इंटरनेट मानक Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. “जान1 का अ+धकार, जी1 का अ+धकार” “प0रा1 को छोड न’ 5 तरफ” Mazdoor Kisan Shakti Sangathan Jawaharlal Nehru “The Right to Information, The Right to Live” “Step Out From the Old to the New” IS 15988 (2013): Seismic Evaluation and Strengthening of Existing Reinforced Concrete Buildings - Guidelines [CED 39: Earthquake Engineering] “!ान $ एक न’ भारत का +नम-ण” Satyanarayan Gangaram Pitroda ““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee”” “!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै” Bhartṛhari—Nītiśatakam “Knowledge is such a treasure which cannot be stolen” IS 15988: 2013 Hkkjrh; ekud izcfyr daØhV ds cus Hkouksa ds HkwdEih; ewY;kadu vkSj lnq <h+ dj.k fn'kk funsZ'k — Indian Standard SEISMIC EVALUATION AND STRENGTHENING OF EXISTING REINFORCED CONCRETE BUILDINGS — GUIDELINES ICS 91.120.25 © BIS 2013 B U R E A U O F I N D I A N S T A N D A R D S MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG NEW DELHI 110002 February 2013 Price Group 8 Earthquake Engineering Sectional Committee, CED 39 FOREWORD This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Earthquake Engineering Sectional Committee had been approved by the Civil Engineering Division Council. This standard is intended to reduce the risk of death and injury that may result from the damaging effects of earthquake on building which predate the current seismic codes [IS 1893 (Part 1) : 2002 ‘Criteria for earthquake resistant design of structures: Part 1 General provisions and buildings’, IS 4326:1993 ‘Code of practice for earthquake resistant design and construction of buildings’ and IS 13920 : 1993 ‘Ductile detailing of reinforced concrete structures subjected to seismic forces — Code of practice’] or have not been designed for earthquake forces. This standard describes a set of key steps and procedures for the assessment of the expected seismic performance of existing building in the event of a design level earthquake and where found necessary, strengthening of existing structural systems and elements for improved seismic performance. Seismic forces for evaluation criteria of existing buildings are different from those meant for the design of new buildings. Appropriate modifications are made to address the issues of reduced serviceable life and acceptable risk for higher importance. Further, to account for uncertainty in the reliability of available information about the existing structure and the condition of structure components, strength calculations need to be suitably modified. For deficient buildings, a broad outline for the design seismic strengthening measures has been developed and the interface with current design codes in general terms has been identified. In the formulation of this standard, assistance has been derived from the following publications: ATC 33.03 Guidelines for seismic evaluation of existing buildings, Applied Technology Council, CA. Eurocode 8 Design provisions for earthquake resistance of structures : Part 3, CEN, Brussels, 2001. FEMA 178 NEHRP Handbook for the seismic evaluation of existing buildings, Building Seismic Safety Council, Washington, D.C., 1992. FEMA 154 Rapid visual screening of buildings for potential seismic hazards: A Handbook, Fedral Emergency Management Agency, Washington DC, USA, 1998. FEMA 310 Handbook for the seismic evaluation of buildings: A Prestandard, Federal Emergency Management Agency, Washington DC, USA, 20C. FEMA 356 Prestandard and commentary for the seismic rehabilitation of building, Federal Emergency Management Agency, Washington DC, USA, 20C. The assessment and improvement of the structural performance of earthquake risk buildings — Draft for General Release, New Zealand National Society for Earthquake Engineering for Building Industry Authority, New Zealand, 1996T. ASCE 31-03 Seismic evaluation of existing buildings, American Society of Civil Engineers, Reston, VA, 2003. ASCE 41-06 Seismic rehabilitation of existing buildings, American Society of Civil Engineers, Reston, VA,2006. Seismic assessment and retrofit of reinforced concrete buildings, International Federation of structural Concrete (Fib), Laussance, Switzerland 2003. Uniform code for building conservation, International Conference of Building Officials, Whittier, CA, USA, 1991. Post-earthquake damage evaluation and strength assessment of buildings, under seismic conditions, Volume 4, UNDP/UNIDO, Vienna, 1985. International existing building code (IBC), International Code Council, Illinois, 2006. (Continued on third cover) IS 15988 : 2013 Indian Standard SEISMIC EVALUATION AND STRENGTHENING OF EXISTING REINFORCED CONCRETE BUILDINGS — GUIDELINES 1 SCOPE deformation of a structural member or system. 1.1 This standard is particularly concerned with the 3.4 Column (or Beam) Jacketing — A method in seismic evaluation and strengthening of existing which a concrete column or beam is covered with a buildings and it is intended to be used as a guideline. steel or reinforced concrete jacket in order to strengthen and/or repair the member by confining the concrete. 1.2 This standard provides a method to assess the ability of an existing building to reach an adequate level of 3.5 Components — The basic structural members that performance related to life-safety of occupants. constitute a building including beams, columns, slabs, Therefore, the emphasis is on identification of braces, walls, piers, coupling beams and connections. unfavourable characteristics of the building that could 3.6 Deformation — Relative displacement or rotation result in damage to either part of a building or the entire at the ends of a component or element or node. structure. 