Seismic Detailing of RC Structures (IS:13920-1993)

Seismic Detailing of RC Structures (IS:13920-1993) Sudhir K Jain Indian Institute of Technology Gandhinagar

November 2012

1

Outline 

This lecture covers: 



Covers important clauses of IS13920 With particular emphasis on Buildings 

Many important clauses applicable to buildings may not be discussed in this lecture in detail.

Sudhir K. Jain, IITGN

Seismic Design of Buildings / November 2012

Slide 2

How to ensure ductility  

Correct collapse mechanism Adequate ductility at locations likely to form hinge in collapse mechanism 



Need sufficient member ductility to ensure adequate structural ductility.

Prevent brittle failure mechanisms to take place prior to ductile yielding



Slide 3

Collapse Mechanism 

Storey Mechanism 



Columns require too much ductility Columns are difficult to make ductile



Slide 4

Collapse Mechanism 

Beam – Hinge Mechanism (Sway Mechanism) 

 

Preferred mechanism Ensure that beams yield before columns do Strong Column –Weak Beam Design



Slide 5

R C Members  Bond Failure: Brittle  Shear Failure: Brittle  Flexural Failure

 Brittle: if over-reinforced section (compression failure)  Ductile: if under-reinforced section (tension failure)

 Hence, Ensure that

 Bond failure does not take place  Shear failure does not precede flexural yielding  Beam is under-reinforced.



Slide 6

Failure of RC Section 

Yielding of tension bars 

 



Ductile Tension failure Under-reinforced section

Crushing of compression concrete   

Brittle Compression failure Over-reinforced section



Slide 7

R C Section 

Tension failure more likely if: 

  



Less tension reinforcement More compression reinforcement Higher grade of concrete Lower grade of steel Lower value of axial compression



Slide 8

Section ductility increases as  

  

Grade of concrete improves Grade of steel reduces Tension steel reduces Compression steel increases Axial compression force reduces 

Generally, columns are less ductile than beams



Slide 9

Capacity Design Concept Brittle Link 



Ductile Link

The chain has both ductile and brittle elements. To ensure ductile failure, we must ensure that the ductile link yields before any of the brittle links fails.



Slide 10

Capacity Design Concept (contd…)

 

Assess required strength of chain from code. Apply suitable safety factors on load and material 



 

Design/detail ductile element(s).

Assess upper-bound strength of the ductile element Design brittle elements for upper-bound load Ensures that brittle elements are elastic when the ductile elements yield.



Slide 11

Capacity Design Concept (contd…)



For instance, in a RC member 

 

Shear failure is brittle Flexural failure can be made ductile Element must yield in flexure and not fail in shear



Slide 12

Capacity Design of Frames  Choose yield mechanism  Locate desirable hinge locations  Estimate reasonable design seismic force on the building  Design the members at hinge locations (upper bound type)  Assess the member forces at other locations under the action of “capacity” force  Design other locations for that force; need not detail these for high ductility



Slide 13

Materials in RC Members  Concrete and steel have very different

characteristics  Steel ductile: strain capacity: ~12% to 25%  Concrete brittle: strain capacity: ~0.35%

HYSD

Mild Steel

20-25% Sudhir K. Jain, IITGN


0.35% Slide 14

Confinement of concrete 

Considerably improves its strain capacity

Stress-strain relationship for concrete proposed by Saatcioglu and Razvi, (1992) Sudhir K. Jain, IITGN


Slide 15

Confinement of Column Sections

Fig. from Paulay and Priestley, 1992



Slide 16

Main Steps  Weak Girder – Strong Column Philosophy  Shear Failure Prevented by Special Calculations (Capacity Design Method)  Good Development Length  Regions Likely to have Hinges Confined with Closely-spaced and Closed Stirrups



Slide 17

Applicability of Code (Cl. 1.1.1)  Originally, this code was applicable for:  All structures in zones IV or V  Structures in zone III with I > 1.0  Industrial structures in zone III  More than 5-storey structures in zone III

 After the Bhuj earthquake, the code made

applicable to all structures in zones III, IV and V.  Even though the code title says “structures”, it was written primarily for buildings. Sudhir K. Jain, IITGN


Slide 18

Background Materials 

The code emerged from the following. These also provide commentary:  Medhekar M S, Jain S K and Arya A S, "Proposed Draft for IS:4326 on Ductile Detailing of Reinforced Concrete Structures," Bulletin of the Indian Society of Earthquake Technology, Vol 29, No. 3, September 1992, 15 - 35.

