Fire and Life Safety Design

Prescriptive vs Performance-Based Approach

Prescriptive approach:

• Follow code provisions directly (IBC, Approved Document B, NBC, etc.)
• Advantages: straightforward, well-understood, defensible
• Limitations: may be overly conservative, may not address novel building configurations, does not account for specific building use patterns

Performance-based approach:

• Establish fire safety objectives, define design fire scenarios, demonstrate through engineering analysis that objectives are met
• Methods: computational fire modeling (FDS, CFAST), evacuation modeling (Pathfinder, STEPS, Simulex), structural fire analysis (FEA with fire curves), smoke control analysis (CFD)
• Advantages: can optimize design, accommodate complex geometries, provide a more accurate risk assessment
• Limitations: requires specialist fire engineer, peer review, AHJ acceptance

Hybrid approach (most common):

• Follow prescriptive code for most provisions
• Use performance-based analysis for specific challenges: atrium smoke control, travel distance extensions, reduced fire resistance periods, alternative egress strategies

Fire Strategy Report Content

A fire strategy report (also called fire safety strategy, fire engineering report, or basis of fire safety design) should include:

1. Building description: use, occupancy, construction, geometry
2. Applicable codes and standards
3. Design fire scenarios (size, location, growth rate)
4. Compartmentation strategy and fire resistance schedule
5. Means of escape: travel distances, exit widths, evacuation strategy
6. Fire detection and alarm design
7. Sprinkler/suppression system scope
8. Smoke control strategy
9. Structural fire protection specification
10. Fire service access and facilities
11. Building management fire safety provisions
12. Drawings: compartmentation plans, egress plans, fire service access plans

Section 2: Compartmentation

Purpose

Fire compartmentation divides a building into distinct zones that contain fire and smoke for a defined period, allowing occupants to evacuate, limiting property damage, and providing safe zones for defend-in-place strategies.

Compartment Sizing

IBC (US):

• No explicit maximum compartment size for most occupancies (controlled by construction type and area per floor)
• I-2 smoke compartments: max 2090 m² (22,500 ft²) per smoke compartment per IBC 407.5.1
• I-3 smoke compartments: max 2090 m² (22,500 ft²) per IBC 408.6

BS 9999 (UK): Maximum compartment sizes by risk profile:

Approved Document B (England & Wales): Maximum compartment sizes by purpose group:

(*Flats: each dwelling is a compartment; common corridor/stair protected)

Fire-Rated Construction Types

Fire Barrier (IBC 707):

• Continuous from floor slab to underside of floor or roof slab above (deck, not ceiling)
• Used for: occupancy separation, exit enclosures, shaft enclosures, horizontal exits
• Supports: must be supported by construction with equal or greater FRR
• Continuity: all joints, penetrations, and openings must be protected

Fire Partition (IBC 708):

• May terminate at underside of a fire-rated floor/ceiling or roof/ceiling assembly
• Used for: dwelling unit separation, sleeping unit separation, corridor walls
• Less stringent than fire barrier (can stop at ceiling if ceiling is rated)

Fire Wall (IBC 706):

• Structurally independent (remains standing if structure on either side collapses)
• Extends from foundation to roof (through roof unless roof is fire-rated)
• Creates separate buildings for code purposes
• Rating: 2 hr (A, B, E, F-2, M, R, S-2, U), 3 hr (F-1, H-3/4/5, S-1), 4 hr (H-1, H-2)

Smoke Barrier (IBC 709):

• Continuous membrane from floor to floor, outside wall to outside wall
• Smoke-tight construction with smoke dampers at duct penetrations
• 1-hour minimum fire resistance
• Used for: I-2 smoke compartments, I-3 smoke compartments, ambulatory care smoke compartments

Fire Door Ratings

IBC/UL system (US):

British Standard / European system:

Fire Door Furniture and Ironmongery

• Self-closing device: overhead closer (EN 1154 / UL 228) mandatory on all fire doors; min closing force per accessibility requirements
• Intumescent strips: 10 mm x 4 mm or 15 mm x 4 mm strips in rebate or door edge; expand at 150-200°C to seal gap between door and frame
• Smoke seals (cold smoke): flexible blade or brush seal at head and jambs; required for FD__S designations and IBC smoke doors
• Hinges: steel butt hinges, minimum 3 per door leaf; no rising butts on fire doors
• Locks and latches: must not compromise fire rating; typically tested as part of fire door assembly
• Hold-open devices: electromagnetic hold-open connected to fire alarm; releases on alarm or power failure; door closer then closes door
• Panic hardware: fire-rated panic bar or touchpad (IBC requires on assembly and educational exit doors for occupant loads > 50)

Glazed Fire Barriers

Types of fire-rated glass:

• Integrity-only glass prevents flame and hot gas passage but radiates heat. Maximum area limited to prevent radiant heat exposure on escape routes.
• Insulating glass (intumescent gel between layers) blocks both flame and radiant heat. Can be used for full fire barriers including in escape routes.

