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Guidelines for Seismic Evaluation of Existing Buildings. Federal Emergency Management Agency. International Conference of Building Offices. 2.1. Washington. Building Seismic Safety Council. 2001 (3) FEMA 178 -NEHRP Handbook for the seismic evaluation of existing buildings. (10) Uniform Code for Building Conservation.1. (6) (7) FEMA 356 . CA. 1988. New Zealand. USA. (5) FEMA 310 -Handbook for the Seismic Evaluation of Buildings – A Prestandard. USA. SCOPE 2. UNDP/UNIDO. This document provides a method to assess the ability of an existing building to reach an adequate level of performance related to life safety of occupants. Washington DC.
.2. Federal Emergency Management Agency. USA. C. Volume 4. 2.03 . New Zealand National Society for Earthquake Engineering for Building Industry Authority. Federal Emergency Management Agency.Prestandard and commentary for the Seismic Rehabilitation of Building. CEN. 20C The Assessment and Improvement of the Structural Performance of Earthquake Risk
Buildings .1. (9) Seismic Evaluation of Existing Buildings. Therefore. Brussels. This document is particularly concerned with the seismic evaluation and strengthening of the existing reinforced concrete buildings and it is intended to be used as a guideline. 1996T (8) Post-Earthquake Damage Evaluation and Strength Assessment of Buildings under Seismic Conditions.6 In developing this document.(75% complete draft). IS:13920-1993 and the following shall apply. USA. 1991. CA. 2003. D. USA. 3. TERMINOLOGY 3.Draft for General Release. assistance has been derived from the following publications: (1) ATC 33. American Society of Civil Engineers. Washington DC. 1998. the emphasis is on identification of unfavorable characteristics of the building that could damage either part of the building or the entire structure. (2) Eurocode 8-Design Provisions for Earthquake Resistance of Structures-Part 3. (4) FEMA 154 -Rapid Visual Screening of Buildings for Potential Seismic Hazards: A Handbook. Applied Technology Council. Washington DC. For the purpose of this guideline. Whittier. 1992. the definitions given in IS:1893 (Part I) -2002.
if it deforms such that the maximum lateral displacement measured from the chord of the deformed shape at any point of the diaphragm is more than 1.5 times the average displacement of the entire diaphragm.or deformation-controlled.9.8. 3. coupling beams. Demand The amount of force or deformation imposed on an element or component. Capacity The permissible strength or deformation for a structural member or system. 3. 3. Column (or Beam) Jacketing A method in which a concrete column or beam is covered with a steel or concrete jacket in order to strengthen and or repair the member by confining the concrete. 3.10.3. slabs. and connections.Infill A panel of masonry placed within a steel or concrete frame. Panels that are in tight contact with a frame around its full perimeter are termed shear infills.6.11. columns. or rotation corresponding to a displacement due to a structural degree of freedom. braces. including beams. deformation. torque.
. displacement.7.5. piers.2. Acceptance Criteria Limiting values of properties such as drift. 3. Components The basic structural members that constitute a building. Action An internal moment.3. typically horizontal. shear. 3. and inelastic deformation used to determine the acceptability of a component. 3.Flexible Diaphragm A floor diaphragm shall be considered to be flexible. Panels separated from the surrounding frame by a gap are termed isolated infills. walls. 3. axial load. 3.4. D e f o r m a t i o n Relative displacement of rotation of the ends of a component or element or node. of a component or element or node. Displacement The total movement. strength demand. designated as force.
Life Safety Performance Level Building performance that includes significant damage to both structural and nonstructural components during a design earthquake.12.Knowledge Factor A factor to represent the uncertainty about the reliability of the available information about the structural configuration and present condition of materials and components of the existing building. mechanical or electrical components of a building that are permanently installed in. the load travels from the diaphragm through connections to the vertical lateral-force-resisting elements. but the level of risk for life-threatening injury and entrapment is low.16. shear walls. though at least some margin against either partial or total structural collapse remains. 3. a building system. Injuries may occur. finally.Overturning Action resulting when the moment produced at the base of vertical lateral-force-resisting elements is larger than the resistance provided by the foundation's uplift resistance and building weight. or are an integral part of.14.Occupancy Risk Factor A factor to represent acceptable risk of damage for existing buildings as a function of occupancy and importance. to soil.