 Medhekar M S and Jain S K, "Seismic Behaviour, Design, and Detailing of R.C. Shear Walls, Part I: Behaviour and Strength," The Indian Concrete Journal, Vol. 67, No. 7, July 1993, 311-318.  Medhekar M S and Jain S K, "Seismic Behaviour, Design, and Detailing of R.C. Shear Walls, Part II: Design and Detailing," The Indian Concrete Journal, Vol. 67, No. 8, September 1993, 451-457. Sudhir K. Jain, IITGN


Slide 19

Concrete Grade 

Originally, as per Cl.5.2: buildings more than 3 storeys high, minimum concrete grade shall preferably be M20. 



Now, word “preferably” has been dropped.

Most codes specify higher grade of concrete for seismic regions than that for non-seismic constructions. Examples: 



ACI allows M20 for ordinary constructions, but a minimum of M25 for aseismic constructions. Euro code allows M15 for non seismic, but requires a min grade of M20 for low-seismic and M25 for medium and high seismic regions.



Slide 20

Steel Grade (Cl. 5.3) 



Originally, the code required that steel reinforcement of grade Fe415 or less only be used. Higher grade of steel reduces ductility. Hence, there is usually an upper limit on grade of steel required.



Slide 21

Steel Grade (Contd…) 

Recently, the code relaxed this requirement. Cl.5.3 now reads as 5.3 Steel reinforcements of grade Fe415 (see IS 1786:1985) or less only shall be used.

However, high strength deformed steel bars, produced by the thermo-mechanical treatment process, of grades Fe500 and Fe550, having elongation more than 14.5 percent and conforming to other requirements of IS 1786:1985 may also be used for the reinforcement. 

Thus, higher grades of steel are now allowed in the Indian code subject to the above restrictions on ductility of bars.



Slide 22

Steel Grade (Contd…) 

ACI has two additional requirements on steel reinforcement: 

Actual yield strength must not exceed specified yield strength by more than 120 MPa. 





The shear or bond failure may precede the flexural hinge formation. If the difference is very high, the capacity design concept will not work.

Ratio of actual ultimate strength to actual yield strength should be at least 1.25. 

To develop inelastic rotation capacity, need adequate length of yield region along axis of the member. This attempts to ensure that.



Slide 23

Flexural Members



Slide 24

Positive Reinforcement 

At a joint face, positive reinforcement should be at least 50% of the negative reinf. Negative steel (At)

Negative steel (At)

Positive steel (Ab



0.5At)

Positive steel (Ab

0.5At)

Two reasons: 



Need adequate compression reinforcement to ensure ductility. Seismic moments are reversible.  See next slide.



Slide 25

Reinforcement Elsewhere (Cl. 6.2.4) 

Steel at top and bottom face anywhere should be at least 25% of max negative moment steel at face of either joint.

8 Nos 20

12 Nos 20 Min 3 Nos 20

Min 4 Nos 20


Min 6 Nos 20


Slide 26

Reinforcement (Contd…)



Reasons: 



Actual moments away from joint may be higher than the design moment. We do not want to reduce large amount of steel abruptly away from the joint.



Slide 27

External Joint of Beam with Column 

Very important to ensure adequate anchorage of beam bars in the column



Slide 28

External Joint (Contd…) 

Notice the top bar of beam is shown to go into column well below soffit of the beam. 





One would cast the columns up to beam soffit level before fixing the beam reinforcement. Problem arises since Indian code does not require minimum column width. 



This is a problem in the construction.

If column is wide enough, this will not be a problem.

Seismic codes generally require column width to be at least 20 times the largest beam bar dia. 

More on column width later in the section on joints.



Slide 29

Lap Splice (Cl. 6.2.6) 

Lap length 



development length in tension

Due to reversal of seismic loads, the bar could be in compression or tension.

Lap splice not to be provided 

 

Within a joint Within a distance of 2d from joint face Within a quarter length of member where yielding may occur due to seismic forces. 

Lap splices are not reliable under cyclic inelastic deformations and hence not to be provided in the critical regions.



