LESSON 2.8 — Disaster-Resistant Construction
A. Standard Map
| Topic | Governing Source | Exam Focus |
|---|---|---|
| Hazard, vulnerability, risk, exposure — definitions | NDMA / standard disaster management | Precise definitional distinctions; not interchangeable |
| India seismic zonation | IS:1893 Part 1:2016 | Zones II–V; Zone I merged into II; Delhi = IV; NE India = V |
| Seismic design principles | IS:1893:2016; IS:13920:2016 | Three-stage approach; ductility; strong column–weak beam |
| IS:13920 — ductile detailing | IS:13920:2016 | Zones III, IV, V; column min. 230 mm; confinement |
| Soft storey — most tested vulnerability | IS:1893 + exam history | Definition; why it fails; remedy |
| Cyclone-resistant construction | NBC 2016; NDMA; GATE-1992 | Roof pitch ≥ 22°; overhang ≤ 457 mm; hip roof best |
| Flood-resistant construction | NBC 2016; NDMA | Wet vs dry flood-proofing; plinth elevation |
| Fire-resistant construction | NBC 2016 Part 4; IS:3809 | Compartmentation; rated assemblies; awareness level only |
Scope note: This lesson provides code-anchored principles. Deep structural design theory (column reinforcement, slab design) belongs to Chapter 8-B1 (Building Structures). Lesson 2.8 covers definitions, zonation, selection principles, and exam-level code facts.
B. Mechanism in Words
- Natural and human-made events (earthquakes, cyclones, floods, fires) present hazards — potential threats.
- A building’s vulnerability determines how badly it responds to a given hazard — the same hazard causes more damage in a poorly constructed building.
- Risk is the intersection of hazard and vulnerability — a highly hazardous area with resilient buildings carries less actual risk than a moderately hazardous area with fragile construction.
- Disaster-resistant design reduces vulnerability at three stages: planning (building geometry and siting), structural design (member sizing and connection detailing), and construction (quality of execution).
- Codes (IS:1893, IS:13920, NBC) prescribe minimum requirements at each stage — compliance converts the general principle of resilience into specific, measurable design decisions.
C. Core Concept Explanations
C1. Precise Definitions — Hazard, Vulnerability, Risk, Exposure
| Term | Definition | What it is NOT |
|---|---|---|
| Hazard | A naturally or human-induced phenomenon with the potential to cause harm — earthquake, flood, cyclone, fire; threat exists regardless of whether damage occurs | Not the same as disaster or risk |
| Vulnerability | The degree to which a system (building, community, infrastructure) is susceptible to damage from a given hazard; depends on construction quality, design, materials, and preparedness | Not the same as exposure or hazard |
| Risk | The probability of harmful consequences resulting from the interaction of hazard and vulnerability; Risk = Hazard × Vulnerability | Risk can be zero if either hazard or vulnerability is zero |
| Exposure | The presence of people, assets, or systems in hazard-prone locations; a building in a flood zone is exposed regardless of whether it is vulnerable | Not the same as risk — you can be exposed but not vulnerable |
| Disaster | An actual event that causes damage and disruption exceeding local coping capacity; requires external assistance for recovery | A hazard that does not cause disaster-level damage is not a disaster |
Exam Anchor:
– Hazard ≠ Disaster: Hazard = potential; Disaster = actual event with overwhelming damage.
– Vulnerability ≠ Hazard: Vulnerability = how susceptible; Hazard = what threatens.
– An area can have HIGH HAZARD and LOW RISK if buildings are well designed (low vulnerability).
C2. Additional Terminology
| Term | Definition | Exam Relevance |
|---|---|---|
| Liquefaction | Saturated cohesionless (sandy, gravelly) soil loses shear strength under earthquake vibration; soil behaves like a liquid; buildings settle, tilt, or sink | Common seismic failure mode near rivers, reclaimed land, coastal areas |
| Vibrofloatation (Vibroflotation) | Ground improvement technique; a vibrating probe densifies loose granular soil in place; reduces liquefaction potential | Mitigation for liquefaction-prone sites |
| Soft Storey | A storey with significantly less lateral stiffness or strength than the storeys above it; typically the open ground floor of a residential building used for parking | Most frequently tested seismic vulnerability in GATE/UPSC |
| Ductility | Ability of a structural member or system to undergo large inelastic deformations without sudden collapse; enables energy dissipation through yielding | IS:13920 prescribes ductile detailing requirements |
| RVS (Rapid Visual Screening) | A quick field method to identify buildings likely to be seismically hazardous; based on visual inspection without structural analysis; produces a score | Used by authorities after earthquakes to prioritise inspection |
| Infill wall | Non-structural masonry wall built between columns; can modify a building’s seismic response (adding stiffness and mass); often ignored in analysis — a major design error | Source of irregular stiffness distribution; contributor to torsional failures |
Soft storey — mechanism: When columns of an open ground floor (stilt parking, commercial space) are more flexible than the stiff upper floors (with brick infill walls), earthquake inertia forces concentrate in the soft ground floor columns. These columns undergo excessive deformation (storey drift) and fail, causing the entire structure above to collapse pancake-style. The Bhuj earthquake (2001) demonstrated this failure mode extensively in Gujarat.