3.7 Demand — The amount of force or deformation 2 REFERENCES imposed on an element or component. The following standards contain provisions, which 3.8 Displacement — The total movement, typically through reference in this text, constitute provisions of horizontal, of a component or element or node. the standard. At the time of publication, the editions indicated were valid. All standards are subject to 3.9 Flexible Diaphragm — A floor diaphragm shall revision and parties to agreements based on this be considered to be flexible, if it deforms such that the standard are encouraged to investigate the possibility maximum lateral displacement measured from the of applying the most recent editions of the standards chord of the deformed shape at any point of the indicated below: diaphragm is more than 1.5 times the average displacement of the entire diaphragm. Diaphragms of IS No. Title wood construction and of similar material or elements 456 : 2000 Code of practice for plain and which are not connected together for seismic loading reinforced concrete (fourth revision) are considered as flexible diaphragms. Cast-in-situ RC 1893 (Part 1) : Criteria for earthquake resistant floor systems are usually not flexible diaphragms. 2002 design of structures: Part 1 General 3.10 Infill — A panel of masonry placed within a steel provisions and buildings or concrete frame. Panels separated from the 13920 : 1993 Ductile detailing of reinforced surrounding frame by a gap are termed isolated infills. concrete structures subjected to A panel in tight contact with a frame around its full seismic forces — Code of practice perimeter is termed a shear infill. 3 TERMINOLOGY 3.11 Knowledge Factor — A factor to represent the For the purpose of this standard, the definitions given uncertainty about the reliability of the available in IS1893 (Part 1), IS13920 and the following shall information about the structural configuration and apply. present condition of materials and components of the existing building. 3.1 Acceptance Criteria — Limiting values of properties such as drift, strength demand, and inelastic 3.12 Lateral Force Resisting System — The deformation used to determine the acceptability of a collection of frames, shear walls, bearing walls, braced component. frames and interconnecting horizontal diaphragms that provide earthquake resistance to a building. 3.2 Action — An internal moment, shear, torque, axial load, developed in a member due to externally applied 3.13 Life Safety Performance Level — Building load/displacement on the structure. performance that includes significant damage to both structural and non-structural components during a 3.3 Capacity — The permissible strength or design earthquake, where at least some margin against 1 IS 15988 : 2013 either partial or total structural collapse remains. 3.25 Required Member Resistance (or Required Injuries may occur, but the level of risk for life- Strength) — Load effect acting on an element or threatening injury and entrapment is low. connection, determined by structural analysis, resulting from the factored loads and the critical load 3.14 Load-Bearing Wall — A wall designed to carry combinations. an imposed vertical load in addition to its own weight, together with any lateral load. 3.26 Rigid Diaphragm — A floor diaphragm shall be considered to be rigid, if it deforms such that the 3.15 Load Path — The path that seismic forces acting maximum lateral displacement measured from the anywhere in the building, take to the foundation of the chord of the deformed shape at any point of the structure and, finally, to the soil. Typically, the load diaphragm is less than 1.5 times the average travels from the diaphragm through connections to the displacement of the entire diaphragm. Reinforced vertical lateral-force-resisting elements, and then concrete monolithic slab-beam floors or those proceeds to the foundation. consisting of pre-fabricated/pre-cast elements with 3.16 Masonry — The assemblage of masonry units, adequate topping reinforced screed can be taken as mortar, and possibly grout and/or reinforcement. Types rigid diaphragms. of masonry are classified herein with respect to the 3.27 Secondary Element — An element that does not type of the masonry units, such as brick/clay-unit affect the ability of the structure to resist earthquake- masonry or concrete masonry. induced deformations. They may or may not actually 3.17 Non-structural Component — Architectural, resist any lateral force. mechanical or electrical components of a building that 3.28 Seismic Demand — Seismic hazard level and are permanently installed in, or are an integral part of commonly expressed in the form of a ground shaking a building. response spectrum. Structural actions (force)/ 3.18 Out-of-Plane Wall — A wall that resists lateral deformation in members of the building are computed forces applied normal to its plane. due to design earthquake. 3.19 Overturning — An action resulting when the 3.29 Seismic Evaluation — An approved process or moment produced at the base of a vertical lateral-force- methodology of evaluating deficiencies in a building resisting element is larger than the resistance provided which prevent the building from achieving life safety by the foundation’s uplift resistance and building weight. objective. 