1
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 4
Collapse Mechanism
Storey Mechanism
Columns require too much ductility
Columns are difficult to make ductile
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 5
Collapse Mechanism
Beam – Hinge Mechanism (Sway Mechanism)
Preferred mechanism
Ensure that beams yield before columns do
Strong Column –Weak Beam Design
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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%
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 15
Confinement of concrete
Considerably improves its strain capacity
Stress-strain relationship for concrete proposed by Saatcioglu and Razvi, (1992)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 16
Confinement of Column Sections
Fig. from Paulay
and Priestley, 1992
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 24
Flexural Members
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 28
External Joint of Beam with Column
Very important to ensure adequate anchorage of beam bars in the column
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 32
Web Reinforcement
Most important requirement in seismic regions
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 33
Web Reinforcement (Contd…)
Several actions by web reinforcement:
Shear force capacity
Confinement of concrete
Lateral support to compression reinforcement
bars
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 34
Web Reinforcement (Contd…)
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 35
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 36
Hoops as per ACI318
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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!
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 44
Capacity Design for Shear (Contd…)
MSA MHB
L
MSA + MHB
LShear force =
MHA MSB
L
MHA + MSB
LShear force =
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 45
Capacity Design for Shear (Contd…)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 48
Detailing Reqmnts for Beams
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 49
Columns
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 51
Locations of Laps in Columns
Region for
lap splices
Bending Moment Diagram
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 54
Detailing at Lap Locations
Hoops to be provided over entire splice length
at spacing not exceeding 150 c/c.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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).
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 56
Transverse Reinforcement
If length of any side of hoop exceeds 300mm,
cross tie to be provided (Cl. 7.3.2)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 57
Transverse Reinforcement (Contd…)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 58
As per ACI318
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 61
Design Shear Force for Column
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 64
Column End at Footing
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
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!!
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 70
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 72
Rectangular Hoops
0.118.0
k
g
y
ck
shA
A
f
fShA
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 74
Beam Column Joints
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 78
RC Detailing Handbook of BIS
Incorrect Practice
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 80
Cracks in Joint Region
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 81
Type of Joints
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 82
Geometric Description of Joints
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 85
Confinement of Concrete Core
Core concrete acts as compression strut, and
It carries shear force.
shell
core
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 86
Compression Strut
Compression Strut
Moment Moment
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 90
Shear Force in Joint
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 91
Shear Force in Joint (Contd…)
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
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.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 93
Anchorage for Longitudinal Bars
Joints should be capable of providing anchorage to beam and column bars.
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 94
External Joints
ACI has standard hooks. Hence, the column width is checked to ensure anchorage.
l
c
by
dh
f
dfl
'65
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 95
Bar Stresses
Gravity Loads Under LateralLoads
Lateral Loads
Sudhir K. Jain, IITGNSeismic Design of Buildings / November 2012
Slide 96
Internal Joints
Codes usually requi
• Seismic Codes usually require that
20DiameterBar Beam
thColumn Wid