C3. India’s Seismic Zonation — IS:1893 Part 1:2016
India is divided into four seismic zones (II, III, IV, V). Zone I was merged into Zone II in the 2002 revision of IS:1893 and does not exist as a separate zone in the current standard.
| Zone | Severity | Zone Factor (Z) | Representative Regions |
|---|---|---|---|
| Zone II | Low | 0.10 | Most of peninsular India; Deccan plateau; parts of Kerala and Tamil Nadu coast |
| Zone III | Moderate | 0.16 | Parts of UP, MP, Maharashtra; coastal areas; parts of Rajasthan |
| Zone IV | High | 0.24 | Delhi; parts of Himachal Pradesh; Jammu; Bihar; parts of Gujarat |
| Zone V | Very High | 0.36 | Northeast India; Kutch (Gujarat); Kashmir Valley; parts of HP and Uttarakhand; Andaman and Nicobar Islands |
Source: IS:1893 Part 1:2016, Seismic Zoning Map of India.
Exam Anchor:
– Delhi = Zone IV (High hazard)
– Northeast India + Kutch = Zone V (Very High hazard)
– Zone I is deprecated — merged into Zone II. No current Zone I exists.
– 59% of India’s land area is vulnerable to moderate or severe seismic hazard (MSK intensity VII or higher).
Zone factor Z is used in the earthquake force calculation:
– Seismic base shear V_B = A_h × W
– Design horizontal seismic coefficient A_h = (Z/2) × (I/R) × (Sa/g)
– Z = Zone factor; I = Importance factor; R = Response reduction factor; Sa/g = Spectral acceleration
Exam awareness level: Know the formula structure; GATE seldom asks for full numerical seismic force calculation at Part A level.
C4. Three Stages of Earthquake-Resistant Design
Earthquake-resistant design operates at three sequential stages:
| Stage | Actions | Key Decisions |
|---|---|---|
| 1. Planning | Building geometry, configuration, siting | Regular plan shape; symmetric layout; avoid soft storey; avoid floating columns; avoid re-entrant corners |
| 2. Structural Design | Structural sizing, connection detailing | IS:13920 ductile detailing for zones III–V; strong column–weak beam principle; sufficient lateral stiffness |
| 3. Construction | Quality of execution | Correct reinforcement placement and cover; proper concrete compaction and curing; no unauthorised modifications to structure |
Strong column–weak beam principle: Under earthquake loading, plastic hinges (zones of yielding) should form in the beams, not in the columns. A beam failure is local and repairable; a column failure can cause progressive total collapse. IS:13920 enforces this through design requirements for moment capacities.
Plan regularity — IS:1893 irregularity triggers:
| Irregularity type | Threshold |
|---|---|
| Re-entrant corner (projections) | Projections > 15% of plan dimension in either direction |
| Mass irregularity | Seismic weight of any floor > 150% of adjacent floor weight |
| Vertical geometric irregularity | Setback in plan dimension > 25% between adjacent storeys |
| Stiffness irregularity (soft storey) | Storey lateral stiffness < 70% of the storey above |
C5. IS:13920:2016 — Ductile Detailing Requirements
Applicability: IS:13920 applies to all RCC buildings in seismic zones III, IV, and V. Buildings in zones I and II (low hazard) need not comply, though good practice is to apply ductile detailing universally.
