Posted: 23 March 2017
The determination considers the assessment required under section 115 when there is a change of use to only part of a building, particularly in respect of Building Code clause B1 Structure, and whether a detailed seismic assessment (DSA) is required for the whole building.
The six-storey-high building is constructed predominantly of cast in-situ reinforced concrete, with concrete floor slabs supported on two-way reinforced concrete moment frames. The existing uses of the ground level under the Change of Use Regulations were classified as crowd activities. The building work proposed to change the use of the level to uses that are classified under sleeping activities.
An initial evaluation procedure (IEP) carried out by structural engineers reported the building was not potentially earthquake-prone and had a National Building Standard (NBS) of less than 67 per cent.
The building consent authority (BCA) refused to grant a building consent to convert the educational facility to six apartments, on the basis that it required a detailed seismic assessment for the entire building and proof that the building has a structural performance level of 80 per cent NBS or more.
The conversion of the ground level constitutes a change of use to which section 115(a) applies. The BCA’s approach was that the change of use required a structural assessment of the entire building.
In a previous determination (2015/070), where a change of use to the top level of a multi-level was proposed, the conclusion was reached that the levels below effectively acted as the foundation to that unit. It was determined that structural strength/serviceability of the levels below the subject unit needed to be included in the structural assessment to check if they were sufficient to meet the performance requirements of clause B1.
In this determination, the proposed change of use and alterations were limited to the ground floor only, and therefore the ground level and the foundations that support it, including the basement level, required assessment.
The determination discussed the ‘as nearly as is reasonably practicable’ (ANARP) test, concluding that the cost of a DSA and any work associated with the structural upgrade of the whole building would be disproportionate to the cost of the proposed building work. However, the IEP had identified that there was a lack of seismic gaps between the moment frames and block infill partition throughout the building and steps to address the lack of separation could be considered reasonably practicable.
The determination discussed the requirements of section 112(b) for the alterations that could be proposed. Section 112(b) requires that the building work carried out must not reduce the level of compliance of other parts of the building, and in this case installing seismic separation at the ground level but not at higher levels would introduce a non-uniformity of stiffness and concentrate earthquake deformation at the ground level. Therefore, the structural alterations would likely reduce the earthquake resistance of the building as a whole and this does not meet the test of section 112(b).
The determination found the BCA incorrectly exercised its powers of decision in refusing to grant the building consent on that basis of requiring a DSA for the whole building. The BCA’s decision was reversed, requiring a new decision be made taking into account the analysis set out in the determination.
Determination 2017/006 in full.
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