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

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era

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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