IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 04 | September 2016 ISSN (online): 2349-6010
Seismic Performance Assessment of Different Types of Eccentric Braced Systems Sreeshma. K. K M. Tech. Student Department of Civil Engineering FISAT, Kerala, India- 683577
Nincy Jose Assistant Professor Department of Civil Engineering FISAT, Kerala, India- 683577
Abstract Steel structures are exhibit ductile behaviour when subjected to transient lateral loading, caused by wind or earthquake action. Steel bracings are lateral load resisting system used in seismic region. In eccentrically braced frames, a distance is either created between the two ends of the bracing members, or between the bracing member and the column, or between the bracing member and the beam. The created distance is called link. The main function of the link is to provide a weak section in the frame which provides plastic deformation capacity and the energy released by the earthquake. EBF configuration is similar to traditional braced frames with the exception that at least one end of each brace must be eccentrically connected to the frame. The energy dissipation is achieved through the yielding of a beam segment called the link, while the other frame members, including outer beam segments, braces, and columns, should remain essentially elastic. Present study deals with eccentrically braced frames with short link. Assess the performance of different types of eccentrically braced frames by performing seismic analysis over the structure. The variation of performance with change in link length, height of the building was also considered in this study. Keywords: Eccentrically Braced Frames, Plastic Deformation, Seismic Analysis _______________________________________________________________________________________________________ I.
INTRODUCTION
Steel structures are widespread; they exhibit ductile behavior when subjected to lateral loading, caused by wind or earthquake action. There are three main types of frames in steel structures, Moment frames, truss moment frames, and braced frames. Braced frames are of two types-concentrically braced frames (CBF) and eccentrically braced frames (EBF). The concentric bracings increase the lateral stiffness of the frame and they usually decrease the lateral drift. However, increase in the stiffness may attract a larger inertia force induced due to earthquake. Further, while the bracings decrease the shear forces and bending moments in columns and they increase the axial compression in the columns to which they are connected. Eccentrically braced frames (EBFs) are relatively a new lateral force resisting system developed to resist seismic events in a suitable manner. Eccentric bracings minimize the lateral stiffness of the system and improve the energy dissipation capacity. The energy dissipation is achieved through the yielding of a beam segment called the link and the other frame members, including outer beam segments, braces, and columns, should stay essentially elastic. The main function of link is to provide a weak section in the frame which provides plastic deformation capacity and dissipate the energy released by the earthquake. EBF systems with shear yielding links are stable and exhibit more ductility than in the case of EBF systems with flexural yielding links, since internal shear force is constant along the length of the link. This present study concentrated on eccentrically braced frames with shear links. The performance of different types eccentrically braced frames is evaluated and it compared with concentric and normal steel building. Linear static, linear dynamic and non- linear static analysis were performed over the structure and its performance are evaluated based on storey displacement, storey drift, time period, ductility and energy dissipation. It is also evaluate the frame performance with two different heights and with different shear link lengths by conducting same seismic analysis. II. LITERATURE REVIEW Significant researches were carried out on seismic behaviour of eccentrically braced frames and a few published works on seismic response steel framed structures are reviewed in this section. Muthumani K studied the seismic performance of steel moment resisting frames with eccentric braces. It has been observed that the performances of building 6 storeys, 9 storey and 15 storey with eccentric configuration has minimum displacement. SandaKoboevic et al studied the global seismic response of 3and 8-story eccentrically braced frames (EBFs), designed for western and eastern North American locations. Different models did not consistently predict either the magnitude of maximum deformation or their location over the height of structure, and large record-to-record variations were observed when examining 84th percentile results. M. A. Musmar investigated the effect of link on eccentrically braced frames. The study incorporated applying lateral loading to several types of eccentrically loaded frames. All eccentric frames in this study exhibit lateral stiffness close to that of concentrically braced frames and demonstrate ductility similar to that of the Moment Frame hence EBFs are efficient laterally stiff framing systems with significant energy dissipation capability to accommodate large seismic and wind forces.
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Seismic Performance Assessment of Different Types of Eccentric Braced Systems (IJIRST/ Volume 3 / Issue 04/ 023)
III. MODELING IN ETAB Modeling and analysis of the tank is performed in ETAB 2015. Two group of models were modeled in ETAB 2015.One group consist of 6 number of 6 storey models and another group consist of 6 number of 10 storey models. Each group consist of four types of steel eccentric braced frames such as inverted- v braced frame, diagonally braced frame, Chevron braced frame(V braced frames), Zipper type frame, one concentrically-x braced frame and one normal moment resisting frame. Concentric x configuration was chosen because various studies show that among different types of concentric configuration concentric x configuration show better performance. The geometric details and the section details of the building are provided in table. 1 and 2 respectively. Table – 1 Geometric Details of the building Type of building Steel Plan area of building 24x18 No bays in X direction 5 No bays in Y direction 3 Number of stories 10 and 6 Bay width 6m Total height 30m(10 storey),18m(6 storey) Storey height 3m Table - 2 Section Properties Column ISWB350 Beam and links ISHB350 Bracing ISB 91.5X91.5X5.4,ISNB250H
Fig. 1: ETAB model of MRF and V-type EBF
Fig. 2: ETAB model of inverted V-type EBF and Diagonal EBF
IV. LOADS CONSIDERED IN THE STUDY The loads applied on building consist of dead load, live load, wind load and earthquake load .It is assumed that building is situated in Kerala that means in moderate seismic region and is a commercial steel building. So loads are considered from
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Seismic Performance Assessment of Different Types of Eccentric Braced Systems (IJIRST/ Volume 3 / Issue 04/ 023)
conditions corresponding IS: 875-1987 and IS: 1893(Part 1)-2002.The gravity loads such as super dead load, live load applied on structures are 2 KN/m2 ,3 KN/m2 respectively and dead loads acting on a structure will calculate the software itself. V. ANALYSIS RESULTS Static Analysis The top storey displacement and structural weight are taken as parameter from this analysis. Top storey displacement and structural weight of CBF, EBF and MRF are studied and compared. Also the top storey displacement and structural weight of four different types of EBF such as vtype, inverted v type, d-type and z type also studied and compared. The results obtained for 10 storey models are given below in figure 3,4,5,6.The results for 6 storey also same as that of 10 storey.
