ANALYTICAL INVESTIGATION ON SEISMIC STRENGTHENING OF RC FRAME USING STEEL CAGE AND VISCOUS DAMPER

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 5, May 2018, pp , Article ID: IJCIET_09_05_070 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed ANALYTICAL INVESTIGATION ON SEISMIC STRENGTHENING OF RC FRAME USING STEEL CAGE AND VISCOUS DAMPER SRM University, kakantkulathur, Chennai, India ABSTRACT: The aim of the present paper is to consider the conduct and proficiency of reinforced concrete (RC) columns strengthened by steel caging and viscous damper which does not conceive excess time to install, does not affect the functioning of the structure and improves the global behavior of the structure. Steel caging method is usually utilized for the seismic strengthening of reinforced concrete (RC) segments of rectangular cross-section. The steel caging comprises of 4 angle sections set at corners and held together by battens at interims along the height. Steel caging acts as passive confinement to the column. Strengthening of RC segment utilizing steel cage is genuinely least demanding accessible method. This strengthening technique requires a constrained space around the segment area when compared to steel jackets. It additionally requires less fire insurance than wrapping with FRP. This reinforcing technique is extremely efficient in gaining axial load carrying capacity of the strengthened column. This paper explores the use of steel caging and viscous dampers as an alternative method for the seismic rehabilitation of non-ductile concrete buildings. Linear static time-history analyses of strengthened frames verified excellent seismic performance of strengthened frame in terms of enhanced lateral strength, stiffness, and energy dissipation and reduced damages in RC frame members under selected ground motions. Keywords: seismic strengthening, RC frame, viscous damper, steel caging, batten Cite this Article:, Analytical Investigation on Seismic Strengthening of RC Frame Using Steel Cage and Viscous Damper, International Journal of Civil Engineering and Technology, 9(5), 2018, pp editor@iaeme.com

2 1. INTRODUCTION Solid structures not outlined as per progressed seismic codes exhibit a noteworthy quake chance since they don't have the malleability required to survive the removals actuated by substantial tremors. Consequently, the seismic retrofit of these buildings typically involves the addition of new, stiff structural elements to reduce earthquake-induced displacements. This approach often requires significant strengthening of the structure and typically involves extensive and expensive foundation work that intrudes on building operations. A reinforced concrete (RC) building must have adequate strength, stiffness, redundancy, and ductility to perform satisfactorily in a major earthquake. The behavior of conventional RC framed buildings subjected to seismic loading primarily depends on good detailing of its components in regions of expected plastic deformation. The nonductile framed buildings, generally constructed before the development of rigorous seismic design and detailing provisions, are often severely damaged or collapsed during strong earthquakes. Seismic strengthening usually involves an increase in strength, stiffness, ductility, or combination of these parameters of frame members. There are essentially two ways to improve the seismic performance of concrete moment frames. One method is to improve the deformation capacity of the beams and columns. This can be achieved by applying external confining and shear reinforcement, or jackets made out of steel or composite materials. This is not always possible however, particularly since it is impractical to apply external confinement to beams that are cast monolithically with a concrete floor slab. Strengthening of deficient columns is one of the widely accepted techniques used to enhance the seismic performance of moment resisting RC frames. Several conventional techniques of column strengthening are available such as concrete jacketing, steel jacketing and composite jacketing. Each strengthening technique has its own advantages and limitations depending on practicability, cost-effectiveness, minimal reduction in useable floor area, and effectiveness in enhancing the desired structural properties. The increase in lateral strength and/or stiffness of existing frames beyond a certain limit may not be always feasible by column strengthening using conventional techniques. Passive energy dissipation devices can be successfully employed to enhance overall seismic performance of frames by minimizing the seismic demand imposed on deficient frame members. Different types of devices are available now-a-days using energy dissipation and damping characteristics due to metal hysteresis, friction, viscous fluid, and visco-elastic materials. These devices can be successfully integrated with the steel framed structures for enhanced seismic performance. However, the most challenging part is to integrate such devices with the existing RC frames for effective utilization of their energy dissipation and damping potential. Hence, there is a need to develop an effective, fast, and practical upgrading scheme for deficient RC frames using energy dissipation devices. This paper explores the use of steel caging and viscous dampers as an alternative method for the seismic rehabilitation of non-ductile concrete buildings. The dampers dissipate energy in proportion to velocity not by displacement and therefore do not cause large increases in earthquake forces. TEST PROGRAM: RC moment resisting frames subjected to lateral loads under seismic loads undergo sway and the columns bend in double curvature. Thus a simple frame of two storey and single bay from a multistory building is considered to study the behavior of the steel caging and viscous damper. This frame is used for experimental study as well. Four different column cross sections were selected in favor to the experimental setup. The frame is subjected to linear time history analysis for both conditions Normal Frame (NF) and editor@iaeme.com