Firestopping (Penetration Seals)

Every penetration through a fire-rated assembly must be sealed with a listed firestop system:

• Pipe penetrations: intumescent pipe collars (combustible pipes -- PVC, PE, PP); mineral wool + intumescent sealant (metal pipes with insulation)
• Cable penetrations: intumescent block, putty, or pillows; cable transit frames for large cable bundles
• Duct penetrations: fire damper (integrity only) or fire/smoke damper (integrity + smoke sealing); rated to match wall rating
• Structural joints: fire-rated joint sealant (silicone or intumescent) with mineral wool backing

Cavity barriers:

• Required in concealed spaces (above ceilings, within walls) to prevent unseen fire spread
• Maximum distance between cavity barriers: 20 m in any direction (ADB); IBC Section 718 requires draft stopping in floor/ceiling spaces of combustible construction at max 280 m² (3,000 ft²)
• At compartment wall/floor junctions extending into concealed spaces

Section 3: Means of Egress Design

Number of Escape Stairs

IBC (US):

• Based on occupancy, travel distance, and floor area
• Minimum 2 exits from each floor (with single-exit exceptions per Table 1006.2.1 for small, low-hazard spaces)
• 3 exits for 501-1,000 occupants per floor; 4 exits for > 1,000

BS 9999 / Approved Document B (UK):

• Single stair permitted for buildings up to 11 m (ADB) in certain occupancies with limited floor area and travel distance
• Number of stairs based on maximum travel distance from any point to nearest stair AND stair capacity calculation

Travel Distances -- International Comparison

IBC (US), sprinklered:

BS 9999 (UK), Risk Profile B1 (office, normal risk):

Approved Document B (England & Wales):

Comparison note: UK travel distances are significantly shorter than IBC because the UK does not generally mandate sprinklers in the same way as the IBC (sprinkler credit is a specific extension of the base distance, not a near-universal requirement). When sprinklers are provided, UK distances increase but still remain shorter than IBC.

Stair Width Calculation

IBC method:

• 7.6 mm (0.3 in) per occupant for stairways
• 5.1 mm (0.2 in) per occupant for other egress components
• Minimum stair width: 1118 mm (44 in) for 50+ occupants; 914 mm (36 in) for < 50 occupants

BS 9999 / ADB method:

• Stair width based on number of persons per floor and number of floors served
• ADB Table 5: for simultaneous evacuation, stair width accommodates total population minus ground floor
• BS 9999 Table 13: discharge rates per unit width (0.6 m per person per minute on stairs at 1100 mm width)

Minimum stair widths:

Protected Stairways

Escape stairs must be enclosed in fire-rated construction to protect occupants during evacuation:

• IBC exit stairway enclosure (Section 1023):

  • 1-hour enclosure for 4 or fewer stories connected
  • 2-hour enclosure for more than 4 stories connected
  • Opening protectives: 1-hour doors for 2-hour enclosure; 0.75-hour or 1-hour doors for 1-hour enclosure

• ADB protected stairway:

  • Walls: fire resistance equal to building's fire resistance period (30-120 min depending on height)
  • Doors: FD30S minimum (self-closing, smoke-sealed)
  • Ventilation: 1.0 m² natural ventilator at top of stair (or mechanical pressurization) for buildings > 18 m

Firefighting Shafts

Required in buildings > 18 m height (UK) or > 23 m / 75 ft (IBC high-rise):

• Fire service elevator (fire lift): min 1100 x 1400 mm cab (UK BS EN 81-72); primary power + secondary (generator) power; waterproof elevator pit and landing; controls override to fire service operation
• Firefighting lobby: min 5 m² (UK); fire-rated enclosure at every floor (2-hour walls, FD60S doors); contains dry/wet riser outlet
• Dry riser: 100 mm diameter rising main with inlet at ground level and outlets at every floor; buildings 18-60 m height (UK). Fire service connects pumping appliance at ground level.
• Wet riser: permanently charged pressurized system; buildings > 60 m (UK) or per IBC standpipe requirements (Class I in high-rise exit stairways)

Scissor Stairs

Two interlocking stairs within a single structural shaft, separated by fire-rated construction:

• Each stair is independently fire-rated (1 or 2 hr) from the other
• Separate smoke lobbies at each level for each stair
• Common in high-rise residential where site constraints prevent two separate stair cores
• IBC: scissor stairs are treated as separate exits only if each has independent exit discharge and the separating construction meets exit enclosure requirements
• UK/BS 9999: accepted where demonstrated that failure of one stair does not compromise the other

Disabled Refuge Areas

Areas of temporary safety for persons unable to use stairs:

• Location: at each floor landing of every escape stair (IBC Section 1009.3; ADB Section 4)
• Size: minimum 900 x 1400 mm (IBC: 760 x 1220 mm / 30 x 48 in clear floor space) per wheelchair space; clear of stair discharge path
• Communication: two-way intercom to fire command center / building management (IBC Section 1009.8)
• Number: one per stair per floor minimum; two per floor if > 200 occupants on that floor
• Not required in fully sprinklered buildings per IBC exception (occupants use elevator for evacuation under emergency evacuation plan)
• Signage: International Symbol of Accessibility + "Area of Rescue Assistance"

Evacuation Strategies

Simultaneous evacuation:

• All occupants evacuate immediately upon alarm
• Standard for most building types (office, retail, education)
• Total evacuation time must be less than available safe egress time (ASET > RSET)

Phased evacuation:

• Fire floor and floor immediately above evacuate first
• Remaining floors evacuate in sequence (above fire floor, then below)
• Common in high-rise office buildings; requires voice/alarm communication system with zoned messaging
• Requires at minimum: sprinklers, smoke detection, compartmentation between floors, voice alarm

Progressive horizontal evacuation:

• Occupants move horizontally through fire/smoke barriers to an adjacent compartment on the same floor
• Standard for I-2 (hospitals) and I-3 (detention) -- defend-in-place
• Each smoke compartment must have capacity to hold its own occupants plus occupants from one adjacent compartment
• Smoke compartment max 2090 m² (22,500 ft²) per IBC 407.5.1

Stay-put (residential):

• UK residential strategy: occupants in flats not affected by fire remain in their flats (each flat is a fire compartment)
• Fire-affected flat occupants escape via protected corridor and stair
• Requires: fire-rated flat entrance doors (FD30S), fire-rated corridor/lobby walls, fire-rated stair enclosure, automatic detection in common areas
• Post-Grenfell review: simultaneous evacuation capability being required alongside stay-put as a backup (Building Safety Act 2022)

Section 4: Structural Fire Protection

Fire Resistance Periods

IBC Table 601 (see building-codes SKILL.md Section 2 for full table)

ADB / BS 9999 minimum fire resistance periods:

Protection Methods

Concrete cover to reinforcement:

• Fire resistance achieved by minimum concrete cover over steel reinforcement
• 20 mm cover: approximately 1 hour FRR (depends on member type, loading)
• 25 mm cover: approximately 1.5 hours
• 35 mm cover: approximately 2 hours
• 50 mm cover: approximately 3-4 hours
• Governed by Eurocode 2-1-2, ACI 216.1, or BS 8110 Part 2

Intumescent coatings (thin-film for structural steel):

• Applied as paint-like coating (0.25-5 mm dry film thickness)
• Expands 20-50 times original thickness at 200-250°C, forming insulating char
• Fire ratings achievable: 30-120 minutes (up to 180 min for some products)
• Advantages: thin profile preserves exposed steel aesthetic; can be applied off-site or on-site
• Limitations: requires controlled application conditions; some products vulnerable to humidity; maintenance access needed for recoating (25-year typical life)
• Specification: dry film thickness (DFT) per fire rating, section factor (Hp/A) of the steel member; higher Hp/A (thinner steel) requires thicker coating

Board encasement:

• Calcium silicate board (e.g., Promat): 15-60 mm thickness for 30-240 min FRR
• Vermiculite/gypsum board: 12-50 mm thickness for 30-180 min FRR
• Advantages: precise, consistent, factory-quality finish; can be installed in any weather
• Limitations: visible box profile around structural members; time-consuming installation for complex geometries

Spray-applied fire protection (SFRM):

• Cementitious (Portland cement + mineral aggregate): density 240-350 kg/m³
• Mineral fibre (slag wool + cement binder): density 190-320 kg/m³
• Typical thickness: 10-40 mm for 1-3 hours
• Advantages: fastest application method, conforms to complex geometries, lowest cost per hour of fire rating
• Limitations: rough finish (must be concealed above ceilings), overspray containment, cannot be applied below ~4°C, fragile if exposed

Concrete encasement (full):

