Fire safety in timber buildings

Timber is used in many types of residential, commercial, and industrial construction. Fire safety is essential in all these buildings, regardless of the materials.

Because wood burns, many people assume that all timber buildings are at a higher risk of fire damage with poor fire behaviour. However, timber buildings can be designed with excellent fire safety for the protection of the occupants and any adjacent properties.

Both the type of timber and how it is used dictate what needs to be done to ensure that timber buildings are as fire-resistant and safe as possible. Timber structures tend to fall into two distinct categories: mass timber structures and light timber structures.

• Mass timber structures are those where the principal structural elements are beams, columns, or panels made from sawn timber, glue laminated timber (glulam), laminated veneer lumber (LVL), or cross laminated timber (CLT). Mass timber buildings have inherent fire resistance because surface charring of the wood allows an insulating layer to form, providing protection to the underlying timber.
   
• Light timber framing consists of timber stud and joist construction, typical of New Zealand house construction. In light timber framed structures, appropriate protective lining materials (e.g. gypsum plasterboards) are used to provide excellent fire resistance.

Construction management to minimise fire risk during construction (before fire protection coatings, plasterboard, or sprinklers are installed) is critical for large timber construction of light frame or mass timber.

Designing buildings for fire safety is a complex topic requiring the integration of a large number of requirements, and an understanding of the basic principles of fire behaviour. 

A threat to life can occur in the early stages of a fire, when flames spread rapidly on combustible surfaces, or content causes occupants to be trapped or overcome by smoke. 

At later stages of a fire, fire resistance is required to prevent fire spread or structural collapse, which could threaten occupants or firefighters elsewhere in the building or in adjacent buildings.

Figure 1 shows the time temperature curve for a fire developing in a typical room with no firefighting intervention. Table 1 shows the significance of the periods in Figure 1.

The transition between the growth period and the burning period is known as flashover when temperatures rise suddenly and all exposed combustible materials ignite and burn. The gas temperatures in the burning period may be up to 1000°C. The duration of the burning period, often referred to as a fully developed fire, is limited by the amount of fuel and the available ventilation through broken windows and other openings. 

Once most of the available fuel has been consumed, the fire enters the decay period, and temperatures start to drop. Burnout is the time when the compartment temperatures revert to near ambient temperatures. 

In timber buildings designed for burnout, it is important to have firefighter intervention at the end of the fire, to extinguish any smouldering combustion in exposed or protected timber. 

Most designs require building elements to be provided with fire resistance (of 30 or 60 minutes) to prevent the spread of fire and to ensure structural stability in the burning period of a fully developed fire. 

Fire resistance is normally determined through full-scale testing by exposing a test specimen to the time temperature curve shown in Figure 2. Calculations are able to replace some fire resistance tests, but some testing will always be required. 

A comparison of Figures 1 and 2 shows that the standard fire resistance test is not the same as a real fire because the time-temperature curve is different, and there is no decay period. However, fire resistance is a useful tool for designers, code officials, and material suppliers to ensure that a certain level of fire safety is provided. 

Three-pronged fire resistance

If a building is to be effective in fire resistance, it must meet three criteria: structural adequacy, integrity, and insulation—always specified in that order.

• To meet the structural adequacy criterion, a structural element must carry applied loads for the duration of the test fire without structural collapse. Learn more about structural fire resistance in the sections on charring and structural resistance below.
   
• The integrity criterion is intended to test the ability of a barrier to contain a fire to prevent fire from spreading from the room of origin. To meet this criterion, the specimen must not develop any cracks or fissures which allow smoke or hot gases to pass through the assembly.
   
• The insulation criterion is intended to test the ability of a barrier to contain a fire to prevent fire from spreading from the room of origin. To meet this criterion, the temperature of the cold side of the fire barrier must not exceed an average temperature increase of 140°C or a local maximum increase of 180°C.

Table 2 shows which criteria must be achieved (shown with an X) for different construction elements. Note that traditional fire-resistant glazing need only meet the integrity criterion because it is not load bearing and it cannot meet the insulation criterion, but some new glazing systems can also provide insulation. 

As an example, a typical load bearing wall may have a fire resistance rating (FRR) of 60/60/60, which means a one-hour rating for structural adequacy, integrity, and insulation. If the same wall is non-load bearing, the FRR would be - /60/60. 

A fire door with a glazed panel may have a FRR of - /30/ - , which means that this assembly has an integrity rating of 30 minutes, with no fire resistance for structural adequacy or insulation.

Fire resistance is normally obtained through full-scale fire resistance testing at an accredited fire laboratory. Most international fire resistance tests are very similar, so it is often possible to have the result of an overseas fire test be accepted in New Zealand, with an expert opinion from an appropriate organisation.

Timber can be ‘self-insulating’

Did you know that, under the right conditions, large pieces of timber are self-limiting in how they burn during a fully developed fire? When large timber members are exposed to a severe fire, the wood decomposes into pyrolysis gases, which react with oxygen to generate flames, heat combustion products and a solid char layer. The layer of charred wood helps to insulate the solid wood below. In many cases, the char layer does not burn away because there is insufficient oxygen in the flames near the surface for oxidation of the char to occur. During the decay period of the fire, when oxygen levels increase, oxidation of char can result in smouldering combustion and some continuing erosion of the char thickness. 

The layer of heated wood below the char layer is about 35 mm thick, and the inner core remains at ambient temperatures. The heated wood above 200°C is known as the pyrolysis zone because this wood is undergoing thermal decomposition into gaseous pyrolysis products, accompanied by loss of weight and strength, as shown in Figure 3. For structural calculations, the heated wood below the char layer can be accounted for with a zero-strength layer, which is specified as 7.0 mm thick in AS/NZS 1720.1.

In New Zealand, the Building Act specifies legal requirements for fire safety, referring to Clauses C1-C6, Protection from Fire, in the New Zealand Building Code (NZBC) in relation to building fire safety. The Building Code provides goals, objectives, and performance requirements as shown conceptually in Figure 4. It can be seen that there are three alternative approaches to demonstrating fire safety, all of which must be carried out by suitably qualified professional fire engineers.

• Prescriptive design follows the Acceptable Solution C/AS1 or C/AS2. These acceptable solutions provide sets of rules that are used to meet the requirements of the NZBC for most simple building designs.
   
• The approved calculation method is set out in the Verification Method C/VM2, Framework for Fire Safety Design. Among other requirements, C/VM2 allows the use of a “time equivalent formula” to calculate the duration of standard fire exposure, which is equivalent to a burnout of all the fuel in a firecell. This time equivalent formula was developed for steel structures, so it may be less accurate when used for timber structures.
   
• A full performance-based design, or specific fire engineering design, can be offered as an Alternative Solution to meet the requirements of the NZBC. Some critical applications, such as tall timber buildings, may require performance-based design. Design for complete burnout in a timber building may not be possible without guaranteed sprinkler control or some level of firefighter intervention.