Guidance for Territorial Authorities and Property Owners on Initial Seismic Assessments

Transcription

1 Guidance for Territorial Authorities and Property Owners on Initial Seismic Assessments New Zealand Society for Earthquake Engineering November 2013

2 ISA Guide for TAs and Owners November

3 Contents 1. Introduction and Scope 3 2. Overview of the Types of Seismic Assessments 3 3. Terminology 5 4. The Context of an Initial Seismic Assessment Background 4.2 Screening and Prioritisation 4.3 Building Usage and Importance Level 4.4 Critical Structural Weaknesses 4.5 Post-1976 Buildings 5. The Format and Delivery of an Initial Seismic Assessment Report Who Prepares and Authorises an ISA Report? 5.2 What Should an ISA Report Cover? 6. Outcomes of an Initial Seismic Assessment Report and Next Steps 10 ISA Guide for TAs and Owners November

4 1. Introduction and Scope This document provides a practical explanation of the process of Initial Seismic Assessments undertaken for Territorial Authorities (TAs) and property owners. Seismic assessments are typically undertaken for buildings that fall within the scope of the earthquake-prone provisions of the Building Act 1 that is, commercial buildings and residential buildings that are of two or more storeys and contain three or more household units. The process of seismic evaluation of buildings is technical in nature, and should be undertaken by experienced and competent structural engineers. This overview accompanies the updated version of Section 3 of the NZSEE Guidelines for the Assessment and Improvement of the Seismic Performance of Buildings in Earthquakes 2, released in November In that update, the new term Initial Seismic Assessment is introduced, referring to the overall process of establishing what the seismic performance characteristics of a building are. The Initial Evaluation Procedure, or IEP, remains the principal engineering tool for undertaking an Initial Seismic Assessment. This document has been prepared by members of the governance group established by the Ministry of Business, Innovation and Employment and EQC to oversee the review and update of the 2006 NZSEE Guidelines. This group includes representatives from territorial authorities and Local Government New Zealand. 2. Overview of the Types of Seismic Assessments There are two broad types of seismic assessments: Initial Seismic Assessment Detailed Seismic Assessment It is important to appreciate that these forms of seismic assessment are in fact part of a continuum of approaches. The relationship between Initial and Detailed seismic assessments is represented diagrammatically in Figure ISA Guide for TAs and Owners November

5 Exterior inspection only Access to drawings Simple structural analysis Complex structural analysis Cost/reliability/confidence Level of judgement required ISA DSA Figure 1: The Continuum of Seismic Assessment The key aspects associated with Initial and Detailed seismic assessments can be summarised as follows: Initial Seismic Assessment (ISA) The Initial Seismic Assessment (or ISA) is a high level screen to indicate the likely seismic performance of a building. It provides a broad indication of the expected performance of a building taking into account its type and age of construction, local seismicity and ground conditions. It may be commissioned by the owner of a building or portfolio of commercial buildings to establish where their buildings sit on the earthquake performance spectrum, and as part of their risk management practices. It may also be undertaken by Territorial Authorities as part of a programme to identify buildings that are believed to be potentially earthquake prone. This describes a building that has been assessed as having its ultimate capacity exceeded in a moderate earthquake (defined in the regulations as being one third of the size of the design level earthquake for that site - 33% of New Building Standard (%NBS) is a proxy for this definition). The Initial Evaluation Procedure (IEP) is a nationally standardised engineering tool for carrying out an Initial Seismic Assessment. The output of an ISA is expressed as a % of New Building Standard or by a grading level. ISA Guide for TAs and Owners November