. 3.Load Path A path that seismic forces take to the foundation of the structure and. 3.3. braced frames and interconnecting horizontal diaphragms that provide earthquake resistance to a building. and then proceed to the foundation.17.Nonstructural Component Architectural. 3. bearing walls.13. 3.18.Plan Irregularity Horizontal irregularity in the layout of vertical lateral-force-resisting elements. 3. Typically.19.Lateral Force Resisting System The collection of frames.15. 3. producing a mismatch between the center of mass and center of rigidity that typically results in significant torsional demands on the structure.
and formulas derived from accepted principles of structural mechanics. or (2) field tests or laboratory tests of scaled models. determined by structural analysis. 3.20. Reinforced concrete monolithic slab-beam floors or those consisting of prefabricated/precast elements with topping reinforced screed can be taken as rigid diaphragms.Primary Component Those components that are required as part of the building’s lateral-force-resisting system.22.3.27.Primary Element An element that is essential to the ability of the structure to resist earthquake-induced deformations.24.Required Member Resistance (or Required Strength) Load effect acting on an element or connection.Redundancy Quality of having alternative paths in the structure by which the lateral forces are resisted.21.Rigid Diaphragm A floor diaphragm shall be considered to be rigid.
. 3.5 times the average displacement of the entire diaphragm. 3. resulting from the factored loads and the critical load combinations. 3.Secondary Element An element that does not affect the ability of the structure to resist earthquake-induced deformations.Pounding Two adjacent buildings coming into contact during earthquake excitation because they are too close together and/or exhibit different dynamic deflection characteristics. allowing for modeling effects and differences between laboratory and field conditions. 3.Secondary Component Those components that are not required for lateral force resistance. if it deforms such that the maximum lateral displacement measured from the chord of the deformed shape at any point of the diaphragm is less than 1.Probable or Measured Nominal Strength The capacity of a structure or component to resist the effects of loads. allowing the structure to remain stable following the failure of any single element. 3. They may or may not actually resist some lateral forces.28. 3.23.25. as determined by (1) computations using specified material strengths and dimensions. 3.26.
33. 3. or 50% of the nominal height of the typical columns at that level. etc.29.34.Short Column The reduced height of column due to surrounding parapet. stiffness.Strengthening Measures Modifications to existing components. infill wall.32.Seismic Demand Seismic hazard level commonly expressed in the form of a ground shaking response spectrum. or moment that can be resisted by a component. 3. 3.30. that correct deficiencies identified in a seismic evaluation as part of a strengthening scheme. etc.Strength The maximum axial force. 3. geometry.31.Vertical Irregularity A discontinuity of strength.Strengthening Strategy A technical approach for developing strengthening measures for a building to reduce its earthquake vulnerability. 4. or installation of new components. 3.Seismic Evaluation An approved process or methodology for evaluating deficiencies in a building which prevent the building from achieving life safety objective.Strengthening Method A procedural methodology for the reduction of building vulnerability.3.Strong Column-Weak Beam The capacity of the columns in any moment frame joint must be greater than that of the beams. LIST OF SYMBOLS The symbols and notations given below apply to the provisions of this document: Ac Total cross-sectional area of columns Ahm Modified seismic coefficient An Area of net mortared/grouted section 23
. or mass in one story with respect to adjacent stories.37. 3.36. infill wall.35. is less than five times the dimension of the column in the direction of parapet. It may also include an estimate of permanent ground deformation. 3. to ensure inelastic action in the beams. 3. shear force.
Inadequate treatment of non-structural components like infill masonry walls. .Main Causes of damage of RC buildings observed in earthquake 1. Inadequate detailing of reinforcement in beams.1 . Poor quality of construction materials and technology. 5. etc.Aw Cp D fck Fo Fwx H H' L ml mo mk nc nf t vu VB Vc Vca Vcb Vd Vj Vwx Wd
Total area of shear walls in the direction of loading Horizontal force factor In-plane width dimension of masonry Characteristic strength of concrete Axial force due to overturning Force applied to a wall at level x Total height of the building Least clear height of opening on either side of pij Length of the building Factor for reduced useable life Occupancy risk factor Knowledge factor Total number of columns Total number of frames Thickness of wall Unit shear strength of the diaphragm Base shear Column shear force Total shear capacity of cross walls in the direction of analysis immediately above the diaphragm level being investigated Total shear capacity of cross walls in the direction of analysis immediately below the diaphragm level being investigated Diaphragm shear Story shear at level. 3.Evaluation Criteria 5. staircases. Total shear force resisted by a shear wall at the level under consideration Total dead load tributary to a diaphragm
τcol Average shear stress in concrete columns τwall Average shear stress in walls 5. particularly from ductility considerations. columns. and the joints. 24
. Lack of good design in planning lateral load resisting system such as moment resistant frames. Inadequate diaphragm action of roofs and floors. water tanks on roofs. 4. frames with shear walls or with infill walls. beam-column joints. 2.