Slide 30

Lap Splice (Contd…)



Wherever longitudinal bar splices are provided: 

Hoops @ not more than 150 mm c/c should be provided over the entire splice length

Ld = development length in tension db = bar diameter



Slide 31

Web Reinforcement 

Most important requirement in seismic regions



Slide 32

Web Reinforcement (Contd…)



Several actions by web reinforcement: 

 

Shear force capacity Confinement of concrete Lateral support to compression reinforcement bars



Slide 33

Web Reinforcement (Contd…) 

Vertical hoops 





Closed stirrups 



Open stirrups cannot confine concrete

135 degree hooks 





Shear direction may reverse during earthquake shaking Hence, inclined bars not effective.

As against normal 90 degree hooks Provides good anchorage to stirrups

10 dia extension ( 75 mm) 



As against 4 dia extension Provides good anchorage.



Slide 34

Web Reinforcement (Contd…)



Two pieces allowed: 





U-stirrup and a cross tie Both with 135 degree hooks at either end.

This is more conservative than the ACI Code 

See next slide for ACI provision.



Slide 35

Hoops as per ACI318



Slide 36

Spacing of Hoops 

Hoop spacing over 2d length at either end of beam not to exceed  

d/4 8 times dia of smallest longitudinal bar Spacing >d/4 >8db

2d


2d

2d


Slide 37

Spacing of Hoops (Contd…)



But, hoop spacing need not be less than 100 mm 





Also, close spacing of hoops over 2d on either side of any other location where flexural yielding is likely Elsewhere, hoop spacing to not exceed d/2 



To ensure space for needle vibrator.

As against 3d/4 permitted by IS:456

First hoop should be placed within 50 mm of the joint face.



Slide 38

Shear Design 

Shear reinforcement to be designed for: 



Factored shear forces as per calculations for applied design loads. Shear forces that will develop when flexural yielding takes place at either end of the beam 

Capacity design concept to ensure shear failure (brittle failure) will not precede the flexural yielding.



Slide 39

Capacity Design for Shear 

Cantilever Beam Example 

 

 

Factored design load 100 kN, Height of 5m Design moment at base =100 x 5 = 500 kNm Design for this moment. Generally, the actual reinforcement may be somewhat higher than calculated. 

100kN (Factored Design Load)

5m

Say the moment capacity of the section is 600 kNm (instead of 500 kNm).



Slide 40

Cantilever example (Contd…) 









Design assumes steel stress as 0.87fy (due to partial safety factor of 1.15) But, steel can take upto say 1.25fy (due to strain hardening). Hence, section can take moment upto about 860 kNm (= 600x1.25/0.87). When moment at base is 860 kNm, the shear force must be 172 kN (= 860/5). Hence, to prevent shear failure prior to flexural yielding, design shear force is 172 kN 

As against 100 kN factored shear force!



Slide 41

Capacity Design (Contd…)



Ratio 1.25 / 0.87 = 1.44 has been rounded off to 1.4 in the code (Cl. 6.33)



Slide 42

Capacity Design for Shear 

Consider beam part of a frame. EQ Force

Sagging

Hogging

EQ Force

Hogging



Sagging

Flexural yielding will be in sagging at one end and hogging at the other end, and vice versa.



Slide 43

Capacity Design for Shear (Contd…) MSA

MHB

L

MSA + MHB

Shear force =

L

MHA

MSB

L Shear force =


MHA + MSB L


Slide 44

Capacity Design for Shear (Contd…)



Slide 45

Example D L a

D L b

V

V

1.2 D 2

L

61.5 kN

' M pa

' M pb

L ' M pa

231

' M pb

105

295

(Va)min = 61.5 -105 = - 45.5 kN (Vb)max = 61.5 + 105 = 166.5 kN Sudhir K. Jain, IITGN


Slide 46

Example (Contd…)

' M mb

M pa M pa

303

M 'pb

209

L

102

(Va)max = 61.5 + 102 = 163.5 kN (Vb)min = 61.5-102 = 40.5 kN Design shear reinforcement for these shear force values as usual. Sudhir K. Jain, IITGN


Slide 47

Detailing Reqmnts for Beams



Slide 48

Columns



Slide 49

Location of Lap Splices  All laps should be only in the central half of the column height.  Seismic moments are maximum in columns just above and just below the beam: hence, reinforcement must not change at those locations.

 Seismic moments minimum in the central half of the column height.

 Hence, reinforcement should be specified from mid-storey-height to next mid-storey-height. Sudhir K. Jain, IITGN


Slide 50

Locations of Laps in Columns

Region for lap splices

Bending Moment Diagram Sudhir K. Jain, IITGN


Slide 51

Lap Splices  Should be proportioned as tension splices.  Columns may develop substantial moments.  The moments are reversible in direction.  Hence, all bars are liable to go under tension.



