Last reviewed 16 Sept 2026 · 10 min read
Fire
Fire triangle and tetrahedron
Combustion requires fuel, heat and oxygen — the fire triangle. The fire tetrahedron adds the uninhibited chemical chain reaction. Fire is extinguished by removing any element: cooling (heat), smothering (oxygen), starving (fuel) or chemical inhibition (chain reaction).
Stages of fire in a room
- Ignition (incipient stage).
- Growth — fire spreads, temperature rises, smoke accumulates.
- Flashover — sudden transition when all combustible surfaces ignite almost simultaneously (room becomes fully involved) — survival unlikely after this.
- Fully developed fire — maximum burning rate and temperature (can exceed about 1000 °C).
- Decay — as fuel is consumed.
Smoke and toxic gases (carbon monoxide, hydrogen cyanide from plastics) cause most fire deaths; hence smoke control and early escape are vital.
Behaviour of building materials in fire
| Material | Behaviour |
|---|---|
| Stone | Granite cracks and disintegrates (different expansion of minerals, quartz); limestone and marble calcine at high temperature; sandstone (fine-grained) behaves moderately; stones are non-combustible but may spall |
| Bricks (burnt clay) | Good fire resistance — already burnt at high temperature; brick walls are excellent fire barriers |
| Concrete | Good fire resistance (non-combustible, low conductivity); may spall (explosive spalling in dense, high-strength concrete with high moisture) and loses strength at high temperatures; adequate cover protects reinforcement |
| Steel | Non-combustible but loses strength rapidly — mild steel retains only about half its strength at around 550–600 °C; large thermal expansion causes distortion; needs fire protection (concrete encasement, fire boards, sprayed insulation, intumescent paints) |
| Timber | Combustible, but large sections perform well — the surface chars, insulating the core which retains strength; small sections burn quickly; fire-retardant treatment improves performance |
| Glass | Ordinary glass cracks and falls out in fire; wired glass and special fire-resistant glazing maintain integrity |
| Aluminium | Melts at about 660 °C — poor fire resistance for structural use |
| Gypsum (plaster, boards) | Good fire protection — chemically combined water is released as steam, absorbing heat |
| Plastics | Combustible; many produce dense toxic smoke and burning droplets; fire-retardant grades needed |
| Asbestos cement | Non-combustible but shatters; health hazards restrict use |
Fire resistance
Fire resistance is the ability of an element to perform its function in a standard fire test for a specified period (e.g. 1, 2 or 4 hours), judged by three criteria:
- Stability (load-bearing capacity) — does not collapse.
- Integrity — does not allow flames or hot gases to pass through cracks/openings.
- Insulation — the unexposed face does not exceed specified temperature rises.
Fire resistance of RCC members depends on member dimensions and concrete cover to reinforcement (IS 456 gives minimum dimensions and covers for different fire ratings). Fire tests in India follow IS 3809.
Fire-safe planning
National Building Code of India (Part IV — Fire Protection) and local fire regulations govern fire safety, including:
Classification by occupancy
Buildings are grouped by use — e.g. residential, educational, institutional (hospitals), assembly (theatres, halls), business (offices), mercantile (shops), industrial, storage and hazardous — with stricter requirements for high occupancy and high hazard. High-rise buildings (commonly defined as 15 m or more in height) need additional provisions.
Key measures
- Compartmentation — dividing buildings into fire compartments with fire-resisting walls, floors and self-closing fire doors to limit spread of fire and smoke.
- Means of escape:
- Adequate number, width and distribution of exits based on occupant load.
- Limits on travel distance to an exit.
- Enclosed fire-resisting staircases, protected lobbies, pressurisation of staircases in tall buildings, no escape through lifts in fire (except designated evacuation lifts).
- Refuge areas in high-rise buildings at specified floor intervals.
- Exit doors opening in the direction of escape, panic bars, illuminated exit signs and emergency lighting.
- Fire lifts for firefighters in tall buildings.
- Structural fire protection of steel and adequate cover to reinforcement.
- Access for fire tenders — open spaces around buildings, adequate road width and turning, hard standing.
- Control of combustible materials — interior finishes with limited surface spread of flame, fire-stopping of shafts and service penetrations.
- Smoke management — smoke vents, exhaust systems, smoke-stop doors.
Fire detection and fire-fighting installations
| System | Function |
|---|---|
| Fire alarm systems | Smoke detectors (early detection), heat detectors (kitchens, dusty areas), flame detectors, manual call points, sounders and control panels |
| Automatic sprinkler systems | Heat-activated sprinkler heads discharge water over the fire — very effective in controlling fires |
| Wet riser | Vertical pipe kept permanently charged with water from pumps/tanks, with landing valves on each floor — tall buildings |
| Dry riser | Vertical pipe kept empty, charged by fire brigade pumps through an inlet at ground level |
| Down-comer | Riser fed from an overhead tank |
| Hose reels | First-aid fire-fighting on each floor |
| Fire hydrants | External hydrants on mains for fire brigade |
| Fire extinguishers | Portable (water, foam, CO₂, dry chemical powder) for small fires |
| Special systems | CO₂ flooding, clean-agent gas, foam (fuel stores), water mist |
| Fire water storage | Dedicated tanks and fire pumps (with standby) |
Lighting
Natural lighting (daylighting)
Daylight factor (DF):
( = illuminance at a point indoors; = simultaneous illuminance outdoors on a horizontal plane under an unobstructed overcast sky.)
Components of daylight factor:
- Sky component (SC) — light received directly from the sky.
- Externally reflected component (ERC) — light reflected from external surfaces (buildings, ground).
- Internally reflected component (IRC) — light reflected from internal surfaces (walls, ceiling, floor).
Good daylighting design: adequate window area and height (higher windows throw light deeper), window distribution, light-coloured interior surfaces, clerestory windows, skylights and light shelves, shading to control glare and heat, orientation (north light is uniform).
Artificial lighting
- Illuminance is measured in lux (lumen/m²). Recommended levels depend on tasks — e.g. general residential areas about 100–300 lux, offices and classrooms about 300–500 lux, and higher for detailed work (IS 3646 gives recommended levels).
- Luminous efficacy (lumen per watt): LED lamps are much more efficient than CFLs and incandescent lamps.
- Consider uniformity, glare control, colour rendering and energy efficiency (controls, daylight sensors).
= number of luminaires; = required illuminance (lux); = area (m²); = lumens per luminaire; = utilisation factor (room shape and reflectances); = maintenance factor (dirt, lamp depreciation).