Timber Construction: Fire Safety

The document covers fundamental fire safety aspects, including the natural combustion behavior of timber, fire resistance strategies, material comparisons, and holistic fire safety engineering concepts.

Fire Safety and Fire Resistance

Fire safety in timber construction requires understanding both material properties and engineering strategies. Key concepts include fire resistance, natural combustion, and hybrid solutions combining timber with other materials.

Basic Definitions

• Fire Resistance (REI): Load-bearing (R), Integrity (E), and Insulation (I) properties of construction elements under fire exposure. Verification is typically denoted as REI 90/120, representing resistance for 90 to 120 minutes.

• Fire Safety Engineering: Strategies include controlling the fall-off of charring layers, preventing fire regrowth, and ensuring compartmentation with smoke detection and sprinklers.

Material Comparisons in Fire Safety

Different construction materials react differently under high temperatures:

Material	Flammability	Fire Resistance	Key Characteristics
Timber	Flammable	Natural combustion (0.8mm/min)	Self-insulating through charring
Steel	Non-flammable	Needs protection or cladding	Loses strength above 500°C
Concrete	Non-flammable	Spalling risk above 200°C	Compression strength loss at high temperatures (600°C)
Gypsum	Non-flammable	0.5mm/min charring	Maintains structure at lower temperatures

Temperature Influence on Steel and Concrete

• Steel loses 50% of its strength at 500°C, leading to rapid structural failure.

• Concrete undergoes spalling and loses compressive strength at temperatures above 600°C.

• Timber, however, maintains its load-bearing capacity for extended periods due to the insulating char layer.

Natural Combustion and Fire Resistance

Wood combustion creates a charred layer that serves as insulation, keeping the inner core cool and maintaining load-bearing capacity. The charring rate varies with the wood species and type:

Wood Species	Combustion Rate (mm/min)
Softwood & Beech	0.8
Engineered Wood	0.7
Oak	0.5
Massive Cladding	0.9

Fire Resistance of Engineered Wood Products

Engineered wood products like Cross-Laminated Timber (CLT) and Glued Laminated Timber (GLT) maintain fire resistance through controlled charring and predictable burn rates. The first layer of CLT chars, while subsequent layers maintain structural integrity.

Hybrid Solutions

Timber-concrete hybrids leverage the insulating properties of wood and the compressive strength of concrete. CLT-concrete hybrids offer fire resistance while providing additional structural support and insulation.

North American Equivalent to REI

In North America, the equivalent to the European REI classification system (which stands for Resistance, Entity, and Insulation) is the ASTM E119 and UL 263 standards for fire resistance ratings. These standards specify the fire resistance duration of building elements (in hours), but the way they are defined and tested differs somewhat from the European approach.

Breakdown of Equivalents:

1. R (Load Bearing Capacity) - Measures the ability of a structure to maintain its load-bearing function under fire exposure. In North America, this is generally referred to as the "Structural Integrity" during a fire test (ASTM E119/UL 263).

2. E (Integrity) - Measures the ability to prevent the passage of flames and hot gases through a separating element (like a wall or floor). In North America, this is typically measured by the "Fire Separation" rating.

3. I (Insulation) - Measures the ability to limit the temperature rise on the non-exposed side of a construction element. In North America, this aspect is covered as part of the "Temperature Rise" criteria in ASTM E119.

Example Comparison:

• REI 90/120 (Europe) - This means that the structure has a fire resistance rating of 90 to 120 minutes, maintaining load-bearing capacity (R), integrity (E), and insulation (I) for that duration.

• 1-hour or 2-hour fire rating (North America) - This means the element can withstand fire exposure for 1 or 2 hours, maintaining structural integrity and limiting temperature rise as specified in ASTM E119.

While the European REI system explicitly breaks down the three aspects of fire resistance, the North American standards often bundle these criteria together, focusing on the overall fire resistance time and structural performance.

Holistic Fire Safety Engineering

Holistic fire safety design considers the complete building lifecycle, integrating prevention, detection, and resistance measures. Key strategies include:

• Risk Analysis: Identifying potential hazards and weak points.

• Integrated Design: Accounting for fire resistance and safety in early planning stages.

• Hybrid Approaches: Utilizing combinations of timber and concrete for optimal performance.

Tested Performance

Fire resistance testing has demonstrated that:

• Natural timber constructions can withstand fire for up to 90 minutes.

• Hybrid timber-concrete systems can achieve fire resistance of up to 300 minutes (5 hours).

• Timber elements provide structural, insulation, and combustion management, making them multifunctional in fire scenarios.

Conclusion

Timber is a safe, efficient, and sustainable material for construction, particularly when holistic fire safety engineering principles are applied. Integrating hybrid solutions and understanding natural combustion behavior ensures that timber remains a viable and resilient option for modern building designs