What Seismic Risk Means for Buildings
Seismic risk is the probability that earthquake ground motion will cause damage, loss of function, or collapse of a building. It depends on three factors: hazard (how likely and how strong), vulnerability (how the building responds), and exposure (what's inside — people, assets, operations).
Bangladesh sits at the junction of the Indian, Eurasian, and Burmese tectonic plates. The country has experienced major earthquakes historically and is classified as moderate-to-high seismic hazard. Yet the majority of Dhaka's building stock was designed with minimal or no seismic detailing.
Applicable Standards & Codes
Seismic Standards in Bangladesh Context
- adopted BNBC 2020 Part 6, Chapter 2 (Seismic) — Defines seismic zones, design response spectra, and detailing requirements for Bangladesh
- referenced ASCE 7 (Seismic Loads) — BNBC seismic load methodology is derived from ASCE 7 with Bangladesh-specific zone factors
- referenced ACI 318 Chapter 18 (Seismic Detailing) — Seismic detailing provisions for reinforced concrete structures
- referenced FEMA P-154 (Rapid Visual Screening) — Methodology used by some organisations for preliminary seismic vulnerability assessment
| Standard | Scope | Publisher |
|---|---|---|
| BNBC 2020, Chapter 2 (Part 6) | Seismic design for Bangladesh — zone map, response spectra, detailing | HBRI / PWD |
| ASCE 7-22, Chapters 11–23 | Seismic design criteria — ground motion, analysis, non-structural | ASCE |
| ASCE 41-17 | Seismic evaluation and retrofit of existing buildings | ASCE |
| FEMA P-154 | Rapid Visual Screening (RVS) of buildings for seismic hazards | FEMA |
| ISO 3010 | Seismic actions on structures — basis of design | ISO |
| ACI 318-22, Chapter 18 | Earthquake-resistant concrete structures — special detailing | ACI |
Bangladesh Seismic Zones
BNBC 2020 divides Bangladesh into four seismic zones:
| Zone | Seismic Zone Coefficient (Z) | Areas | Hazard Level |
|---|---|---|---|
| Zone 1 | 0.12 | Southwest Bangladesh (Khulna, Barisal) | Low |
| Zone 2 | 0.20 | Central Bangladesh (Dhaka, Comilla) | Moderate |
| Zone 3 | 0.28 | North-central (Mymensingh, Tangail) | Moderate–High |
| Zone 4 | 0.36 | Northeast/North (Sylhet, Rangpur, Chittagong Hills) | High |
Dhaka is Zone 2 (Z = 0.20) — moderate seismic hazard. This means buildings must be designed for earthquake forces equal to at least 20% of the PGA (Peak Ground Acceleration) at bedrock.
Key Metrics & Acceptance Criteria
| Parameter | Standard / Source | Good Practice | Red Flag |
|---|---|---|---|
| Inter-storey drift ratio | BNBC 2020 / ASCE 7 | ≤ 0.02 (2% of storey height) | > 0.025 or no analysis available |
| Soft storey indicator | ASCE 7 §12.3.3.1 | Stiffness ratio between adjacent storeys < 0.7 = soft storey | Open ground floor with no infill or bracing |
| Plan irregularity | BNBC / ASCE 7 Table 12.3-1 | Torsional irregularity ratio < 1.2 | > 1.4 (extreme torsional irregularity) |
| Lateral force-resisting system | ASCE 7 Table 12.2-1 | Moment frames with R ≥ 5, Shear walls with R ≥ 5 | Unreinforced masonry bearing walls (R = 1.5) |
| Ductile detailing | ACI 318 Ch. 18 | Conforming to Special or Intermediate moment frame detailing | Ordinary moment frame in Zone 2+ without justification |
| Non-structural restraint | ASCE 7 Chapter 13 | Ceilings, MEP, partitions braced for seismic | Unrestrained suspended ceilings, unreinforced partitions |
Common Seismic Vulnerability Indicators
- Soft storey (pilotis) — open ground floor for parking with no structural walls or bracing. Extremely common in Dhaka residential and mixed-use buildings. The single most dangerous seismic vulnerability.
- Short column effect — partial-height infill walls creating short, stiff column segments that attract high shear forces. Common where window openings create half-height walls adjacent to columns.
- Heavy overhangs — cantilevered upper floors heavier than lower floors shift the centre of mass upward, increasing seismic demand.
- Plan irregularity — L-shaped, T-shaped, or U-shaped buildings with re-entrant corners concentrate stress at the junctions.
- Vertical irregularity — setbacks, transfer beams, or changes in structural system between floors.
- No seismic detailing — concrete buildings designed without ductile detailing (closely spaced stirrups, proper lap lengths, confined joints) fail in a brittle manner.
What to Inspect on Site
- Ground floor configuration — is it open (parking)? Are there structural walls or bracing? This is the #1 seismic risk factor in Dhaka.
- Building shape — regular rectangular plan is best. L, T, U shapes create stress concentrations.
- Number of storeys vs. apparent structural system — a 10+ storey building on columns alone (no shear walls) is a concern.
- Visible column-beam joints — look for inadequate joint confinement (wide stirrup spacing, lack of cross-ties).
- Infill walls — are masonry infill walls engaging short columns? Are they separated from the frame with seismic gaps?
- Neighbouring buildings — are buildings too close, risking pounding during earthquakes? Separation should be at least 1% of building height.
- Non-structural elements — heavy cladding, water tanks on roof, unreinforced parapets — all become projectiles in earthquakes.
