Comparative Study on Concentric Steel Braced Frame Structure due to Effect of Aspect Ratio using Pushover Analysis

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1 247 Comparative Study on Concentric Steel Structure due to Effect of using Pushover Analysis Kiran Kamath 1, Shruthi 2, Shashikumar Rao 2 1 Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal, 2 Post Graduate Student, Structural Engineering, Department of Civil Engineering, Manipal Institute of Technology, Manipal, ABSTRACT The present study focuses on the effect of different aspect i.e. H/B ratio, where H is the total height of the building frame and B is the base width of the building frame, on the seismic performance of the steel frame structures. Here, height of the building is kept constant and the base width is varied. In the present study, seven different aspect ranging from 1.0 to 3.75 have been considered for the ten storey steel frame building with concentric bracing i.e. X bracing and without bracing system. Two types of frames are considered for the study, one with similar steel sections for maximum strength required for beam and column and the other with varying steel sections conforming to the strength and serviceability requirements to withstand the specified loading. For this analytical study, ETABS is used and the comparison between the performances of bare frames with different aspect is made using pushover curves. Roof displacement, base shear carried and performance point are the parameters used to identify the seismic performance of the frames. It is inferred that provision of bracings to the frame structure increased the base shear carrying capacity, performance point and reduced the roof displacement for all types of aspect considered. Keywords -, Pushover analysis, Steel, X, Type 1 Section, I. INTRODUCTION In last decades Steel structure plays an important role in the construction industry. It is necessary to design a structure to perform well under seismic loads. Shear capacity of the structure can be improved by introducing Steel bracings in the structural system. Bracings can increase the energy absorption of structures or decrease the demand forced by earthquake loads. With the addition of bracings to the structures with amplified energy dissipation may safely resist forces and deformations triggered by robust ground waves. Bracings can be used as retrofit as well. Design of such structure should have good ductility property to perform well under seismic loads. To evaluate ductility and other properties for bracing Push over analysis is performed. In earthquake resistant design, structures are usually designed for lower levels of seismic forces and allowed to undergo nonlinear response under severe ground motion. Hence, a nonlinear static pushover analysis has become popular in recent years to determine parameters such as initial stiffness, yield load, yield displacement, maximum base shear and maximum displacement. Khan and Khan, (2014) studied on the typical 15 th storey regular steel frame building with different pattern of bracing system. This building is designed for various types of concentric bracings like V, X, Diagonal and Exterior X and performance of each frame is carried out through nonlinear pushover analysis. Three types of sections i.e. ISA, ISMC, ISMB are used to compare for same patterns of bracing and results are compared with roof displacement and performance point [1]. Kalibhat et al., (2014) studied on the effect of a provision of concentric bracings on the seismic performance of the steel frames with two different types of concentric bracings (viz. X and inverted-v type bracing) for the different storey levels. They found that inclusion of bracing increased the base shear capacity and decreased the roof displacement and also reduced the inter storey drift. The lateral storey displacements of the building are reduced by the use of inverted-v bracing in comparison to the X bracing system [2]. Vijayakumar and Babu, (2012) estimated the behavior of G+2 reinforced concrete bare frame subjected to earthquake forces in zone III. The reinforced concrete structures were analyzed by nonlinear static analysis using SAP2000 software. The results obtained in terms of pushover demand, capacity spectrum and plastic hinges gave an insight into the real behavior of structures. Most of the hinges have developed in the beams in the form of immediate occupancy, Life safety, Collapse prevention and few in the columns. The column hinges have limited the damage [3]. Poluraju and Rao, (2011) assessed the performance of G+3 building using pushover analysis. Observations showed that properly designed frame will perform well under seismic loads. It was found that the hinges developed more in the beams than the columns; thereby column had limited damage [4]. Kadid and Yahiaoui, (2011) examined the seismic behaviour of RC buildings strengthened with different types of steel braces, X-braced, inverted V braced, ZX braced, and Zipper braced and nonlinear pushover

