IS 13920 1993 ductile detailing of RCC structures subjected to seismic forces

1. Earthquake Resistant Design of StructuresEarthquake Resistant Design of Structures
IS 13920-1993 : Ductile detailing of RC structures
subjected to seismic forces – code of practice

12. 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

13. Zoning Map (under revision)
 Ductility – to enable the structure to absorb
energy during earthquakes to avoid sudden
collapse
 Details for ductility in IS 13920

14. Roof-top DisplacementRoof-top Displacement
V/W(Acceleration)V/W(Acceleration)
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

16. 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

17. Principles of ductility
 Avoid shear failure
 Avoid compression failure
 Ensure continuity
 Confine the critical areas where hinge can form.

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Fig.1 Anchorage of Beam Bars
in an External Joint

25. Fig.2 Lap, Splice in Beam

26. Fig.3 Beam Web Reinforcement

27.  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

28.  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

29. Fig.5 Beam Reinforcement

30. Typical Details of
Reinforcement of Main Beams

31. 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

32.  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

33. 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

34. 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.

35. 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

36. 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

37. Fig.6 Reinforcement requirement for
Column with More Than 100 mm
Projection Beyond Core

38. Fig.7 Transvers Reinforcement
in Column

39. Fig.7A

40. Fig.8 Calculation of Design
Shear Forces for Column

41. Fig.9 Column and Joint
Detailing

42. Typical Section of Column

43. Fig 10 Provision of Special Confining
Reinforcement in Footing

44. Shear Walls
 Minimum thickness 150mm
 Preferably 200 mm with 2 layer steel
 Minimum steel 0.0025 inch in each direction
 Check for shear

45. Boundary Elements
 To be designed as columns
Minimum steel 0.8%
Maximum steel 6%

46. Coupled shear wall
 Provide diagonal steel
– As = Vu / (1.74 fy sin α )
 Openings
– Provide the interrupted beams on either side

47. Fig.11 Special Confining Reinforcement
Requirement for Columns Under
Discontinued Walls

48. Fig.12 Columns with Varying
Stiffness

49. Conclusions
 India has a well developed code
 Problem lies in compliance
 Introduce earthquake engineering in
curriculum
 Update knowledge
 Registration of engineers

51. THANK YOUTHANK YOU
Jose Kurian