Advanced Retrofitting Methods and Techniques for RC Building: State of an Art

Transcription

1 Advanced Retrofitting Methods and Techniques for RC Building: State of an Art Miss. Swati Sanjay Nibhorkar * M. E (Structure) Scholar, Civil Engineering Department, G. H. Raisoni College of Engg and Management, Amravati, Maharashtra, India. Swati.nibhorkar@raisoni.net Prof. B. H. Shinde Assistant Professor, Civil Engineering Department, G. H. Raisoni College of Engg and Management, Amravati, Maharashtra, India. Abstract: A higher degree of damage in a building is expected during an earthquake if the seismic resistance of the building is inadequate. The decision to strengthen it before an earthquake occurs depends on the building s seismic resistance. The structural system of deficient building should be adequately strengthened in order to attain the desired level of seismic resistance.the seismic retrofitting of reinforced concrete buildings not designed to withstand seismic action is considered. After briefly introducing how seismic action is described for design purposes, methods for assessing the seismic vulnerability of existing buildings are presented based on the literature survey carried out. The existing building can be retrofitted using various techniques like Jacketing existing beams, columns, or joints, Use of Fibre Reinforced Cement, confinement of column by embedded composite grid etc. So, in this paper, efforts are made to describe thedifferent retrofitting techniques available and its suitability for particular conditions. The traditional methods of seismic retrofitting are reviewed and their weak points are identified. Modern methods and philosophies of seismic retrofittingare also reviewed. Keywords: retrofitting, RC Building, Seismic zone, Earthquake, Strengthening. A 1.0 INTRODUCTION higher degree of damage in a building is experienced during an earthquake if the seismic resistance of the building is inadequate. The decision to strengthen it before an earthquake occurs depends on the building s seismic resistance. The structural system of deficient building should be adequately strengthened in order to attain the desired level of seismic resistance. The recent earthquakes in Turkey and India have highlighted the structural inadequacy of those building stocks with respect to seismic loads and design. Building owners and occupiers are now aware of how vulnerable their buildings are and may wish to undertake strengthening work. In India there are thousands of apartment buildings vulnerable to severe damage in a moderate or larger earthquake [1]. The seismic hazard in the areas, where those earthquakes have occurred, has been known for a long time because of similar events that occurred in the past. It is therefore legitimate to ask why constructions vulnerable to earthquakes exist if people and institutions knew of the seismic hazard. Several causes may have contributed to the creation of such a situation. Thus the solution on this hazard is necessary. Retrofitting of any existing building is a complex task and requires skill, retrofitting of Reinforced Concrete (RC) buildings is particularly challenging due to complex behavior of the RC composite material. The behavior of the buildings during earthquake depends not only on the size of the members and amount of reinforcement, but to a great extent on the placing and detailing of the reinforcement. The construction practices in India result in severe construction defects, which make the task of retrofitting even more difficult. It is not surprising that in areas long known to be subject to the seismic hazard it is not infrequent to find constructions vulnerable to earthquakes. Theseconstructions need to be retrofitted to allow them to withstand the effects of the earthquake groundmotion expected at the site considered. In the following sections some advancedmethods and techniques used for the evaluationof the seismic resistance and vulnerability of reinforced * Corresponding Author

