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2.2

Part 2.2 Structural provisions

Part 2.2 Structural provisions

Part 2.2 need not be complied with if, for the purposes of H1D2(b) only, the Deemed-to-Satisfy Provisions of H1D3 to H1D11 relating to structural elements are complied with.

The resistance of a building or structure must be greater than the most critical action effect resulting from different combinations of actions, where—

  1. the most critical action effect on a building or structure must be determined in accordance with 2.2.3 and the general design procedures contained in AS/NZS 1170.0; and
  2. the resistance of a building or structure is determined in accordance with 2.2.4.

Explanatory information

A building or structure must be designed to resist the most critical effect resulting from different combinations of actions, taking into consideration—

  • the probability of simultaneous occurrence of two or more actions; and
  • the levels of reliability of the structure when subject to combined actions; and
  • the characteristics of the action.

Determining the levels of reliability of the structure when subject to combined actions should be consistent with the levels of reliability implicit in the design events for natural phenomenon. When designing for the maximum combined actions, a principle frequently adopted is that the maximum is likely to occur when at least one of the actions is at its maximum value.

NCC Blurbs

The magnitude of individual actions must be determined in accordance with the following:

  1. Permanent actions:
    1. the design or known dimensions of the building or structure; and
    2. the unit weight of the construction; and
    3. AS/NZS 1170.1.
  2. Imposed actions:
    1. the known loads that will be imposed during the occupation or use of the building or structure; and
    2. construction activity actions; and
    3. AS/NZS 1170.1.
  3. Wind, snow and earthquake actions:
    1. the applicable annual probability of design event for safety, determined by—
      1. assigning the building or structure an Importance Level in accordance with Table 2.2.3a; and
      2. determining the corresponding annual probability of exceedance for safety in accordance with WA Table 2.2.3b; and
    2. for wind actions, AS/NZS 1170.2 or AS 4055, except where varied by WA Part 2.3; and
    3. for snow and ice actions, AS/NZS 1170.3; and
    4. for earthquake actions, AS 1170.4.
  4. Actions not covered in (a), (b) and (c) above:
    1. the nature of the action; and
    2. the nature of the building or structure; and
    3. the Importance Level of the building or structure determined in accordance with Table 2.2.3a; and
    4. AS/NZS 1170.1.
  5. For the purposes of (d) the actions include but are not limited to—
    1. liquid pressure action; and
    2. ground water action; and
    3. rainwater action (including ponding action); and
    4. earth pressure action; and
    5. differential movement; and
    6. time dependent effects (including creep and shrinkage); and
    7. thermal effects; and
    8. ground movement caused by—
      1. swelling, shrinkage or freezing of the subsoil; and
      2. landslip or subsidence; and
      3. siteworks associated with the building or structure; and
    9. construction activity actions.
Table 2.2.3a Importance Levels of buildings and structures
Importance Level Building types
1 Buildings or structures presenting a low degree of hazard to life and other property in the case of failure.
2 Buildings or structures not included in Importance Level 1.
Insert WA table (2.2.3b) 2.2.3 as follows:
WA Table 2.2.3b: Design events for safety – annual probability of exceedance
Importance Level Non- cyclonic wind Cyclonic wind other than wind Region D north of the Tropic of Capricorn Cyclonic wind in wind Region D north of the Tropic of Capricorn Snow Earthquake
1 1:100 1:200 1:250 1:100 1:250
2 1:500 1:500 1:1000 1:150 1:500
Figure 2.2.3 Wind regions
image-2-2-3-wind-regions.svg

Explanatory information: Permanent and imposed actions

Permanent actions include the dead loads of the building or structure. These include the load imposed by the building’s components inclusive of the forces imposed by the floors, walls, roofs, suspended ceilings, etc.

Imposed actions include live loads on the building or structure. These include the load arising from construction activity and the intended use or function of the building or structure.

Explanatory information: Application of AS 1170.4

There are certain limitations on the application to domestic structures such as Class 1a and Class 1b buildings in Appendix A of AS 1170.4. These limitations include building height, roof slope, etc. For additional information refer to Appendix A of AS 1170.4.