Fig. 3: Storey displacement for CBF,EBF,MRF(10 storey)
Fig. 4: Storey displacement for four types of EBF (10 storey)
Comparison of the structural weight of 10 storied and 6 storied CBF, EBF and MRF are shown in below table 3.It is observed that structural weight of EBF is in between CBF and MRF. The structural weight of V-TYPE is higher than other configuration. Table – 3 Structural weight (kN) TYPE 10 storey MRF 10545.3 V-type EBF 10830.25 Inverted V-type EBF 10814 D-type EBF 10758 Z-type EBF 10828 CBF 11018
6 storey 6381.7 6581 6574 6528 6580 6714
Response Spectrum Analysis The results of response spectrum analysis are shown in following graph. The storey displacement, storey drifts and time period were taken as parameter for this analysis.
Fig. 5: Storey displacement for CBF, EBF, MRF (10 storey)
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Seismic Performance Assessment of Different Types of Eccentric Braced Systems (IJIRST/ Volume 3 / Issue 04/ 023)
Fig. 6: Storey displacement for four types of EBF (6 storey)
Fig. 7: Time period for CBF, EBF, MRF
Push Over Analysis The results of pushover analysis are shown in the following table. The hinge formation at the final step of push is also depicted. The push is provided in the x-direction. The ductility and energy dissipation of building can be calculated from the push over curves. The area under pushover curves gives the energy dissipated by the building. The area under push over curve is high means the building have high energy dissipation capacity during earthquake . Table – 4 Ductility and energy dissipation for 10 storey Model Ductility Energy Dissipation (Kn-M) MRF 8.03 4846 V- TYPE EBF 2.35 2269 IV-TYPE EBF 2.19 2187.6 Z-TYPE EBF 4.06 2554.5 D-TYPE EBF 2.16 2383.5 CBF 1.27 1807 Table - 5 Ductility and energy dissipation for 10 storey Model Ductility Energy Dissipation (Kn-M) MRF 7 2735 V- TYPE EBF 2.54 1142 IV-TYPE EBF 2.1 1091 Z-TYPE EBF 4.94 1579 D-TYPE EBF 2.5 1207.5 CBF 1.52 773.5
VI. CONCLUSIONS The three dimensional analysis done in ETABS 2015 provided approving results in the form of acceptable control of parameters. The results showed that MRF have the lowest lateral strength and lateral stiffness and high deformation capacity. Building braced either eccentrically or concentrically the lateral load resisting capacity of the building improved. The performance of EBF is in between CBF and MRF. The displacement, drift, structural weight, time period, energy dissipation, ductility all shows that EBF is improvement over both CBF and MRF and also there is no variation in performance of EBF with the height. The decrease in time period of the building can be used as means to evaluate the behavior of the building during a seismic event. The decrease in time period of the building is due to increase in building stiffness that is facilitated by providing either CBF or EBF. But EBF shows less time period than CBF. Push over curves gives good result about energy dissipation and ductility. The energy dissipation will vary with height, building with 10 storey have high energy dissipation than 6 storey. The displacement, drift, ductility, energy dissipation, time period are maximum for Z-type as compared to other three types, Z-type showed the better performance, so it is the best configuration.
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REFERENCES Xue Ming Han, P.Eng., P.E, “Eccentrically braced frame design for moderate seismic regions”, The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China [2] M. A. Musmar“,Effect of link on eccentrically braced frames”, Journal of Engineering Sciences, Assiut University, January 2012 [3] SandaKoboevic, Jonathan Rozon, and Robert Tremblay, “SeismicPerformance of Low-to-Moderate Height Eccentrically Braced Steel Frames Designed for North American Seismic Conditions”, American Society of Civil Engineer2012. [4] ArdeshirDaneshmand,BehrokhHosseiniHashemi,“Performance of intermediate and long links in eccentrically braced frames”, Journal of Constructional Steel Research 70 (2012) 167–176, sciencedirect [5] Boaco, P.P. Rossi, “Seismic behavior of eccentrically braced frames”, Engineering Structures 31 (2009) 664674, science direct. [6] M. N. Chimeh& P. Homami, “Efficiency of bracing systems for seismic rehabilitation of steel structures”,15 WCEE 2012 [7] GülYiğitsoy,CemTopkaya, “Stability of beam in eccentrically braced frame”, Journal of Constructional Steel Research 96 (2014) 14–25 [8] M. Naghipour, N. Javadi, A. “Investigation of RBS Connection Ductility in Eccentrically Braced Frame,Procedia Engineering 14 (2011) [9] Mazen Ali Musmar, “Effect of link dimension on D type eccentric steel frames”,American Journal of Engineering and Applied Science ,2013 [10] Ramya A, Muthumani K, Nafeez Ahmed L, “Performance of eccentrically braced frames under the action of lateral load”,IJRET: [1]
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