BIBLIOGRAPHIC REFERENCE - GNS Science 2012-024.pdf · BIBLIOGRAPHIC REFERENCE Uma, S.R., 2012....

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BIBLIOGRAPHIC REFERENCE Uma, S.R., 2012. Achieving acceptable performance levels in the seismic design of buildings, GNS Science Report 2012/24. 21 p.

S.R. Uma, GNS Science, PO Box 30368, Lower Hutt © Institute of Geological and Nuclear Sciences Limited, 2012 ISSN 1177-2425 ISBN 978-1-972192-07-8

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GNS Science Report 2012/24 ii

CONTENTS ABSTRACT ............................................................................................................................. III KEYWORDS ........................................................................................................................... III 1.0 INTRODUCTION .......................................................................................................... 1

1.1 Performance-based approach ........................................................................... 1 1.2 Building Regulatory System .............................................................................. 2

2.0 PERFORMANCE –BASED EARTHQUAKE ENGINEERING (PBEE): CURRENT STATE OF PRACTICE ................................................................................................. 3

2.1 Performance Levels .......................................................................................... 3 2.2 Tolerable Impact Levels (New Zealand Building Code) .................................... 4 2.3 Limitations in First Generation PBEE Methods ................................................. 6

3.0 DESIGN STANDARDS ................................................................................................ 7

3.1 Acceptable Solutions and Alternative Solutions ................................................ 7 3.2 Relationship between Acceptable Solutions and the Performance

Requirements .................................................................................................... 7 3.3 Limit States of Design ....................................................................................... 8

3.3.1 Consideration of sources of uncertainty ................................................ 9

4.0 FACTORS AFFECTING PERFORMANCE OUTCOMES .......................................... 10

4.1 Barriers to Performance-based Design ........................................................... 10

5.0 TOWARDS ACHIEVING TOLERABLE PERFORMANCE LEVELS ......................... 11

5.1 Advanced Methods of Reliability Analyses ..................................................... 11 5.2 FEMA 349 ....................................................................................................... 12 5.3 ATC 58 ............................................................................................................ 12

6.0 PROPOSED MEASURES TO IMPROVE PERFORMANCE-BASED DESIGN IN NEW ZEALAND ......................................................................................................... 13

6.1 Communication of Earthquake Risk (ATC 58-1) ............................................. 14 6.2 Methodology Proposed by ATC 58 ................................................................. 14

6.2.1 Stages of development ....................................................................... 15

7.0 SUMMARY ................................................................................................................. 17

8.0 ACKNOWLEDGEMENTS .......................................................................................... 17

9.0 REFERENCES ........................................................................................................... 17

FIGURES

Figure 1 Performance system model (PMS) .................................................................................................... 2 Figure 2 Seismic performance design objective matrix as per SEAOC Vision 2000 [1995] ............................. 4 Figure 3 Typical load-deformation curve .......................................................................................................... 9

TABLES Table 1 Tolerable impact levels related to building importance level and earthquake intensity ...................... 5 Table 2 Categories of consequences .............................................................................................................. 5 Table 3 Key structural performance ................................................................................................................ 6 Table 4 Action plans to develop and implement performance-based seismic design ................................... 12 Table 5 Proposed tasks for conducting seismic assessment of NZ building types ....................................... 16

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GNS Science Report 2012/24 iii

ABSTRACT

Failures of structures during earthquakes, especially recent events, provide lessons to the structural engineering profession. In addition, they stimulate engineering professionals to revisit existing design provisions and to make improvements wherever necessary. More importantly, uncertainties associated with the occurrence of natural hazards, structural capacity and ability of existing buildings to withstand future hazards make the profession more challenging. A natural consequence of uncertainty is risk, and so a primary purpose of the structural engineering profession is to manage that risk and to maintain the safety of buildings and other facilities at socially acceptable levels. A well-established framework includes various components such as: (i) a building regulatory system to express the public’s expectations in terms performance and cost; (ii) supporting documents such as design standards that include loading and material standards, prescriptions of magnitudes of forces for design, and methods of analyses to determine resistance. The New Zealand Building Code adopts a performance-based approach and sets the objectives and goals related to functional and performance requirements of a building structure, including construction, demolition and alteration work. References are made to design standards and guidelines which include acceptable solutions and criteria for the design. However, the current design procedures are not able to fully address the performance expectation of the client or building owner in terms of acceptable levels of performance or acceptable levels of loss. This report discusses the present status of the performance-based approach adopted in New Zealand design practices, identifies gaps in implementing performance-based design, and the way forward to achieve tolerable impact levels in buildings under earthquake loading.

