Ductile Detailing for Earthquake Resistant R C Structures Dr. S. K. PRASAD Professor of Civil Engineering S.J. College of Engineering Mysore 570 006 1
Ductile Detailing Objective To provide adequate toughness and ductility to resist severe earthquake shocks without collapse IS 13920 : 1993 (Reaffirmed 2003) Code of Practice for Ductile detailing of reinforced concrete structures subjected to seismic forces 2
Ductile Detailing Where is this required? Structure in Seismic Zone IV or V. Structure in Seismic Zone III with Importance factor (I) greater than 1.0. Structure in Seismic Zone III and is an industrial structure, and Structure is located in Seismic Zone III and is more than five storeys high. 3
Ductile Detailing Design of structures : IS : 456 2000 (modified by the provisions of IS 13920 : 1993) All structural buildings Grade of concrete: Minimum f ck = 20 MPa Grade of steel : Fe 415 or less shall be used 4
Flexural member Two types of Damage Flexure Shear Flexural failure is preferred 5
1 Ductile Detailing Flexural members Factored axial stress under earthquake loading 0.1 fck. Preferably width to depth ratio > 0.3. Width, b 200 mm. Depth, D ¼ of clear span. 6
1 Ductile Detailing Flexural members LONGITUDINAL REINFORCEMENT At least two bars throughout the member length at both top and bottom. Tension steel ratio on any face at any section Maximum steel ratio on any face at any section Positive steel at a joint face steel at that face. half the negative 7
Flexural detailing 8
1 Ductile Detailing Flexural members LONGITUDINAL REINFORCEMENT Contd. Steel provided at each of top and bottom face of member at any section along its length 1/4 of maximum negative steel provided at the face of either joint 9
1 Ductile Detailing Flexural members LONGITUDINAL REINFORCEMENT Contd. External Joint For both the top and bottom bars of the beam Anchorage length = L d + 10 dia - allowance for 90 degree bends. 10
Anchorage of beam bars in exterior joints 11
1 Ductile Detailing Flexural members LONGITUDINAL REINFORCEMENT Contd. Internal Joint For both faces of beam, bars shall be taken continuously through the column. 12
Failure at internal joint Shear failure of R C beam column joint during the 1985 Mexico earthquake when beam bars are 13 passed outside the column cross section (EERI)
1 Ductile Detailing Flexural members LONGITUDINAL REINFORCEMENT Contd. Splicing In region of splicing of longitudinal bars Hoops to be provided over the entire splice length, at a spacing not exceeding 150 mm Lap length shall not be less than the bar development in tension 14
Splicing or lapping of longitudinal bars 15
1 Ductile Detailing Flexural members Splicing of Longitudinal Reinforcement Contd. Lap splices shall not be provided 1. Within a joint 2. Within a distance of 2d from joint face, and 3. Within a quarter length of the member where flexural yielding may generally occur under the effect of earthquake forces. Not more than 50 percent of the bars shall be spliced at one section. 16
1 Flexural members WEB REINFORCEMENT Shall consist of vertical hoops. Closed stirrup having a 135 0 hook with a 10 dia extension ( min of 75 mm) that is embedded in the confined core 17
Vertical Stirrups 18
Vertical Stirrups 19
1 Flexural members WEB REINFORCEMENT In compelling circumstances, it may also be made of TWO pieces of reinforcement; a U stirrup with a having a 135 0 hook and a 10 dia extension ( min of 75 mm), and a crosstie. 20
1 Flexural members WEB REINFORCEMENT Crosstie bar having a 135 0 hook and a 10 dia extension ( min of 75 mm) at each end. The hooks shall engage peripheral longitudinal bars. Minimum bar dia for hoops For spans less than 5 m is 6 mm For spans more than 5 m is 8 mm Contribution of bent up bars and inclined hoops to resist shear shall not be considered. 21
1 Flexural members WEB REINFORCEMENT 22
1 Flexural members WEB REINFORCEMENT 23
1 Flexural members Spacing of hoops a) At either end of the beam Over a length of 2d, spacing shall not exceed d/4 8 times the dia of smallest longitudinal bar Minimum spacing is 100 mm First loop 50 mm from joint face 24
1 Flexural members Spacing of hoops b) On either side of a section where flexural yielding may occur Over a length of 2d, spacing shall not exceed d/4 8 times the dia of smallest longitudinal bar c) Elsewhere Spacing d/2 25
1 Columns and Frame Members Factored axial stress under earthquake loading 0.1 fck. Minimum Dimension 200 mm. In frames which have beams of span > 5m, Minimum dimension 300 mm For columns having unsupported length > 4m, Minimum dimension 300 mm Preferably b/d ratio > 0.4. 26
