Geotechnical Engineering Design and the New Zealand Building Code System

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Geotechnical Engineering Design and the New Zealand Building Code System

The Earthquake Geotechnical Engineering Practice Series (the Modules), published jointly by MBIE and NZGS during 2016 and 2017, is currently being updated. Feedback from users is now being incorporated and new research and information is being incorporated to continue to assist design professionals to improve practice. Feedback has been received showing there was a need to provide greater clarity about the Building Code system and where the Modules fit into the system. This article is an advance copy of what is intended to be included in Module 1 when it gets published later this year.

1.5.1 Building Code System

Figure 1 provides an overview of the design process and the interaction with legal requirements. Getting a clear understanding of client requirements is always the starting point. Client requirements may well exceed the minimum performance levels set in the Building Code, depending on their objectives and purposes for the building, they cannot be set at a lower level than the Building Code. Once the design brief is developed and agreed, decisions can be made about the design approach, the type of ground investigations needed, the process necessary for estimating ground motion parameters, and whether a simplified or more complex dynamic non-linear analysis is appropriate.
The design is undertaken following a check for legal minimum requirements, with close collaboration of all other parties, including structural designers and the Building Consent Authority, BCA.

The regulatory triangle on the right-hand side of Figure 1 describes the Building Code system, providing the hierarchy for the minimum legal requirements and compliance pathways. Refer to Building Code Handbook for greater detail. The diagram also illustrates that Section 175 guidance can be issued by MBIE to better inform and provide greater clarity for designers and BCAs. Guidance can be targeted at three levels: Building Act provisions; Building Code performance requirements; or for design solutions, including Alternative Solutions.

Figure 1 Design Process and Legal Context

The Building Act: 2004, (B.A. s.17), requires that all building work must comply with the Building Code. The Building Code ‘prescribes functional requirements for buildings and the performance criteria with which buildings must comply in their intended use’ (B.A. s.16). The performance-based Building Code is detailed in Schedule 1 of Building Regulations 1992. Internationally there has been a strong move towards performance-based Building Codes. They allow for innovation in methods and materials used, greater flexibility to meet client requirements and quicker uptake of new knowledge. The Objective, Functional Requirement, and Performance is stated in the Code and it is up to the designer to meet these outcomes rather than hiding behind some prescriptive design exercise of ‘just following the rules’.

There are different pathways for demonstrating compliance with the Code. Prescriptive Acceptable Solutions and Verification Methods provide a ‘deemed to comply’ pathway and Alternative Solutions provide an opportunity for the designer to demonstrate directly that the design meets the performance requirements set in the Code. Acceptable Solutions and Verification Methods are not fully comprehensive, and most projects have an Alternative Solution element, requiring additional analysis and considerations.

Designs following the Acceptable Solutions and Verification Methods published by MBIE, eg Verification Method B1/VM1, or Acceptable Solution B1/AS1, must be accepted by the Building Consent Authority, BCA, when making Building Consent decisions. Verification Methods and Acceptable Solutions usually reference specific New Zealand or international Standards, eg NZS 1170.5 is referenced in B1/VM1 and NZS 3604 is referenced in B1/AS1.

Being a performance-based code, Alternative Solutions are permitted, indeed should be encouraged, provided the designer can demonstrate that the design will meet minimum Code performance requirements. This is particularly the case for geotechnical engineering as considerable complexities are often encountered but are not considered in the prescriptive pathway. Real caution is necessary if using the prescriptive pathway, as the relevant Verification Method, B1/VM4, has a narrow scope of application, and some aspects are thought to no longer meet current design practice. This is an area being reviewed by MBIE.

The BCA must issue a Consent if it is ‘satisfied on reasonable grounds that the provisions of the building code would be met if the building work were properly completed in accordance with the plans and specifications that accompanied the application’ (B.A. s.49).

Following s.175 guidance that has a legal status, helps BCAs satisfy the ‘reasonable grounds’ test. Section 19(2)(b) of the Building Act specifically provides for BCAs to take into account Section 175 guidance when making consent decisions. However, it is important that the designer recognises the limits of their competence and experience and that they are capable of undertaking the project design requirements. This capability will be a factor the Building Consent Authority considers when assessing a building consent application and applying the ‘reasonable grounds’ test.

1.5.2 Section 175 Guidance

Given the complexity and uncertainties associated with earthquake geotechnical engineering problems, as well as the unique features of each site and consequent large variety of soil characteristics and ground conditions encountered, geotechnical practice frequently relies on guidance documents rather than prescriptive standards. Such guidance documents provide appropriate support for geotechnical engineers and designers, and hence are the accepted norm internationally. For this reason, it was decided that publishing s.175 guidance was the best means of getting better consistency and improving general practice in earthquake geotechnical engineering in New Zealand. This decision followed consultations and advice from practitioners and the New Zealand Geotechnical Society who advised that, with the exception of some limited applications and straightforward low-risk situations, prescriptive ‘deemed to satisfy’ Verification Methods or Acceptable Solutions are not able to address complex issues requiring engineering evaluation and judgement.

As guidance, the Modules provide general principles using the latest research knowledge so that practitioners are aware of and focus on key issues in the assessment, rather than producing detailed calculation methods, often available in textbooks and journal publications. Some worked examples have been produced to provide more detail. Guidance can also be updated periodically more readily than Verification Methods and Acceptable to incorporate new advances in the rapidly evolving field of earthquake geotechnical engineering.

The building process can be complex. There are numerous site conditions, products, design methods and building systems that can be used to carry out a construction project. Sound engineering decision making is required throughout the course of the project to provide the client with a robust outcome. This will include specifying the type and scope of field investigations and laboratory testing of the soils appropriate to the level of risk, interpretation of soils and the site, modelling, analysis and interpretation as well as on-going monitoring throughout construction to verify assumptions made in the design. Good design is not a ‘box ticking’ exercise. It is difficult to address all these aspects in prescriptive documents; sound and experienced engineering judgement is required throughout.

The power to issue guidance was introduced into the 2004 Building Act following the “leaky building crisis”. Section 175 of the Building Act enables the Chief Executive of MBIE to develop and publish guidance to assist all parties involved to comply with the Act or perform their functions under the Act. The 2002 Hunn investigation into the ‘leaky building crisis’ concluded there had been systemic failure. One factor was the inadequacy of the Building Code and the accompanying Acceptable Solutions and Verification Methods (Hunn 2002). The report recommended the development of “guidelines on the interpretation of the Building Act and companion documents to provide an educational and reference document” and that “It should provide guidance on the interpretation of the Building Code provisions for Objective, Functional Requirement, and Performance”.

Guidance is therefore now an integral part of the Building Code system. Following guidance does not automatically mean acceptance by the Building Consent Authority when processing consent applications. However, Section 175 Guidance does have regulatory status and BCAs can rely on it when making reasonable ‘grounds decisions’ on code compliance.


 

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