KEYWORDS

Seismic performance design objectives; Performance levels; New Zealand design practice; Reliability; Risk-based design.

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GNS Science Report 2012/24 1

1.0 INTRODUCTION

In New Zealand the building industry introduced a performance-based approach in the early 1990s. The Building Act 1991 set out a new legislative framework for building controls in New Zealand. It required design in accordance with the Building Regulations and the Building Code in a performance-based structure. A more stringent Building Act was framed in 2004 which essentially adopted 1991 performance-based structure. The Building Regulations 1992 are those regulations called up by the Building Act 2004. Regulators are responsible to the public and are empowered to represent public interest and expectations for how buildings and facilities are expected to perform. The First Schedule to the Building Regulations is known as the New Zealand Building Code. In accordance with the Building Act, the Building Code sets the objectives and goals related to functional and performance requirements of a building structure including construction, demolition and alteration work. References are made to Standards which describe methods to be used to demonstrate the performance outcomes of the structure. The objectives of the report include: (i) discussion on the performance-based approach used in the current New Zealand Building Code; (ii) identification of current gaps in knowledge and practices; and (iii) review of existing methodologies to improve performance-based goals in design practices.

1.1 Performance-based approach

Performance-based approaches in building codes deal with goals and objectives to be achieved. The goals and objectives can be related to building performance levels expressed in terms of life-safety, functionality and amenity of the building. These performance levels can be expressed in terms of structural damage and/or non-structural damage, or in terms of loss, casualties and downtime that are more meaningful to the client or stakeholders. Prior to the performance-based codes, design practice followed prescriptive codes that gave definitive solutions in terms of material, design and construction methods without stating goals and objectives. A few advantages of codes following the performance-based approach are highlighted below: (i) New technologies can be adopted as long as they are able to demonstrate their

performance in compliance with the stated goals and objectives. (ii) Innovative approaches are encouraged to find optimum ways to meet performance

criteria, including the cost. (iii) The approach is transparent in clearly stating the goals and objectives in terms of

performance to be achieved. With the prescriptive approach, in contrast, there are no clearly stated performance achievements related to the design procedures suggested

The prescriptive approach, even though less scientific, reflects practices that have demonstrated acceptable performance over time. It should be noted that a performance-based approach requires efforts to demonstrate the ability of the products and services to meet the goals and objectives. As long as the efforts are not onerous in terms of cost and complicated testing procedures, the performance-based approach is more promising than the prescriptive-based solutions.

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1.2 Building Regulatory System

Performance-based regulations specify outcomes rather than specific solutions. The totality of the building regulatory system is captured in a performance system model (PMS). It is typically formatted as a hierarchical structure (Figure 1) in which the top level contains goals and objectives expressed as qualitative statements. Functional statements are stated to satisfy the objectives. Further, operative and performance requirements that satisfy the functional requirements, and thereby the objectives, are provided. Note that the above requirements are considered to be high-level and are descriptive and qualitative. The building code addresses the above requirements. Following this, performance criteria satisfying the operative and performance requirements are stated within the standards. Standards provide quantitative information to enable achievement of the desired performance criteria. In this regard, acceptable solutions and verification methods are included within the standards. Acceptable solutions are usually “deemed to comply” solutions that generally include the former prescriptive solutions, i.e. the solutions that society has accepted over time as being “acceptable” or “appropriate.” Verification methods are intended to verify the required performance of new alternative solutions.

Figure 1 Performance system model (PMS)

Some standards are referred to as mandatory documents by the Building Code. Some supporting documents remain as guidance documents without being treated as mandatory. The combination of building regulations, enforcement mechanisms, standards, guidance documents and related support measures form the building regulatory system.