2 248 II. analysis is carried out. For the analysis purpose they considered three and six story RC building with different patterns of bracing system with different cross sections as mentioned above. Conclusions showed that adding braces influenced the global capacity of the buildings in terms of strength, deformation and ductility in comparisons with bare frame. They found that the X and Zipper bracing systems performed better than other braces depending on the type and size of the cross section [5]. Inel and Ozmen, (2006) carried out push over analysis using SAP [2000] comparing the performance of the building for default hinge properties and user defined hinge properties. They have concluded that the result obtained from user defined hinge properties are more accurate than of default hinges [6]. Maheri and Hadjipour, (2003) conducted experiments on scaled models of ductile RC frames with steel X and knee bracing system. Experiment results signifies that the yield and strength capacity of a ductile RC frame can be increased and its global displacements can be reduced to the desired levels by directly adding either X or a knee bracing system to the frame. Both X and knee bracing systems may be used to design or retrofit for a damage-level earthquake, whenever designing or retrofitting for a collapse-level earthquake is considered knee bracing is a effective system [7]. From previous work we can observe that many experimental and analytical works have been done in the area of the pushover analysis of the RC frames and few works on steel frames with different types of bracing systems. Since no work is done on aspect of steel frames with different types of bracing systems. Hence, the present work is focused on the effect of different aspect on the seismic performance of the steel frames with X bracing systems using ETABS and results are analyzed through pushover analysis. PUSHOVER ANALYSIS Linear elastic analysis gives a good indication of elastic capacity of structures and indicates where the first yielding will occur but it cannot predict failure mechanisms and accounts for redistribution of forces due to progressive yielding. Among different approaches described in ATC-40, Nonlinear Static Pushover analysis is very popular because of its simplicity and ability to estimate component and system level deformation demands with acceptable accuracy without intensive computational and modeling effort as dynamic analysis. Pushover analysis is a static, nonlinear procedure in which the magnitude of the structural loading is incrementally increased in accordance with a certain predefined pattern. Pushover analysis may be categorized as displacement controlled pushover analysis when lateral movement is executed on the building and its equilibrium designates the forces. In the same way, when lateral forces are enforced, the analysis is termed as force-controlled pushover analysis. The target displacement or target force is projected to signify the III. maximum displacement or maximum force expected to be qualified by the structure during the design earthquake. Response of structure beyond full strength can be bent on only by displacement controlled pushover analysis. Hence, in the present study, displacement-controlled pushover method is used for analysis of structural steel frames. A plot of the total base shear versus top roof displacement in a building is attained by this analysis that would specify any early failure or weakness. The analysis is performed up to failure, thus it permits purpose of collapse load and ductility capacity. A typical pushover curve is shown in Figure 1. Force versus displacement is plotted for gradually increasing lateral loads till failure. Beyond elastic limit, different states such as Immediate Occupancy (IO), Life Safety (LS), Collapse prevention (CP), >E collapse are defined as per ATC 40 and FEMA 356. Figure 1: Typical pushover curve DESCRIPTION OF STEEL FRAME STRUCTURES (i) (ii) (iii) Figure 2: (i) Steel bare frame, (ii), (iii) for aspect ratio1.0 In the present study, a 2- bay two dimensional steel frame structures with one bay X braced frame, two bay X braced frame and structure without bracing with different aspect

