is 13920 1993 ductile detailing of rcc structures subjected to seismic forces – code of practice
TRANSCRIPT
Earthquake Resistant Design of Structures Earthquake Resistant Design of Structures
IS 13920-1993 : Ductile detailing of RC structures
subjected to seismic forces – code of practice
Performance criteria
Moderate Earthquake– Without structural damage– Could occur a number of times in the life span– Code based design seismic coefficients
Large earthquake– Without collapse– May occur once in the life of the structure– Not catered by the codal design seismic co-efficient– Additional resistant by incorporating details for
ductility
Zoning Map (under revision)
Ductility – to enable the structure to absorb
energy during earthquakes to avoid sudden
collapse
Details for ductility in IS 13920
Roof-top DisplacementRoof-top Displacement
V/W
V
/W
(Ac
cel
era
tio
n)
(Ac
cel
era
tio
n)
Low-Strength; Low-Stiffness; BrittleLow-Strength; Low-Stiffness; Brittle
Moderate Strength and Stiffness; DuctileModerate Strength and Stiffness; Ductile
High-Strength; High-Stiffness; BrittleHigh-Strength; High-Stiffness; Brittle
Need for ductility
Earthquake resistant design – costs money Cost increases geometrically for no damage
design Codes adopts lower coefficient
– reduction factor Provisions for durability for once in life
earthquake Design criteria is no-collapse design IS-13920 – 1993 detailing for ductility
Principles of ductility
Avoid shear failure Avoid compression failure Ensure continuity Confine the critical areas where hinge can form.
IS 13920-1993
Applicable for structures located in
– Zones IV and V– Zone III and I > 1– Zone III and is an industrial structure– Zone III and more than five stories
Critical zones in R.C. Frames
Where plastic hinge can form and requires proper confinement:
Ends of beams upto length of 2d– Large negative moments and shears
Moment reversal is possible Ends of columns
– – about 1/6 of the clear height Beam column joints
– Reversible local shear– Causes diagonal cracking
Beams
Width to depth ratio > 0.3
Width not less than 200mm
Depth not greater than 0.25 times span
Minimum number of bars: 2
Detailing of Beams
Member size proportions– Web width 200mm –
• For proper detailing and confinement
– Overall depth D 0.25 of clear span Longitudinal reinforcement
– Minimum longitudinal steel = 0.24 (fck)/fy• Equals .00259 for M20 and F415
– Maximum long steel on any face, 0.025
Detailing of Beams
– Minimum compression steel, 0.5 Ast• Ensures tensile failure
– Minimum two bars (equal to trim) throughout the length of beam at top and bottom
– Full bond length = Ld + 10 times dia. of bar– Splice near quarter-span points, only 50%,
• Lap length = Ld• Confined within stirrups spaced @ 150 mm
Detailing of Beams
Transverse reinforcement– Transverse stirrups designed to ensure shear
capacity exceeds the flexure load capacity– Spacing of stirrups
• at ends upto 2d d/4, 8 times dia. of smallest bar, > 100 mm
• Elsewhere d/2
Fig.1 Anchorage of Beam Bars in an External Joint
Fig.2 Lap, Splice in Beam
Fig.3 Beam Web Reinforcement
Minimum percentage of steel = 0.24 fck / fy
Maximum Steel Ratio
0.25 times ‘+’ steel at support +0.5 times ‘-’ steel
Minimum steel ratio 0.25 ‘-‘ steel ratio at joint
Development length: ld + 10 dia. Splicing
Hoops at 100mm c/c
No laps at joints within 2 dia or 1/4th span
Not more than ½ the bars to be lapped Web reinforcement
Bent-up bars cannot take shear
Fig.5 Beam Reinforcement
Typical Details of Reinforcement of Main Beams
Columns
Minimum dimension not less than 200mm
- do - not less than 300mm for span > 5m or height >
4m
Footing stirrup shall continue 300mm into
footing
Special Ductility Provision
Ash = 0.09 S Dk (fck / fy) [ Ag / Ak – 1 ] for circular
= 0.18 S h (fck / fy) [ Ag / Ak – 1 ] for rectangular
Detailing of Columns
Member size proportions– Minimum side dimensions
• b 200 mm and
• b 300 mm if beam span exceed 5m or unsupported column height exceeds 4 m.
– Preferable ratio of sides, b/d > 0.4, D is larger side dimension
Detailing of Columns
Longitudinal Reinforcement– Splice not more than 50% at any section
• Within middle half height
– Proper detailing where columns area extends more than 100 mm beyond confined core. (Fig. 6 of code)
• If extended portion is non structural provide minimum long and transverse steel as per IS 456.
Detailing of Columns
Transverse Reinforcement– Transverse tie
• Closed hoops• Ends bent through 135° with length 10 dia of stirrps as is
crucial to ensure adequate dimension
– Special confinement steel in the end region of column for a length larger of:
• 450 mm• 1/6 of clear height• Longer lateral dimension (D) of the column
Detailing of Columns
– Specing(s) of special confining reinforcement at end regions
• S b/4, b is the smaller dimension• 100 mm & 75 mm
– Spacing elsewhere b/2, b is smaller dimension– Area of cross section of bar forming special confining
hoop shall be calculated as per clause• 7.4.7 for spiral• 7.4.8 for rectangular stirrups
Fig.6 Reinforcement requirement for Column with More Than 100 mm
Projection Beyond Core
Fig.7 Transvers Reinforcement in Column
Fig.7A
Fig.8 Calculation of Design Shear Forces for Column
Fig.9 Column and Joint Detailing
Typical Section of Column
Fig 10 Provision of Special Confining Reinforcement in Footing
Shear Walls
Minimum thickness 150mm
Preferably 200 mm with 2 layer steel
Minimum steel 0.0025 inch in each direction
Check for shear
Boundary Elements
To be designed as columns
Minimum steel 0.8%
Maximum steel 6%
Coupled shear wall
Provide diagonal steel– As = Vu / (1.74 fy sin )
Openings
– Provide the interrupted beams on either side
Fig.11 Special Confining Reinforcement Requirement for Columns Under
Discontinued Walls
Fig.12 Columns with Varying Stiffness
Conclusions
India has a well developed code
Problem lies in compliance
Introduce earthquake engineering in
curriculum
Update knowledge
Registration of engineers
THANK YOUTHANK YOU
Jose Kurian