• Steel members fully encased in concrete
• Provides 2-4 hours FRR depending on cover thickness
• Heavy, expensive, rarely used for fire protection alone in modern construction
• Still common where structural composite action (steel-concrete) is desired

Mass Timber Fire Design

Charring method (Eurocode 5-1-2 / NDS/AWC):

• Standard charring rate: 0.65 mm/min for softwood (spruce, pine, fir)
• Charring rate for glulam/CLT: 0.65-0.70 mm/min
• Zero-strength layer: 7 mm below char line (wood heated but not charred has reduced strength)
• Effective residual section = original section - char depth - zero-strength layer

Design process:

1. Determine required fire resistance period (e.g., 90 minutes)
2. Calculate char depth: 0.65 mm/min × 90 min = 58.5 mm per exposed face
3. Add zero-strength layer: 58.5 + 7 = 65.5 mm per exposed face
4. Residual section = original - 65.5 mm per exposed face
5. Verify structural capacity of residual section under fire load combination (typically dead + 0.5 × live, reduced from ambient design loads)

Type IV-A (encapsulated mass timber):

• All mass timber surfaces protected by noncombustible material
• Typically 2 layers of 16 mm (5/8 in) Type X gypsum board (32 mm total)
• Gypsum provides ~60 minutes protection before charring begins
• Total fire resistance = gypsum protection time + residual timber capacity

Type IV-C (exposed mass timber):

• Timber may be fully exposed; fire resistance from oversized section alone
• Architectural advantage: warm, exposed wood aesthetic
• Requires larger timber dimensions to account for full charring

Section 5: Smoke Control

Purpose

Smoke is the primary cause of death in building fires. Smoke control systems maintain tenable conditions on escape routes and in occupied spaces during the time needed for evacuation.

Natural Smoke Ventilation (NSHEV -- Natural Smoke and Heat Exhaust Ventilation)

Sizing:

• General rule: free ventilation area = 5% of floor area of the smoke reservoir, or 1.5 m² per 200 m² of floor area (whichever is greater)
• Smoke reservoir depth: minimum 3 m (10 ft) from ceiling to bottom of smoke layer (maintain clear layer height above 2.5 m for tenability)
• Inlet air: low-level openings at least equal in area to exhaust openings
• Location: at highest point of smoke reservoir (roof vents, high-level windows)

Automatic opening:

• Actuated by: smoke detection (most common), fusible link (thermal), fire alarm signal
• Override: manual release at ground level and adjacent to ventilator
• Fail-safe: opens on power failure (unless wind/weather concerns require closed default)

Mechanical Smoke Extract

• Extract rate: determined by fire engineering analysis based on design fire size and smoke production rate
• Typical rates: 4-10 air changes per hour for corridor extract; higher for atrium and large-volume spaces
• Fans: rated for 300°C for 60 minutes (minimum) or 400°C for 120 minutes (for extended fire exposure); per EN 12101-3
• Ductwork: fire-rated or within fire-rated shaft; dampers at compartment boundaries
• Make-up air: must be provided to replace extracted smoke; typically low-level inlets at 75-80% of extract rate to maintain slight negative pressure in fire zone

Stair and Lobby Pressurization

• Maintains positive pressure in escape stairs and lobbies to prevent smoke infiltration
• Pressure differential: 50 Pa (0.2 in w.g.) with all doors closed (BS EN 12101-6); 12.5 Pa minimum, 87 Pa maximum (IBC)
• Air velocity through open door: 0.75 m/s minimum (some codes require 1.0 m/s) to resist smoke flow
• Fan sizing: accounts for leakage through closed doors, walls, and when one door is open on the fire floor
• Door opening force: must not exceed 133 N (30 lbf) at the door handle with pressurization system operating (IBC 1010.1.3)
• Compensation: variable-speed fans or pressure-relief dampers to adjust for doors opening/closing

Smoke Curtains and Fire Shutters

Smoke curtains (fixed or automatic):

• Fabric barriers descending from ceiling to contain smoke within a defined reservoir
• Deployed by: gravity (fail-safe), motor-driven from smoke detection signal
• Drop time: typically 30-60 seconds to full deployment
• Rating: per EN 12101-1 (D class for ambient smoke barriers, DH class for hot smoke)

Fire shutters:

• Steel or composite rolling/sliding shutters providing fire separation
• Rating: 1-4 hours (integrity); some provide insulation rating
• Deployed automatically on fire alarm; manual override required
• Used where fire doors are impractical (wide openings, service counters, conveyor openings)
• Activation: fusible link (thermal), fire alarm signal, or both

Atrium Smoke Control

Atriums require specific smoke control analysis due to large interconnected