OER for ICT workshop of IIT Bombay on "Seismic Design Codes in India"

1. Seismic Design Codes in India
RC1225_008
VIJAYA G. S [TEAM LEAD]
S. BHAVANISHANKAR,
H. C. MUDDARAJU,
CHETHAN. K

2. Seismic Design Codes in India
• Seismic codes help to improve the behaviour of
structures so that they may withstand the
earthquake effects without significant loss of life
and property.
• Countries around the world have procedures
outlined in seismic codes to help design
engineers in the planning, designing, detailing
and constructing of structures.

3. An Earthquake-Resistant Building has four
virtues in it, namely:
• Good Structural Configuration: Its size, shape and structural system
carrying loads are such that they ensure a direct and smooth flow of
inertia forces to the ground.
• Lateral Strength: The maximum lateral (horizontal) force that it can
resist is such that the damage induced in it does not result in collapse.
• Adequate Stiffness: Its lateral load resisting system is such that the
earthquake-induced deformations in it do not damage its contents
under low-to-moderate shaking.
• Good Ductility: Its capacity to undergo large deformations under
severe earthquake shaking even after yielding, is improved by
favourable design and detailing strategies.
Seismic codes cover all these aspects.

4. Indian Standards on Earthquake Engineering
1. IS 1893 (Part IV): 2005 - Criteria for Earthquake
Resistant Design of Structures (Industrial Structures
including stack-like structures)
2. IS 1893 (Part I): 2002 - Criteria for Earthquake
Resistant Design of Structures (General Provisions and
Buildings)
3. IS 4326: 1993 -Code of Practice for Earthquake
Resistant Design & Construction of Buildings.
4. IS 13827: 1993 - Guidelines for improving Earthquake
Resistance of Earthen Buildings
5. IS 13828: 1993 - Guidelines for Improving Earthquake
Resistance of Low Strength Masonry Buildings
6. IS 13920:1993 - Code of Practice for Ductile Detailing
of Reinforced Concrete Structures Subjected to Seismic
Forces.
7. IS 13935: 1993 - Guidelines for Repair and Seismic
Strengthening of Buildings.

5. Indian Standard
IS: 1893 (Part1) -2002
Criteria for
Earthquake Resistant Design

6. ZONE
II 5 – 6 Low
III 6 – 6.5 Moderate
IV 6.5 – 7 Severe
V ≥ 7 Very severe

7. BRIEF OVERVIEW ON PROVISIONS OF
IS 1893: 2002
 Earthquake motion causes vibration of the structure
leading to inertia forces.
 Structure must be able to safely transmit the horizontal
and the vertical inertia forces generated in the super
structure through the foundation to the ground.
 Earthquake-resistant design requires ensuring that the
structure has adequate lateral load carrying capacity.
 Seismic codes will guide a designer to safely design the
structure for its intended purpose.

8. Contd
Seismic codes are unique to a particular region or
country.
In India, IS1893 is the main code that provides outline
for calculating seismic design force.
This force depends on the mass and seismic coefficient of
the structure
The seismic coefficient of the structure depends on
properties like seismic zone in which structure lies,
importance of the structure, its stiffness, the soil on which
it rests, and its ductility.

9. Criteria for choice of seismic analysis

10. Basic Approach
Seismic Design Philosophy
1. To ensure that structures posses at least a minimum
strength to withstand minor earthquakes (<DBE)without
damage.
2. Resist moderate earthquakes (DBE) without
significant structural damage though some non-
structural damage may occur and that
3. Structures withstand a major earthquake (MCE)
without collapse.
Note:- Actual forces that appear on structures during
earthquakes are much greater than the design forces
specified. Hence, ductility arising from inelastic material
behavior and detailing and over-strength are relied upon
to account for this difference in actual and design lateral
loads.

11. Load Combination
6.3.1 Load Combinations
When earthquake forces are considered on a structure,
these shall be combined where the terms DL, IL and EL
stand for the response quantities due to dead load,
imposed load and designated earthquake load
respectively.
Partial safety factors for limit state design of reinforced
concrete, the following load combinations shall be
accounted for:
1.5 (DL + IL)
1.2{DL + IL±EL)
1.5(DL±EL)
0.9DL±l.5EL.

12. Static analysis
The design horizontal seismic coefficient Ah for a
structure is determined by the following
expression:
Ah = Z I Sa ------------(1)
2 R g
where,
Z= Zone factor
I = Importance factor
R= Response reduction factor
Sa /g= Average response acceleration coefficient

13. ZONE FACTOR Z
Zone factors are given in Table, refers
to the zero period acceleration value for
the maximum credible earthquake
(MCE) in a zone. It is divided by a
factor of 2 in order to reduce the
maximum credible earthquake (MCE)
zone factor to the factor for design
basis earthquake (DBE).