It may be mentioned that buildings designed as per 1984 version will in general not need retrofitting except those on stilts (soft first storey) and those using 230 mm or thinner columns will need retrofitting.4 . U will not be taken less than 0. response reduction factor are to be taken directly from IS: 1893.5 where Trem = Remaining useful life of the building Tdes = Design useful life of the building. 1966 & 1962 versions. The useable life factor U. Since the provisions of this code are strongly correlated with the design criteria of new buildings contained in IS: 1893. which will make the buildings susceptible to the above mentioned damage. Buildings designed to 1975 version of the code may be found deficient to a small extent. reference shall always be made to the current edition of IS: 1893. This section defines the minimum evaluation criteria for the expected performance of life. Modification to seismic forces as given in IS: 1893 and to material strengths will be applicable to both preliminary and detailed assessments described in this document. 5.7 in any case.Lateral Load Modification Factor The lateral force determined for strength related checks needs to be modified for reduced useable life.safety of existing buildings with appropriate modification to IS: 1893 seismic force. 1975. Alternatively. 5.The evaluation criteria are aimed at determining the weakness/deficiencies in design and construction. which is applicable for the seismic design of new buildings. is to be multiplied to the lateral force (base shear) for new building as specified in IS: 1893-2002 (Part – 1). 2. Note:1. Engineer incharge may use his discretion in regard to retrofitting decision. U will be determined as U = (Trem /Tdes )0.2 The seismic performance of existing buildings is evaluated in relation to the performance criteria in use for new buildings. 5. a site-specific seismic design criteria developed along the principles described in IS: 1893 may be used. All existing structural elements must be able to carry full other non-seismic loads in accordance with the current applicable codes related to loading and material strengths. it is seen that buildings designed accordingly from time to time. By comparing the requirements of the versions of IS:1893 of 2002 with 1984.3 -
Basic inputs for determination of seismic forces such as seismic zone. 3. building type. will be found deficient to some extent. 25
75 members. and verification of structural 0. Factor U may be applied in all cases (except in a building of critical safety. The probable material strengths need to be multiplied with a Knowledge Factor. including simple calculations.Evaluation Process Existing buildings that were not designed in accordance with the principles and philosophies of earthquake codes and buildings that predate the current seismic code as described in the following sections would be assessed.6.85 0. 5. robustness. they all need to be further modified for the uncertainty regarding the reliability of available information. Building designed to earlier code versions.1 A preliminary evaluation of building is carried out which involves broad assessment of apparent physical condition. Stability of infill walls under out of plane lateral earthquake must be ensured in each case.e. Table 1: Knowledge factor. structural integrity and strength of structure. such as modification to structure or materials testing undertaken of existing Documentation as above in (1) but no testing of materials. 26
. if desiredU may be taken as 1.6 . K as defined in Table 1.4. i.90 0. No Description of Building 1 2 3 4 5 6 Original construction documents available..80
Documentation as in (4) and limited inspection. originally specified values for materials and minor deterioration of original condition Incomplete but useable original construction documents and no testing. i.5 . 5.e.. or materials test results with large variation Little knowledge of details of a component 0.00 0. These can also be assessed from the values given in the original building documents. using originally specified values for materials. unless over designed and those not designed for earthquake forces will generally need retrofitting.70
5. This document recommends that probable strengths are either based on actual tests or the default values given in the subsequent sections of this document. and present condition of the component. including post-construction activities. Documentation as above in (1) but no testing of materials. 5. Probable or measured nominal strengths are best indicator of the actual strength and can only be obtained by field or lab tests on a series of samples.Modified Material Factor Strength capacities of existing building components should be based on the probable material strengths in the building. K 1. However.0). K S. 6.