6 Detailed Seismic Assessment (DSA) A Detailed Seismic Assessment typically follows as the next step after an Initial Seismic Assessment where more information or reliability is sought. It is a more detailed quantitative appraisal undertaken by earthquake engineers to establish the seismic performance of a building. It can be triggered by an ISA score near 34%NBS where more reliability is required to determine the EQP status, or be used directly by an owner seeking to understand the seismic performance of their building. Engineers use a range of techniques to quantitatively assess the performance level of the building as part of a DSA, with the results again expressed as %NBS or by a grading level. Building owners should note that TAs typically require a DSA to respond to an ISA assessment which suggests that a building is potentially earthquake prone, unless that outcome is accepted. In some cases additional information may become available that clearly indicates a higher score is appropriate from the original ISA, which can then be revised. A DSA is also required to support a building consent application to undertake seismic strengthening works on an existing building. For owners of buildings which are likely to be under or near the 34%NBS threshold, it could be more cost effective to obtain a DSA directly, as a DSA will provide more certainty and avoid the potential for having to pay for both an ISA and a DSA. 3. Terminology There is a range of terminology associated with the seismic assessment of buildings, and the following table provides some explanation for these. IEP ISA DSA DDE %NBS CSW Initial Evaluation Procedure This is the principal engineering tool to identify potentially Earthquake Prone Buildings as part of an Initial Seismic Assessment. Initial Seismic Assessment This is intended to be a coarse evaluation involving as few resources as reasonably possible and is the recommended first step in the process of determining whether a building may be earthquake prone. Detailed Seismic Assessment This is a quantitative assessment and report by structural engineers, which involves calculations and potentially modeling of the structure. Detailed Damage Evaluation (previously referred to as Detailed Engineering Evaluation) This evaluation is only undertaken in a post-earthquake (recovery) context. It encompasses both qualitative and quantitative approaches. % of New Building Standard This will be a percentage which describes the seismic capacity of the building relative to New Building Standards for a not less than 50 year design life. Critical Structural Weakness A CSW is any weakness in the structure that could potentially influence its performance/capacity at any level of earthquake shaking. In an IEP, a critical structural weakness may be rated as insignificant, significant or severe. ISA Guide for TAs and Owners November

7 EPB Earthquake Prone Building Describes a building that has been assessed as having its ultimate capacity likely to be exceeded in a moderate earthquake (defined in the regulations as being one third of the size of the design level earthquake for that site - 33%NBS is a proxy for this definition) 4. The Context of an Initial Seismic Assessment 4.1 Background An Initial Seismic Assessment should be undertaken by an experienced Structural Engineer who is competent in assessing the performance of a building during a seismic event. A site visit will be undertaken to assess the site, type of construction of the building and materials used. An ISA can be carried out with varying levels of information. With reference to Figure 1, an ISA can be completed solely on the basis of an exterior inspection, through to involving a detailed review of the drawings. Use of drawings will allow a review of interior details such as stairs, column detailing and floor construction type. It is a strong recommendation that drawings be reviewed if the building is designed after 1976, when modern seismic codes took effect. It is however acknowledged that plans of the building will in some cases not be able to be sighted, for various reasons. In such circumstances, the engineer will need to undertake specific physical investigations. The Initial Evaluation Procedure (IEP) is a high-level screening tool that allows building owners and Territorial Authorities to draw initial conclusions about the seismic performance of building stock. Significant assumptions are built into the calculation and formulas within the process. Accordingly, an ISA report is not: A report that should be used to support the sale of a property A document of confidence in supporting an investment decision A document that might support a change of use decision that might give rise to additional strengthening requirements While an ISA report may refer to a %NBS as the outcome or result, it should also detail an alpha rating as described in the following table: Percentage of New Building Standard (%NBS) > <20 Letter Grade A+ A B C D E ISA Guide for TAs and Owners November