3 Analytical Investigation on Seismic Strengthening of RC Frame Using Steel Cage and Viscous Damper Strengthened Frame (SF). Then the similar scheme is implemented to a multistory frame of cross section 450x450mm. 2. MATERIALS: Steel caging is made up of angle sections welded together by steel battens. No epoxy materials are considered for the interaction between steel cage and column. The experimental study involves in the usage of various materials and testing equipment. The materials used in the present project are 1. Concrete (M20) 2. Angle section 3. Batten (mild steel) 4. Viscous damper Angle section: The columns were strengthened by four steel angles and battens which were welded to the longitudinal angles at equal intervals.. The angles used in analytical investigation were 35mm equal angles and having thickness of 5mm. Batten: The angle sections are connected by mild steel battens of 5mm thick. Mild steel is a type of steel with low carbon. The amount of carbon content present in mild steel is 0.05% to 0.25%. Fewer amounts of carbon constitute for higher ductility, since the battens are subjected to tension mild steel is preferred. It acts as passive confinement to the column. Viscous damper: They were originally developed as shock absorbers for the defense and aerospace industries. In re-cent years, they have been used extensively for seismic application for both new and retrofit construction. During seismic events, the devices become active and the seismic input energy is used to heat the fluid and is thusly dissipated. Subsequent to installation, the dampers require minimal maintenance. They have been shown to possess stable and dependable properties for design earthquakes. Viscous dampers consist of a cylinder and a stainless steel piston. The cylinder is filled with incompressible silicone fluid. The damper is activated by the flow of silicone fluid between chambers at opposite ends of the unit, through small orifices. Properties of viscous damper used are a) Damping coefficient = 0.5 b) Damping exponent = 50 c) Effective stiffness = 2000kN/mm 3. ANALYTICAL STUDY: Modeling: In the present project linear and nonlinear static behavior of the frame is analyzed using software package SAP2000. Four set of frames were analyzed by linear time history analysis. Each set consists of two frames which are Nominal frame (NF), fully strengthened frame (FF). Partially strengthened frame consists of only steel cage where as fully strengthened frame consists of steel cage and viscous damper. Each frame is of single bay single storey with Column height 1m, Beam length 1.5m. The cross section of beam and column is varied for each set of frames a. 100x100mm (C100) b. 150x150mm (C150) c. 200x200mm (C200) d. 250x250mm (C250) editor@iaeme.com

4 Figure 1 3D view of the experimental model Each frame is made up of primarily Reinforced Concrete column and the strengthened frames are provided with additional steel caging. The strengthened frame consists of four steel angles at four corners and the angle sections are then connected by a weld with mild steel battens. This caging acts as a passive confinement to the column. Due to the excessive axial load the column under goes lateral strain and tends to fail. The steel caging controls this failure and provides more ductility to the column. Figure 2 skeletal structure of specimen SAP 2000 Section designer: Section Designer is a coordinated utility, incorporated with SAP2000, CSi Bridge, and ETABS, that enables the demonstrating and investigation of custom cross sections. Section Designer is valuable for the assessment of part properties and nonlinear reaction, including nonlinear pivot and PMM-pivot conduct. Adjustment elements might be appointed to reenact broken segment conduct. These section definitions would then be able to be appointed to frame objects. Nonstandard or composite areas of self-assertive geometry might be made in Section Designer and after that fused into a base model. Areas may incorporate at least one materials and a client characterized rebar design. All segments are thought to be non-compact editor@iaeme.com