14. Importance Factor I

15. Response Reduction Factor

16. Fundamental Natural Period Ta(sec)
For moment-resisting frame building without brick infill
panels may be estimated by the empirical expression :
For all other buildings, including moment resisting frame buildings
with brick infill panels, may be estimated by the empirical expression:
h = Height of building, in meters. And
d = Base dimension of the building at the plinth
level, in meters along considered direction of lateral
force

17. Average Response Acceleration
Coefficient (Sa
/g)
RES PONS E S PEC TRA FOR ROC K AND S OIL S ITES FOR 5 PERC ENT DAMPING
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
PERIOD (s )
T Y P E I(R O C K O R HA R D
T Y P E II(ME D IUM
T Y P E III
SpectralAccelerationSa/g
Rock
Medium soil
Soft Soil

18. Sa /g= Average response acceleration coefficient, In case
design spectrum is specifically prepared for a structure at a
particular project site, the same may be used for design at
the discretion of the project authorities.
For Rocky or Hard soil sites
1+15T 0.00≤T≤0.10
Sa/g 2.50 0.10≤T≤0.40
1.00/T 0.40≤T≤4.00
For Medium soil sites,
1+15T 0.00≤T≤0.10
Sa/g 2.50 0.10≤T≤0.55
1.36/T 0.55≤T≤4.00
For Soft soil sites
1+15T, 0.00≤T≤0.10
Sa/g 2.50 0.10≤T≤0.67
1.67/T 0.67≤T≤4.00

19. “The Design acceleration spectrum for vertical motions,
when required, may be taken as two-thirds of the design
horizontal acceleration spectrum.”
The design acceleration Spectrum has been prepared as
per IS 1893 –2002
Zone V
Assuming damping as 5%
The type of soil as Soft soil
Zone factor: For Zone V = 0.36
Importance factor I = 1.00
Response reduction factor R = 1.00

21. The total design seismic base shear (VB)along
any principal direction shall be determined by
following expression.
V B=Ah * W -------(3)
where,
W is the total weight of the building calculated
using the structural details

22. DISTRIBUTION OF DESIGN FORCE

23. Dynamic analysis
Dynamic analysis shall be performed to obtain the design
seismic forces, and its distribution to different levels along
the height of the building and to the various lateral load
resisting elements under any of the following
conditions:
For regular buildings, if the height is greater than 40 m
in Zones IV and V or greater than 90 m in Zone II and III
For irregular buildings, if height is more than 12 m in
Zones IV and V and more than 40 m in Zones II and III.

24. IS:13920 – 1993
Ductile Detailing of Reinforced
Concrete Structures subjected
to Seismic Forces –
Code of Practice

25. Gravity loading due to self weight and contents on buildings
causes RC frames to bend resulting in stretching and
shortening at various locations. Tension is generated at
surfaces that stretch and compression at those that shorten
(Figure b).
Under gravity loads, tension in the beams is at the bottom
surface of the beam in the central location and is at the top
surface at the ends.
Earthquake loading causes tension on beam and column
faces at locations different from those under gravity loading
(Figure c); The level of bending moment, (figure d) due to
earthquake loading depends on severity of shaking and can
exceed that due to gravity loading. Thus, under strong
earthquake shaking, the beam ends can develop tension on
either of the top and bottom faces. Since concrete cannot carry
this tension, steel bars are required on both faces of beams to
resist reversals of bending moment. Similarly, steel bars are
required on all faces of columns too.

26. Earthquake
shaking reverses
tension and
compression in
members-
[reinforcement is
required on both
faces of members]

27. Location and amount of longitudinal steel bars in
beams - [these resist tension due to flexure]
BEAMS

28. Location and amount of vertical stirrups in beams
– IS:13920-1993 limit on maximum spacing ensures
good earthquake behaviour.

29. Details of lapping steel reinforcement in seismic
beams-[IS 13920-1993]

30. Steel reinforcement in seismic beams –[stirrups with
135o
hooks at ends required as per IS:13920-1993]

31. COLUMNS
Steel
reinforcement in
columns –
closed ties at close
spacing improve
the performance of
columns under
strong earthquake
shaking

32. Steel
reinforcement in
seismic columns-
closed ties with
135o
hooks are
required as per IS
13920-1993

33. Placing vertical
bars and closed
ties in columns-
[column ends and
lap lengths are to
be protected with
closely spaced ties]

34. Closed loop steel ties in beam-column joints –
such ties with 135o
hooks resist the ill effects of
distortion of joints

35. Anchorage of Beam Bars in an External Joint

36. Conclusions
• Countries like Japan, New Zealand and USA with a
history of earthquakes have well developed earthquake
codes.
• Development of building codes in India started rather
early, the first formal seismic code, namely IS 1893, was
published in 1962.
• Today, we have a fairly good range of seismic codes
covering a variety of structures, ranging from mud or
low strength masonry houses to modern buildings.
• However, the key to ensuring earthquake safety lies in
having laws that enforces and implements these design
code provisions in actual constructions.