8 While not earthquake-prone, buildings that lie between 34%NBS and 67%NBS are regarded as being earthquake risk buildings, as they pose a higher risk than new buildings. 4.2 Screening and Prioritisation The term screening in the context of an ISA can be likened to a sieve. It is a process that allows buildings to be assessed against broad criteria and assumptions to identity those buildings whose seismic performance in an event is likely to be rated at or below 34%NBS. For TAs seeking to undertake a programme of seismic evaluation across all buildings in the community, a preliminary desktop screening exercise is appropriate to identify which buildings need assessment (and which clearly don t) in accordance with the earthquake-prone building policy. Prioritisation of the assessments then follows this screening process, with the scope and extent of the ISA programme to be undertaken depending on the objectives and priorities of the TA under current legislation. Similarly, owners of a portfolio of buildings may wish to undertake similar screening, in accordance with their organisational policies. The new Section 3 of the NZSEE Guidelines outlines a Preliminary Screening Process that can assist with this step. Following the completion of an ISA programme, both TAs and building owners should then focus on those buildings rated 34%NBS and below, which should typically then have a DSA undertaken. 4.3 Building Usage and Importance Level The actual use of a building has an impact on its required seismic capacity. These uses are defined as Importance Levels, or IL for short, as follows: IL 1 - IL 2 - IL 3 - IL 4 - This describes the lowest level of people occupancy. Walkways and outbuildings fall into this category. Describes buildings which don t fall into the other categories eg normal office buildings. Describes buildings where large numbers of people can congregate in one space. Conference centres and hotels fall into this category. Buildings that must be operational immediately after an earthquake or other hazard event. Each IL category effectively increases the required seismic performance for a building - i.e. a building assessed as 26-31%NBS at IL 2 reduces to 20-25%NBS at IL 3 because of the higher occupant risk. ISA Guide for TAs and Owners November

9 4.4 Critical Structural Weaknesses A primary aspect of the ISA is the identification and qualitative assessment of the effects of any aspects of the structure and/or its parts that would be expected to reduce the performance of the building in earthquakes, thereby increasing the life safety risks to occupants and/or adversely impacting on neighbouring buildings and people. These deficiencies in the building are referred to as critical structural weaknesses (CSWs). Any party reading an ISA report should look for any indication by the engineer of the presence of a significant Critical Structural Weakness (CSW), as they have the potential to adversely impact on seismic performance. It is important to note that buildings may have Critical Structural Weaknesses that are assessed as being insignificant, with relatively minor adverse impact on the overall performance. 4.5 Post-1976 Buildings Buildings designed and constructed using seismic design codes from 1976 onwards need to be approached from a slightly different perspective when undertaking an Initial Seismic Assessment. They are unlikely to be earthquake-prone in terms of the Building Act definition, but can contain critical structural weaknesses that could lead to a sudden, non-ductile mode of failure at levels of seismic loading less than current design levels. It is important that buildings that may be earthquake risk buildings - that is, lie between 34%NBS and 67%NBS - with unacceptable failure modes be identified. The greater use and availability of computer programs for structural analysis and architectural developments led to the adoption of sometimes quite complex structural configurations and lateral load paths in buildings of this era. Whereas for earlier buildings it might have been possible to identify a generic structural form from an exterior inspection, it is often difficult to pick this for post-1976 buildings. This is particularly the case for mixed-use buildings involving competing structural layouts, with for example accommodation, office and car parking. These structures typically feature offset columns or other transfer structures causing irregular steps in the load path that may or may not have been appropriately taken into account in the original design. It is therefore important that ISAs on post-1976 buildings involve both a full interior inspection and a review of available structural documentation. Additional procedures for identifying critical vulnerabilities in buildings, including across a population of buildings, are under still under development. ISA Guide for TAs and Owners November