1 . 5. no further action is required.3 A detailed evaluation when required will include numerical checks on stability and integrity of the whole structure as well as the strength of each member.5. overall stability and integrity are acceptable. Else a detailed evaluation is required unless exempted.General
Preliminary evaluation is a quick procedure to establish actual structural layout and assess its characteristics that can affect its seismic vulnerability. -PRELIMINARY EVALUATION 6.2 If the results of preliminary evaluation for strength.6. It is a very approximate procedure based on conservative parameters to identify the potential earthquake risk of a building and can be used 27
.6. A flow diagram summarizing various steps of the evaluation process is shown in Figure 1. Conventional design calculations for these checks will use modified demands and strengths.
6. especially location of masonry infill walls. floor and roof diaphragm connection to frames.Acceptability Criteria
A building is said to be acceptable if it meets ALL the configuration-related checks as well as global level checks on axial and shear stress as outlined in the following sections. The following information either needs to be confirmed or collected during the visit: (a) General information: Number of storeys and dimensions.Load Path The structure shall contain at least one rational and complete load path for seismic forces from any horizontal direction so that they can transfer all inertial forces in the building to the foundation.4.
.2 -Geometry No change should be made in the horizontal dimension of lateral force resisting system of more than 50% in a storey relative to adjacent stories. Method is primarily based on observed damage characteristics in previous earthquakes coupled with some simple calculations.3 .to screen buildings for detailed evaluation.4 -Configuration-Related Checks 6.4. excluding penthouses and mezzanine floors. damage from past earthquakes. 6. year of construction (b) Structural system description: Framing vertical lateral force-resisting system. alterations and additions that could affect earthquake performance (h) Architectural features that may affect earthquake performance.Weak Storey The strength of the lateral force resisting system in any storey shall not be less than 80% of the strength in the storey above. 6.2 . and to determine the condition of the building and its components.4.Site Visit
A site visit will be conducted by the design professional to verify available existing building data or collect additional data. 6. basement and foundation system (c) Building type as in IS 1893 (Part1) (d) Site soil classification as in IS 1893 (Part 1) (e) Building use and nature of occupancy (f) Adjacent buildings and potential for pounding and falling hazards (g) General conditions: Deterioration of materials.1 .3 . 6.
The number of bays of moment frames in each line shall be greater than or equal to 2.4. proper restoration will need to be taken before or along with retrofitting.Deterioration of Concrete There should be no visible deterioration of the concrete or reinforcing steel in any of the vertical or lateral force resisting elements.4.4.4. or shall be anchored to the lateral-force-resisting elements of the main structure.4.7.11 . 6. infill wall.4.4. 6.4. or 50% of the nominal height of the typical columns in that storey.5 .3 of IS:1893 (Part-1) – 2002.Mezzanine/loft/sub-floor – Interior mezzanine/loft/sub-floor levels shall be braced independently from the main structure.4 -Soft Storey The stiffness of lateral load resisting system in any storey shall not be less than 70% of the stiffness in the storey above or less than 80% of the average stiffness of the three storeys above. 6.Torsion The estimated distance between a storey center of mass and the storey centre of stiffness shall be less than 30% of the building dimension at right angles to the direction of loading considered. 6.7 . 6. In case it is there.9 . etc. 6.10 .Redundancy The number of lines of moment frames in each principal direction shall be greater than or equal to 2.11. penthouses. and mezzanine floors need not be considered in mass irregularity.4.12 . 6.6. etc.8 .6 .Mass There shall be no change in effective mass more than 100% from one storey to the next.Short Columns The reduced height of a column due to surrounding parapet.Vertical Discontinuities All vertical elements in the lateral force resisting system shall be continuous from the roof to the foundation. Light roofs.
. infill wall. 6. shall not be less than five times the dimension of the column in the direction of parapet.Adjacent Buildings The clear horizontal distance between the building under consideration and any adjacent building shall be greater than the gap width specified in Cl.
(6... b) A building is 6 storeys and higher. if any of the following conditions are met: a) The building fails to comply with the requirements of the Preliminary Evaluation.1 .. d) Buildings with inadequate connections between primary structural members.. fck is characteristic cube strength of concrete: nc Vj τcol = nc ....6 .3)
where...5. nc = total no... of frames in the direction of loading VB = base shear H = total height L = length of the building.5 -Strength-Related Checks Approximate and quick checks shall be used to compute the strength and stiffness of building components.