3.20 Plan Irregularity — Horizontal irregularity in 3.30 Short Column — The reduced height of column the layout of vertical lateral-force-resisting elements, due to surrounding parapet, infill wall, etc, is less than producing a mismatch between the center-of-mass and five times the dimension of the column in (a) the center-of-rigidity that typically results in significant direction of parapet, infill wall, etc, or (b) 50 percent torsional demands on the structure. of the nominal height of the typical columns at that 3.21 Pounding — Two adjacent buildings impacting level. during earthquake excitation because they are too close 3.31 Strength — The maximum axial force, shear together. force, or moment that can be resisted by a component. 3.22 Primary Element — An element that is essential 3.32 Strengthening Measures — Modifications to to the ability of the structure to resist earthquake- existing components, or installation of new induced deformations. components, that correct deficiencies identified in a 3.23 Probable or Measured Nominal Strength — seismic evaluation as part of a strengthening scheme. The strength of a structure or a component to resist the 3.33 Strengthening Method — A procedural effects of loads, as determined by: (a) computations methodology for the reduction of earthquake using specified material strengths and dimensions, and vulnerability of the building. formulas derived from accepted principles of structural mechanics; or (b) strength field tests or laboratory tests 3.34 Strengthening Strategy — A technical approach of scaled models, allowing for modelling effects and for developing strengthening measures for a building differences between laboratory and field conditions. to reduce its earthquake vulnerability. 3.24 Redundancy — Provision of alternative load 3.35 Strong Column-Weak Beam — The capacity of paths in a structure by which the lateral forces are the column in any moment frame joint must be greater resisted, allowing the structure to remain stable than that of the beams, to ensure inelastic action in the following the failure of any single element. beams. 2 IS 15988 : 2013 3.36 Vertical Irregularity — A discontinuity of V = shear contribution of concrete con strength, stiffness, geometry, or mass in one storey with V = shear contribution of FRP sheet FRP respect to adjacent stories. V = base shear B 4 SYMBOLS V = shear force contribution of steel in a s reinforced concrete beam The symbols and notations given below shall apply to V = storey shear at level j the provisions of this standard: j V = allowable shear force A = total cross-sectional area of columns u c 5 EVALUATION CRITERIA A = gross area of the reinforced concrete section g A = steel to be provided in the jacket 5.1 General s A = area of shear transfer reinforcement vf The seismic performance of existing buildings is A = cross-section area of a single bar evaluated in relation to the performance criteria in use vf for new buildings. This section defines the minimum A = area of shear wall w evaluation criteria for the expected performance of life A = total area of shear walls in the direction of wall safety of existing buildings with appropriate loading modification to IS 1893 (Part 1) seismic force which bf = width of flange is applicable for the seismic design of new buildings. d = diameter of stirrup h 5.2 Since the provisions of this standard are strongly E = modulus of concrete correlated with the design criteria of new buildings c f = characteristic strength of concrete contained in IS 1893 (Part 1), reference shall always ck be made to the current edition of IS 1893 (Part 1). All F = axial force due to overturning o existing structural elements must be able to carry full f = yield strength of steel y other non-seismic loads in accordance with the current H = total height applicable standards related to loading and material strengths. I = gross moment of intertia of reinforced g concrete section 5.3 Basic inputs for determination of seismic forces K = knowledge factor such as seismic zone, building type, response reduction factor are to be taken directly from IS 1893 (Part 1). L = length of the building Alternatively, a site-specific seismic design criteria L = development length of bar in tension d developed along the principles described in IS 1893 M = moment (Part 1) may be used. Modification to seismic forces n = total number of columns as given in IS 1893 (Part 1) and to material strengths c will be applicable to both preliminary and detailed n = total number of frames in the direction of f assessments described in this standard. loading P = axial load 5.4 Lateral Load Modification Factor P = strength in axial compression The lateral force determined for strength related checks ac P = minimum yield strength in tension for the needs to be modified for reduced useable life. The y braces useable life factor U, is to be multiplied to the lateral force (base shear) for new building as specified in t = thickness of flange f IS 1893 (Part 1). U will be determined as T = remaining useful life of the building rem U = (T /T )0.5 T = design useful life of the building rem des des where τ = average shear stress in concrete columns col τ = average shear stress in walls T = remaining useful life of the building; and wall rem t = thickness of jacket T = design useful life of the building. j des U = useable life factor U will not be taken less than 0.7 in any case. µ = coefficient of friction NOTES η = efficiency factor 1 By comparing the requirements of the revisions of IS 1893 of 2002 with 1984, 1975, 1966 and 1962 revisions, it is seen V = total shear capacity of reinforced concrete that buildings designed accordingly from time to time, will be beam found deficient to some extent. 3 IS 15988 : 2013 2 It may be mentioned that buildings designed as per IS 1893 5.6.1 A preliminary evaluation of building is carried will in general not need retrofitting except those on stilts (soft out. This involves broad assessment of its physical first story) and those using 230 mm or thinner columns will condition, robustness, structural integrity and strength need retrofitting. of structure, including simple calculations. 