Seismic Assessment and Building Strengthening

December 16, 2013

Building owners around the country are under pressure to demonstrate adequate building resistance to earthquake loads. This generally happens in one of several ways:

  1. In order to provide either building finance and/or insurance, insurers and funders insist on seismic assessments to assess earthquake risk.
  2. Tenants ‘vote with their feet’ by moving to buildings that are considered less of a seismic risk.
  3. Territorial Authorities carry out an Initial Seismic Assessment of buildings considered to be potential risks and then ask owners to carry out further tests.

An Initial Seismic Assessment (ISA) provides a broad indication of the expected performance of a building taking into account its type and age of construction, local seismicity, ground conditions and usage. Depending on the outcome, a building owner may then be advised to carry out a Detailed Seismic Assessment (DSA) which may ultimately result in the requirement for seismic strengthening to the building.

A DSA typically follows an ISA where more information or reliability is sought. It is a more detailed quantitative appraisal than the ISA that seeks to establish the seismic performance of a building.

Initial Evaluation Procedure (IEP)

The principal ‘tool’ used by engineers as part of an ISA to identify potentially earthquake-prone buildings and draw initial conclusions about their seismic performance is an IEP.

It is essential that an ISA be carried out, or supervised by, New Zealand Chartered Professional Engineers (CPEng) or equivalent who have sufficient relevant experience in the design and evaluation of buildings for earthquake effects to exercise the degree of judgment required and who have had specific training in the Initial Evaluation Procedure. – “Guidance for Territorial Authorities and Property Owners on Initial Seismic Assessments” ­­ MBIE.

Prendos engineers are IPENZ registered and trained and experienced in using IEP procedures, specifically the IEP methodology set out in the New Zealand Society of Earthquake Engineering Guidelines 2006 which provides for a:

  • Review of all available property information
  • Site visit to carry out a visual inspection
  • Record of overall measurements if not available from existing records

The subsequent ISA/IEP report includes:

  • Description of the building
  • Executive summary of salient report findings including:
  • Evidential structural weaknesses
  • Overall rating of the structure (percentage of New Building Standard (%NBS))
  • Relevant appendices
  • Assessment calculation sheet
  • Selected marked-up drawings

Detailed Seismic Assessment

Following a review of all available property records and a site inspection, a typical Detailed Engineering Evaluation (DEE)/DSA report includes:

  • Executive summary
  • Description of methodology
  • Detailed description of the bracing elements
  • Summary of results
  • Critical members %NBS
  • Overall rating of the structure
  • Initial scope of remedial works to achieve building owners desired overall %NBS rating
  • Rough order of costs to achieve building owners desired overall %NBS rating


Cathy Thomas - Chartered Structural Engineer, head of Structural Engineering.   Cathy has over 20 years experience in the construction industry.  Her various roles include technical, project management, civil, structural and environmental design.  Projects that she has been involved with span:

  • Shopping centres
  • Low rise commercial developments (precast concrete and structural steel)
  • Complex architecturally designed residential and commercial buildings
  • Service stations – civil and environmental 
  • School buildings – new and upgrading existing buildings
  • Aluminium sign structures

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