(6.Shear Stress in RC Frame Columns The average shear stress in concrete columns. of frames in the direction of loading...1) where. nf = total no. of columns. nf= total no.. such as poorly designed and/or constructed joints of pre-cast elements. 30
. Vj = storey shear at level j and Ac = total cross-sectional area of columns.25fck....3 . c) Buildings located on incompetent or liquefiable soils and/or located near (less than 12 km) active fault trace and/or with inadequate foundation details..6.4of this document..5.... 6. 6.Axial Stress in Moment Frames Axial stress in the columns of moment frames at base due to overturning forces alone (Fo) as calculated using the following equation shall be less than 0.Recommendation for Detailed Evaluation A building is recommended to undergo a detailed evaluation as described in Section 6. τcol.
H L.5.. 6.. The seismic base shear and storey shears for the building shall be computed in accordance with IS 1893-2002 (Part-1) and the modification factor U as per cl.nf Ac ………..1 √fck where. computed in accordance with the following equation shall be limited to 0...
Lap splices shall be located only in the central half of the member length. (g) Stirrup Spacing—The spacing of stirrups over a length of 2d at either end of a beam shall not exceed (a) d/4. If more than 50 percent of the bars are spliced at one section.7. The lap length shall not be less than the bar development length in tension. (b) within a distance of 2d from joint face. and shall be in accordance with provisions of IS: 13920 for shear design of beams and columns. Alternatively. The first hoop shall be at a distance not exceeding 50 mm from the joint face. (a) No Shear Failures — Shear capacity of frame members shall be adequate to develop the moment capacity at the ends.3 Ld where Ld is the development length of bar in tension as per IS 456: 2000.The parallel legs of rectangular hoop shall be spaced not more than 300 mm centre to centre.1 .Longitudinal bars shall be spliced only if hoops are located over the entire splice length. the provision of a crosstie should be there. at a spacing not exceeding 150 mm. If the length of any side of the hoop exceeds 300 mm. (d) Column-Bar Splices . the lap length shall be 1. (c) Beam Bars . Not more than 50 percent of the bars shall preferably be spliced at one section. (h) Joint Reinforcing— Beam-column joints shall have ties spaced at or less than 150 mm. or (b) 8 times the diameter of the smallest longitudinal bar. Any deficiency should be considered in choosing the response reduction factor R in detailed evaluation and in the retrofit design. The hooks shall engage peripheral longitudinal bars. (e) Beam. . In case of beams vertical hoops at the same spacing as above shall also be located over a length equal to 2d on either side of a section where flexural yielding may occur under the effect of earthquake forces. Elsewhere. (f) Column-Tie Spacing .DETAILED EVALUATION 7. however. the beam shall have vertical hoops at a spacing not exceeding d/2. Lap splices shall not be located (a) within a joint.At least two longitudinal top and two longitudinal bottom bars shall extend continuously throughout the length of each frame beam. a pair of overlapping hoops may be located within the column. (b) Strong Column/Weak Beam – The sum of the moment of resistance of the columns at any joint shall be at least 1. Hoops shall be located over the entire splice length at spacing not exceeding 150 mm centre to centre.bar Splices .Checking Original Design Details The following details shall be checked in the original design. It should be proportioned as a tension splice.
. and (c) within a quarter length of the member where flexural yielding may occur under the effect of earthquake forces. it need not be less than 100 mm. Not more than 50 percent of the bars shall be spliced at one section. At least 25% of the longitudinal bars located at the joints for either positive or negative moment shall be continuous throughout the length of the members.1 times the sum of the moment of resistance of the beams along each principal plane of the frame joints.
7.3 . positive connection will need to be provided during retrofitting. Note:. b) Available seismic resistance The available seismic resistance (ASR) of a building is expressed quantitatively by the earthquake force under which the first of the columns of any building storey will reach its ultimate limit strength. These are defined below: a) Minimum seismic resistance The required minimum seismic resistance (MSR) is expressed quantitatively by the design seismic coefficient as per IS: 1893-2002.2 . The probable strengths determined from conventional methods and applicable codes shall be modified with appropriate knowledge factor K given in table 1. If not. (b) Masonry Joints — The mortar shall not be easily scraped away from the joints by hand with a metal tool. when the remaining structure remains in the undamaged state.Detailed Evaluation Procedure That is.(j) Stirrup and Tie Hooks .. The overall lateral force (or Base shear) on the building is obtained by multiplying the design seismic coefficient and the total weight of the building. (c) Cracks in Infill Walls — There shall be no existing diagonal cracks in infill walls. Seismic demand on critical individual components shall be determined using seismic analysis methods described in IS 1893-2002 (part-1) for lateral forces prescribed therein with modification for reduced Useable life (i. (d) Cracks in Boundary Columns —There shall be no existing diagonal cracks in concrete columns that encase masonry infills. which includes (i) the basic seismic coefficient for the zone (ii) the fundamental period of the building.The beam stirrups and column ties shall preferably be anchored into the member cores with hooks of 135° and 6d extension. described in Section 5.RC Frames with Masonry Infill Wall The infill walls should be checked for the following criteria: (a) Masonry Units — There shall be no visible deterioration of masonry units. factor U).
