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Earthquake Issues

Mason Reed explains what is involved in a structural assessment for earthquake prone buildings and the process for remediation.

By: Mason Reed

1 April 2019

Earthquake-prone buildings form a significant portion of the existing commercial and industrial building stock in New Zealand. Some buildings did not perform well during recent earthquake events (for example, the CCTV building in Christchurch). As such, earthquake resilience of buildings in New Zealand is a hot topic. Building owners and businesses are being reminded of their social responsibility to provide a safe environment to the building users.

The New Zealand Building Act 2004 provides regulations relating to the assessment and identification of “earthquake prone buildings”. (More information relating to the Act can be found on the following website: https:// www.building.govt.nz/managingbuildings/ managing-earthquake-pronebuildings/ how-the-system-works/).

The principal objective in the Act is the preservation of life. The Act focuses on the most vulnerable buildings, in terms of public safety, and typically applies to non-residential buildings.

For consulting engineers, this essentially means that any nonresidential building that has been identified by the local council as being potentially “earthquake prone” will need to be assessed, and the building assigned a rating. This assessment should identify the critical structural elements under seismic loading and determine the “percentage of New Building Standard” (%NBS) for the structure.

The Act provides set time frames for commercial building owners to assess their buildings, which vary depending on the relative seismic risk of the region. Territorial Authorities will generally notify building owners as to the requirement for an earthquake prone assessment, and will provide the timeframe for the work to be completed. Methods of structural assessments are either of the following:

“ISA” (Initial Seismic Assessment), is the most simplistic, cheap, and rapid assessment method/tool providing a generic rating to a building, based on known mechanisms of potential structural weaknesses. Typically, ISA’s are a qualitative assessment, which may include some simplistic calculations. These are generally appropriate to first scoping out whether a building is earthquake prone/earthquake risk, and may be suitable for purposes such as obtaining finance to purchase a property.

“DSA” (Detailed Seismic Assessment), is a detailed analysis of a building and its critical parts, and provides much more useful and accurate information. The level of assessment and investment is higher than an ISA, and is both qualitative and quantitative. Much of the design calculations and assessment can be utilised in any required strengthening design, so investment in this process can provide cost and time savings later on.

Depending on the use of the building, and the resulting calculated %NBS, the building may require earthquake strengthening to bring the building up to an appropriate standard. Should the building require strengthening, we recommend an initial “concept strengthening” design scheme be developed, in order to provide the asset owner with enough information to make commercial decisions about how they want to proceed. At this time, it is prudent for the designer to take on feedback from the client and their contractor as to the likely costs, outcomes and practicalities of the proposed strengthening scheme.

Earthquake strengthening projects need to give careful consideration to the client and/or building users’ operational requirements, to minimise business disruption and impacts on any businesses. Upfront discussions, and in-depth understanding of the client’s requirements are paramount for any engineer. For commercial multi-units with different titles, an early discussion with the adjoining unit owners around assessing the building as a whole, and apportioning costs is strongly recommended where possible.

Early engagement and “buy in” for the proposed strengthening scheme with a contractor will typically yield construction efficiencies and cost savings for the project. This approach should be promoted, particularly for strengthening works, as it is a proven process for a successful project. In following columns for this magazine, we will touch on the notion of “resilience” in design, and the benefits around utilising this as a performance measure, over and above the minimum regulatory requirements, so as to ensure that buildings are able to be returned to full “functionality” soon after any major earthquake event.

Withers Tsang & Co Phone 09 376 8860, www.wt.co.nz YOUR ACCOUNTANT NICK ASHFORD ‘Is it worth changing your residential rental to short term accommodation?’ asks Nick Ashford. THE COUNCIL CONUNDRUM – Fraser Thomas is a multi-discipline engineering and land surveying firm. We have offices in Auckland (Highbrook and Mt Eden) and also an office in Christchurch. We are soon opening offices in Hawke’s Bay and Queenstown. We offer a “one-stop shop” for engineering and land surveying services required for building and land developments. This year the company is celebrating 50 years in business. If you require any engineering or land surveying services, call Mason on 021 979296 to discuss your project.

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