The Building Act also includes provisions for existing buildings, with the intent that people can use buildings safely and the seismic performance of the building stock should improve over time. The Building Act does not require existing buildings to achieve compliance with the current Building Code. Upgrading of existing buildings is not required unless they are considered dangerous or earthquake-prone (Building Act sections 121 and 124).
However, new building work undertaken when the building is being altered needs to comply fully with the Building Code. The existing building should perform no worse than it did prior to the alterations taking place (Building Act section 112). The structural aspects of the existing building may therefore need to be upgraded if the building is undergoing a change of use (Building Act sections 114 and 115).
In terms of the legislative framework, earthquake strengthening is considered the same as standard building work, and all the same building control processes apply. Generally, strengthening work will be a specifically engineered design for each case. It is the responsibility of the building owner or designer to prove that all the requirements of the Building Code are met. This may be done using Verification Methods, peer review, physical tests or special analysis, for example.
Change of use
Every building or part of a building is deemed to have been built for an intended use, which relates to fire purpose groups – see Schedule 2 of the Building (Specified Systems, Change the Use and Earthquake-prone Buildings) Regulations 2005. The different uses relate to Fire Purpose Groups. The Building Act therefore has specific requirements when the intended use of a building is to change.
When a building changes its intended use, its structural performance must be upgraded if the Building Code requirements for the intended use are more onerous than before the change. This must be to a standard that is “as nearly as is reasonably practicable” to that of a new building that would be used for the same purpose, including earthquake resistance. The question of what is reasonably practicable is a trade-off between the advantages and disadvantages of upgrading the building to comply with any particular provision of the Building Code.
A building owner intending to change the use of an existing building must give written notice to the building consent authority. In order that building upgrade requirements are proportionate and not excessively costly for owners, building consent authorities need to balance issues of safety and community interests against the cost of improvements.
Building standards have developed considerably over the years, and many existing buildings fall short of the standards now required for new buildings. The objective of the Building Act regarding earthquake-prone buildings is to target the most vulnerable buildings and thereby reduce the level of risk to people.
To do this, the Building Act establishes the concept of earthquake-prone buildings and provisions to improve these buildings over time. The Building Act defines an earthquake-prone building as follows:
(1) A building is earthquake prone for the purposes of this Act if, having regard to its condition and to the ground on which it is built, and because of its construction, the building—
(a) will have its ultimate capacity exceeded in a moderate earthquake (as defined in the regulations); and
(b) would be likely to collapse causing—
(i) injury or death to persons in the building or to persons on any other property; or
(ii) damage to any other property.
(2) Subsection (1) does not apply to a building that is used wholly or mainly for residential purposes unless the building—
(a) comprises 2 or more storeys; and
(b) contains 3 or more household units.
The Building Act (section 122) defines a moderate earthquake as one that would generate shaking at the site of the building that is of the same duration as, but that is one-third as strong as, the earthquake shaking (determined by normal measures of acceleration, velocity and displacement) that would be used to design a new building at the site. This definition was introduced via regulations and came into effect in 2005.
In other words, current legislation is directed at improving existing buildings (with the exception of small multi-unit residential buildings) that are less than one-third of the strength required for a new building. This requirement is often referred to as the one-third rule. The definition is critical for making decisions about the need to carry out remedial work on existing buildings to reduce health and safety risks associated with the collapse of buildings from earthquakes.
New building standard
The one-third rule is also commonly described as “33 percent NBS” or 33 percent of the new building standard rule. The NBS refers to the full strength of the earthquake that would be used to design a new building on the same site under the current design requirements of the Building Code – the so-called design earthquake.
Before the modern Building Act came into effect in 2004, New Zealand legislation did include some provision for upgrading ‘earthquake risk buildings’ (section 66 of the Building Act 1991), although it was limited to unreinforced masonry and concrete buildings. These buildings typically perform poorly in New Zealand’s major earthquakes, as do early concrete and steel structures designed before the first seismic design code came into force in 1936 (NZS 95 Model Building By-Law).
Prior to 1936 and the introduction of this standard, wind loading was the only lateral force considered during the design of structures, and in many cases, no lateral forces were considered at all.
One-third is considered a reasonable balance between a requirement to upgrade all non-complying buildings and the previous position where only unreinforced masonry buildings were considered to pose a risk during an earthquake. The 33 percent NBS corresponds to approximately 20 times the risk of the building reaching a similar condition to that which a new 100 percent NBS building would reach in a full design earthquake. Given this risk, many believe strengthening to two-thirds NBS is reasonable and economically achievable for most buildings, although it exceeds the statutory minimum requirement.
The Building Act requires all local authorities to develop, consult on and publish a policy for managing earthquake-prone buildings in its region and to review this policy every 5 years. This policy should include information on how building work is prioritised and how the policy applies to heritage buildings. However, the Building Act does not stipulate the degree to which an earthquake-prone building should be strengthened, unless a change of use is also involved.
The Building (Earthquake-prone Buildings) Amendment Bill is currently being considered by the Local Government and Environment Select Committee. Once finalised, this legislation is likely to set the framework for strengthening of New Zealand’s existing building stock for the next generation.