Fire Load and Compartmentation in Multi-Storey Factories

Fire safety in a multi-storey manufacturing building rests on three independent systems: preventing ignition, limiting spread, and getting people out. The third one is the only one with no second chance, and it is the one most often compromised by later modifications.

Fire load density

q = sum( m_i x H_i ) / A     [MJ per m^2]

  m_i = mass of combustible material i (kg)
  H_i = calorific value of material i (MJ/kg)
  A   = floor area (m^2)

A garment store room holding 30 tonnes of mixed cotton-polyester in 400 square metres carries roughly (30,000 x 22) / 400 = 1,650 MJ per square metre. That is a high fire load, comparable to a warehouse classification, and it should not be treated as ordinary factory occupancy in the design.

What fire load drives

• Required fire resistance rating of compartment walls and floors — commonly 60, 120 or 240 minutes as load rises.
• Maximum compartment size — higher loads demand smaller compartments.
• Sprinkler design density — hazard classification follows from the load and the storage arrangement, including storage height.
• Smoke ventilation requirements.

Compartmentation, and the three ways it fails

A compartment is only rated if it is complete. In practice, the rating is defeated by:

1. Unsealed service penetrations. Every cable tray, pipe and duct crossing a fire wall must be fire-stopped with a tested system. A single unsealed 100 mm penetration can defeat a two-hour wall in minutes. Audit penetrations after every maintenance project — this is where the damage happens.
2. Doors wedged open. A fire door held open by a fire extinguisher is not an unusual sight and it is a complete failure of the system. Fit hold-open devices linked to the fire alarm if doors must stay open operationally.
3. Vertical openings. Unprotected lift shafts, cable risers, conveyor openings between floors and open stairwells turn a single-floor fire into a whole-building fire. Protected shafts and self-closing doors at every level are what stop this.

Evacuation arithmetic

Total evacuation time = detection + alarm + pre-movement + travel

Stair flow capacity  = 1.0 to 1.3 persons/second per metre of effective width
Effective width      = clear width - 0.30 m (edge allowance)

For a floor with 400 occupants using a 1.5 m clear stair: effective width 1.2 m, flow about 1.2 x 1.15 = 1.38 persons per second, so 400 people take roughly 290 seconds to pass a point — before adding the time to reach the stair and before merging flows from other floors. Multi-floor merging is what turns an acceptable single-floor calculation into a dangerous building-wide one.

Pre-movement time is the most underestimated term. In an unprepared building it frequently exceeds three minutes, dominating everything else. Drills reduce it more than any physical change to the building.

The provisions that actually get people out

• At least two independent, remote exits from every floor, with travel distance to the nearest exit within code limits (commonly 30-45 m for factory occupancy without sprinklers).
• Exit doors opening in the direction of travel, never locked during occupancy. Collapsible gates on stairs are the single most lethal common violation.
• Illuminated exit signage visible from any point on the escape route, with battery backup.
• A refuge or protected lobby arrangement where the building height requires it.
• Drills at least twice a year, at different times, including at least one on the night shift if the plant runs one.

What to check this month

Walk every escape route from the furthest workstation to the assembly point, at the pace of the slowest worker, with a stopwatch. Note every locked door, every stored pallet narrowing a corridor, every missing sign. That walk finds more real risk than a document review of the same building ever will.