 298 P a g e

3 Fig.1: Plan of building showing bracings at location L1 Fig.2: Plan of building showing bracings at location L2 Fig.3: Plan of building showing bracings at location L3 Represents bracing members The building had 5 bays of 4m each in X direction and 3 bays of 4m each in Z direction. The height of each storey was 3.00 m. The building was to be used for residential purpose and the ductile detailing of steel was done as per Indian standard code (IS) 13920:1993[1] for special moment resisting frame. The grade of concrete used for beams was M30 and for columns was M40. The grade of steel used was Fe415. Member Properties TABLE I Thickness of slab Size of Beams Size of Columns Size of Bracings m 0.45x0.3 m 0.45x0.45 m Double Angle sections placed back to back of optimal size 299 P a g e

4 Loading- Various types of load considered are discussed in succeeding sections. Dead Load- The dead load on all floors was considered as 6 KN/m 2. The wall load of inner 4.5 thick brick wall was taken as 7.5 KN/m and that of outer 9 thick brick walls was taken as 12 KN/m (after deductions for openings). Live Load- The live load had been taken as 3.00 KN/m 2 for intermediate floors and 1.5 KN/m 2 for roof. Seismic Load- As per IS [2], the dynamic analysis was performed using Response Spectrum Method. In response spectrum method design parameters for horizontal seismic coefficient were as below: Z, zone factor = 0.36 I, importance factor = 1 R. response reduction factor = 5 Damping ratio = 0.05 Load Combinations- The different load combinations to be analyzed were as per IS 875(Part 3):1987 III. ANALYSIS OF RESULTS The building frames have been analyzed using response spectrum method in STAADPRO-2007, which is based on stiffness matrix method of analysis. Various parameters like Fundamental time period, Base shear, Lateral displacements, Storey shears, Bending moment, Shear force and axial force in various members have been compared in the succeeding sections. 3.1 Time Period of building From Table II it is evident that with the addition of bracing members the fundamental time period of the building decreases. Building with X bracing at location L1 gives minimum time period of seconds as compared to seconds of unbraced building. It has been seen that the percentage reduction for six storeyed building varies from 3.7 to 35% with maximum reduction in X bracing at location L1. TABLE II Fundamental Time Period (secs) of building 3.2 Spectral acceleration of building From Table III it has been concluded that the maximum increase is seen for X braced building with value of and the minimum increase for inverted V braced building with the value of spectral acceleration as So there is 28.50% increase in X bracing at L1 location and 3.85% increase for K bracing at L3 location. Time Period(secs) for 6 storeyed building Unbraced L1 L2 L3 X Brace V Brace Inverted V Brace K Brace P a g e

5 TABLE III Spectral acceleration building Spectral acceleration for 6 storeyed building Unbraced L1 L2 L3 X Brace V Brace Inverted V Brace K Brace Base Shear of building It is evident that the base shear increases with the addition of bracing members. Fig. 4 showed that the maximum increase is seen in X braced building. This is because with the increase in spectral acceleration the horizontal seismic coefficient (A h ) increases. The maximum increases is recorded for X bracing at location L1 of 84.82% and minimum of 10.11% for inverted V bracing for L3 location. Fig.4: Base Shear of building 3.4 Lateral Displacement at various height Typical nodes at storey height of 0m, 3m, 6m, 9m, 12m, 15m, 18m are selected. For comparative study displacements are checked at these nodes for all load combinations. The results were compared and it is seen that the displacement increased with the increase in storey height but the braced building showed less displacements as compared to unbraced building (refer Fig.5-8). The maximum displacement for building with X bracing at location L1 is mm as compared to mm in unbraced building. So there is 39% decrease for X bracing at L1 location. In V, inverted V and K braced building the reduction is up to 25% for all heights. The maximum percentage reduction is observed at 12 m height. 301 P a g e

6 Fig. 5: Lateral Displacement in X-braced building Fig. 6: Lateral Displacement in V-braced building Fig. 7: Lateral Displacement in inverted V-braced building Fig. 8: Lateral Displacement in K-braced building 302 P a g e

7 3.5 Storey Drift of Building As per IS1893:2002 [2], the storey drift in any storey due to minimum specified design force shall not exceed times the storey height. From Fig. 9-12, it is found that all the results are within permissible limits. It is observed that drift increased up to height of 6 m and then showed a considerable decrease. It is very clear that drift is more when no bracing members are provided but when bracing member are provided than drift decreases. By comparison X bracing showed least drift as compared to other cases. The maximum drift of 0.27 cm is observed for building with X-bracing at location L1. So there was 58.39% reduction for this case. Similarly, in V, inverted V and K braced building maximum percentage reduction of 55.9 %,12.84% and % is observed for location L2. Fig. 9: Storey Drift in X-braced building Fig. 10: Storey Drift in V-braced building Fig. 11: Storey Drift in inverted V-braced building 303 P a g e

8 Fig. 12: Storey Drift in K-braced building 3.6 Axial Forces in bracings The results of axial forces in bracing members are shown in Table IV to VI for all cases. The members selected are designated as A1, A2, A3, B1, B2, B3, C1, C2, and C3. The member designation of A, B, C for the three locations L1, L2, L3 is shown in Fig. 1-3 and 1 represents the members on ground level, 2 represents members on third floor level and 3 represents members on top floor level. It is evident that axial force is more for the cases which showed less bending moment, shear force and vice versa. This is because the lateral loads are transferred from columns and beams to bracing members. After comparison (refer table IV-VI) it can be stated that X braced building carried maximum axial force in bracings at all the three locations L1, L2, L3. TABLE IV Axial Force(KN) in Bracing members at L1 Location Bracing X Brace V Brace Inv. V Brace K Brace A A A B B B C C C TABLE V Axial Force(KN) in Bracing members at L2 location `Bracing X Brace V Brace Inv. V Brace K Brace A A A B B B C C C P a g e

9 TABLE VI Axial Force(KN) in Bracing members at L3 location Bracing X Brace V Brace Inv. V Brace K Brace A A A B B B C C C Bending moment and Shear force in columns Bending moment and Shear force at the base of various typical columns is studied for all load cases. It is found that bending moment and shear force is more when no bracings are provided but when bracing members are provided a considerable decrease is seen. This is due to the transfer of lateral loads from columns to bracing members. It is observed that X braced building has least forces in columns with percentage reduction up to 62% and 41% in bending moment and shear force respectively. IV CONCLUSIONS It has been seen that the performance of the building enhanced with the provision of bracings in the framed system. It is recommended that the building with X bracing at exterior frame in alternate bays showed the best performance. The K bracings are the least preferred bracing type. REFERENCES 1. IS 13920:1997, Ductile detailing of reinforced concrete structures subjected to seismic forces-code of practice. 2. IS 1893(part 1):2002, Criteria for earthquake resistant design of structures, part 1general provisions and buildings. 3. IS 875(Part 5):1987, Code of Practice For Design Loads (Other Than Earthquake) For Buildings And Structures Part 5 Special Loads And Combinations. 4. Maheri, M.R. and Sahebi, A. (1997). Use of Steel Bracing in Reinforced Concrete Frames, Engineering Structures, 19(12), Desai J. P., Jain A. K. and Arya A. S., Seismic response of R. C. braced frames, Computers and Structures Volume 29 No.4, pp , Ravikumar G. and Kalyanaraman V., Seismic design and retrofit of RC multistoried buildings with steel bracing, National Program on Earthquake Engineering Education, P a g e