2 concrete buildings will be described together withtraditional and innovative techniques of seismic retrofitting of the same structures. This paper presents a brief review of the available methods and techniques for retrofitting of RC building. In this paper, efforts are also made to describe the different retrofitting techniques available and its suitability for particular conditions. There are three sources of deficiencies [2] in a building, which have to be accounted for by the retrofitting engineer: (i) Inadequate design and detailing (ii) Degradation of material with time and use, and (iii) Damage due to earthquake or other catastrophe. The retrofit engineer is expected to estimate the deficiency resulting from all the three sources, suggest a retrofit scheme to make up for the deficiencies and demonstrate that the retrofitted structure will be able to safely resist the future earthquake forces expected during the lifetime of the structure. 2.0 DOCUMENTS DEVOTED TO ASSESSMENT AND RETROFIT OF EXISTING STRUCTURES The Indian Standard [3] focuses on providing guidance on reinstating damaged or weak elements by rebuilding or strengthening. General principles as well as some common strengthening techniques are discussed. For example, details of encasing reinforced concrete members to improve strength, and methods of improving floor and roof diaphragm action are provided. The Indian Standard appears to concentrate on reinstating or upgrading gravity load paths of reinforced concrete members, rather than improving seismic resistance. Its value therefore lies in implementationof rapid repairs, probably in most cases to non-engineered structures. While the lack of emphasis on the need for engineering evaluation, analysis and design of seismically deficient structures may be addressed in a future revision.it might be argued that much repair and retrofitting will be undertaken without professional engineering advice. This highlights the value of practical and detailed documentation in the form of manuals for contractors who can then retrofit a limited range of common building types. Such a document has been recently produced by a NGO for the Gujarat reconstruction [4]. The document [11] highlights a higher degree of damage in a building is expected during an earthquake, if the seismic resistance of the building is inadequate. The decision to strengthen it before an earthquake occurs depends on the building s seismic résistance. The structural system of deficient building should be adequately strengthened, in order to attain the desired level of seismic resistance.the guidelines are intended to provide a systematic procedure for the seismic evaluation of buildings, which can be applied consistently to a rather wide range of buildings. This document also discusses some cost effective strengthening schemes for existing older buildings where identified as seismically deficient during the evaluation process. The detailed study on United Nations Industrial Development Organization (UNIDO) manual, Federal Emergency Management Agency (FEMA 310), Structural Engineering Research Center (SERC) and New Zealand Draft Code is carried out by the author. Author recommended that seismic evaluation procedures for buildings are a combination of configuration-related and strength-related checks. Though there have been no significant differences in which the configuration related assessments are carried out, there is considerable degree of non-uniformity in the manner strength related assessments are carried out. All the documents except UNIDO manual provide explicit checking criteria for the configurationally induced irregularities. Strength checks are performed either at global (structure) or local (element) level or at both levels as in FEMA 310. At the global level, there is no shear stress check given by UNIDO manual and New Zealand Draft Code and no component level analysis is performed in UNIDO manual and SERC report. The review of various evaluation procedures indicates clearly that FEMA 310 and New Zealand Draft Code are more suitable for use in buildings of developing countries, which are not only difficult to classify in certain type buildings but also their capacities cannot be estimated with significant confidence. FEMA 310 provides a more generalized approach to seismic evaluation, which is thorough and provides several levels of assessment with varying degree of complexity suitable for a large class of structures. FEMA172, 1992 [5]handbook describes techniques that engineers can use to solve a variety of seismic rehabilitation problems in existing buildings, a broad spectrum of building types and building components of both structural

3 and nonstructural. Techniques are illustrated with sketches, and the relative merits of the techniques are also discussed. FEMA172,1992 handbook and its supporting report FEMA173, 1989 [6] and FEMA174, 1989 [7] documents information need to develop a seismic rehabilitation program and establish priorities for rehabilitation. Through the presentation of nationally applicable guidelines, it helps local jurisdictions to make informed decisions about rehabilitating seismically hazardous existing buildings, and it includes a review of the relevant technical and societal issues and a procedure to resolve the issues. FEMA308, 1999 [8] consists practical guidance for the repair, upgrade of earthquake damaged concrete and masonry wall buildings. Target audiences are design engineers, building owners, officials, insurance adjusters, and government agencies. The publication contains sections on most important aspect of performance based design, repair technologies, categories of repair, and nonstructural considerations. The last section includes repair guides, which provides outline specifications for typical repair procedures. In this publication FEMA315, 1998 [9] discusses the mission, history, and results of FEMA is Existing Building Program (EBP) which provides four objectives and 25 tasks to be carried out through the EBP. The fourobjectives are: 1. Promote seismic rehabilitation and advance the implementation of previously developed materials. 2. Monitor the use of and refine existing materials 3. Develop new seismic rehabilitation tools, and 4. Consider new program directions for the EBP. Estimated costs for the next ten to fifteen years and guidelines for plan implementation are also included. The publication also describe administrators with information to assess the seismic vulnerability of school buildings, hospital building, office building, apartment building, retail building and to implement a program of incremental seismic rehabilitation. Increase in the number of seismically resistant buildings in all areas of identified earthquake risk. Also FEMA 356, 2000 [10] can be used as an evaluation tool, the more traditional evaluation resultingfrom the application of FEMA 310 may be more appropriate for a developing country. Both documents take a rigorous approach to determining existing structural conditions by specifying the as-built information required, including exposure of primary reinforced concrete connections to ascertain the standard of reinforcement detailing. Uncertainties associated with minimum data collection are accounted for in the analysis by application of a Knowledge Factor. This approach might be redundant for buildings whose existing structures are discounted completely, due to serious constructional deficiencies, and in whom additional structures are inserted to resist lateral loading. The documents also outline potential geological hazards and provide guidance onassessment and mitigation. 3.0 IDENTIFICATION OF SEISMICALLY DEFICIENT BUILDING Existing buildings [1] can become seismically deficient when (a) seismic design code requirements are upgraded since the design of these buildings with an older version of the code; (b) seismic design codes are deficient, (c) engineering knowledge makes advances rendering insufficient the previous understanding used in their design, and (d) designers lack understanding of the seismic behavior of structures. Indian buildings built over the past two decades are deficient because of the reasons stated in (b), (c), and (d) above. Concept of pushover analysis [12] that is becoming a popular tool in the profession for design of new buildings, seismic evaluation of existing buildings and developing appropriate strategy for seismic retrofitting of buildings. It is shown how this analytical technique can be useful in deciding seismic retrofitting strategy and techniques. Pushover analysis of the structures (Lakshmanan D, 2006) using SAP 2000 evaluating the various repair strategies for use in the improvement of the seismic performance of RC structures are highlighted. The behaviors of repaired beams of beam column joints are discussed in detailed. It is observed that inherent deficiencies in the detailing of the beam-column joints get reflect even after repair, though the performance factors indicate significant improvement. Two of the logical extensions show that the repair would not be as effective in these cases.