Key prescriptions:
| Element | IS:13920 Requirement |
|---|---|
| Minimum column dimension | 230 mm in any plan direction |
| Minimum beam width | 200 mm |
| Beam depth-to-width ratio | ≤ 4 |
| Longitudinal reinforcement in beams | Min 0.24 √(f_ck)/f_y at any section; top and bottom |
| Confinement reinforcement (stirrups) | Closely spaced stirrups in plastic hinge regions (end zones of beams and columns) |
| Stirrup spacing in confinement zone | Smallest of d/4 or 8 times the diameter of smallest longitudinal bar, or 100 mm |
| Beam-column joint | Joint must be confined; avoid eccentric loading on joints |
Source: IS:13920:2016 — Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
Exam Anchor: IS:13920 = zones III, IV, V. Minimum column dimension = 230 mm. These are the two most tested IS:13920 facts.
C6. Cyclone-Resistant Construction
Cyclone loads on buildings are primarily wind pressure, suction, and debris impact. The structural and architectural decisions that matter most are:
Roof design — the most critical element in cyclone resistance:
| Parameter | Requirement | Source |
|---|---|---|
| Minimum roof pitch | ≥ 22° | NBC 2016; NDMA cyclone guidelines |
| Maximum overhang at verges and eaves | ≤ 457 mm (18 inches = approximately 460 mm) | NBC 2016; NDMA |
| Best roof form for cyclone resistance | Hip roof (all four sides slope inward) | GATE-1992 |
| Worst roof form | Gable (triangular end walls create high wind pressure zones) | GATE-1992 |
Mechanism — why hip roof is better than gable:
A hip roof has no exposed vertical triangular gable end. Wind pressure on a vertical gable wall creates uplift forces at the roof-wall connection. A hip roof has sloping surfaces on all sides — wind deflects over the slope rather than striking a flat face, reducing net uplift.
Exam Anchor (GATE-1992): Hip roof > Gable roof for cyclone resistance. This is one of the most cited historical GATE questions on cyclone design.
Additional cyclone-resistant measures:
| Measure | Purpose |
|---|---|
| Square or circular plan | Minimises exposed face to wind; reduces pressure differential |
| Straps and holddowns at roof-wall connections | Prevent roof from lifting off the walls (common failure in cyclones) |
| Anchor bolts for roof trusses to ring beam | Ties roof structure to masonry wall structure |
| Reinforced concrete ring beam at roof level | Distributes lateral forces; ties walls together |
| Protected openings (shutters, storm shutters) | Prevents wind entering the building and creating internal pressure |
| Reduced window-to-wall area | Minimises glazing that can shatter under wind/debris impact |
Bearing capacity in flood-prone soil: Soil bearing capacity in a flood-prone or saturated ground condition is approximately HALF of its dry ground value. Foundation design in cyclone-prone coastal areas must account for this.
C7. Flood-Resistant Construction
Two primary strategies:
| Strategy | Approach | Application |
|---|---|---|
| Dry flood-proofing | Seal the building to prevent water from entering; waterproof walls, floors, and openings up to the design flood level | Buildings that must remain operational during flooding; historic structures that cannot be elevated |
| Wet flood-proofing | Allow water to enter the building deliberately; reduce buoyancy and differential pressure on the structure; use flood-resistant materials inside | Basements; lower floors of buildings where sealing is impractical or too expensive |
Key flood-resistant design principles:
| Measure | Purpose |
|---|---|
| Plinth elevation above design flood level | Primary defence — keep the occupied floor above the highest expected flood mark |
| Reinforced concrete or masonry plinth | Prevent plinth from washing away or settling |
| Waterproof cement plaster (1:2 ratio) on substructure | Reduces capillary absorption and moisture penetration |
| Pressure relief valves in basement walls | Prevents differential hydrostatic pressure from rupturing walls if building is dry-proofed |
| Flood-resistant material selection | Avoid materials degraded by water: standard gypsum board, particleboard, standard brick at plinth level; use dense concrete blocks, natural stone, or vitrified tile below flood mark |
| Venting below elevated buildings | Allows water to flow freely under raised structures on stilts; reduces hydrostatic pressure on foundation |
Wet vs dry flood-proofing: The key difference is intent. Wet proofing accepts water entry and designs for it. Dry proofing excludes water. Both are legitimate strategies depending on building use and feasibility.
C8. Fire-Resistant Construction (NBC 2016 Part 4 — Awareness Level)
Scope: Fire resistance design at the level of compartmentation principles and rated assembly identification. Detailed fire service design belongs to Chapter 6 (Services).