10 5. The Format and Delivery of an Initial Seismic Assessment Report 5.1 Who prepares and authorises an ISA Report? An Initial Seismic Assessment is primarily a qualitative process, expected to be undertaken by experienced structural engineers. It requires considerable knowledge of the earthquake behaviour of buildings, and judgement as to key attributes and their effect on building performance. It is therefore, essential that this level of assessment 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 Had specific training in the objectives of and processes involved in the initial evaluation procedure. While engineering graduates may undertake and process the initial assessment, the final report must be reviewed and signed off by a Chartered Professional Engineer with competency and experience in seismic design and assessment. 5.2 What should an ISA Report Cover? The reporting of the results of the ISA should be appropriate for the particular circumstances. It is recommended that when ISA reports are sent out to building owners and/or tenants they include explanatory information such as description of the building structure, the results of the ISA, the level of knowledge that the assessment was based on and the limitations of the process. Suggested template letters are provided for this purpose in Section 3 of the NZSEE Guidance document one for reports being sent by engineers who are engaged directly by building owners, and another for TAs to send to building owners following their assessment process. When the results of a TA-initiated ISA are being reported, building owners must be advised of the limitations of the process employed. The result from an ISA process is typically reported in terms of: 1. As a %NBS 2. As a grade referred to in section 4.1 of this document An ISA report should highlight specific areas of potential concern in relation to the seismic performance of the building. This provides a clear pointer for the areas that a subsequent DSA should focus on. If the IEP is used, the report is in a prescriptive format (i.e. every IEP will canvass identical detail), covering the following topics: a. Description of the building This will include the building name (if any), address, a photo and will describe its general construction with a building outline. ISA Guide for TAs and Owners November

11 b. Determine baseline NBS Assessment of the baseline NBS, given the age of building and the applicable design standards at the time of construction It will consider applicable soil conditions that are likely to impact on building ductility (i.e. the building s flexibility in a seismic event) c. Performance Assessment Ratio (PAR) This aspect of the IEP discusses the configuration and structural features of the building, and attempts to identify aspects that are likely to reduce the seismic performance of the building such as: Whether it is long and narrow or otherwise irregular in shape Irregular distribution of walls within the building Whether this building could pound or hit its neighbour in a seismic event Whether the land platform is flat or sloping Column height, number of columns and any significant changes in building mass at height In pre-1976 buildings, the presence of non-ductile columns, gap and ledge stairs and hollowcore floors d. Determination of %NBS e. Grading Based on previous inputs this will produce a %NBS score In accordance with the table of grades in section 4.1 of this document, a grade will be assigned corresponding to the %NBS f. Earthquake Prone (or not) If the number in d) above is 34%NBS or below, the building is potentially earthquake prone If an IEP is undertaken, the results will be detailed in Table IEP- 4 of the IEP report. 6. Outcomes of an Initial Seismic Assessment Report and Next Steps The result from an ISA process must be considered an initial or interim result, given the typically limited time inputs from engineers. It therefore reports a potential status for the building Accordingly, from the results and rating from an ISA report, the following steps are suggested: 1. If the rating is <34%NBS, the owner may either wish to go straight to a strengthening (or other) solution, or commission a DSA. A DSA will either establish that the building warrants a higher rating and may not be earthquake-prone, or will confirm that it is earthquake-prone. In the latter case, the DSA is the first step towards developing a strengthening solution. 2. If the ISA rating is over 34%NBS but less than 67%NBS, the building owner will need to reflect that the building still poses a much higher risk than an equivalent new structure. Accordingly, it is recommended that the owner commissions a DSA. ISA Guide for TAs and Owners November

12 3. If significant Critical Structural Weaknesses are found, it is recommended that the owner obtain a DSA regardless of the ISA rating. Even though a building may not be earthquake-prone, it is important that the potential failure mode, which may be brittle and hence sudden, is understood. Even though a building may not be earthquake-prone in terms of the Building Act, the building owner may still have difficulty attracting anyone to rent it or occupy it. Owners should consider their long term uses or intentions for the building, which in some circumstances may require planning for strengthening in the future. If significant Critical Structural Weaknesses are identified by the engineer, building owners should ask for further advice. If their building is declared earthquake prone by the identification of a significant CSW, correction of these elements may be all that is needed to take them over 34%NBS and therefore be no longer considered earthquake-prone. Building owners are encouraged to ask questions of their engineer or Territorial Authority, as they are there to help. Please contact the New Zealand Society for Earthquake Engineering s Executive Officer exec@nzsee.org.nz if you require further information on Initial Seismic Assessments. ISA Guide for TAs and Owners November