Chapter 12 of 13

Earthquake resistant design of structures

In the UPSC ESE Civil syllabus under RCC Design · 3 parts

Earthquake-Resistant Design Philosophy & Building Configuration · Seismic Zoning & Design Seismic Loads (IS 1893) · Ductility & Ductile Detailing (IS 13920)

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (31 sections)

Part 1 of 3

Earthquake-Resistant Design Philosophy & Building Configuration

Last reviewed 16 Sept 2026 · 7 min read

Objectives of earthquake-resistant design

Designing a structure to remain elastic during the strongest possible earthquake is uneconomical. Earthquake-resistant design therefore aims at:

No damage under frequent minor shaking.
Repairable damage (mainly non-structural) under moderate shaking — the design basis earthquake (DBE).
No collapse (life safety) under rare major shaking — the maximum considered earthquake (MCE).

The structure is allowed to yield and dissipate energy in controlled locations during strong shaking — "earthquake-resistant", not "earthquake-proof".

Performance levels (performance-based design)

Level	Description
Operational / Immediate occupancy	Negligible damage; building usable immediately
Life safety	Significant damage but adequate margin against collapse; occupants safe
Collapse prevention	Severe damage; structure just stands

Performance-based seismic design targets specified performance levels for given hazard levels, verified by nonlinear analyses (e.g. pushover analysis, nonlinear time history).

Key structural properties

Property	Role
Strength	Resist lateral forces without failure
Stiffness	Limit deformations/drift to control damage to non-structural elements and P-Δ effects
Ductility	Ability to deform inelastically without significant loss of strength — dissipates energy; allows design for reduced forces
Redundancy	Multiple load paths — failure of one element does not cause collapse
Overstrength	Actual strength greater than design strength
Damping	Energy dissipation

FormulaDuctility factor

μ = \frac{Δ _{u}}{Δ _{y}}

$Δ_{u}$ = ultimate displacement; $Δ_{y}$ = yield displacement. The response reduction factor R in codes reflects ductility, overstrength and redundancy.

Part 2 of 3

Seismic Zoning & Design Seismic Loads (IS 1893)

Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 10 min read

Seismic zoning of India

India is divided into seismic zones based on expected intensity of shaking. Under IS 1893 (Part 1): 2016, there are four zones:

Zone	Seismic risk	Zone factor Z	Approx. MSK intensity basis
II	Low	0.10	VI (or less)
III	Moderate	0.16	VII
IV	Severe	0.24	VIII
V	Very severe	0.36	IX and above

Zone V includes the north-east, parts of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, the Rann of Kachchh, north Bihar and the Andaman and Nicobar Islands (2016 map).
The zone factor Z corresponds to the maximum considered earthquake (MCE) in the zone; Z/2 represents the design basis earthquake (DBE).

NoteCode revision

The Bureau of Indian Standards has been revising IS 1893 (Part 1), including the seismic zonation map (a revised map with a new highest-hazard zone has been reported). Most examinations continue to use the 2016 provisions summarised here; check the current edition and zone map before use in design.

Design philosophy (IS 1893)

Minor earthquakes (more frequent, less than DBE) — structures resist without damage.
Moderate earthquakes (DBE) — no significant structural damage, though some non-structural damage may occur.
Major earthquakes (MCE) — no collapse (life safety).

Structures are therefore designed for forces much lower than elastic forces of the MCE, relying on ductility, redundancy and overstrength — accounted for by the response reduction factor R.

Design parameters

Soil types (for spectra)

Type	Soil	Typical SPT N
I	Rock or hard soil — well-graded gravels, dense sands	N > 30
II	Medium or stiff soil	N 10–30
III	Soft soil	N < 10

Importance factor I

Structure	I
Important and community buildings — hospitals, schools, emergency buildings (fire stations, police), telephone exchanges, power stations, large assembly buildings	1.5
Residential/commercial buildings with occupancy more than 200 persons	1.2
All other buildings	1.0

Response reduction factor R (examples)

Lateral load resisting system	R
RC ordinary moment resisting frame (OMRF)	3.0
RC special moment resisting frame (SMRF)	5.0
Steel OMRF / SMRF	3.0 / 5.0
Ordinary RC structural walls	3.0
Ductile RC structural walls	4.0
Dual system — ductile structural walls with SMRF	5.0

(Unreinforced masonry and other systems have lower values; OMRFs are not permitted in higher zones for buildings as per code restrictions.)