Explanatory information: Importance Levels (Table 2.2.3a)

Table 2.2.3a provides a generic description of building types to which Importance Levels have been assigned. The “Importance Level” concept is applicable to building structural safety only. More specific examples are provided in the following list. The examples are indicative and not exhaustive.

  • Importance Level 1: Isolated minor Class 10a buildings and Class 10b structures.
  • Importance Level 2: Class 1 buildings; Class 10a buildings and Class 10b structures associated with Class 1 buildings.

Importance Levels must be assigned on a case by case basis and relate to the hazards to human life and other property in the event of the structure’s failure. For example—

  1. Importance Level 1 is for minor isolated structures that rarely contain people, are not required as part of normal infrastructure and present a low risk to life and other property.
  2. Importance Level includes domestic housing and structures intended to contain reasonable numbers of people under normal operations.

Explanatory information: Construction in cyclonic areas

The intent of building construction in cyclonic areas (see Figure 2.2.3) is to ensure the structure has sufficient strength to transfer wind forces to the ground with an adequate safety margin to prevent collapse of the building and the building being lifted, or slid off its foundations.

To resist these forces it is necessary to have—

  • an anchorage system, where the roof is connected by the walls to the footings by a chain of connections; and
  • a bracing system to prevent horizontal collapse due to wind forces; and
  • continuity of the system where each structural element is interlocked to its adjoining structural element throughout the building.

Explanatory information: Anchorage

Anchorage of the system is achieved by using a variety of connectors. Each connector must be capable of carrying the uplift force, because the ability of the building to resist the wind forces is directly related to its weakest link.

Explanatory information

In Western Australia state variations apply to wind regions B and D, this includes wind region B2 as referenced in AS/NZS 1170.2.

The state variation for wind region B or B2 will ensure that designers consider the combination of peak external pressures and increased internal pressures in design of buildings and use a cyclonic (C) classification instead of non-cyclonic (N) classification. The definition of design wind speed is varied in WA Schedule 1 to identify that wind Region B is a C classification in Western Australia. Other changes have also been made to reflect this.

In addition to a variation to clause H1D7 of NCC Volume Two, and clauses 2.2.3 and 2.2.4 of the ABCB Housing Provisions Standard - a variation is made to the application of AS/NZS 1170.2 and AS 4055 (when used as a primary referenced document, secondary or subsequent referenced document). Refer to WA Schedule 2 and WA Part 2.3.

The state variation for wind region D applies only to those parts of region D located north of the Tropic of Capricorn. The 2021 edition of AS/NZS 1170.2 includes a reduction in design wind speeds for wind region D. The variation will retain similar design wind speeds for wind region D as the 2011 edition of AS/NZS 1170.2 previously referenced in the National Construction Code.

NCC Title

Determination of individual actions

NCC State
WA
NCC Variation Type
Replacement
NCC SPTC Current
Determination of individual actions

The magnitude of individual actions must be determined in accordance with the following:

  1. Permanent actions:
    1. the design or known dimensions of the building or structure; and
    2. the unit weight of the construction; and
    3. AS/NZS 1170.1.
  2. Imposed actions:
    1. the known loads that will be imposed during the occupation or use of the building or structure; and
    2. construction activity actions; and
    3. AS/NZS 1170.1.
  3. Wind, snow and earthquake actions:
    1. the applicable annual probability of design event for safety, determined by—
      1. assigning the building or structure an Importance Level in accordance with Table 2.2.3a; and
      2. determining the corresponding annual probability of exceedance for safety in accordance with Table 2.2.3b; and
    2. for wind actions, AS/NZS 1170.2 or AS 4055; and
    3. for snow and ice actions, AS/NZS 1170.3; and
    4. for earthquake actions, AS 1170.4.
  4. Actions not covered in (a), (b) and (c) above:
    1. the nature of the action; and
    2. the nature of the building or structure; and
    3. the Importance Level of the building or structure determined in accordance with Table 2.2.3a; and
    4. AS/NZS 1170.1.
  5. For the purposes of (d) the actions include but are not limited to—
    1. liquid pressure action; and
    2. ground water action; and
    3. rainwater action (including ponding action); and
    4. earth pressure action; and
    5. differential movement; and
    6. time dependent effects (including creep and shrinkage); and
    7. thermal effects; and
    8. ground movement caused by—
      1. swelling, shrinkage or freezing of the subsoil; and
      2. landslip or subsidence; and
      3. siteworks associated with the building or structure; and
    9. construction activity actions.
Table 2.2.3a Importance Levels of buildings and structures
Importance Level Building types
1 Buildings or structures presenting a low degree of hazard to life and other property in the case of failure.
2 Buildings or structures not included in Importance Level 1.
Table 2.2.3b Design events for safety—annual probability of exceedance
Importance Level Non-cyclonic wind Cyclonic wind Snow Earthquake
1 1:100 1:200 1:100 1:250
2 1:500 1:500 1:150 1:500
Figure 2.2.3 Wind regions
image-2-2-3-wind-regions.svg