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2.0 PERFORMANCE –BASED EARTHQUAKE ENGINEERING (PBEE): CURRENT STATE OF PRACTICE

Performance-based earthquake engineering (PBEE) is related to the assessment of system-level performance of a structure to seismic excitation as well as detailed design of its structural features to achieve prescribed performance goals. SEAOC Vision 2000, 1995 was the first document to approach robust performance-based seismic design. Further, publications such as FEMA 273, 1997, FEMA 274, 1997 (as the commentary for FEMA 273) addressed rehabilitation of existing structures, followed by comprehensive guidelines for PBEE, as FEMA 356, 2000. Target building performance levels as mentioned in FEMA 356 were described in terms of the range of damage to structural and non-structural components. Structural and non-structural damage states are sometimes described in terms of engineering limit states (for example, drift values) which are assumed to correspond to the various performance levels for a particular component. It should be noted that these limits are only indicative of the range that exists for the limit states that typical structures undergo and should not be considered as acceptance criteria for post-earthquake damage assessment on a specific building. The performance levels are related to the post-earthquake condition of a building in terms of indicative measures of damage or losses, including repair costs, to the structure. In the New Zealand Building Code (Draft 8), building performance levels are referred to in terms of tolerable impact levels (TIL) and are categorised into mild, moderate, high/severe and very severe impacts. They essentially relate to the performance levels of structural and non-structural systems in a building affecting amenity, functionality, life-safety and downtime criteria, which are often expressed as economic loss. However, there are no quantitative details given to model the above mentioned qualitative measures.

2.1 Performance Levels

The assessment procedures from earlier guidelines [SEAOC, 1995; FEMA 273, 1997; FEMA 356, 2000] identify building performance level as the combination of structural and non-structural components performance levels to form a complete description of an overall damage level. The component damages are described in discrete forms in terms of one or multiple structural response indices, usually given by maximum (peak) responses, for example, maximum roof displacement, maximum inter-story drift, and peak floor acceleration. Structural damage designations are (S-1 to S-5) and non-structural damage designations are (N-A to N-D). A performance objective is typically defined when a set of structural and non-structural performance levels is coupled with different intensities of seismic input. For example, with reference to Figure 1, a ‘Basic objective’ for buildings having importance level 2, is for the fully operational performance level to be achieved at 43-year return period intensity earthquake shaking, and life-safety is expected to be satisfied at 475-year return period intensity earthquake shaking. In probabilistic terms it can be expressed as 50% exceedence in 30 years corresponds to a ‘Frequent event’ and 10% exceedence in 50 years to a ‘Rare event’. Buildings are designed for a chosen objective that should satisfy related performance requirements.

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Figure 2 Seismic performance design objective matrix as per SEAOC Vision 2000 [1995]

2.2 Tolerable Impact Levels (New Zealand Building Code)

The main objectives of the New Zealand Building Code for Structure are: (i) to safeguard people against unacceptable injury caused by structural failure, representing ‘life-safety’ performance criteria; and (ii) to safeguard people from an unacceptable loss of amenity and functionality, representing serviceability performance criteria. Within the Building Code, the impact (performance) levels are expressed at high level and in qualitative terms relating to the damage suffered by the structure, non-structural components and contents, and also downtime period. Functional and performance requirements of a structure are related to two limit states mentioned before. To satisfy the life-safety criteria, various measures in terms of loss of stability and strength and avoidance of progressive collapse at earthquake intensities with low probability of occurrence are stated. The importance of accounting for variability and uncertainty in design and construction methods is emphasized. It is stated that there shall be a low probability of demand exceeding structural capacity. The ‘amenity and functionality’ criteria are to be satisfied at earthquake intensities with higher probabilities of occurrence. However, for buildings with higher importance level (say, IL4 buildings where post-earthquake functionality is to be maintained), the amenity and functionality levels are set at earthquake intensities different from ordinary buildings with importance level 2 (IL2). A performance objective matrix is presented for buildings with different importance levels in Table 1 which relates tolerable impact levels (TIL) to intensities of earthquake. Impact levels are categorised into performance affecting: (i) amenity (TIL1) indicating mild impact; (ii) functionality (TIL2) indicating moderate impact; and (iii) strength and stability (TIL4 for earthquake and TIL3 for other types of natural hazards) indicating severe impact, at various specified intensities of earthquake. The impact at collapse condition is referred to as TIL5 indicating very severe impact. However, in New Zealand, the design addresses only the life-safety issue with maximum impact corresponding to TIL4. The impacts are described in terms of consequences and key structural performance as listed in Table 2 and Table 3. Note that there is no direct mapping of key structural performance to the likely consequences or quantification of these measures.