Design horizontal seismic coefficient

FormulaDesign acceleration coefficient

A_{h} = \frac{Z}{2} \cdot \frac{I}{R} \cdot \frac{S _{a}}{g}

For structures with $T \leq 0.1$ s, $A_{h}$ is not taken less than $Z /2$ , whatever the value of $R$ (IS 1893: 2016).
$S_{a} / g$ = design spectral acceleration coefficient for 5% damping, multiplied by a damping factor for other damping ratios.

Design acceleration spectrum (5% damping)

For the equivalent static method (IS 1893: 2016):

Soil type	$S_{a} / g$
I (rock/hard)	$2.5$ for $T < 0.40$ s; $\frac{1.00}{T}$ for $0.40 \leq T < 4.00$ s; $0.25$ for $T \geq 4.00$ s
II (medium)	$2.5$ for $T < 0.55$ s; $\frac{1.36}{T}$ for $0.55 \leq T < 4.00$ s; $0.34$ for $T \geq 4.00$ s
III (soft)	$2.5$ for $T < 0.67$ s; $\frac{1.67}{T}$ for $0.67 \leq T < 4.00$ s; $0.42$ for $T \geq 4.00$ s

For the response spectrum method, the initial branch rises linearly as $1 + 15 T$ for $T < 0.10$ s, then follows the same values.

Softer soils give larger spectral values at longer periods.
Damping multiplying factors (to the 5% spectrum), e.g. 2% → 1.4; 5% → 1.0; 7% → 0.9; 10% → 0.8.
Commonly adopted damping: 5% for RC and masonry, 2% for steel structures.

Fundamental natural period

FormulaApproximate fundamental period (IS 1893: 2016)

Bare RC moment resisting frame buildings (without infill): $T_{a} = 0.075 h^{0.75}$
Bare RC–steel composite MRF buildings: $T_{a} = 0.080 h^{0.75}$
Bare steel MRF buildings: $T_{a} = 0.085 h^{0.75}$
All other buildings (including MRF with masonry infill, structural walls): $T_{a} = \frac{0.09 h}{d}$

$h$ = height of building (m); $d$ = base dimension (m) along the direction of shaking.

Seismic weight

Seismic weight of each floor = full dead load + appropriate percentage of imposed load:
- 25% of imposed load for loads up to 3 kN/m²;
- 50% for loads above 3 kN/m².
Imposed load on roofs need not be considered.
Weights of walls and columns are divided between the floors above and below.
Total seismic weight $W$ = sum of floor seismic weights.

Equivalent static (seismic coefficient) method

FormulaBase shear and distribution

Design base shear

V_{B} = A_{h} W

Minimum design base shear (IS 1893: 2016): $V_{B} \geq ρ W$ , with $ρ$ = 0.7% (Zone II), 1.1% (III), 1.6% (IV), 2.4% (V).

Vertical distribution (lateral force at floor $i$ ):

Q_{i} = V_{B} \frac{W _{i} h _{i}^{2}}{\sum _{j = 1}^{n} W _{j} h _{j}^{2}}

$W_{i}$ = seismic weight of floor $i$ ; $h_{i}$ = height of floor $i$ from the base.

The parabolic distribution approximates the first mode with some higher-mode effect.
Storey shear at any level = sum of lateral forces above that level.

Part 3 of 3

Ductility & Ductile Detailing (IS 13920)

Last reviewed 16 Sept 2026 · 9 min read

Ductility in reinforced concrete

Ductility is the ability to undergo large inelastic deformations without substantial loss of strength.

