Wind Hazard in Bangladesh
Bangladesh is one of the most cyclone-exposed countries in the world. The Bay of Bengal funnels cyclones directly into the Bangladesh coastline, with Chittagong and Cox's Bazar being among the highest-risk areas globally. BNBC 2020 provides wind load requirements that reflect this hazard, with basic wind speeds ranging from 195 km/h inland to 260 km/h on coastal islands.
BNBC 2020 Wind Speed Map — Detailed
| Region | Basic Wind Speed V (km/h) | V (m/s) | Wind Zone | Cyclone Exposure |
|---|---|---|---|---|
| Dhaka | 195 | 54.2 | Zone I | Low — inland |
| Rajshahi | 195 | 54.2 | Zone I | Very low — far inland |
| Sylhet | 195-210 | 54.2-58.3 | Zone I-II | Low — nor'easters possible |
| Khulna | 210-225 | 58.3-62.5 | Zone II-III | Moderate — Sundarbans buffer |
| Chittagong City | 225-245 | 62.5-68.1 | Zone III-IV | High — direct cyclone path |
| Cox's Bazar | 245-260 | 68.1-72.2 | Zone IV | Very high — exposed coast |
| Coastal Islands | 260 | 72.2 | Zone V | Extreme — Hatiya, Sandwip, Bhola |
| Chattogram Hill Tracts | 195-210 | 54.2-58.3 | Zone I-II | Low — sheltered by hills |
Exposure Categories
| Category | Description | Typical Location | Wind Effect |
|---|---|---|---|
| A | Urban centre, dense buildings ≥10m height for ≥800m | Central Dhaka (Motijheel, Gulshan) | Lower — buildings create wind shelter |
| B | Suburban, scattered buildings, some trees | Dhaka outskirts, secondary cities | Moderate |
| C | Open terrain, few obstructions | Rural areas, industrial zones, open fields | Higher — no wind shelter |
| D | Flat, unobstructed coastal areas | Beachfront, river estuaries, offshore | Highest — direct wind exposure |
Importance Factor for Wind
| Occupancy Category | Importance Factor (I) | Examples |
|---|---|---|
| I — Low hazard | 0.87 | Agricultural, minor storage |
| II — Normal | 1.00 | Offices, retail, residential, factories |
| III — Substantial hazard | 1.15 | Schools, assembly >300, power stations |
| IV — Essential | 1.15 | Hospitals, fire stations, emergency shelters |
Cyclone-Specific Building Considerations
For Coastal Buildings (Zone III-V)
- Roof connections: Hurricane straps/clips connecting rafters to top plates — most common failure is roof blowoff.
- Glazing protection: Impact-resistant glazing or shutters for openings — missile debris is a major failure mechanism.
- Cladding attachment: Enhanced fastener patterns and closer spacing for metal cladding.
- Wind-borne debris region: BNBC 2020 designates coastal zones as wind-borne debris regions requiring impact-resistant openings.
- Flood + wind combination: Coastal buildings face simultaneous storm surge and wind — foundation must resist both.
- Generator protection: Backup power must be above flood level and wind-protected.
For Inland Buildings (Zone I-II)
- Nor'easters (Kalbaishakhi): Seasonal storms with gusts up to 100-130 km/h — below design wind but can damage poorly maintained buildings.
- Roof maintenance: Loose roofing materials are dangerous in even moderate wind events.
- Signage and facades: Poorly attached signage, AC units, and facade panels become projectiles.
Wind-Related Due Diligence Checklist
| Item | What to Check | Red Flag |
|---|---|---|
| Structural design wind speed | Which wind speed was used in the design? | Design wind speed lower than current BNBC requirement |
| Roof system | Roof type, attachment method, condition | Corroded fasteners, loose sheets, no hurricane straps (coastal) |
| Glazing | Window/curtain wall system, frame integrity | Non-impact-rated glazing in cyclone zone, cracked seals |
| Cladding | Attachment system, corrosion, deflection | Loose panels, corrosion at fasteners, inadequate spacing |
| External attachments | Signage, AC units, antennas, facade features | Poorly anchored items that become missiles |
| Water ingress history | Evidence of wind-driven rain, leaks during storms | Water staining, mould, damaged finishes near openings |
References
- BNBC 2020 Part 6 Chapter 2 — Wind Loads
- ASCE 7 — Minimum Design Loads (Wind Provisions)
- NOAA — Tropical Cyclone Data
- Bangladesh Meteorological Department
- ReliefWeb — Bangladesh Cyclone History
Insights & Guidance
- BNBC 2020 wind speeds: 195 km/h (Dhaka, inland) to 260 km/h (coastal islands).
- Chittagong and Cox's Bazar are Zone III-IV (225-260 km/h) — among the highest wind zones globally.
- Exposure categories A-D significantly affect wind forces — a building on an open coastal site (D) experiences much higher forces than one in dense urban Dhaka (A).
- Roof blowoff is the most common cyclone damage mode — check roof connections and condition.
- Wind-driven rain and water ingress cause more damage than structural wind failure in most cyclones.
Wind damage is the most common natural hazard damage to buildings in Bangladesh. Understanding BNBC wind requirements, cyclone zones, and what to check during due diligence protects investments and occupant safety.
- Roof blowoff — inadequate roof-to-structure connections, especially in coastal buildings.
- Facade missile damage — loose AC units, signage, or panels become projectiles.
- Water ingress — wind-driven rain enters through glazing failures, damaging contents.
- Structural calculations with wind speed and exposure category
- Roof connection details (hurricane straps for coastal buildings)
- Cyclone preparedness plan for coastal facilities
- Roof condition — loose sheets, corroded fasteners?
- External attachments — AC units, signage properly anchored?
- Evidence of past water ingress around windows/openings?
- What wind speed was used in the structural design?
- Is there a cyclone preparedness plan?
- When was the roof last inspected?
- Coastal buildings — structural engineer with wind/cyclone design experience.
- Roof assessment — roofing specialist or structural engineer.