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Table 1 Tolerable impact levels related to building importance level and earthquake intensity

Building importance level (IL)

Return period

Probability of exceedence in 50

years (%) IL1 IL2 IL3 IL4 IL5

5000 1 TIL4

2500 2 TIL4

1000 5 TIL4 TIL2

500 10 TIL4 TIL2

250 18 TIL2 TIL1

100 39 TIL4 TIL2 TIL1

50 65 TIL1

25 87 TIL1

Note: Amenity, TIL1; Functionality, TIL2; Strength & stability (Other hazards, TIL3; Earthquakes TIL4)

Table 2 Categories of consequences

TIL 1 Mild

TIL 2 Moderate

TIL 3 High

TIL 4 Severe

TIL 5 Very severe

Occupational safety

Safe to occupy

Safe to occupy

Safe to occupy after clearance

by authority

Building unsafe to occupy for

one year

Building unsafe to

occupy more than one year

Structural damage

Little or no damage

Minor damage

Moderate but repairable damage to structure

Major damage but repairable

Major and extensive damage,

irreparable

Fabric damage Minor

damage to fabric

Moderate damage

Damage, mostly

repairable, some

replacement required

Major damage, some

repairable, most requiring

replacement

Major and extensive damage.

irreparable

Contents damage

Some affected Affected Most affected Most seriously

affected Contents not salvageable

Functional continuity Maintained

Function affected

less than 1 hour

Function affected for up

to 7 days

Extensively affected Ceases

Evacuation Easy Easy

Evacuation with no

difficulty; Access un-

inhibited

Unassisted evacuation possible;

Deaths/ Casualties

No deaths/ No injuries

No deaths / injuries unlikely

No deaths; some injuries.

Some deaths likely; moderate

number of injuries

Moderate number of

deaths/ high number of

injuries

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Table 3 Key structural performance

TIL 1 (Mild)

TIL 2 (Moderate)

TIL 3 (High)

TIL 4 (Severe)

TIL 5 (Very

severe) TIL 6

(Extreme)

Structural integrity

Fully maintained Maintained

Maintained except for

minor areas

Not maintained

for significant

parts

Not maintained

for most parts

Gone

Stability Fully maintained Maintained


minor areas

Not maintained

for significant

parts

Not maintained

for most parts

Gone

Support Fully maintained Maintained


minor areas

Not maintained

for significant

parts

Not maintained

for most parts

Gone

Progressive collapse None None Unlikely Possible Extensive Complete

Damage / Loss of amenity

Not significant Minor Moderate Significant Extensive Total

Damage to other

properties Unlikely Possible Likely Moderate Significant Extensive

2.3 Limitations in First Generation PBEE Methods

Documents like FEMA 356 have addressed building target performance levels in terms of structural and non-structural damage. The procedures are focussed on assessing the performance of the individual structural and non-structural components that comprise a building, as opposed to the global performance of the building as a whole. In FEMA 356, system performance is related to structural component criteria and it is judged based on the most critical (localised) component in the structure. Thereby the nonlinear interaction of structural components is not effectively considered in the assessment of building performance. Most significantly, the reliability of the procedures in delivering the design performance has neither been characterised nor quantitatively and rationally evaluated. Such PBEE methods heavily rely on discrete component-level acceptance criteria, as opposed to probabilistic system-level performance metrics. Qualitative descriptions of structural damage are not helpful in quantifying performance measures that are directly useful to building stakeholders. However, some recent works [e.g. Haselton et al., 2008] provides a methodology to quantify performance in terms of repair costs, life-safety, and post-earthquake functionality or downtime and to express as probabilistic measures. These measures are more meaningful to building stakeholders and owners.

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3.0 DESIGN STANDARDS

The purpose of design standards is to provide detailed criteria for acceptability or compliance with the intent of the regulations. Standards support scientifically rigorous design methods and recommendations in order to link explicitly to both the performance goals and operative requirements stated as regulatory requirements. Design standards that include loading and materials standards enable engineering professionals to design buildings that exhibit consistency in expected performance when designed according to the procedures and the recommendations within the standards.

