seismic detailing of rc structures (is:13920-1993) frame design of buildings... · seismic...

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

Sudhir K Jain

Indian Institute of Technology Gandhinagar

November 2012

Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012

Slide 2

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.


Slide 3

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 4

Collapse Mechanism

Storey Mechanism

Columns require too much ductility

Columns are difficult to make ductile


Slide 5

Collapse Mechanism

Beam – Hinge Mechanism (Sway Mechanism)

Preferred mechanism

Ensure that beams yield before columns do

Strong Column –Weak Beam Design


Slide 6

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 7

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 8

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 9

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 10

Capacity Design Concept

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.

BrittleLink

DuctileLink


Slide 11

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 12

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 13

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 14

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% 0.35%


Slide 15

Confinement of concrete

Considerably improves its strain capacity

Stress-strain relationship for concrete proposed by Saatcioglu and Razvi, (1992)


Slide 16

Confinement of Column Sections

Fig. from Paulay

and Priestley, 1992


Slide 17

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 18

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.


Slide 19

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.


Slide 20

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 21

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 22

Steel Grade (Contd…)

Recently, the code relaxed this requirement. Cl.5.3 now reads as5.3 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 23

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 24

Flexural Members


Slide 25

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

Two reasons: Need adequate compression reinforcement to ensure

ductility.

Seismic moments are reversible.

See next slide.

Positive Reinforcement

Negative steel (At)

Positive steel (Ab 0.5At)Positive steel (Ab 0.5At)

Negative steel (At)


Slide 26

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

Min 3 Nos 20

Min 4 Nos 20

12 Nos 20

Min 6 Nos 20


Slide 27

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 28

External Joint of Beam with Column

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


Slide 29

External Joint (Contd…)

Notice the top bar of beam is shown to go into column well below soffit of the beam. This is a problem in the construction.

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. 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 30

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 31

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 32

Web Reinforcement

Most important requirement in seismic regions


Slide 33

Web Reinforcement (Contd…)

Several actions by web reinforcement:

Shear force capacity

Confinement of concrete

Lateral support to compression reinforcement

bars


Slide 34


Vertical hoops Shear direction may reverse during earthquake

shaking

Hence, inclined bars not effective.

Closed stirrups Open stirrups cannot confine concrete

135 degree hooks 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 35


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 36

Hoops as per ACI318


Slide 37

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

2d 2d 2d

Spacing >d/4

>8db


Slide 38

Spacing of Hoops (Contd…)

But, hoop spacing need not be less than 100 mm

To ensure space for needle vibrator.

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

As against 3d/4 permitted by IS:456

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


Slide 39

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 40

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. Say the moment capacity of the

section is 600 kNm (instead of 500 kNm).

5m

100kN (Factored Design Load)


Slide 41

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 42

Capacity Design (Contd…)

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


Slide 43

Capacity Design for Shear

Consider beam part of a frame.

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

EQ Force

HoggingSagging

EQ Force

Hogging Sagging


Slide 44

Capacity Design for Shear (Contd…)

MSA MHB

L

MSA + MHB

LShear force =

MHA MSB

L

MHA + MSB

LShear force =


Slide 45

Capacity Design for Shear (Contd…)


Slide 46

Example

kNLD

VV LD

b

LD

a 5.612

2.1

105

''

L

MM pbpa

231'

paM295'

pbM

(Va)min = 61.5 -105 = - 45.5 kN

(Vb)max = 61.5 + 105 = 166.5 kN


Slide 47

Example (Contd…)

102L

MM 'mbpa

303M pa209M '

pb

(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.


Slide 48

Detailing Reqmnts for Beams


Slide 49

Columns


Slide 50

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.


Slide 51

Locations of Laps in Columns

Region for

lap splices

Bending Moment Diagram


Slide 52

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 53

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 54

Detailing at Lap Locations

Hoops to be provided over entire splice length

at spacing not exceeding 150 c/c.


Slide 55

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 56

Transverse Reinforcement

If length of any side of hoop exceeds 300mm,

cross tie to be provided (Cl. 7.3.2)


Slide 57

Transverse Reinforcement (Contd…)


Slide 58

As per ACI318


Slide 59

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 60

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 61

Design Shear Force for Column


Slide 62

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 63

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 64

Column End at Footing


Slide 65

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.


Slide 66

Confinement Reinf. Area

Area of cross section of circular hoops or spirals

to be not less than:

0.109.0

k

g

y

ck

kshA

A

f

fSDA


Slide 67

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.


Slide 68

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.


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 69

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 70

More Example


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 71

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 72

Rectangular Hoops

0.118.0

k

g

y

ck

shA

A

f

fShA


Slide 73

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 74

Beam Column Joints


Slide 75

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 76

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 77

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 78

RC Detailing Handbook of BIS

Incorrect Practice


Slide 79

Issues

Serviceability Cracks should not occur due to

Diagonal compressionm

Joint shear

Strength Should be more than that in adjacent members

Ductility Not needed for gravity loads

Needed for seismic loads

Ease of Construction Should not be congested.


Slide 80

Cracks in Joint Region


Slide 81

Type of Joints


Slide 82

Geometric Description of Joints


Slide 83

Moment Strength Ratio

Moment strength ratio to ensure Strong Column – Weak Beam

Columns should have higher moment capacity than the beams

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

0.1M

M

)beams(n

)cols(n


Slide 84

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 85

Confinement of Concrete Core

Core concrete acts as compression strut, and

It carries shear force.

shell

core


Slide 86

Compression Strut

Compression Strut

Moment Moment


Slide 87

Confinement

Provided by the beams (and slabs) around the joint, and

By the reinforcement:

Longitudinal bars (from beams and columns,

passing through the joint), and

Transverse reinforcement

Col.

Plan


Slide 88

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 89

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 90

Shear Force in Joint


Slide 91

Shear Force in Joint (Contd…)


Slide 92

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 93

Anchorage for Longitudinal Bars

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


Slide 94

External Joints

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

l

c

by

dh

f

dfl

'65


Slide 95

Bar Stresses

Gravity Loads Under LateralLoads

Lateral Loads


Slide 96

Internal Joints

Codes usually requi

• Seismic Codes usually require that

20DiameterBar Beam

thColumn Wid

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