5 Analytical Investigation on Seismic Strengthening of RC Frame Using Steel Cage and Viscous Damper Figure 4 custom cross section using sap 2000 Building model: Strengthening scheme proposed in the paper is applied to a multistory building. Similar to the above mentioned frame a building frame is considered for the analysis. Which comprises of 4 storey, 3 bays in x-direction, 2 bays in y-direction. Height of each storey is 3m, width of each bay is 6m. the cross section of the column is 450x450mm and cross section of the beam is 300x300mm and the angle section used is 60x60x6mm, mild steal battens of 6mm are used. Properties of viscous damper are Damping coefficient = 0.5, Damping exponent = 50, Effective stiffness = 2000kN/mm. Figure 3 building model Analysis: Present models are analyzed under linear and nonlinear conditions. Load cases used for analysis are 1. Time history ( linear ) 2. Pushover (nonlinear ) editor@iaeme.com

6 Displacement 3. RESULTS AND DISCUSSION: Time history analysis: Time-history analysis provides for linear or nonlinear evaluation of structural response under loading which may vary according to the specified time function. This system does not require dynamic investigation, in any case, it represent the dynamics of whole building approximately. The static technique is the most straightforward one-it requires less computational efforts and depends on define given in the code of practice. To begin with, the design base shear is processed for the entire building, and it is then appropriated along the height of the building. The lateral forces at each floor levels are appropriated to each horizontal load opposing components. Moment capacity of the strengthened frame increased significantly for each cross section. Table represents the maximum moment carrying capacity of each frame. Cross section (mm) Table 1 moment carrying capacity Nominal frame Moment capacity[knm] Strengthened frame % difference C C C C Displacement of the top storey decreased for the strengthened frame. Table represents the displacements of the top storey of each frame. Cross section (mm) Table 2 Displacement Nominal frame Displacement[mm] Strengthened frame % difference C C C C The graph of maximum displacement of the top storey the difference between nominal and strengthened is decreasing because the steel angles used for strengthening for all cross section was 35 x 35 x 5mm. increasing angle section for larger cross section would give better result x x x x250 nominal strengthened Figure 4 Displacement curves of nominal and strengthened frame editor@iaeme.com

7 Base shear (kn) Analytical Investigation on Seismic Strengthening of RC Frame Using Steel Cage and Viscous Damper Analytical results for building model: Moment carrying capacity of strengthened frame is kn, Normal frame is kn Capacity increased by 19.14%. Max Displacement of joint 22, strengthened frame is 0.376m, Normal frame is m, Max displacement decreased by 34%. The graph represents the drift ratios of normal and strengthened frame.it can be observed that the base shear of strengthened frame less compared to the strengthened frame strengthened normal Drift ratio Figure 5 drift ratio curve Pushover analysis: The pushover analysis is a method to observe various damage states of a frame. The part of non-straight proportionate static (pushover) analysis is as a rule increasingly perceived as a simplest method for the assessment of the seismic reaction of structures. Pushover analysis is subsequently progressively being considered inside current seismic codes, both for design of new structures and for evaluation of existing ones. This technique accepts an arrangement of incremental lateral load along the height of the structure. Local nonlinear impacts are demonstrated and the structure is pushed until the point of collapse. With the expansion in the magnitude of loads, frail connections and failure modes of the structures are found. At each progression, the base shear and the top displacement can be plotted to create the pushover curve.this technique is moderately basic and gives data on the strength, deformation and ductility of the structure and dispersion of demand. This grants to distinguish the critical members prone to seismic tremors by the development of plastic hinges. To demonstrate pushover analysis, a lateral load versus deformation curves for the frame is required. The outcomes from a pushover analysis will give the load versus deflection curve. Besides, the pushover analysis gives just the base shear versus roof displacement conduct of a building. The seismic study of a structure has been possible with the nonlinear static procedure (NSP) or pushover analysis depicted in FEMA-356 and ATC-40. Table 3 and 4 represent various pattern of hinge formation for both the nominal frame and strengthened frame. It is observed that the nominal frame under goes deformation at less amount of base shear than that of strengthened frame. Also it is to be noted that the hinge formation is severe from D to E in the Nominal frame where it is not the case in strengthened frame which interprets that the strengthened frame is ductile than nominal frame editor@iaeme.com