Slide 52

No of bars to be lapped  Code does not allow more than 50% of the bars to lapped at the same location.

 For buildings of normal proportions, it means:  Half the bars to be spliced in one storey, and the other half in the next storey.

 Construction difficulties.  The clause appears to be very harsh.  It should allow all bars to be lapped at the same location but with a penalty on the lap length.



Slide 53

Detailing at Lap Locations  Hoops to be provided over entire splice length at spacing not exceeding 150 c/c.



Slide 54

Transverse Reinforcement  A hoop must be (Cl. 7.3.1):  Closed stirrup  Have 135 degree hook  Have 10 dia extension (but not less than 75mm) at each end which is embedded in core concrete.

 10 dia extension: difficulties in construction  ACI now allows 6 dia extension (subject to a minimum of 75 mm).



Slide 55

Transverse Reinforcement  If length of any side of hoop exceeds 300mm, cross tie to be provided (Cl. 7.3.2)



Slide 56

Transverse Reinforcement (Contd…)



Slide 57

As per ACI318



Slide 58

Spacing of Hoops (Cl. 7.3.3)  Spacing of hoops anywhere not to exceed half the least lateral dimension of the column.

 Except where confinement reinforcement is needed: closer spacing will be needed there.



Slide 59

Shear Design  Column to be designed for larger of  Calculated factored shear force.  Shear force by capacity design concept assuming plastic hinge forms at the beams on either side.  It is assumed in this clause that the columns will not yield before the beams do (Strong Column – Weak Beam Design)

 However, recall that our code does not have the clause for strong column – weak beam design.



Slide 60

Design Shear Force for Column



Slide 61

Special Confining Reinf.  Must be provided over a length lo from each joint face. Length lo must be larger of:  Larger lateral dimension of the column  1/6 of the clear span of member  450mm



Slide 62

Special Confining Reinf. (Contd…) 

If point of contraflexure not within middle half of the member clear height: 

Special confining reinforcement should be provided over full column height.



Slide 63

Column End at Footing



Slide 64

Spacing of Special Conf. Reinf.  Spacing of hoops for special confinement reinforcement  Not to exceed ¼ of minimum column dimension.  But need not be less than 75mm nor more than 100 mm.

 The above spacing is really for buildings.  For large bridge piers, may allow larger spacing  AASHTO: minimum spacing of 100mm  Japanese code: minimum spacing of 150mm  Indian code needs to incorporate this. Sudhir K. Jain, IITGN


Slide 65

Confinement Reinf. Area  Area of cross section of circular hoops or spirals to be not less than:

Ash


fck 0.09SDk fy

Ag Ak

1.0


Slide 66

Example:  Column dia: 300 mm  M20 concrete, Fe415 reinforcement  Spacing of confinement reinforcement should not exceed 300/4 = 75, or 100mm and cannot be less than75mm.  Hence, spacing of confinement reinf. = 75 mm  Assuming clear cover of 40mm:  Core dia (Dk) is 220mm; Ak=38,000 sq.m  Overall dia = 300mm; Ag=70,700 sq.m Ash = 0.09 x 75 x 220 x (20/415) x [(300/220)2 - 1] = 61.5 sq.mm

Hence, 10 mm dia bars are needed. Sudhir K. Jain, IITGN


Slide 67

Another Example:  Same as earlier: change column dia to 200mm.  Stirrup spacing will still be 75mm.  Core dia is 120mm Ash = 0.09 x 75 x 120 x (20/415) x [(200/120)2 - 1] = 69.4 sq.mm

Need 10 mm stirrups.

 Same as earlier: change column dia to 150mm.  Stirrup spacing will still be 75mm.  Core dia is 70mm Ash = 0.09 x 75 x 70 x (20/415) x [(150/70)2 - 1] = 81.8 sq.mm Need 12 mm dia stirrups!!



Slide 68

Confinement Reinforcement  The last term in bracket tends to increase as the column size reduces.

 For very small sections, you will get larger dia bars.

 Can be a problem in the detailing of boundary elements of shear walls.