3 249 has been modeled and analyzed using ETABS. Two types of frames are considered for the study, one with similar steel sections (Type 1) for maximum strength required for beam and column and the other with varying steel sections (Type 2) confirming to the strength and serviceability requirements to withstand the specified loading. Structural configuration of different types of framed structures of aspect ratio 1.0 is shown in the Figure 2. The building consists of G+9 stories. All columns in all models are assumed to be fixed at the base for simplicity. The height of each floor is 3.0m. Live load on floor is taken as 3kN/m 2 and on roof is 1.5kN/m 2. Floor finish on the floor is 1kN/m 2. Weathering course on roof is 2kN/m 2. In the seismic weight calculation only 25 of floor live load is considered. The unit weights of concrete and masonry are taken as 25kN/m3 and 20kN/m3 respectively. The building is steel moment resisting frame with concentric bracing considered to be situated in seismic zone III. The medium type of soil is considered and time period of the building in X-direction is considered based on base dimension of the building as per IS code The sizes used for beam is Girder Section1, column is Girder Section2 and that of X bracing is ISWB600 for Type 1 section. Beam and column sizes for Type 2 sections as per SP 6 (1) 1964 are tabulated in Table 1. Girder Section1: Web plate (800x12) mm, Flange angle (150x150x18) mm, Flange plates (400x40) mm Girder Section2: Web plate (800x12) mm, Flange angle (150x150x18) mm, Flange plates (500x32) mm Girder Section3: Web plate (800x12) mm, Flange angle (150x150x18) mm, Flange plates (400x16) mm IV. RESULTS AND DISCUSSION Linear static and pushover analysis is conducted on all the models for seismic loads defined as per IS (Part-I) using ETABS. The pushover analysis provides an insight into the structural aspects, which controls the performance during earthquakes. It also provides data on the strength and ductility of a building. The results obtained from analysis are compared and discussed as follows. Figure 3: Pushover curves for bare frame and one bay X braced frame structures of Type 1 section with different aspect Table 1: Beam, column and bracing sizes of Type 2 section for different aspect Base Width B in m Beam Size Column Size X Brace Girder Section1 Girder Section2 ISWB Girder Section3 ISWB600 C.P 32mm ISMB ISMB550 C.P 40mm ISMB 600 C.P 25mm ISWB600 C.P 32mm ISWB400 C.P 32mm ISMB 500 ISMB ISMB 600 ISWB 600 ISMB ISMB 450 ISWB 600 ISMB ISMB 400 ISWB 550 ISMB 400 C.P: Cover Plate Figure 4: Pushover curves for bare frame and two bay X braced frame structures of Type 1 section with different aspect From Figures 3 and 4, Steel Bare, frame and braced frame of Type 1 section with aspect ratio 1.0 is showing 34, 49 and 49 better performance in terms of performance base force when compared to aspect ratio 3.75 respectively. It is also found that for aspect 1.5 and 1.25, performance base force has marginally increased when compared to aspect ratio 1.0

4 250 because of negligible variation in mass. As bracing is introduced to the bare frame structures it increased the performance of the base force and ductile behaviour of the structure. Figure 7: Pushover curves for bare frame, one bay X braced frame and two bay X braced frame structures of Type 1 section for aspect 1.0 Figure 5: Pushover curves for bare frame and one bay X braced frame structures of Type 2 section with different aspect Figure 6: Pushover curves for bare frame and two bay X braced frame structures of Type 2 section with different aspect From Figures 5 and 6, Steel Bare, frame and braced frame of Type 1 section with aspect ratio 1.0 is showing 92, 88 and 91 better performance in terms of performance base force when compared to aspect ratio 3.75 respectively. It is also found that higher the aspect lesser is the performance base force for all types of frame structures. Figure 8: Pushover curves for bare frame, one bay X braced frame and two bay X braced frame structures of Type 1 section for aspect 3.75 From Figures 7 and 8, it is found that as bracing is introduced to the structure maximum base force increased considerably for all aspect considered. It is also found that two bay X braced frame is showing higher performance base force than other types of structures. From Tables 2 and 3, it can be observed that as aspect ratio increased performance of base shear decreased considerably from aspect ratio 1.0 to 3.75 for both type of sections. As aspect ratio increased roof displacement decreased considerably from aspect ratio 1.0 to 3.75 for Type 1 section and that of Type 2 section increased considerably. It is also observed that bracing enhances the base shear carrying capacity and reduced the roof displacement. Two bay X braced frame is showing better performance than bare frame and one bay X braced frame structures.