1 Columns and Frame Members LONGITUDINAL REINFORCEMENT Lap Splicing Shall be provided only in the central half of the member length Length = Tension splice Hoops to be provided over the entire splice length Spacing of hoops 150 mm Not more than 50 percent of the bars shall be spliced at one section. 27
1 Columns and Frame Members LONGITUDINAL REINFORCEMENT Any area that extends more than 100 mm beyond the confined core due to architectural requirements shall be detailed as follows: Structural Minimum longitudinal and transverse reinforcement as per IS 13920 : 1993 Non-structural as per IS 456-2000 28
1 Columns and Frame Members TRANSVERSE REINFORCEMENT Circular columns Rectangular columns - Spiral or circular hoops - Rectangular hoops Closed Stirrups 135 0 hook with a 10 dia extension ( min of 75 mm) that is embedded in the confined core Spacing of parallel legs of rectangular hoops 300 mm Provide crosstie if the length of any side of the hoop is > 300 mm 29
Ties or hoops 30
1 Columns and Frame Members TRANSVERSE REINFORCEMENT Closed Stirrups 31
1 Columns and Frame Members TRANSVERSE REINFORCEMENT Alternative Closed Stirrups A pair of overlapping hoops may be provided Spacing of hoops b/2, where b = least dimension 32
Shear failure Large spacing of ties and lack of 135 o hook ends caused brittle failure during 2001 Bhuj earthquake spacing 33
Cross ties 34
Columns and Frame Members TRANSVERSE REINFORCEMENT Special Confining Reinforcement Shall be provided over a length l 0 from each joint face towards midspan l 0 on either side of any section where flexural yielding may occur under the effect of earthquake forces The length of l 0 shall not be less than larger lateral dimension of the member 1/6 of clear span of the member, and 450 mm 35
Columns and Joint Detailing 36
Beam column joint Critical - Need to be designed properly 37
Beam column joint Closed loop ties in beam column joints will resist the ill effects of distortion of joints 38
Beam column joint Providing horizontal ties at joints Three stage procedure 39
Reinforcement details for Shear Wall
Columns and Frame Members TRANSVERSE REINFORCEMENT Special Confining Reinforcement Column terminates into a footing or mat, special confining reinforcement shall extend at least 300 mm into the footing or mat. 41
Columns and Frame Members TRANSVERSE REINFORCEMENT Special Confining Reinforcement Spacing of hoops used as special confining reinforcement 1/4 of minimum member dimension. minimum 75 mm shall not be more than 100 mm 42
Columns and Frame Members TRANSVERSE REINFORCEMENT Special Confining Reinforcement Area of cross section, A sh, of the bar forming circular hoops or spiral is Area of cross section, Ash, of the bar forming rectangular hoops is 43
Joints of Frames TRANSVERSE REINFORCEMENT Special confining reinforcement as required at the end of the column shall be provided through the joint as well. For joints which have beams framing into all vertical faces of it where each beam width is at least 3/4 of the column width provide half the special confining reinforcement required at the end of the column. Spacing of hoops 150 mm. 44
Shear Walls Resist lateral forces coming on structures Thickness 150 mm Reinforcement in longitudinal and transverse directions Minimum pt = 0.25% of gross area If wall thickness is > 200 mm, reinforcement shall be provided in two curtains. Diameter of bars 1/10 of wall thickness. Spacing should be the least of l w /5 3 t w, and 450 mm where, l w is the horizontal length of wall, and t w is the thickness of the wall. 45
Joints of Frames TRANSVERSE REINFORCEMENT Special confining reinforcement as required at the end of the column shall be provided through the joint as well. For joints which have beams framing into all vertical faces of it where each beam width is at least 3/4 of the column width provide half the special confining reinforcement required at the end of the column. Spacing of hoops 150 mm. 46
IS - 13920: 1993 Requirements of detailing R C structures to give adequate toughness and ductility to resist earthquake shocks better without collapse. Particularly necessary in structures located in Zones 3, 4 and 5. Distinction between Toughness & Resilience. Steps to enhance ductility and toughness in R C structures
Horizontal earthquake force 48
Structural action under earthquake 49
Earthquake shaking reverses tension and compression in members. Reinforcement is required in both faces of members 50
Columns should be stronger than beams Two distinct designs of building that result in different earthquake performances 51
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