AS NZS 1720.4:2019 pdf download

07-27-2021 comment

AS NZS 1720.4:2019 pdf download.Timber structures Part 4: Fire resistance of timber elements.
Section 2 Design method
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.
2.4 Insulation
The insulation of a building element shall be determined using a standard fire test or by calculation of the residual cross-section after the nominated fire resistance period.
Where the calculation of the effective residual cross-section is used, and after the nominated fire resistance period, the minimum residual width or thickness of an element providing separation shall be 23 mm when calculated in accordance with Clause 2.7. NOTE I This minimum is based on empirical data for the heat affected region below the char layer and where gaps have no effect. NOTE 2 The above calculation assumes the calculation achieves the minimum thickness of 30 mm when the 7 mm of uncharred timber is accounted for.
2.7 Determining the effective residual section
The effective residual section shall be determined by subtracting the calculated effective depth of charring dc from all fire-exposed surfaces of the timber element, as shown in Figure 2.7. When determining the effective residual section of rectangular elements, corner charring shall be ignored as shown in Figure 2.7. Charring of solid timber floors shall be in accordance with Clause 9.5.4 of NZS 3603:1993. For hollow timber box beams with interior voids that will not be exposed to fire, the effective residual section shall be determined in the same way as for a solid section.
2.8 Barrier junctions
If an unprotected timber element is incorporated in a fire-resisting barrier and the unprotected timber element is detailed to prevent gaps from forming at the junction of the barrier and the timber element, the effect of the barrier junction on the effective residual section when determining fire resistance for structural adequacy shall be taken into account. NOTE The effect of the barrier junction on the effective residual section is shown in Figure 2.8.
Section 3 Assessment of joints with metal connectors
3.1 Unprotected connectors
The fire resistance for structural adequacy of joints fabricated with unprotected metal connectors shall be established by test in accordance with a standard fire test. In the absence of test information, the fire resistance for structural adequacy of joints fabricated with unprotected metal connectors shall be regarded as negligible.
3.2 Protected metal connectors
3.2.1 General The fire resistance for structural adequacy of metal connectors may be improved by the means of protection given in Clause 3.2.2. Clause 3.2.3. or Clause 3.2.4.
3.2.2 Embedding Where protection is provided by means of embedding, the metal connectors shall be fully embedded within a timber element to a depth equal to the calculated effective depth of charring in accordance with Clause 2.6. as shown in Figure 3.2.2. The residual holes resulting from such embedment shall be filled by timber plugs glued into place.AS NZS 1720.4 pdf download.

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