. and (iii) the importance factor of the building. the Available Seismic Resistance should at least be equal to the Minimum Seismic Resistance required.4 of the document. restoration will be needed before or during retrofitting (e) Wall Connections — All infill walls shall have a positive connection to the frame to resist out-of-plane forces.In case (a) to (d) exist.e. and there shall be no areas of eroded mortar. 7.
3 Perform a linear equivalent static or dynamic analysis of lateral load resisting system of the building for the base shear calculated as in 7.4 -
Evaluate acceptability of each component by comparing its probable strength with the seismic actions.General This section outlines seismic strengthening options and strategies at a general level.3.5 Estimate the storey drifts and decide whether it is acceptable in terms of the requirements of IS 1893-2002 (Part-1).3.2.3.2 Calculate the total lateral force (design base shear) in accordance with IS: 1893 and multiply it with U the factor for reduced useable life (see 5. and describes a methodology for the design of the strengthening measures as modifications to correct/ reduce seismic deficiency identifying during the evaluation procedure discussed in Section 7. The engineer has to ensure that the failure of these few elements will not lead to loss of stability or initiate progressive collapse.4).2 and determine the resulting member actions for critical components. carried out upto the collapse load. all critical elements of the lateral force resisting elements have strengths greater than computed actions and drift checks are satisfied. 7. 8.1 .1 Estimate the probable flexural and shear strengths of the critical sections of the members and joints of vertical lateral force resisting elements. This needs to be verified by a non-linear analysis such as pushover analysis. 7. b) Except a few elements. .4 . 7.SEISMIC STRENGTHENING 8. These calculations shall be performed as per respective codes for various building types and the strength modified with knowledge factor (K).3. 7.Acceptability Criteria A building is said to be acceptable if either of the following two conditions are satisfied: a) All critical elements of lateral force resisting elements have strengths greater than computed actions and drift checks are satisfied. 7.3.
One such measure is jacketing of RC columns. 8. If such deficient members are small in number. stiffness and mass result in poor seismic performance.
8.1 . Simple removal of such discontinuities may reduce seismic demand on other structural components to acceptable levels.1 Existing buildings with a sufficient level of strength and stiffness at the global level may have some members (or components). which lack adequate strength.2 .2.2.Eliminating or Reducing Structural Irregularities 8. The chosen seismic strengthening scheme should increase the redundancy of lateral load resisting elements to avoid collapse and overall instability. Often these irregularities exist because of discontinuity of structural members. stiffness or ductility. 8. an economical and appropriate strategy is to modify these deficient members alone while retaining the existing lateral-force resisting system.1.2. stiffness and/or ductility of deficient members and their connections.2. 8.2
. Strengthening measures could include such as jacketing columns or beams.Strengthening at Member Level 8.Seismic Strengthening Options and Strategies
Seismic strengthening for improved performance in the future earthquakes can be achieved by one of several options discussed in this section.2Member level modification can be undertaken to improve strength.2.1.2. Braced frames and shear walls can also be effectively used to balance stiffness and mass distribution within a storey to reduce torsional irregularities.2.1 Irregularities related to distribution of strength.3Member level strengthening measures that enhance ductility of the member without significantly increasing its strength/stiffness are often useful when analysis indicates that a few members of the lateral-load resisting system are deficient.2.2.2 An effective measure to correct vertical irregularities such as weak and/or soft storey is the addition of shear walls and braced frames within the weak/soft storey. 34
. 8.1.8. which improves the member level ductility by increased confinement.