3 Buildings designed to earlier code revisions of IS 1893 may be found deficient to a small extent. Engineer incharge may 5.6.2 If the results of preliminary evaluation for use his discretion in regard to retrofitting decision. strength, overall stability and integrity are acceptable, 4 Building designed to earlier code revisions of IS 1893, unless no further action is required. Else a detailed evaluation over designed and those not designed for earthquake forces will generally need retrofitting. is required unless exempted. 5 Factor U may be applied in all cases (except in a building of NOTE — Single or two storey buildings (not housing essential critical safety, if desired U may be taken as 1.0). services required for post-earthquake emergency response) of total floor areas less than 300 sq. m may be exempted from 5.5 Modified Material Factor detailed evaluation even when a preliminary evaluation indicates deficiencies and where seismic retrofitting is carried Strength capacities of existing building components out to remedy those deficiencies. shall be based on the probable material strengths in 5.6.3 A detailed evaluation includes numerical checks the building. Probable or measured nominal strengths on stability and integrity of the whole structure as well are best indicator of the actual strength and may only as the strength of each member. Conventional design be obtained by field or lab tests on a series of samples. calculations for these checks shall use modified It is recommended that probable strengths are either demands and strengths. A flow diagram summarizing based on actual tests or the default values given in the various steps of the evaluation process is shown in subsequent clauses. These may also be assessed from Fig.1. the values given in the original building documents. However, they all need to be further modified for the 6 PRELIMINARY EVALUATION uncertainty regarding the reliability of available information, and present condition of the component. 6.1 General The probable material strengths need to be multiplied The preliminary evaluation is a quick procedure to with a Knowledge Factor, K as defined in Table 1. establish actual structural layout and assess its characteristics that may affect its seismic vulnerability. Table 1 Knowledge Factor, K It is a very approximate procedure based on Sl No. Description of Building K conservative parameters to identify the potential (1) (2) (3) earthquake risk of a building and may be used to screen buildings for detailed evaluation. Method is primarily i) Original construction documents available, 1.00 including post-construction activities, such as based on observed damage characteristics in previous modification to structure or materials testing earthquakes coupled with some simple calculations. undertaken of existing structure ii) Documentation as in Sl No. (i) but no testing 0.90 6.2 Site Visit of materials, that is using originally specified values for materials A site visit shall be conducted by the design iii) Documentation as in Sl No. (i) no testing of, 0.80 professional to verify available existing building data that is originally specified values for materials or collect additional data, and to determine the and minor deterioration of original condition condition of the building and its components. The iv) Incomplete but useable original construction 0.70 documents and no testing following information either needs to be confirmed or v) Incomplete or no documents available but 0.70 collected during the visit: extensive testing and inspection done to establish current strength of load resisting a) General information — Number of storeys members and dimensions, year of construction. vi) Documentation as in Sl No. (iv) and limited 0.60 b) Structural system description — Framing inspection, and verification of structural members, or materials test results with large vertical lateral force-resisting system, floor variation and roof diaphragm connection to walls, vii) Little knowledge of details of a component 0.50 basement and foundation system. c) Building type and site soil classification as in 5.6 Evaluation Process IS 1893 (Part 1). d) Building use and nature of occupancy. Existing buildings not designed in accordance with the e) Adjacent buildings and potential for pounding principles and philosophies and requirements of current and falling hazards. seismic standards as described in the following clauses shall be assessed. f) General conditions — Deterioration of 4 IS 15988 : 2013 Preliminary Evaluation (For Overall Seismic Vulnerability Assessment) a) Site visit and collection of data in (see 6.2) b) Configuration-related checks ( Load path, geometry, weak/soft storey, vertical discontinuities, mass irregularity, torsion, adjacent buildings, short columns) (see 6.4) c) Strength-related Check (see 6.5 ) 1. Determination of modified demand lateral force considering occupancy risk factor and factor for useable life. 2. Shear stress Check in columns and walls. 3. Axial stress check in moment frame columns. Acceptability Yes Criteria Satisfied (see 6.6) Detailed Evaluation (For Primary Lateral-Force Resisting System) a) Component Strength using Knowledge Factor (see 7.2.1) b) Elastic Static/Dynamic Analysis for Modified Lateral Force (see 7.2.2.and 7.2.3) c) Comparing Probable Component Strengths with Expected Seismic Demands Acceptability Criteria Satisfied (see 7.3) Yes No Retrofit Recommended Retrofit Not Recommend d FIG.1 FLOW CHART SUMMARIZING EVALUATION PROCESS 5