Part B1 Structural provisions
BP1.1 Structural Reliability
A building or structure, during construction and use, with appropriate degrees of reliability, must—
perform adequately under all reasonably expected design actions; and
withstand extreme or frequently repeated design actions; and
be designed to sustain local damage, with the structural system as a whole remaining stable and not being damaged to an extent disproportionate to the original local damage; and
avoid causing damage to other properties, by resisting the actions to which it may reasonably expect to be subjected.
The actions to be considered to satisfy (a) include but are not limited to—
permanent actions (dead loads); and
imposed actions (live loads arising from occupancy and use); and
wind action; and
earthquake action; and
snow action; and
liquid pressure action; and
ground water action; and
rainwater action (including ponding action); and
earth pressure action; and
differential movement; and
time dependent effects (including creep and shrinkage); and
thermal effects; and
ground movement caused by—
swelling, shrinkage or freezing of the subsoil; and
landslip or subsidence; and
siteworks associated with the building or structure; and
The Objective of this Part is to—
safeguard people from injury caused by structural failure; and
safeguard people from loss of amenity caused by structural behaviour; and
protect other property from physical damage caused by structural failure; and
safeguard people from injury that may be caused by failure of, or impact with, glazing.
The Objective is based on the belief that people should not be subject to risk of injury from a building suffering structural failure—BO1(a). Nor should there be any amenity loss caused by structural behaviour—BO1(b). Additionally, other property should not be at risk of physical damage caused by structural failure—BO1(c) and people should be safeguarded from injury due to failure or impact with glazing —BO1(d).
The term “structural behaviour” as used in BO1(b) can describe deflections, creep, vibration, settlement and the like. Problems with structural behaviour fall short of actual structural failure.
A building could have excessive deflection of a window lintel which causes the glass to shatter. This could interfere with the building’s use without causing it to collapse.
“Loss of Amenity” refers to the loss of a person’s ability to use a building in the manner intended.
Structural deflections could cause a building’s doors to stick, and thus detract from a person’s ability to move about the building.
A building or structure is to withstand the combination of loads and other actions to which it may be reasonably subjected.
Glazing is to be installed in a building to avoid undue risk of injury to people.
Glazing in a building should not cause injury to people due to its failure or people impacting with it because they did not see it.
BP1.1 consists of two parts:
- performance attributes that a building is required to have; and
- a list of actions to be considered in association with these attributes.
BP1.1(a) uses the term "with appropriate degrees of reliability” which can be judged with due regard to the possible consequences of failure and the expense, level of effort and procedures necessary to reduce the risk of failure. The measures that can be taken to achieve the appropriate degree of reliability include:
- choice of a structural system, proper design and analysis;
- implementation of a quality policy;
- design for durability and maintenance; and
- protective measures.
Degrees of reliability of structural elements can be quantified in terms of probabilities of failure with the use of probabilistic models for actions and resistances.
BP1.1(a)(i) is concerned with the serviceability limit states of buildings in terms of local damage, deformation and vibration. Expected actions are actions with high probabilities of occurrence. The acceptable level of serviceability is subjective. The design for serviceability depends to a large extent on professional judgement. The risk of serviceability failure is, historically, of the order of 10-1 to 10-2.
BP1.1(a)(ii) is concerned with the ultimate limit states of buildings in terms of strength and stability. Extreme actions are actions with low probability of occurrence. Repeated actions are actions, with high frequencies of occurrence in a given time period, that may cause fatigue or other cumulative failures. The notional probability of failure of structural elements is of the order of 10-3 to 10-4 for a 50 year reference period. The probability of structural failure is historically of the order of 10-6 per year.
BP1.1(a)(iii) is concerned with consequences of unspecified actions and is often referred to as "structural robustness". It includes, but is not limited to, progressive collapse. Ways to improve structural robustness include providing redundancies, minimum resistances, protective measures, etc.
BP1.1(a)(iv) is concerned with damage to other properties, which may be caused by reasons other than structural if BP1.1(a)(i) to (iii) are met.
List of actions
BP1.1(b) lists actions to which a building “may reasonably be subjected”. All possible actions cannot be listed. “Engineering judgement” may need to be used to determine all likely actions and in accessing the likely effects of those actions.
Buildings and structures should be able to withstand the effects of wind, rain or snow. However, they would not be expected to withstand the impact of a crashing aeroplane.
BP1.1(b)(xiv) uses the defined term “construction activity actions”. The term only refers to construction activities that may have an effect on the final design such as stacking or propping. The safety of the building during construction is normally controlled by occupational health and safety authorities.
BP1.2 Structural resistance
The structural resistance of materials and forms of construction must be determined using five percentile characteristic material properties with appropriate allowance for—
known construction activities; and
type of material; and
characteristics of the site; and
the degree of accuracy inherent in the methods used to assess the structural behaviour; and
action effects arising from the differential settlement of foundations, and from restrained dimensional changes due to temperature, moisture, shrinkage, creep and similar effects.
BP1.2 states the principles for the determination of the structural resistance of materials and forms of construction.
It should be noted that the construction activities referred to in BP1.2(a) may be more than those contained in the defined term of “construction activity actions”. For example, welding of structural steel might cause distortion or change the characteristics of the steel, and hence need to be accounted for. For this reason, the defined term has not been used in BP1.2(a).