4 The nonlinear static analysis of RC building [13] is performed using pushover approach before and after retrofitting. The comparison of strength parameters and pushover curve indicates that there is increase in ductility. As regards to stiffness of the building, it is seen that it remains more or less same up to linear stage, while in nonlinear stage every point increase after retrofitting. The strength of the building is correlated with base shear and the net enhancement in strength after retrofitting. This paper [14] introduces a method of design of structural upgradation using FRC and discusses the design of enhancement of RCC elements with FRC, a strategy of upgradation of RCC frames and use of the developed strategy of upgradation for retrofitting of RCC frames based on Capacity Spectrum Method. 4.0 IMPROVEMENT OF EXISTING RC BUILDINGS Three levels of improvement of existing RC frame buildings [1] are possible, namely (a) repair, (b) restore, and (c) strengthen. The consequence of any prescribed method of the retrofitting are (a) adding brick masonry walls in all possible bays in ground storey, (b) jacketing of all RC columns in ground storey only, (c) adding steel diagonal braces in some bays in ground storey, and (d) infilling existing RC frame with RC structural walls in some bays in ground storey only. In all cases, foundation strengthening may be essential. On the seismic capacity of the building should be quantitatively evaluated its effectiveness from the points of view of strength, stiffness and ductility are shown in Fig 1 Sometimes, a retrofit scheme may have better performance than the damaged structure, but still may be poor; the retrofit scheme that assures at least a basic ductility is preferable to the others. This paper [16] concludes that to identify an efficient retrofitting method for existing open ground storey RC frame buildings. A two dimensional RC frame is designed with nonductile. Detailing is subjected to nonlinear static pushover analysis. The RC frame is retrofitted by three methods. a) Concrete jacketing of columns in the ground storey, b)brick masonry infill in the ground storey, and c) RC structural wall in the ground storey panel of all the methods studied the use of structural wall in the ground storey panel give the maximum strength and ductility. The objective of this study [17] is to identify an efficient retrofitting method for reinforced concrete buildings. Two buildings: One open ground storey with infills and the other partial open ground storey with infills, which are damaged in the January 2001 Bhuj earthquake, are subjected to static pushover analysis with code specified design shear distribution. The observed failure modes conform to the actual structural damages sustain by the buildings during that earthquake. The selectedmethods of retrofitting are a) Jacketing of columns in the ground storey, b) Structuralwalls in the ground storey of some selective panels, and c) Structural walls for all thestories in some panels. These three basic schemes are used in combination forascertaining an economical method giving the maximum strength and ductility. Of all themethods studied, the combination of column jacketing in ground storey and shear wallthroughout the height of the building with selective strengthening of upper storey framemembers, give the most economic and desirable performance. Fig.1: Relative performance of different retrofit strategies [1]