Compartmentation: The division of a building into fire-resistant zones that contain a fire within one area for a defined time period, preventing it from spreading to adjacent areas. Achieved through:
– Fire-rated walls and floors (fire compartment boundaries)
– Fire doors (self-closing; rated at 0.5, 1, or 2 hours)
– Fire dampers in HVAC ducts crossing compartment boundaries
– Intumescent seals at service penetrations
Fire resistance rating (FRR): The period (in hours) during which a structural element or assembly maintains its load-bearing capacity (stability), integrity (no passage of flames/hot gases), and insulation (limiting temperature rise on unexposed face) under standard fire conditions.
| Element | Typical FRR |
|---|---|
| Load-bearing external wall | 2 hours |
| Internal fire compartment wall | 1–2 hours |
| Fire door | 0.5–2 hours (self-closing) |
| Structural frame (beams/columns) | 1–4 hours depending on building height |
| Basement structure | 4 hours |
Source: NBC 2016 Part 4 — Fire and Life Safety; IS:3809 (Fire Resistance Tests for Structures).
Passive vs active fire protection:
| Type | Definition | Examples |
|---|---|---|
| Passive | Structural features that resist fire spread without requiring activation | Fire compartment walls, fire doors, intumescent seals, rated concrete cover |
| Active | Systems that require activation (automatic or manual) to combat fire | Sprinklers, fire detection systems, fire extinguishers, wet/dry risers |
C9. The Bhuj Earthquake 2001 — Key Lessons
Date: 26 January 2001 (Republic Day); Time: 08:46 IST.
Magnitude: 6.9 Richter / 7.7 Mw (moment magnitude).
Epicentre: Bhuj, Kutch district, Gujarat — Zone V.
Deaths: ~13,805 (official); estimates including missing persons range to ~20,000.
Buildings destroyed: ~230,000.
Structural lessons documented:
| Failure mode | Buildings affected | Design lesson |
|---|---|---|
| Soft storey collapse | Multi-storey RC buildings with open ground floors for parking/shops | Avoid open ground floors in seismic zones; or provide equal stiffness via RC walls |
| Irregular plan | Buildings with re-entrant corners, wings, and setbacks | Use regular, compact plan shapes; seismic joints if plan is complex |
| Floating columns | Columns that start at upper floors, unsupported from foundation | Never use floating columns in seismic zones — provide continuous load path to foundation |
| Poor construction quality | Wide range of buildings | Correct reinforcement cover, laps, and compaction are as critical as design |
| Unreinforced masonry failure | Rural and peri-urban housing | Seismic bands (lintel and sill bands) and pilasters mandatory in zones III–V |
What survived: Traditional Bhunga (circular earthen huts with thatched roof) in rural areas performed well — their circular form, low height, and heavy mud walls dissipated energy through mass and geometry. This does not mean all masonry survived — unreinforced masonry was among the worst performers overall.
Exam Trap: “Masonry structures generally survived the Bhuj earthquake” is an oversimplification. Unreinforced masonry suffered heavily. The Bhunga-style and well-reinforced masonry survived — blanket statements about masonry survival are incorrect.
D. Parameter Table
| Parameter | Value | Source |
|---|---|---|
| Seismic Zone II — Zone Factor Z | 0.10 | IS:1893 Part 1:2016 |
| Seismic Zone III — Zone Factor Z | 0.16 | IS:1893 Part 1:2016 |
| Seismic Zone IV — Zone Factor Z | 0.24 | IS:1893 Part 1:2016 |
| Seismic Zone V — Zone Factor Z | 0.36 | IS:1893 Part 1:2016 |
| Delhi seismic zone | Zone IV | IS:1893 Part 1:2016 |
| Northeast India / Kutch seismic zone | Zone V | IS:1893 Part 1:2016 |
| 59% of India’s land area | Moderate–severe seismic hazard | NDMA |
| IS:13920 — applicable zones | III, IV, V | IS:13920:2016 |
| Minimum column dimension (IS:13920) | 230 mm | IS:13920:2016 |
| Plan irregularity threshold (projection) | > 15% of plan dimension | IS:1893 Part 1:2016 |
| Minimum roof pitch for cyclone | ≥ 22° | NBC 2016; NDMA |
| Maximum roof overhang (eaves/verge) | ≤ 457 mm (≈ 18 inches) | NBC 2016; NDMA |
| Best roof form for cyclone | Hip roof | GATE-1992 confirmed |
| Soil bearing capacity in flood zone | ~50% of dry ground value | NDMA guidelines |
| Bhuj earthquake date | 26 January 2001 | Historical record |
| Bhuj magnitude | 6.9 Richter / 7.7 Mw | Historical record |
| Bhuj zone | Zone V | IS:1893 |
E. Common Confusions
| Confusion | Correct Distinction |
