Initial Seismic Assessments (ISA) and Detailed Seismic Assessments (DSA) of existing buildings.
Building insurers and financiers are increasingly demanding seismic assessments, to assess earthquake risk before providing building cover or finance. Territorial Authorities may also identify buildings considered to be potential risks, and then ask owners to carry out seismic assessments on buildings that are considered earthquake prone. What’s more, tenants are increasingly 'voting with their feet', moving to buildings that are considered less of a seismic risk, so a building with low seismic risk is a highly marketable asset.
ISA, DSA, IEP or DEE?
The terminology can be confusing, but our structural engineers will help demystify it.
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. The principal ‘tool’ used by engineers to carry out an ISA is called in Initial Evaluation Procedure (IEP), so the terms are often used interchangeably.
Depending on the outcome of the ISA, a building owner may then be advised to carry out a Detailed Seismic Assessment (DSA), previously known as a Detailed Engineering Evaluation (DEE), 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, but the assessments can be carried out independently, without the need for the other. A DSA is a more detailed quantitative appraisal than the ISA that seeks to establish the seismic performance of a building.
Does your building need an ISA?
Guidance provided by MBIE to Territorial Authorities and property owners on Initial Seismic Assessments states that:
"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."
Prendos structural engineers are Engineering New Zealand (ENZ) registered and trained, and experienced in using IEP procedures to identify potentially earthquake-prone buildings and draw initial conclusions about their seismic performance.
We have extensive experience with the IEP methodology set out in the New Zealand Society of Earthquake Engineering Guidelines 2006 which provides for:
- 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 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
Accurately assess earthquake performance with a DSA
After an ISA is carried out, it may be advisable to carry out a more detailed evaluation and quantitative assessment with a DSA. A DSA is intended to provide an accurate assessment of the building’s capacity to withstand an earthquake event.
Unlike an ISA, a DSA is likely to require some form of site testing and measurement, as well as analysis and calculation. It includes a comprehensive analysis of the capacities of the existing building elements and systems.
Following a review of all available property records and a site inspection, a typical DSA report would include:
- 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
