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ISA, DSA, and Seismic Strengthening

Design Engineering has experience in Earthquake/Seismic Assessment – Initial Seismic Assessments and Detailed Seismic Assessments for various buildings throughout New Zealand. We has seismic assessment experience in steel portal frame industrial buildings and poly panel industrial buildings.

ISA – Initial Seismic Assessment

What is the Initial Seismic Assessment (ISA)?

A seismic assessment carried out in accordance with Part B of the Engineering Assessment Guidelines.

An ISA uses the buildings age, building materials, seismic zone, subsoil class, building importance level, adjacent buildings and site characteristics to determine if a building is potentially earthquake prone and offer a seismic grade and range of %NBS

An ISA may not be worthwhile for older buildings or buildings with limited information on file. A DSA is recommended for such a building.

DSA – Detailed Seismic Assessment

What is the Detailed Seismic Assessment (DSA)?

It is the seismic assessment carried out in accordance with Part C of the Engineering Assessment Guidelines. It is a comprehensive quantitative assessment of the strength and deformation capability of a building.

DSA- Is a more detailed quantitative tool which involves analysis of existing records, targeted calculations and modelling based upon the level of information available and confidence in its accuracy.

  • Expands on ISA if one has been carried out (not a pre-requisite).
  • Will provide overall %NBS rating of structure and provide initial scope of remedial works to achieve building owners desired %NBS.

We can assist ISAs, DSAs, and general earthquake reports to provide to councils, banks insurance companies, real estate agents.

What is a %NBS Rating?

A %NBS rating indicates the percentage of the New Building Standard that a building achieves in terms of protecting life in earthquakes.

The %NBS rating provides an indication of how well the building protects life when compared with a hypothetical similar new building on that same site that just complies with the minimum standard required by the Building Code.

A %NBS rating allows comparison between buildings as well as against the earthquake-prone building requirements.

A %NBS rating doesn’t predict how the building will perform in a particular earthquake. Earthquakes have a range of different ground-shaking effects. How a certain earthquake affects a specific building at a particular site depends on many factors. These include the earthquake itself, local geological and geotechnical features, the characteristics of that specific building and how all of these factors interact.

This means a %NBS rating does not represent an absolute assessment of risk or safety. For example, a rating of less than 34%NBS does not mean a building poses an imminent risk nor is that building expected to collapse in moderate levels of earthquake shaking. However, that building is expected to present a greater risk to life during earthquake shaking than a building with a significantly higher rating.

Why do you need and Earthquake Assessment?

Territorial authorities are required by the Building Act as of July 2017 to identify potentially Earthquake Prone Buildings under the time line below.

Seismic risk areaPriority buildingsAll other buildings
High2.5 years5 years
Medium5 years10 years

Once a building is identified as earthquake prone the building owner must then undertake the necessary strengthening work to bring the building to an acceptable standard. From the issue of the earthquake prone notice the building owner has the time frames as shown below to carry out strengthening work required.

Seismic risk areaPriority buildingsAll other buildings
High7.5 years15 years
Medium12.5 years25 years

An earthquake assessment will determine if a building is earthquake-prone (ISA and DSA) and the level of work required to strengthen the building to an acceptable standard (DSA).

If you would like to find out more about our services, or you have an opportunity for Design Engineering to assist with your engineering service requirements on any given project, and/or require a free quotation, please contact us