Explanatory information: Permanent and imposed actions

Permanent actions include the dead loads of the building or structure. These include the load imposed by the building’s components inclusive of the forces imposed by the floors, walls, roofs, suspended ceilings, etc.

Imposed actions include live loads on the building or structure. These include the load arising from construction activity and the intended use or function of the building or structure.

Explanatory information: Application of AS 1170.4

There are certain limitations on the application to domestic structures such as Class 1a and Class 1b buildings in Appendix A of AS 1170.4. These limitations include building height, roof slope, etc. For additional information refer to Appendix A of AS 1170.4.

Explanatory information: Importance Levels (Table 2.2.3a)

Table 2.2.3a provides a generic description of building types to which Importance Levels have been assigned. The “Importance Level” concept is applicable to building structural safety only. More specific examples are provided in the following list. The examples are indicative and not exhaustive.

  • Importance Level 1: Isolated minor Class 10a buildings and Class 10b structures.
  • Importance Level 2: Class 1 buildings; Class 10a buildings and Class 10b structures associated with Class 1 buildings.

Importance Levels must be assigned on a case by case basis and relate to the hazards to human life and other property in the event of the structure’s failure. For example—

  1. Importance Level 1 is for minor isolated structures that rarely contain people, are not required as part of normal infrastructure and present a low risk to life and other property.
  2. Importance Level 2 includes domestic housing and structures intended to contain reasonable numbers of people under normal operations.

Explanatory information: Construction in cyclonic areas

The intent of building construction in cyclonic areas (see Figure 2.2.3) is to ensure the structure has sufficient strength to transfer wind forces to the ground with an adequate safety margin to prevent collapse of the building and the building being lifted, or slid off its foundations.

To resist these forces it is necessary to have—

  • an anchorage system, where the roof is connected by the walls to the footings by a chain of connections; and
  • a bracing system to prevent horizontal collapse due to wind forces; and
  • continuity of the system where each structural element is interlocked to its adjoining structural element throughout the building.

Explanatory information: Anchorage

Anchorage of the system is achieved by using a variety of connectors. Each connector must be capable of carrying the uplift force, because the ability of the building to resist the wind forces is directly related to its weakest link.

NCC Blurbs

The following requirements, or any combination of them, must be used to determine the structural resistance of materials and forms of construction as appropriate:

  1. Earthworks: H1D3(1).
  2. Earth retaining structures: H1D3(2).
  3. Termite risk management: H1D3(3).
  4. Concrete construction (including slabs and footings, piled footings and reinforced and prestressed concrete structures): H1D4.
  5. Piled footings: H1D12.
  6. Post-installed and cast-in fastenings in concrete: AS 5216.
  7. Masonry (including masonry veneer, unreinforced masonry and reinforced masonry): H1D5.
  8. Steel construction (including steel framing and structural steel members): H1D6.
  9. Timber construction (including design of timber structures, timber framing and design of nail-plated timber roof trusses): H1D6.
  10. Composite steel and concrete: AS/NZS 2327.
  11. Aluminium construction:
    1. AS/NZS 1664.1.
    2. AS/NZS 1664.2.
  12. Roof construction (including plastic sheeting, roofing tiles, metal roofing and terracotta, fibre-cement and timber slates and shingles): H1D7.
  13. Wall cladding: H1D7.
  14. Glazed assemblies: H1D8.
  15. Barriers and handrails (including stairway and ramp construction):
    1. H5D3; and
    2. AS/NZS 1170.1 for the determination of loading forces on a barrier.
  16. Attachment of decks and balconies to external walls of buildings: H1D11.
  17. Garage doors and other large access doors in openings not more than 3 m in height in external walls of buildings determined as being located in wind region B, C or D in accordance with Figure 2.2.3: AS/NZS 4505.
  18. For high wind areas: requirements listed in (a) to (q) as appropriate or the Northern Territory Deemed to Comply Standards Manual.