BP1.3 Glass installations at risk of human impact
Glass installations that are at risk of being subjected to human impact must have glazing that—
if broken on impact, will break in a way that is not likely to cause injury to people; and
resists a reasonably foreseeable human impact without breaking; and
is protected or marked in a way that will reduce the likelihood of human impact.
Glazing in a building is not always readily visible to all people. It is therefore important to avoid human impact where possible. This may not always be possible. BP1.3 therefore contains three parts:
- if glazing is broken due to human impact, it must fail in so that small pieces will not cause injury to people (BP1.3(a));
- if human impact could occur, the glazing should be of a strength to resist that impact without breaking (BP1.3(b)); and
- to make it more visible, glazing should be marked with a motive or the like ((BP1.3(c))
BP1.4 Buildings in flood areas
The actions and requirements to be considered to satisfy (a) include but are not limited to—
flood actions; and
elevation requirements; and
foundation and footing requirements; and
requirements for enclosures below the flood hazard level; and
requirements for structural connections; and
material requirements; and
requirements for utilities; and
requirements for occupant egress.
BP1.4 only applies to buildings in which people are likely to sleep that are located in a flood hazard area, i.e. a Class 2 or 3 building or a Class 4 part of a building, a Class 9a health-care building or a Class 9c building. A flood hazard area is determined by the appropriate authority (usually the relevant local government) as an area to be affected by flood. The determination is usually via a planning instrument. It is important to note that the NCC provision does not override a provision in a planning instrument which may restrict development in a flood hazard area.
BP1.4 states the principles for the design and construction of the specified buildings in a flood event. The principles include preventing—
- the buildings suffering structural damage or collapse due to the hydrostatic effect (pressure of still water), or the hydrodynamic effect (force of moving water), or debris impact; and
- the buildings from being lifted off foundations or footings due to the buoyancy effect; and
- foundations and footings being affected by scour or erosion caused by moving water; and
- degradation of structural materials as a result of being immersed in water.
BV1 Structural reliability
This Verification Method is applicable to components with a resistance coefficient of variation of at least 10% and not more than 40%. For components with a calculated value less than 10%, then a minimum value of 10% must be used.
the capacity reduction factor ϕ satisfies—
ϕ ≤ Average (ϕG, ϕQ, ϕW,…),
ϕG, ϕQ, ϕW,… are capacity reduction factors for all relevant actions and must contain at least permanent (G), imposed (Q) and wind (W) actions; and
the capacity reduction factors ϕG, ϕQ, ϕW,… are calculated for target reliability indices for permanent action βTG, for imposed action βTQ, for wind action βTW, … in accordance with Equation 1—
= ratio of mean resistance to nominal; and
= ratio of mean action to nominal; and
Cs = correction factor for action; and
CR = correction factor for resistance; and
VS = coefficient of variation of the appropriate action as given in Table BV1.1; and
VR = coefficient of variation of the resistance; and
γ = appropriate load factor for the action as given in AS/NZS 1170.0; and
ϕ = capacity factor for the appropriate action; and
the annual target reliability indices βTG, βTQ, βTW,… are established as follows:
For situations where it is appropriate to compare with an equivalent Deemed-to-Satisfy product, a resistance model must be established for the equivalent Deemed-to-Satisfy product and βTG, βTQ, βTW must be calculated for the equivalent Deemed-to-Satisfy product in accordance with Equation 1. The target reliability indices βTG, βTQ, βTW,…thus established, must not be less than those given in Table BV1.2 minus 0.5.
For situations where it is not appropriate to compare with an equivalent Deemed-to-Satisfy product, the target reliability index β must be as given in Table BV1.2.
Type of action
Target reliability index β
Wind, snow and earthquake action
Application of Table BV1.2:
- Table BV1.2 is applicable for components that exhibit brittle failure similar to concrete as specified in AS 3600.
- For components with creep characteristics similar to timber as specified in AS 1720.1, the target reliability index for permanent action must be increased to 5.0.
- The above target reliability indices are based on materials or systems that exhibit creep or brittle failure characteristics similar to timber and concrete. Table BV1.2 may also be applicable to materials or systems that exhibit creep or brittle failure differently to steel, timber or concrete provided that the creep or brittle nature of the material or system are properly accounted for in the design model.
- The above target reliability indices are also applicable for materials or systems that exhibit ductile failure characteristics.
The resistance model for the component must be established by taking into account variability due to material properties, fabrication and construction process and structural modelling.
BV1 is a means to verify the structural reliability of a structural component or connection in order to meet the requirements of BP1.1 and BP1.2. For further guidance, refer to the ABCB Handbook for Structural Reliability.
BV2 Structural robustness
Compliance with BP1.1(a)(iii) is verified for structural robustness by—
assessment of the structure such that upon the notional removal in isolation of—
any supporting column; or
any beam supporting one or more columns; or
any segment of a load bearing wall of length equal to the height of the wall,
the building remains stable and the resulting collapse does not extend further than the immediately adjacent storeys; and
demonstrating that if a supporting structural component is relied upon to carry more than 25% of the total structure a systematic risk assessment of the building is undertaken and critical high risk components are identified and designed to cope with the identified hazard or protective measures chosen to minimise the risk.
BV2 is a means to verify the structural robustness of a building or structure in order to meet the requirements of BP1.1(a)(iii). For further guidance, refer to the ABCB Handbook for Structural Robustness.