Ductile Detailing for Earthquake Resistant R C Structures

Dr. S. K. PRASAD Professor of Civil Engineering
S.J. College of Engineering Mysore – 570 006
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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
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Structure in Seismic Zone III with
Importance factor (I) greater than 1.0.
Structure in Seismic Zone III and is an
industrial structure, and
is more than five storeys high.
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(modified by the provisions of IS 13920 : 1993)
All structural buildings
Grade of steel : Fe 415 or less shall be used
Flexural member
loading 0.1 fck.
Width, b 200 mm.
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both top and bottom.
steel at that face.
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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
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External Joint For both the top and bottom bars of the beam Anchorage length = Ld + 10 dia - allowance for 90
degree bends.
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Internal Joint For both faces of beam, bars shall be taken continuously through the column.
Failure at internal joint
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Shear failure of R C beam – column joint during the 1985 Mexico earthquake when beam bars are passed outside the column cross section (EERI)
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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
Splicing or lapping of longitudinal bars
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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.
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Closed stirrup having a 1350 hook with a 10 dia
extension ( min of 75 mm) that is embedded in the
confined core
TWO pieces of reinforcement;
• a U – stirrup with a having a 1350 hook and a
10 dia extension ( min of 75 mm), and
• a crosstie.
Flexural members
WEB REINFORCEMENT
Crosstie – bar having a 1350 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.
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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
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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
0.1 fck.
• Minimum dimension 300 mm
• Minimum dimension 300 mm
Preferably b/D ratio > 0.4.
• Shall be provided only in the central half of the
member length
• Spacing of hoops 150 mm
• Not more than 50 percent of the bars shall be spliced at one section.
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Columns and Frame Members
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
Non-structural –
Rectangular columns - Rectangular hoops
• Closed Stirrups
1350 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
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TRANSVERSE REINFORCEMENT
Shear failure
Large spacing of ties and lack of 135 o hook ends caused brittle failure during 2001 Bhuj earthquake spacing
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Special Confining Reinforcement
Shall be provided over a length l0 from each joint face towards midspan
l0 on either side of any section where flexural
yielding may occur under the effect of earthquake
forces
The length of l0 shall not be less than larger lateral dimension of the member
1/6 of clear span of the member, and
450 mm
TRANSVERSE REINFORCEMENT
Closed loop ties in beam column joints will resist
the ill effects of distortion of joints
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Three stage procedure
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Special Confining Reinforcement
Column terminates into a footing or mat, special confining reinforcement shall extend at least 300 mm into the footing or mat.
TRANSVERSE REINFORCEMENT
1/4 of minimum member dimension.
minimum 75 mm
TRANSVERSE REINFORCEMENT
Special Confining Reinforcement
Area of cross section, Ash, of the bar forming circular hoops or spiral is
Area of cross section, Ash, of the bar forming rectangular hoops is
TRANSVERSE REINFORCEMENT
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.
TRANSVERSE REINFORCEMENT
Thickness 150 mm
directions
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
• lw/5 • 3 tw , and • 450 mm where, lw is the horizontal length of wall, and
tw is the thickness of the wall.
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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.
TRANSVERSE REINFORCEMENT
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
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Two distinct designs of building that result in different earthquake performances
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