5 The present paper [18] deals with the seismic retrofitting of an existing fourteen storied RC building frame located in seismic zone IV. The study includes seismic evaluation and retrofitting of RC framed building, by using steel bracing and infill masonry walls. The seismic performance of two retrofitting techniques such as steel bracing (V, diamond and cross pattern) and infill walls are relatively compared. Among three patterns of steel bracing, cross pattern shows better performance than V and diamond bracing patterns. The structural analysis [19] is performed using a suitable computer analysis program. The steps involve developing a computational model of the building, applying the external forces, calculating the internal forces in the members of the building, calculating the deformations of the members and building and finally interpreting the results. First deficiencies observed in existing RC buildings for resisting earthquake identified. It is essential to identify the deficiencies in a building before undertaking retrofit. Identification of the deficiencies is also expected to create awareness for future construction. 5.0 CONCLUSION A number of experimental and analytical studies focused on seismic retrofitting techniques and extensive seismic damage control activities in practice have contributed to the present state of development. Further research should be conducted to improve the selection of appropriate retrofit techniques using criteria based on performance, economy and constructability. There are many seismic retrofit techniques available, depending upon the varioustypes and conditions of structures. Therefore, the selection of the type of intervention is a complex process, and is governed by technical as well as financial and sociologicalconsiderations. The following methods are carried out by most of the researchers which are concrete jacketing of columns of ground floor, brick masonry infill in the ground floor, X and V bracing, shear wall, FRP of beams and columns. All these topics require further research, and it is essential for seismic retrofitting of reinforced concrete structures. Also from the literature it may be suggested that the Nonlinear static analysis must be performed so as to obtain the actual results after performing the analysis of structures. The following factors such as; Cost versus importance of the structure, Available workmanship, Fulfillment of the performance goals of the owner, Level of quality control, Irregularity of stiffness, strength and ductility, Sufficient capacity of foundation system and Repair materials and technology available should also be considered while selecting the seismic retrofitting technique. References [1] Murty CVR, (2002), quantitative approach to seismic strengthening Of RC frame building, Seminar on seismic assessment and retrofitting buildings, pp [2] Yogendra Singh, (2003), Challenges in retrofitting of RC buildings, Workshop on retrofitting of structures, IIT Roorkee, pp [3] Indian Standard, 1993, Repair and Seismic Strengthening of Buildings Guidelines, IS 13835:1993, Bureau of Indian Standards, New Delhi [4] D. D. Ayala and A. W. Charleson, REVIEW OF SEISMIC STRENGTHENING GUIDELINES FOR R. C. BUILDINGS IN DEVELOPING COUNTRIES, Published by Elsevier Science Ltd. All rights reserved 12th European Conference on Earthquake Engineering Paper Reference 820. [5] FEMA172. NEHRP handbook of techniques for the seismic rehabilitation of existing buildings. Building seismic safety council. Washington D.C, (1992). [6] FEMA173. Establishing programs and priorities for the seismic rehabilitation of buildings: Supporting report. Building systems development Inc. Washington, D.C, (1989). [7] FEMA174. Establishing programs and priorities for the seismic rehabilitation of buildings: Handbook.Building systems development Inc. Washington.D.C, 989. [8] FEMA308. The repair of earthquake damaged concrete and masonry wall buildings. Applied Technology Council. Redwood city CA, (1999). [9] FEMA315. Seismic rehabilitation of buildings: Strategic plan Earthquake engineering research institute. Washington, D.C, (1998). [10] FEMA 356, 2000, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, Washington DC [11] DrDurgesh C Rai, (2005), guidelines for seismic evaluation and strengthening of existing building, Provision with commentary and explanatory examples, Indian Institute of

6 Technology Kanpur, Document no- IITK- GSDMA-Earthquake 6, vol. 4. [12] Sudhir K. Jain, Srikant T, (2002), analysis for seismic retrofitting of buildings, The Indian concrete journal, pp [13] Shailesh Kr. Agrawal, Ajay Chourasia, (2003), Nonlinear static analysis for seismic evaluation and retrofit of RC buildings, Workshop on rertofitting of structures, pp [14] Abhijit Mukherjee, Amit R. Kalyani, (2004), Seismic retrofitting of reinforced concrete frames with fiber reinforced composites, Workshop on seismic evaluation and retrofitting of building, pp [15] Richard N. White, Khalid Mosalam. Seismic evaluation and rehabilitation of concrete buildings, Cornell University, Ithaca NY USA, pp [16] KaustubhDasgupta, Murty CVR. Quantitative seismic retrofitting of open ground storey RC frame buildings. [17] Ashutosh V Mahashabde, KaustubhDasgupta, Murty CVR, (2003), Seismic strengthening of gravity load designed RC frame buildings, 4th International conference on earthquake engineering and seismology. [18] Amar Prakash, Thakkar SK, (2003), A comparative study of retrofitting of RC building, Workshop on retrofitting of structures, IIT Roorkee, pp [19] Dr. Meher Prasad A. Structural analysis for seismic retrofit, Handbook on seismic retrofit of building, IIT Madras, pp of 1 year. years. Author s Biography Miss.S.S.Nibhorkar appeared PG from Amravati University. Her area of specialization is Structural Engineering She is working with G.H.Raisoni College of Engineering & Mang. Amravati institution. She has an experience Prof. B. H. Shinde obtained PG from Aurangabad University. His area of specialization is Structural Engineering. He is working with G.H.Raisoni College of Engineering & Mang. Amravati Institution. He has experience of 5