braced or moment frames help reduce inelastic demands on the existing gravity load resisting elements. This technique is most effective for relatively stiff buildings with low profiles and large mass compared to light.8. 8.3.1 .Strengthening At Structural Level 8. 35
.Alternative Strengthening Options Often existing buildings do not have sufficient lateral resistance against earthquake forces and resulting large ductility demands for members are too difficult to be met by them.Design Criteria The performance criteria for the design of strengthening measures shall be same as for evaluation process as defined in Section 5.1 In structures where more than a few critical members and components do not have adequate strength and ductility. However. An overall response of base isolation is reduction in demands on the elements of the structure. 8. 8.3.Supplemental Damping and Isolation Seismic isolation and supplemental damping are rapidly evolving unconventional strategies for improving seismic performance of structures. Energy dissipation helps in overall reduction in displacements of the structure. an effective way is to strengthen the structure so that the overall displacement demands can be reduced. flexible structures. but less costly compared to base isolation.Methods of Analysis and Design for Strengthening 8. Provision of supplemental strength in form of additional lateral force-resisting elements such as shear walls.3 . which may require further strengthening.2.3 Seismic gaps (or movement joints) can be created wherever possible between various parts of a building with irregular plan geometry to separate it into a number of regular independent structures. base isolation is technically complex and costly to implement. However. 8.2.4 . this strategy is technically complex. This technique is most effective in structures that are relatively flexible and have some inelastic deformation capacity. Again.1 .3 .2. It may enhance force demands on some other elements.4. Braced frames and shear walls are an effective means of adding stiffness and strength. care should be exercised to provide sufficiently wide gaps to avoid the problem of pounding.2.
(b) The column size and section details are estimated for P and M as determined above.8. (c) The existing column size and amount of reinforcement is deducted from the values obtained considering the demand. Steel encasement is the complete covering of the existing member with thin plates. steel encasement or wrapping with FRPs. (b) Steel profile jacketing can be done through steel angle profiles placed at each corner of the existing reinforced concrete member and connected together as a skeleton with transverse steel straps.4. in terms of axial load (P) and moment (M) is obtained.5. The procedure for reinforced concrete jacketing is: (a) The seismic demand on the columns. (c) Retrofitting using FRPs involves placement of composite material made of continuous fibers with resin impregnation on the outer surface of the RC member.2 . 8. steel profile jacketing. deficient frame members and joints are identified during detailed evaluation of building.1. Another way is by providing steel encasement.3 .Member Capacities Member capacities of existing elements shall be based on the probable strengths as defined in Section 5 and also used for Detailed Evaluation. 8.1 The.5.5 . (a) RC jacketing involves placement of new longitudinal reinforcement and transverse reinforcement bars in the new concrete overlay around existing member. Where possible.RC Jacketing of Columns Reinforced concrete jacketing improves column flexural strength and ductility. (e) The actual concrete and steel provided in the jacket is as given below: Ac = (3/2) Ac ′ and As = (4/3)As′ where Ac and As = Actual concrete and steel to be provided in the jacket Ac' and As' = Concrete and steel values obtained for the jacket after deducting the existing concrete and steel from their respective required amount. 36
. Closely spaced transverse reinforcement provided in the jacket improves the shear strength and ductility of the column.4.1 . Members requiring strengthening or enhanced ductility can be jacketed by reinforced concrete jacketing.Analysis Options The engineer may choose to perform the same analysis as performed during the evaluation process.Strengthening Options for RC Framed Structures
8. 8. (d) The extra size of column cross-section and reinforcement is provided in the jacket. the deficient members should first be stress-relieved by propping.
(f) The spacing of ties to be provided in the jacket in order to avoid flexural shear failure of column and provide adequate confinement to the longitudinal steel along the jacket is given as: 2 s = fy dh √ fck tj where fy = yield strength of steel fck = cube strength of concrete dh = diameter of stirrup tj = thickness of jacket (g)In order to transfer the additional axial load from the old to the new longitudinal reinforcement. Concrete strength should be at least 5 MPa greater than the strength of the existing concrete. The minimum specifications for jacketing of columns are: (a) Strength of the new materials must be equal or greater than those of the existing column. bent-down bars help in good anchorage between existing and new concrete. Moreover. Figure 3: Reinforced Concrete Jacketing
(i) If the transfer of axial load to new longitudinal steel is not critical then friction present at the interface can be relied on for the shear transfer.
. (j) Dowels which are epoxy grouted and bent into 90° hook can also be employed to improve the anchorage of new concrete jacket. (h) The number of bent-down bars required is given as. hs = width of bent-down bars. Asb = total cross-section of the bent down bars. which can be enhanced by roughening the old surface. ∆P nα = 20 Asb + 10 hs where ∆P = additional axial load to be transferred to the jacket reinforcement. bent down bars are provided which are intermittent lap welded to bars of jacket and longitudinal bars in the existing column exposed for the purpose.