|---|---|
| Hazard = disaster | Hazard = potential threat; Disaster = actual event overwhelming coping capacity. A flood hazard zone does not automatically become a disaster — it becomes one only when damage and disruption are severe enough. |
| Vulnerability = hazard | Vulnerability = susceptibility (building quality, design, materials). Hazard = the threatening phenomenon. A well-built building in a high-hazard zone has low vulnerability even though hazard is high. |
| IS:13920 applies to all zones | IS:13920 ductile detailing is mandatory for zones III, IV, V only. Zones I and II are not required to comply, though it is good practice. |
| Zone I still exists in IS:1893 | Zone I was merged into Zone II in the 2002 revision of IS:1893. The current 2016 standard has only zones II–V. |
| Hip roof has the same wind resistance as gable | Hip roof outperforms gable under cyclone conditions. A hip roof has no exposed vertical gable end — all surfaces slope. Gable ends create high-pressure zones. |
| Wet flood-proofing = waterproofing | Wet flood-proofing deliberately ALLOWS water entry. Dry flood-proofing EXCLUDES water through sealing and barriers. Waterproofing is typically used in dry flood-proofing. |
| Bhuj masonry = blanket survival | The Bhunga circular hut (reinforced + circular plan) survived. Conventional unreinforced masonry suffered heavily. It is incorrect to say “masonry survived Bhuj” without qualification. |
| Soft storey = old/weak building | Soft storey is a structural configuration failure — it commonly occurs in MODERN RC buildings with open ground floors for commercial or parking use. It is independent of building age. |
F. Exam Traps
| Trap | Incorrect Assumption | Correct Answer |
|---|---|---|
| T42 | Zone I seismic zone applies to parts of south India | Zone I was merged into Zone II in IS:1893:2002 and does not exist in the current standard. |
| T43 | Delhi is in seismic Zone V | Delhi is in Zone IV (High). Zone V covers Northeast India, Kutch, and specific Himalayan zones. |
| T44 | IS:13920 applies to buildings in all seismic zones | IS:13920 ductile detailing is mandatory for zones III, IV, V only. |
| T45 | Gable roof is best for cyclone resistance | Hip roof is best for cyclone resistance — no exposed gable end; all surfaces slope. Gable ends create pressure concentration. (GATE-1992) |
| T46 | Roof overhang should be ≥ 457 mm in cyclone zones | Overhang should be ≤ 457 mm. Long overhangs in cyclone zones create large uplift surfaces. |
| T47 | Soft storey is primarily a problem in old buildings | Soft storey is a structural configuration problem — it is most common in modern multi-storey RC buildings with open ground floors. Age is not the determining factor. |
| T48 | Wet flood-proofing seals the building against water entry | Wet flood-proofing allows water to enter deliberately. Dry flood-proofing seals the building. |
G. Answer-Writing Cues
For disaster terminology questions:
“Hazard is a potential source of harm — the threat itself. Vulnerability is the susceptibility of a structure or community to that hazard, determined by construction quality, design, and preparedness. Risk is the product of hazard and vulnerability — a high-hazard area with low-vulnerability buildings carries moderate actual risk. Exposure refers to the presence of people or assets in hazard-prone zones. A disaster occurs when an event exceeds local coping capacity and requires external assistance.”
For seismic design principles:
“Earthquake-resistant design operates at three stages. At the planning stage, regular plan configuration, symmetric layout, and avoidance of soft storeys and floating columns are critical. At the structural design stage, IS:13920 ductile detailing (applicable in seismic zones III, IV, V) ensures adequate deformation capacity through confined reinforcement. At the construction stage, correct reinforcement placement, concrete quality, and curing are equally critical — a well-designed building poorly executed is as vulnerable as a poorly designed one.”
For cyclone resistance:
“Hip roof outperforms gable under cyclone loading — all surfaces slope inward, eliminating the exposed vertical gable end that creates high pressure zones. Minimum roof pitch is 22° per NBC 2016; eave and verge overhangs must not exceed 457 mm (18 inches) to limit uplift forces. Square or circular building plans minimise the exposed face presented to any wind direction.”