FormulaDuctility measures

Curvature ductility $μ_{ϕ} = \frac{ϕ _{u}}{ϕ _{y}}$

Displacement ductility $μ_{Δ} = \frac{Δ _{u}}{Δ _{y}}$ $ϕ_{y} = \frac{ε _{y}}{d - x _{y}}$ ; $ϕ_{u} = \frac{ε _{c u}}{x _{u}}$ — ductility increases when the neutral axis depth at failure is small and the ultimate concrete strain is large.

Factors increasing ductility of RC sections

Confinement by closely spaced transverse reinforcement (hoops, spirals) — increases ultimate compressive strain and strength of concrete.
Lower tension steel ratio (under-reinforced sections) and compression reinforcement.
Lower axial compression (high axial load reduces ductility of columns).
Higher concrete strength (reduces neutral axis depth) and steel with good elongation and a margin between yield and ultimate strength.
Preventing premature shear failure, bond/anchorage failure and buckling of bars.

IS 13920: 2016 — scope

IS 13920 (Ductile design and detailing of reinforced concrete structures subjected to seismic forces — code of practice) applies to RC structures designed as special moment resisting frames and special structural walls; it is mandatory for such systems in higher seismic zones as required by IS 1893.

Materials (outline)

Minimum concrete grade M20 (higher grades for taller buildings as specified).
Reinforcement of high-ductility grades with minimum specified elongation; grades with high yield strength beyond limits are restricted.
Actual yield strength should not greatly exceed the specified value (to preserve the capacity design hierarchy).

Beams (flexural members)

Geometry

Width-to-depth ratio $b / D \geq 0.3$ .
Width $b \geq 200$ mm.
Depth $D \leq$ 1/4 of clear span.
Width not to exceed column width plus limited extensions on each side.

Longitudinal reinforcement

At least two bars continuous at top and bottom.
Minimum tension steel ratio $ρ_{min} = 0.24 f_{c k} / f_{y}$ on any face; maximum steel ratio 2.5%.
Positive steel at the joint face ≥ half the negative steel there.
Steel at any section (top or bottom) ≥ one-quarter of the maximum negative steel at either joint face.
Bars anchored into columns with development length and 90° bends (in exterior joints) beyond the inner face of the column.

Lap splices

Lap splices not within joints, not within a distance of 2d from the joint face, and not in regions of potential plastic hinges.
Not more than 50% of bars spliced at one section.
Hoops over the entire lap length at close spacing (the code limits spacing to 150 mm).

Transverse reinforcement

Closed hoops with 135° hooks and extension of 6 times the bar diameter but not less than 65 mm, embedded in the confined core.
Minimum hoop bar diameter 8 mm (larger for longer spans as per code).
Over a length of 2d from each joint face (and on either side of any section where flexural yielding may occur): hoop spacing ≤ d/4, 6 times the smallest longitudinal bar diameter and 100 mm (IS 13920: 2016; the 2002 edition used d/4 and 8 times the bar diameter).
First hoop within 50 mm of the joint face.
Elsewhere, spacing ≤ d/2.

(Limits are summarised; refer to the code clause for exact wording for each case.)

Capacity-based shear design of beams

FormulaDesign shear in beams

V_{u, a} = V_{a}^{D + L} - 1.4 \frac{M _{u, l im}^{A s} + M _{u, l im}^{B h}}{L _{A B}}, V_{u, b} = V_{b}^{D + L} + 1.4 \frac{M _{u, l im}^{A s} + M _{u, l im}^{B h}}{L _{A B}}

(and the corresponding case for sway to the other side)

$M_{u, l im}^{s}$ , $M_{u, l im}^{h}$ = sagging and hogging moment capacities at the beam ends (with actual reinforcement); $L_{A B}$ = clear span; $V^{D + L}$ = shear due to factored gravity loads (1.2(DL + IL)). Design shear is the larger of this capacity-based shear and the shear from analysis. The contribution of concrete is often ignored in plastic hinge regions under high seismic shear (as per code).

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