Fire Performance Notes

Fire Performance

Fire Behavior

• When wood is heated above its decomposition temperature, it converts into char and gases.
• Some gases are combustible and burn rapidly.
• With enough heat and oxygen, the char combusts slowly, producing embers.
• Evaluating fire behavior is complex due to the numerous factors influencing it at different levels (materials, structural elements, entire structures, and the specific fire scenario).
• Reaction to fire.
• Resistance to fire.

Fire Development Stages

• Ignition: Initial start of the fire.
• Growth: Fire begins to spread and increase in intensity.
• Flashover: A rapid transition to a state of total surface involvement in a fire.
• Burning: Sustained combustion.
• Decay: Fire decreases in intensity due to lack of fuel or oxygen.

Fire Behavior Factors

• Human Behavior: Influences detection, escape, and survival.
• Detection: Use of smoke and heat detectors.
• Active Control: Fire suppression methods like manual extinguishers, sprinklers, and fire service intervention.
• Passive Control: Strategies such as smoke control and managing flammability and surface flame spread.

Fire Stages and Control

• Growth (Fuel Controlled):
  • Focus is on flammability and surface flame spread.
  • Escape is critical.
  • Smoke and heat detectors are used.
  • Fire can be extinguished manually or via sprinklers.
  • Smoke control measures are important.
• Burning (Ventilation-Controlled):
  • External smoke and flame become significant.
  • Fire service manages control.
• Decay (Fuel-Controlled):
  • Emphasis on fire resistance, containment, and preventing collapse.

Pre-Flashover

• The primary fire load comes from the contents, not the structure itself.
• Wall and ceiling linings play a crucial role in fire spread.
• Important factors include ignitability, flame spread, heat release, and smoke production.
• Sprinklers are highly effective in this stage for life and property protection.
• Sprinklers can prevent fires from reaching flashover.
• However, complete reliance on sprinklers is not advisable.

Post-Flashover

• Fire resistance becomes the primary objective.
• Goals are to contain the fire and prevent structural collapse.

Fire Resistance of Wood

• Light timber requires protection, often with gypsum board.
• Heavy timber can be evaluated by calculating the residual section's capacity based on the charring rate.
• Connections need to be protected as necessary.

Timber and Fire

• Wood forms a char layer when exposed to fire.

Density and Combustion Rate

• Higher density wood or wood composites generally have a lower rate of combustion.

Timber Types

• Light timber framing.
• Heavy timber:
  • Sawn timber.
  • Glulam (glued laminated timber).
  • LVL (laminated veneer lumber).
  • Minimum thickness of 90mm.

Fire Design Equations

• Involve calculations for:
  • Residual sections.
  • Duration of the fire event.
  • Loss of load-bearing capacity.
  • Resistance of a timber member in fire.

Residual Section Calculation

• Considers the radius of arris rounding, calculated charring line, and calculated depth of charring.
• The radius of arris rounding ($r$) equals the calculated depth of charring.
• Area lost due to rounding: $A = 0.215r^2$
• The center of gravity of this area lies at a distance of $y = 0.223r$ from either side.

Charring Thickness Calculation

• Formula: $t<em>c = β</em>0 * t_{f,req}$
  • $t_c$ = thickness of charring.
  • $t_{f,req}$ = required standard fire resistance in minutes.
  • $β_0$ = charring rate (0.65 mm / minute for Radiata Pine).
• Example: For a 30-minute Fire Resistance Rating (FRR), $t_c = 0.65 * 30 = 19.5$ mm.

Structural Integrity During Fire

• The residual cross-section must be sufficient to support fire loads without exceeding design strengths.
• Example: An 180x540 wooden beam with a 60-minute fire resistance rating is expected to lose 39 mm on each exposed side.

Residual X-Section

• The remaining cross-section (e.g., 102x501) must resist loads present during the fire (e.g. 60 minutes).

Fire Protection of Connections

• Connections are protected if fasteners are covered with protective plugs or wood/wood-based panels with a minimum thickness of $t_c$.

Key Points

• The timber structure's contribution to the overall fire load is usually small.
• The charcoal layer formed on the surface slows down the burning rate.
• Fire resistance increases with the structural size of the section.
• Glulam and LVL have a fire performance comparable to solid wood sections.
• Heavy timber members can often be repaired after a fire.