AS NZS 1720.4:2019 Timber structures
2.1  General
This section sets out a method for determining the structural adequacy, integrity and insulation of timber elements for a nominated fire resistance period (FRL or FRR). It specifies the method used to calculate the residual cross-section for a timber element following notional charring of the surfaces that have been exposed to a standard fire test for the nominated period
The minimum width or thickness of any timber element before charring shall be 75 mm.
NOTE  There are limitations to char methods when applied to fire-separating barriers where charring of the barrier may result in deflection and gaps forming. In this case, char analysis may not be sufficient in isolation and the integrity and insulation performance should be verified by testing or an advanced calculation method which incorporates these effects.
2.2  Structural adequacy
The combination of actions applied to the structural element during the nominated fire resistance period shall be determined in accordance with AS/NZS 1170.0.
The strength, stability and deformation of the residual portion of the element shall be determined using a standard fire test or calculated in accordance with AS 1720.1, assuming a load duration of 5 h. These calculations shall allow for any loss of structural restraint due to charring of the member under consideration, charring of restraining elements exposed to fire, or loss of strength of fire-affected connections.
NOTE  There are no deflection limits for structural adequacy.
The procedure to determine the residual cross-section after the nominated fire resistance period for use in structural adequacy checks shall be as follows:
(a) Determine the notional charring rate for the timber species in accordance with Clause 2.5.
(b) Determine the effective depth of charring in accordance with Clause 2.6.
(c) Determine the extent of the residual section based on the effective depth of charring in accordance with Clause 2.7.
2.3  Integrity
The integrity of a building element shall be determined using a standard fire test.
NOTE  There are no deflection limits when calculating the fire resistance of structural elements, however,the design of a structural separating element such as a load-bearing wall or a floor-ceiling assembly needs to consider whether large deflections or other deformations will lead to an integrity failure.