H. PYQ Linkage Note
| Topic | Exam Appearance | Pattern |
|---|---|---|
| Hazard vs vulnerability distinction | GATE, UPSC-CPWD | MCQ: definitional; “which term describes susceptibility?” → Vulnerability |
| Seismic zone of Delhi | GATE, UPSC-CPWD | MCQ: “Delhi is in seismic zone ___” → Zone IV |
| Seismic zones count (II–V; no Zone I) | GATE, UPSC-CPWD | MCQ: “How many seismic zones does IS:1893 define?” → 4 (II, III, IV, V) |
| IS:13920 zone applicability | UPSC-CPWD | MCQ: “IS:13920 applies to which seismic zones?” |
| Minimum column dimension (IS:13920) | GATE, UPSC-CPWD | MCQ: “Minimum column dimension per IS:13920 is…” → 230 mm |
| Soft storey — definition and danger | GATE (most tested vulnerability) | MCQ: “Which structural condition causes concentrated damage in earthquakes?” |
| Hip vs gable for cyclone | GATE-1992 | MCQ: “Which roof form is best for cyclone-prone areas?” → Hip |
| Minimum roof pitch for cyclone | UPSC-CPWD, state PSC | MCQ: minimum pitch → 22° |
| Wet vs dry flood-proofing | UPSC-CPWD | MCQ: distinguish the two approaches |
| Bhuj earthquake — date, zone, lessons | GATE, UPSC-CPWD | MCQ: date (26 Jan 2001); zone (V); key structural lesson (soft storey) |
I. Mini-Check — Lesson 2.8 (5 Questions)
Q1 (MCQ): Which of the following best defines “vulnerability” in the context of disaster-resistant design?
(A) The intensity of a natural hazard at a particular location
(B) The probability that a natural hazard event will occur within a given period
(C) The susceptibility of a structure to damage from a hazard, determined by its design and construction quality
(D) The presence of a building or population within a hazard-prone area
A1: (C). Vulnerability = susceptibility to damage from a hazard — a function of the building itself. Hazard intensity = (A); hazard probability relates to (B); presence in hazard zone = (D) which is exposure.
Q2 (MCQ): Per IS:1893 Part 1:2016, Delhi is classified under which seismic zone?
(A) Zone III (B) Zone IV (C) Zone V (D) Zone II
A2: (B) Zone IV. Delhi = Zone IV (High hazard; Z = 0.24). Zone V covers Northeast India and Kutch. Zone III covers moderate hazard areas of peninsular India.
Q3 (MCQ): According to GATE-1992 and NBC 2016 cyclone design guidance, which roof form is most resistant to cyclone wind loads?
(A) Flat roof (B) Gable roof (C) Hip roof (D) Shed (mono-pitch) roof
A3: (C) Hip roof. A hip roof slopes inward on all four sides — there is no vertical gable face to accumulate wind pressure. Gable roofs (B) are the worst — the triangular vertical gable end creates high pressure zones and large uplift at the roof-wall connection.
Q4 (MCQ): IS:13920:2016 (Ductile Design and Detailing of Reinforced Concrete Structures) is mandatory for buildings in which seismic zones?
(A) All zones (I through V) (B) Zones III, IV, and V only (C) Zones IV and V only (D) Zone V only
A4: (B) Zones III, IV, and V only. IS:13920 ductile detailing is triggered by moderate to high seismic hazard. Zone II (which now incorporates old Zone I) does not require IS:13920 compliance, though it is good practice.
Q5 (MSQ): Which of the following statements about seismic-resistant design principles are correct per IS:1893 and IS:13920? Select all that apply.
(A) A plan with projections exceeding 15% of the plan dimension is considered irregular per IS:1893
(B) The minimum dimension of an RC column per IS:13920 is 230 mm in any plan direction
(C) Soft storey buildings typically have a stiffer ground floor than the upper floors
(D) IS:13920 applies to buildings in seismic zones III, IV, and V
(E) Strong column–weak beam design ensures plastic hinges form in beams before columns
A5: (A), (B), (D), and (E).
– (A) ✓ Re-entrant corner projections > 15% trigger irregularity classification per IS:1893.
– (B) ✓ Minimum column dimension = 230 mm per IS:13920.
– (C) ✗ Wrong — a soft storey has a less stiff (weaker, more flexible) ground floor, not stiffer. That reduced stiffness is what makes it dangerous.
– (D) ✓ IS:13920 applies to zones III, IV, V.
– (E) ✓ Strong column–weak beam = plastic hinges form in beams (local, repairable) before columns (which cause collapse).