8 STEP Table 3 Plastic hinge pattern of Nominal Frame at different damage level DISPLACEMENT [mm] BASE SHEAR [KN] A B B IO IO LS LS CP CP C C D D BEYOND E TAL E STEP Table 4 Plastic hinge pattern of Strengthened Frame at different damage level DISPLACEMENT [mm] BASE SHEAR [KN] A B B IO IO LS LS CP CP C C D D E BEYOND E TAL Fig 6 represents the capacity spectrum curve with capacity curve represented in green curve and demand curve shown in orange. The point where the two curves meet is known as Performance point. This point represents the global behavior of the structure, even when the structure seems to have adequate global capacity but the local collapse may occur beyond the performance point editor@iaeme.com

9 Base Shear Analytical Investigation on Seismic Strengthening of RC Frame Using Steel Cage and Viscous Damper Figure 6 Capacity spectrum Fig 7 represents the comparison of the pushover curve of the nominal frame and the strengthened frame. It is observed that the load versus deflection curve of the strengthened frame shows better performance than the nominal frame Pushover curve Displacement NF SF Figure 7 Pushover curve 4. CONCLUSION: Seismic performance of a multi storey frame depends on the stiffness and lateral strength of the column. The two strengthening techniques comprising of steel caging for columns and viscous damper as energy dissipating device used in strengthening of RC frame has proved to be successful in improving the seismic performance of the weak RC frame. Under linear loading the load carrying capacity of the frame increased by 26%. Moment carrying capacity increased 19.14%. Displacement decreased by 34%. Under nonlinear loading the load carrying capacity of the frame increased almost twice. Moment carrying capacity increased 28.6%.displacement decreased by 15%. In each case strengthened frame (SF) improved in lateral strength, lateral displacement, energy dissipation potential than the normal frame (NF). Increasing the area size of angle section with the increase of cross sectional area improved the performance of the strengthened frame (SF) editor@iaeme.com

10 REFERENCES [1] CSI (2006). SAP2000 Basic Analysis Reference Manual-Version 11, Computers and Structures Inc., Berkeley, CA. [2] IS: 456 (2000). Indian Standard Plain and Reinforced Concrete-Code of Practice. Fourth revision, Bureau of Indian Standards, New Delhi. [3] Dipti R. Sahooa and[ Durgesh Rai, M.EERI [A Novel Technique of Seismic Strengthening of Nonductile RC Frame using Steel Caging and Aluminum Shear Yielding Damper]. [4] Pasala Nagaprasad, Dipti Ranjan Sahoo and Durgesh C. Rai [Seismic strengthening of RC columns using external steel cage] [5] Giuseppe Campione [Simplified Analytical Model for R.C. Columns Externally Strengthened with Steel Cages ] [6] Mahmoud F. Belal, Hatem M. Mohamed, Sherif A. Morad [Behavior of reinforced concrete columnsstrengthened by steel jacket] [7] A.M. Tarabia, H.F. Albakry [Strengthening of RC columns by steel anglesand strips] [8] M.SaraswathI and Prof. S.Saranya [Strengthening of RC square column using Steel angles] [9] N. P. Sherje and Dr. S. V. Deshmukh, Preparation and Characterization of Magnetorheological Fluid For Damper In Automobile Suspension. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp editor@iaeme.com