5 251 Table 2: Base Shear for Linear Static Analysis of different frame structures for Type 1 and s Bare B.S in kn Type 1 Section B.S in kn Increa se B.S in kn Incre ase B.S: Base Shear Table 3: Roof Displacement for Linear Static Analysis of different frame structures for Type 1 and s Bare R.D in mm Type 1 Section R.D in mm Decre ase R.D in mm Decre ase Figure 9: Capacity and Demand spectrum curve for bare frame, one bay X braced frame and two bay X braced frame structures of Type 1 section for aspect 3.75 From Figure 9, it can be observed that as bracing is introduced to the structures the performance point increases considerably, where performance point is the intersection between the capacity curve and demand curve. This represents the performance of the building. Table 4: Base shear and roof displacement at performance point for Type 1 section Type 1 Section Performance Point(V(kN),d(mm)) Bare 1.00 (936.69, 35) ( ,23) ( , 14) 1.25 (881.28, 30) ( ,20) ( , 11) 1.50 (829.40, 27) ( ,17) ( , 09) 2.00 (735.60, 23) (839.74,14) (873.44, 7.4) 2.50 (651.09, 21) (669.40,12) (687.98, 7.2) 3.00 (569.07, 20) (557.44,12) (568.46, 7.4) 3.75 (457.63, 18) (448.75,12) (455.31, 8.3) R.D: Roof Displacement

6 252 Table 5: Base shear and roof displacement at performance point for Type 2 section Performance Point(V(kN),d(mm)) Bare 1.00 (936.69, 35) ( , 23) ( , 14) 1.25 (657.23, 38) (914.36,23) ( , 12) 1.50 (488.39, 41) (702.39,24) (821.94,13) 2.00 (372.16, 41) (563.84,27) (664.97,14) 2.50 (266.19, 43) (433.75,32) (551.69,17) 3.00 (175.48, 53) (316.89,36) (446.92,18) 3.75 (135.23, 55) (218.76,41) (322.14,24) From Tables 4 and 5, it can be observed that as aspect ratio increased base shear at performance point decreased considerably from aspect ratio 1.0 to 3.75 for both type of sections. As aspect ratio increased roof displacement at performance point decreased considerably from aspect ratio 1.0 to 3.75 for Type 1 section and that of Type 2 section increased considerably. Two bay X braced frame is showing better performance than bare frame and one bay X braced frame structures. V. CONCLUSIONS The following are the observations from present analysis. 1. As aspect ratio increases, base shear carrying capacity decreases for both type of section considered in the study. 2. As aspect ratio increases, roof displacement decreases for frames with Type 1 section and for Type 2 section it increases considerably. 3. The provision of bracing enhances the base shear carrying capacity of frames and reduces roof displacement undergone by the structures. 4. Steel frame with aspect ratio 1.0 and two bay X braced frame showed better performance in terms of performance base force and performance point when compared to other aspect and bare frame, one bay X braced frame structures considered in the study. REFERENCES [1] Khan M. I. and Khan K.N.,(2014), Seismic Analysis of Steel with Bracings using Pushover Analysis, International Journal of Advanced Technology in Engineering and Science, Volume No. 02, Issue No. 07, Page No [2] Kalibhat M.G., Kamath K., Prasad S. K. and Pai R.R., 2014, Seismic Performance of Concentric Steel s from Pushover Analysis, IOSR Journal of Mechanical and Civil Engineering, Page No [3] Vijaykumar and Babu V. D.L., 2012, Pushover Analysis of Existing Reinforced Concrete d Structures, European Journal of Scientific Research, Volume No. 71, Issue No. 02, Page No [4] Poluraju.P. and Rao N. P.V.S., 2011, Pushover analysis of reinforced concrete frame structure using SAP 2000, International Journal of Earth Sciences and Engineering, Volume No. 04, Issue No. 06, Page No [5] Kadid A. and Yahiaoui D., 2011, Seismic Assessment of RC s, Procedia Engineering, Volume No. 14, Page No [6] Inel M. and Ozmen H.B., 2006, Effects of plastic hinge property in nonlinear analysis of reinforced concrete buildings, Engineering Structure, Volume No. 28, Page No [7] Maheri M.R. and Hadjipour A., 2003, Experimental investigation and design of steel brace connection to RC frame, Engineering Structures, Volume No. 25, Page No [8] IS 800, 2007, General construction in steel, Bureau of Indian Standards, New Delhi. [9] IS 1893(part 1), 2002, Provision on seismic design of buildings, Bureau of Indian Standards, New Delhi. [10] FEMA 356, 2000, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, Washington DC. [11] ATC-40, 1996 Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technical Council, California Seismic Safety Commission, Redwood City, California. [12] SP 6 (1), 1964, Hand book for Structural Engineer, Bureau of Indian Standards, New Delhi. VI. ACKNOWLEDGMENT The authors would like to express their sincere thanks to The Director, and H.O.D, Civil engineering, Manipal Institute of Technology, Manipal for providing necessary facilities required for the present study.