Gap Management: What to Do When Seismic Adequacy Is Unknown
- Rapid Visual Screening (RVS) — use FEMA P-154 methodology. Can be completed in 15–30 minutes per building. Produces a score indicating likelihood of collapse.
- Detailed Seismic Evaluation — per ASCE 41-17 for buildings that fail RVS screening. Requires structural engineer with seismic expertise.
- Retrofit options — steel bracing, FRP wrapping of columns, addition of shear walls, base isolation (for critical facilities). Cost: 5–15% of building replacement value.
- Risk acceptance documentation — if retrofit is not feasible, document the risk formally with management sign-off, enhanced insurance, and emergency preparedness measures.
References & Sources
- BNBC 2020, Part 6 Chapter 2 — Seismic Design Requirements (HBRI, Government of Bangladesh)
- ASCE 7-22 Minimum Design Loads — Seismic Provisions
- ASCE 41-17 Seismic Evaluation and Retrofit of Existing Buildings
- FEMA P-154 Rapid Visual Screening of Buildings for Potential Seismic Hazards
- USGS Earthquake Hazards Program
- ACI 318-22 Chapter 18 — Earthquake-Resistant Structures
Insights & Guidance
- Bangladesh is seismically active — Dhaka is Zone 2 (Z = 0.20), Sylhet/Chittagong Zone 4 (Z = 0.36).
- Soft storey (open ground floor) is the #1 seismic vulnerability in Dhaka buildings.
- Inter-storey drift must not exceed 2% of storey height (0.02 ratio).
- Most pre-2010 Dhaka buildings lack seismic detailing per BNBC 2020 or ACI 318 Chapter 18.
- FEMA P-154 Rapid Visual Screening is a cost-effective first step — 15–30 minutes per building.
- When seismic adequacy is unknown, document the gap formally and establish monitoring.
Bangladesh has not experienced a major urban earthquake in modern times — but seismologists consider it overdue. The 1897 Great Assam Earthquake (M 8.1) caused widespread damage across what is now northeast Bangladesh. Dhaka's soft alluvial soil amplifies ground motion, and its dense building stock — much of it without seismic detailing — creates extreme vulnerability.
For MNC occupiers, seismic risk assessment is a standard component of pre-lease due diligence in seismically active regions. Failing to assess seismic risk exposes organisations to occupant safety liability, business continuity failure, and insurance coverage disputes. The cost of a seismic screening (USD 2,000–5,000 per building) is negligible compared to the consequences.
- Soft storey collapse — open ground floor pancakes, upper floors remain relatively intact but fall as a unit. Most common earthquake failure mode in South Asia.
- Column shear failure — brittle failure from inadequate transverse reinforcement; no warning before collapse.
- Pounding damage — adjacent buildings with insufficient separation collide during shaking, causing localized damage or progressive collapse.
- Non-structural casualties — falling ceilings, toppling partitions, unrestrained equipment cause 50%+ of earthquake injuries in commercial buildings.
- Soil liquefaction — saturated sandy soils in low-lying Dhaka areas lose bearing capacity during shaking, causing building tilting or settlement.
- Business continuity loss — even moderate damage (cracked partitions, broken utilities) can render a building unusable for weeks or months.
- Seismic design certificate — structural engineer's confirmation that the building was designed for the applicable BNBC seismic zone.
- Structural drawings showing seismic detailing — confined joints, closely spaced stirrups (≤ 100 mm in plastic hinge zones), proper anchorage.
- Geotechnical report — soil investigation including site classification (BNBC Site Class SA–SE) and liquefaction potential assessment.
- Seismic evaluation report — for buildings > 15 years old: FEMA P-154 RVS or ASCE 41 evaluation.
- Retrofit documentation — if seismic strengthening was performed: drawings, calculations, completion certificate.
- Emergency preparedness plan — earthquake-specific procedures, evacuation routes, assembly points, post-earthquake inspection protocol.
- Open ground floor with columns only (no walls or bracing) — soft storey risk.
- Building shape — irregular plan (L, T, U) or significant setbacks.
- Heavy water tanks, cooling towers, or equipment on roof without visible bracing.
- Unreinforced masonry parapets or decorative elements at height.
- Adjacent buildings very close (less than 300 mm gap for mid-rise) — pounding risk.
- Visible column-beam joints showing wide stirrup spacing or poor confinement.
- Signs of previous earthquake damage — repaired diagonal cracks, patched concrete.
- Building age and number of storeys — pre-2006 buildings above 6 storeys in Dhaka are high priority for screening.
- Which BNBC seismic zone is this building designed for? Is there documentation confirming this?
- Does the structural design include seismic detailing per ACI 318 Chapter 18 or BNBC equivalent?
- Has a seismic evaluation or Rapid Visual Screening (RVS) been conducted? What was the score/finding?
- What is the lateral force-resisting system — moment frames, shear walls, or dual system?
- Has a geotechnical investigation been conducted? What is the site soil classification?
- Is there a soft storey condition? If yes, has mitigation been implemented or evaluated?
- Are non-structural elements (ceilings, partitions, MEP) seismically restrained?
- Does the building's emergency plan include earthquake-specific procedures?
- Any building in BNBC Zone 2–4 without documented seismic design or evaluation.
- Buildings with visible soft storey condition (open ground floor, parking podium).
- Pre-lease due diligence for buildings above 4 storeys in Dhaka or any height in Zone 3–4.
- After any felt earthquake — even minor shaking warrants post-earthquake structural inspection.
- Before any major renovation or change of use that could alter the building's dynamic response.
- When planning retrofit or strengthening — requires seismic engineering specialist, not general structural engineer.