Slide 69

More Example  Same as earlier: change column dia to 2000mm. Stirrup spacing will now be 100mm. Core dia is 1920mm Ash = 0.09 x 100 x 1920 x (20/415) x [(2000/1920 )2 - 1] = 70.84 sq.mm Need 10 mm stirrups!! Clearly, too small for 2 m dia column.



Slide 70

Confinement Reinforcement

 For very large diameters, the last term in bracket tends to be very small.

 This leads to under-design of large diameter bridge piers.



Slide 71

Rectangular Hoops

Ash


fck 0.18Sh fy

Ag Ak

1.0


Slide 72

Confinement Hoops Thus, equations of Cl. 7.4.7 and Cl. 7.4.8 break down for very large sections and very small sections. This needs to be fixed in the code. IRC draft under discussion provides additional requirements on this.



Slide 73

Beam Column Joints



Slide 74

Joints in RC Frames 

Moment resisting frame has three components 

 



Beams Columns Rigid joint between beams and columns.

Joint is a very important element. 

Earlier, joint was often ignored in RC constructions, even though in steel constructions adequate attention was always paid to the joint.



Slide 75

Codal Provisions  



Provisions in IS:13920 on joints are very weak. Considerable improvements are needed in the next edition. Partly, this is because IS:456 lacks general framework for joint calculations.



Slide 76

Reinforcement in Joint  

Joint too needs to have stirrups like columns do. In most constructions in our country, joints are not provided with stirrups. 



It is often tedious to provide stirrups in joint due to congestion.

In gravity design, there was a practice that bottom beam bars need not be continuous through the joint. 

It is simply not acceptable when building has to carry lateral loads.



Slide 77

RC Detailing Handbook of BIS

Incorrect Practice



Slide 78

Issues 

Serviceability 

Cracks should not occur due to  



Strength 



Should be more than that in adjacent members

Ductility  



Diagonal compressionm Joint shear

Not needed for gravity loads Needed for seismic loads

Ease of Construction 

Should not be congested.



Slide 79

Cracks in Joint Region



Slide 80

Type of Joints



Slide 81

Geometric Description of Joints



Slide 82

Moment Strength Ratio 



Moment strength ratio to ensure Strong Column – Weak Beam Columns should have higher moment capacity than the beams

M n( cols ) M n( beams ) 

1.0

Normally, the codes require this ratio to be at least 1.2



Slide 83

Moment Strength Ratio (Contd…)  

Our code does not have this requirement. Notice that the original draft contained in Medhekar’s paper had this clause 





This clause requires much larger column sizes than prevalent in India. It was felt that this may not be followed in practice and hence it should be deferred for the time being.

It is perhaps time to think of bringing this clause in the code.



Slide 84

Confinement of Concrete Core  

Core concrete acts as compression strut, and It carries shear force. core

shell



Slide 85

Compression Strut

Compression Strut Moment



Moment

Slide 86

Confinement 

Provided by the beams (and slabs) around the joint, and Col. Plan



By the reinforcement: 



Longitudinal bars (from beams and columns, passing through the joint), and Transverse reinforcement



Slide 87

Confinement (Contd…) 



Better to provide more number of smaller dia longitudinal bars in beams and columns. Requirements on transverse reinforcement reduced if joint is confined by beams on all faces.



Slide 88

IS:13920 



Unless the joint is confined by beams, special confinement reinforcement provided in the columns to also be provided in joint. If beams frame on all four faces of the joint, the joint may be provided half the reinforcement given above. This is provided: 



Beam widths are at least ¾ column width.

Spacing of hoops in the joint region not to exceed 150 mm.



Slide 89

Shear Force in Joint



Slide 90

Shear Force in Joint (Contd…)



Slide 91

Shear Strength 

 

Indian code does not require shear strength of joint to be checked. This should be introduced. ACI and other codes provide a formal method to check shear stress within the joint region.



Slide 92

Anchorage for Longitudinal Bars 

Joints should be capable of providing anchorage to beam and column bars.



Slide 93

External Joints 

l dh

ACI has standard hooks. Hence, the column width is checked to ensure anchorage.

f y db

l

65 f ' c



Slide 94

Bar Stresses

Gravity Loads Sudhir K. Jain, IITGN

Lateral Loads

Under Lateral Loads


Slide 95

Internal Joints



Codes usually requi

• Seismic Codes usually require that

Column Width Beam Bar Diameter Sudhir K. Jain, IITGN

20


Slide 96

Seismic Detailing of RC Structures (IS:13920-1993)

Recommend Documents