2 . (e) Minimum diameter of ties should be 8 mm and not less than 1/3 of the longitudinal bar diameter. (c) Minimum jacket thickness should be 100 mm. 8. whereas the spacing close to the joints within a length of 1/4 of the clear height should not exceed 100 mm.2 . It adds significant strength and stiffness to framed structures.Addition of New Structural Elements One of the strengthening methods includes adding new structural elements to an existing structure to increase the lateral force capacity. 8. The design of shear walls shall be done as per IS: 13920. Shear strength of a beam after strengthening: V = Vcon + VS + VFRP where.5.0035 and FRP reaches its maximum strain.1. (d) Lateral support to all the longitudinal bars should be provided by ties with an included angle of not more than 135°.1 Addition of new reinforced concrete shear walls provides a better option of strengthening an existing structure for improved seismic performance. perpendicular to the shear plane.(b) For columns where extra longitudinal reinforcement is not required. the spacing of ties should not exceed the thickness of the jacket or 200 mm whichever is less.5.87 x fy x Asv x (d/Sv) VFRP = Af ff (d/s) Vcon is shear contribution of concrete VS is shear contribution of steel and VFRP is shear contribution of FRP sheet 8. Limit state moment capacity of FRP retrofitted member is given by: Ultimate flexural strength is determined based on the assumption that compressive concrete reaches a strain of 0.2. Vcon = tc x b x D VS = 0. (f) Vertical spacing of ties shall not exceed 200 mm. is given as. The rupture strength of FRP is used as its limiting strength.Fiber Jacketing of a beam Dimensions of FRP jacket is determined assuming composite action between fiber and existing concrete. (a) The shear transfer reinforcement (dowel bars). Shear walls and steel bracing can be added as new elements to increase the strength and stiffness of the structure.
.5. a minimum of 12φ bars in the four corners and ties of 8φ @ 100 c/c should be provided with 135°bends and 10φ leg lengths. Preferably.
Avf n= Avf' where.0 for concrete placed against hardened concrete with surface intentionally roughened.2 Steel diagonal braces can be added to existing concrete frames. = 0. and rectangular tubes shall have an out-to-out width to wall thickness ratio not exceeding 288/√fy. Braces should be arranged so that their center line passes through the centers of the beam-column joints.Avf = Vu η fy μ where.5
(b) The number of bars required for resisting shear at the interface are given as.5.5 Ac (Ac is the area of concrete section resisting shear transfer). μ = Coefficient of friction = 1.2fckAc or 5. For circular sections the outside diameter to wall thickness ratio shall not exceed 8960/fy. (c) In case of Chevron (V) braces. (c) The minimum anchorage length of the grouted-in longitudinal and transverse reinforcement of the shear wall in to the existing components of the building shall not be less than 6 times the diameter of the bars.2. Vu= Allowable shear force not greater than 0. Avf' = cross-section area of a single bar.75 for concrete anchored to asrolled structural steel by headed studs or by reinforcing bars. the beam intersected by braces shall have adequate 39
. η = Efficiency factor = 0. 8. Angle or channel steel profiles can be used. Some of the design criteria for braces are given below: (a) Slenderness of bracing member shall be less or equal to 2500/√fy. (b) The width-thickness ratio of angle sections for braces shall not exceed 136/√fy.
This load shall be calculated using a minimum of yield strength Py for the brace in tension and a maximum of 0. Braces in X-.3 times of load capacity for the brace in compression Pac. which is then placed in frame bay and firmly connected.5.
. V. (e) The brace connection should be adequate against out-of-plane failure and brittle fracture. 8.2. (d) The top and bottom flanges of the beam at the point of intersection of V braces shall be designed to support a lateral force equal to 2% of the beam flange strength fybftf. Typical connection detail is shown in Figure 5.can be arranged inside a heavy rectangular steel rim.3 Prefabricated steel bracing subassemblages as shown in Figure 6 can be used. for ease of construction.strength to resist effects of the maximum unbalanced vertical load applied to the beam by braces.and inverted V.
Ministry of Home Affairs. New Delhi
Tel: 91-11-23093178.s. Arya and Ankush Agarwal
under the GoI-UNDP Disaster Risk Management Programme Email: anand. Website: www.arya@undp. North Block.org.in
.ndmindia. Email: ndmindia@nic.agarwal@undp.in. Tele/fax: 23094019. ankush.Prepared by:
Professor Anand S.
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