Explanatory information

The weight of roof or ceiling insulation, particularly if additional ceiling insulation is used for compliance with the energy efficiency provisions, needs to be considered in the selection of plasterboard, plasterboard fixings and building framing.

NCC Title

Determination of structural resistance of materials and forms of construction

NCC State
WA
NCC Variation Type
Replacement
NCC SPTC Current
Determination of structural resistance of materials and forms of construction

The following requirements, or any combination of them, must be used to determine the structural resistance of materials and forms of construction as appropriate:

  1. Earthworks: H1D3(1).
  2. Earth retaining structures: H1D3(2).
  3. Termite risk management: H1D3(3).
  4. Concrete construction (including slabs and footings, and reinforced and prestressed concrete structures): H1D4.
  5. Piled footings: H1D12.
  6. Post-installed and cast-in fastenings in concrete: AS 5216.
  7. Masonry (including masonry veneer, unreinforced masonry and reinforced masonry): H1D5.
  8. Steel construction (including steel framing and structural steel members): H1D6.
  9. Timber construction (including design of timber structures, timber framing and design of nail-plated timber roof trusses): H1D6.
  10. Composite steel and concrete: AS/NZS 2327.
  11. Aluminium construction:
    1. AS/NZS 1664.1.
    2. AS/NZS 1664.2.
  12. Roof construction (including plastic sheeting, roofing tiles, metal roofing and terracotta, fibre-cement and timber slates and shingles): H1D7.
  13. Wall cladding: H1D7.
  14. Glazed assemblies: H1D8.
  15. Barriers and handrails (including stairway and ramp construction):
    1. H5D3; and
    2. AS/NZS 1170.1 for the determination of loading forces on a barrier.
  16. Attachment of decks and balconies to external walls of buildings: H1D11.
  17. Garage doors and other large access doors in openings not more than 3 m in height in external walls of buildings determined as being located in wind region C or D in accordance with Figure 2.2.3: AS/NZS 4505.
  18. For high wind areas: requirements listed in (a) to (q) as appropriate or the Northern Territory Deemed to Comply Standards Manual.

Explanatory information

The weight of roof or ceiling insulation, particularly if additional ceiling insulation is used for compliance with the energy efficiency provisions, needs to be considered in the selection of plasterboard, plasterboard fixings and building framing.

(1) Structural software used in computer aided design of a building or structure that uses design criteria based on the Deemed-to-Satisfy Provisions of NCC Volume Two and the ABCB Housing Provisions, including its referenced documents, for the design of steel or timber trussed roof and floor systems and framed building systems, must comply with the ABCB Protocol for Structural Software.

(2) The requirements of (1) only apply to structural software used to design steel or timber trussed roof and floor systems and framed building systems for buildings within the following geometrical limits:

  1. The distance from ground level to the underside of eaves must not exceed 6 m.
  2. The distance from ground level to the highest point of the roof, neglecting chimneys, must not exceed 8.5 m.
  3. The building width including roofed verandahs, excluding eaves, must not exceed 16 m.
  4. The building length must not exceed five times the building width.
  5. The roof pitch must not exceed 35 degrees.

(3) The requirements of (1) do not apply to design software for individual frame members such as electronic tables similar to those provided in—

  1. AS 1684 Parts 2, 3 and 4; or
  2. NASH Standard Residential and Low-Rise Steel Framing, Part 2.

Explanatory information

2.2.5 does not apply where a software package simply eliminates manual calculations and the process of the package requires identical methodology as that undertaken manually, e.g. AS 1684 span tables and bracing calculations.