Ductile detailing

Ductile detailing for earthquake resistant RC structures

1. Ductile Detailing for Earthquake Resistant RC Structures - PRESENTED BY S.HARITHA

2. Ductility The ability to sustain large inelastic deformations without significant loss in strength. Objective To provide adequate toughness and ductility to resist severe earthquake shocks without collapse. Ductility = inelastic deformation capacity Ductility  is the ability of  reinforced concrete  members to undergo considerable deflection prior to failure. This characteristic of  reinforced concrete  structural members is crucial as it provides signs of failure and prevents total collapse. This is especially significant in seismic areas.

3. INTRODUCTION The basic approach of earthquake resistant design should be based on lateral strength as well as deformability and ductility capacity of structure with limited damage but no collapse. A ductile materials is one which can undergo large elongations while resisting loads. When applied to reinforced concrete members and structures, ductility means the ability to sustain significant inelastic deformations before collapse. A brittle material fails suddenly while ductile material gives sufficient warning before collapse thus saving many lives. It is very important to incorporate ductility into the structures to make them earthquake resistant. To have sufficient ductility, the designer should pay attention to detailing of reinforcement.

4. 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

5. Principles of ductility Avoid shear failure Avoid compression failure Ensure continuity Confine the critical areas where hinge can form. FACTORS THAT INCREASES DUCTILITY: Using a simple and regular structural configurations Using more redundancy on the lateral load resisting system Avoiding column failures – adopting weak beam - strong column principal in design Using under reinforced beam sections so that they can undergo large rotation before failure.

6. Where it is required? 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. Code book - IS 13920-1993 – IS 456 2000

7. BEAM A beam is a  horizontal structural member  in a building to resist the lateral loads applied to the beams axis. The structural member which resists the forces laterally or transversely applied to the (beam) axis is called a  beam . In it, the loads are acting transversely to the longitudinal axis, which produces the  shear forces  and  bending moment . The lateral load acting on beams are the main cause bending of the beam. They are responsible to transfer a load from the slab to the column. The load distribution pattern is, Slab  |>  Beam  |>  Column  |>  Foundation

8. BEAM FAILURE Beams in RC buildings have two sets of steel reinforcement, namely (a) longitudinal bars placed along the length (b) stirrups, placed vertically at regular intervals along its full length. Longitudinal bars resist bending moment while vertical stirrups resist shear force. Beams sustain two basic types of failures, namely: - Flexural failure - Shear failure

9. FLEXURAL FAILURE As the beam sags under the increased loading, it can fail in two possible ways. It relatively more steel is present on the tension face, concrete crushes in compression, is a brittle failure and is therefore undesirable. If relatively less steel is present on the tension face, the steel yield first and redistribution occurs in the beam until eventually the concrete crushes in compression

10. SHEAR FAILURE A beam may also fail due to shearing action. A shear crack is inclined at 45° to the horizontal. It develops at mid depth near the support and grows towards the top and bottom faces. Closed loop stirrups are provided to avoid such shearing action. Shear failure is brittle, and therefore it must be avoided in the design of RC beams . Shear failure of R C beam – column joint during the 1985 Mexico earthquake when beam bars are passed outside the column cross section (EERI)

11. DUCTILE DETAILING OF BEAM – IS 13920 Clause 6.1 – Page no. 3 Width to depth ratio > 0.3 Width not less than 200mm Depth not greater than 0.25 times span Minimum number of bars: 2 Member size proportions – Web width ≥ 200mm For proper detailing and confinement – Overall depth D ≤ 0.25 of clear span

12. Longitudinal reinforcement – Minimum longitudinal steel = 0.24 (√ fck )/ fy { Equals .00259 for M20 and F415 } – Maximum long steel on any face, 0.025 Minimum compression steel, ≥ 0.5 Ast (Ensures tensile failure) Minimum two bars 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

13. Transverse reinforcement: Transverse stirrups designed to ensure shear capacity exceeds the flexure load capacity Spacing of stirrups at ends up to 2d ≤ d/4, ≤ 8 times dia. of smallest bar, > 100 mm Elsewhere ≤ d/2 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

14. Detailing of Beam

15. COLUMN Columns are defined as vertical load-bearing members supporting axial compressive loads chiefly. This structural member is used to transmit the load of the structure to the foundation. In reinforced concrete buildings beams, floors, and columns are cast monolithically. The bending action in the column may produce tensile forces over a part of cross-section. Still, columns are called compression members because compressive forces dominate their behavior . WHY DO COLUMN FAILS? Inadequate transverse reinforcement in columns and Short column  Inadequate gaps between adjacent buildings  Strong beam–weak column Failures of gable walls Poor concrete quality and corrosion In-plane/out-of-plane effect

16. DUCTILE DETAILING OF COLUMN 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 Ash = 0.18 S h ( fck / fy ) [ Ag / Ak – 1 ] for rectangular

17. 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.

18. 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

19. Spacing S ≤ b/4, b is the smaller dimension 100 mm ≥ & ≥ 75 mm Spacing elsewhere ≤ b/2, b is smaller dimension.

20. Detailing of Column

21. SHEAR WALLS A shear wall is a structural component often provided to multistoreyed or tall buildings or buildings in areas of high wind velocity or seismic activity. The purpose of a shear wall is to resist the lateral loads that are imposed on the structure due to wind, earthquake or sometimes due to hydrostatic or lateral earth pressure. What structures need shear walls? Almost all houses have external shear walls, but internal shear walls are typically found only in larger houses and high-rise buildings subject to lateral winds and seismic forces. The taller the building, the greater the need for internal shear walls and a lateral force resisting system.

22. DUCTILE DETAILING OF SHEAR WALLS Minimum thickness 150mm Preferably 200 mm with 2 layer steel Minimum steel 0.0025 inch in each direction Check for shear Large spacing of ties and lack of 135 o hook ends caused brittle failure during 2001 Bhuj earthquake spacing

23. 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 α ) Openings – Provide the interrupted beams on either side

24. Detailing of Shear wall

25. ADVANTAGES OF SHEAR WALL Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. The overwhelming success of buildings with shear walls in resisting strong earthquakes ; “ we can not afford to build concrete building meant to resist severe earthquakes without shear walls.”

26. Conclusion Specification of steel and concrete to be used are indicated. Special care should be taken to detail the joints so that they keep together under earthquake forces. India has a well developed code problem lies in compliance. Introduce earthquake engineering in curriculum to update knowledge, and to increase the quality of engineers. A complete ductile detailing is designed to resist earthquake , done only through good code provisions (IS-13920-1993).

27. Thank you…