Fire resistance

Charring rate

Calculation based on charring rate for heavy timber sections

NZS 3603 includes a calculation method for fire resistance of exposed timber members is based on the charring rate.

The method applies to sawn timber, glulam and LVL members whose least dimension is at least 90 mm.

After deducting the depth of charring on the exposed surface, the residual section is required to support the fire design loads specified in the Structural Design Actions Standard AS/NZS 1170.

The specified charring rate is 0.65 mm per minute for solid timber or glulam.

Testing at the University of Canterbury has resulted in a proposed charring rate of 0.72 mm per minute for LVL.

Residual cross section of LVL beam after fire exposure.

Source: AH Buchanan

Fire testing of glulam beams - removal from the furnace after the test.

Source: BRANZ

Joints

Joints between timber members

Fully glued timber to timber joints using thermosetting resin adhesives have similar fire performance as solid timber, with charring occurring at a predictable rate on exterior surfaces.

Narrow gaps, less than 5 mm wide between timber members where there is no possibility of hot gases flowing through the gap does not significantly increase the rate of charring through the section.

Intumescent compounds should be used wherever possible in construction gaps.

Steel brackets used for location purposes only do not need additional fire protection.

Components which are stressed under load, such as brackets and bolted or nailed gusset plates should be protected with insulating materials as necessary to achieve the required fire resistance rating.

Source: AH Buchanan

Insulation

Insulating materials that are resistant to heat can be used to protect timber members or joints between timber members, and can be used to delay the onset of char, and therefore extend the fire resistance.

Insulation will reduce the rate of temperature increase of steel fasteners including steel plates, bolts, and nails.

The principal insulating materials available are solid timber, gypsum plasterboard, calcium silicate board and intumescent paint.

Gypsum plaster is particularly useful as a fire resisting material because of the energy required to drive off the water of crystallisation within the material.

Source: Timber Design Guide, 2007.

Frame walls

Timber framed walls and partitions consisting of vertical studs lined with gypsum plaster board have excellent fire resistance, and may be used wherever fire-resisting partitions or fire walls are required.

The lining thickness, method of fixing and stopping, spacing of studs and nogs (dwangs) all affect the fire resistance, and construction must follow the approved specification.

Non-load-bearing timber stud and plasterboard lined partitions have approved fire ratings of up to 4 hours, while load bearing timber stud walls have approved ratings up to 2 hours.

Designers must refer to lining manufacturers’ brochures for specifications and ratings of approved assemblies.

Source: Timber Design Guide, 2007.

Stopping & Detailing

Any fire resisting barrier is only as good as its weakest link.

Any recesses or penetrations must be constructed such that the required fire performance is still achieved.

Whenever alterations are carried out in timber buildings, it is essential that fire resisting linings remain intact or be reinstated to the same standard as before the alteration.

All fire rated timber construction must continue to perform its function in a fire.

This requires carrying the applied loads and not permitting the spread of fire for the required periods.

Careful detailing and sound workmanship is necessary to achieve the integrity required to prevent spread of fire.

Fire stopping in timber frame construction is normally achieved by providing a minimum of 50 mm nominal thickness of timber immediately behind the lining materials at each floor and ceiling level.

A large number of proprietary materials such as intumescent paints and putties, fireproof mastic, and ceramic fibre blankets and ropes are also available for sealing openings.

They must be used following the manufacturers' installation instructions.

Source: Timber Design Guide, 2007.

Connectors

Fire performance of connectors

For timber structural members to continue to perform their function when exposed to a fire, the timber connections between members must be considered in addition to the timber sections alone.

In some cases the connection is the critical component and may contribute to early failure of the structural system for example, steel nail plates.

Other types of connections are less vulnerable, such as the bearing support of a beam fixed to the plate of a wall.

The behaviour of metal fasteners depends on the temperature of the metal, as strength will be lost as the temperature rises.

Metal also conducts heat more rapidly than wood leading to charring of the timber in contact with the metal.

The fire performance of the following types of connectors is detailed in the PDF below:

• Nails, screws and nailed plate connectors.

• Bolted connections.

• Tooth plate connectors.

• Steel shear plate and split ring connectors.

Fire performance of connectors - Printable PDF