3.1 Acceptable Solutions and Alternative Solutions

Acceptable and alternative solutions are an integral part of a well-functioning performance-based regulatory system. Acceptable solutions provide a dimension of stability for those who remain confident in the way things are being done while alternative solutions give the freedom to deal with innovative ideas or difficult rehabilitation. Design standards provide at least one set of acceptable solutions which are deemed to deliver the required performance as required by the Building Code. Also, an alternative solution different from the corresponding acceptable solution is supported provided the new solution demonstrates the expected performance. This is an important feature supported by the regulatory system for those wanting to encourage innovation and the advancement of new technologies. There are two ways these alternative solutions can be assessed for compliance against the code: comparing against the stated acceptable solutions (benchmark approach) or assessed against the objectives and performance requirements (first principles approach). The first official acceptable solutions were usually the old prescriptive codes. So when performance-based regulations were first introduced the old code frequently became the acceptable solution. Over time the identification of these solutions became more explicit and tied to the objectives and performance requirements they were satisfying. What has happened in the transition to performance-based regulations is a majority of the designers and builders continue to want to follow the acceptable solutions they were most familiar with. Even though there is greater flexibility if a performance-based design is chosen, anything more than comparatively minor departures from the acceptable solutions is viewed as a higher risk or more costly approach and is only used in certain kinds of projects. Consequently, when looking at performance-based regulations today, most countries will have some form of a prescriptive option available for their stakeholders [Bergeron et al., 2001].

3.2 Relationship between Acceptable Solutions and the Performance Requirements

In most cases the acceptable solution is deemed to meet the objectives and performance requirements without a precise analytical relationship being established. Such acceptable solutions are frequently deemed to meet the objectives and performance requirements because they are what people are used to and accept as being adequately safe, especially when they formed part of the previous prescriptive code. This kind of ‘benchmark’ approach is more conservative by recognizing that objectives and performance requirements are predominantly qualitative and that the societal performance expectations are reflected in the

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acceptable solutions. This approach tends to be used where a more evolutionary change is being introduced to the system and may perhaps be a reflection of the concern for liability. However, there is a risk in trying to elicit all performance expectations that have become inherent in the acceptable solutions that have evolved in the design standards. In other words, the relationship between the acceptable solutions and the expected performance requirements are neither well proven nor established. Therefore, there is a degree of uncertainty and deviation in performance levels that can be expected to be achieved.

3.3 Limit States of Design

The Building Code specifies various tolerable impact levels related to a number of earthquake intensity levels and the impacts in terms of consequences and key structural performances (ref. Table 2 and Table 3). However, it will be an onerous task on design engineers to satisfy all the stated performance criteria under the specified earthquake intensities. The New Zealand earthquake loading standards NZS 1170.5:2004, aims to satisfy two performance levels related to: (i) life-safety; and (ii) serviceability. The corresponding limit states of design are referred to as ultimate limit state and serviceability limit state. By satisfying these two limit states, it is expected that the intermediate TILs will also be achieved satisfactorily by the building. Figure 3 shows a typical load-deformation curve in which the vertical axis represents the base shear coefficient and horizontal axis represents the drift ratio. The target design level, for example in IL2 buildings, is aimed at an earthquake intensity having a 500-year return period. For a given site condition, the base shear derived from an elastic design spectrum (Ce) is reduced to account for the structural ductility, and the structural performance factor (Sp) which is dependent on the level of ductility. This design level (Cs) possibly denotes the 1st significant yield point in the load deformation curve. This is checked against the serviceability design level which corresponds to an earthquake intensity of 25-year return period. The structure is designed for the greater of the two base shears so derived. Materials standards provide guidance on detailing needed to achieve local (or section) ductility. It is expected that by realising local ductility, there will be a degree of confidence that the structure will achieve the structural ductility (kµ) while resisting the design base shear (ref. Figure 3). Note that the redundancy and expected strengths of materials will give rise to an overstrength factor (Ωo) and result in increased base shear strength. To address uncertainty in loadings on the structure and in the resistance developed (in other words ‘capacity’), a number of factors related to loading as well as the resistance suggested within the standards. Often the factors are based on reliability analyses [Ellingwood et al., 1980] or expert judgement as discussed in the section below.

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Figure 3 Typical load-deformation curve

3.3.1 Consideration of sources of uncertainty

It is impossible to predict precisely the value of each of the individual factors that affect seismic performance. Lots of uncertainties are associated with: (i) the intensity or wave form of future shaking; (ii) the structural models being used; and (iii) incomplete knowledge acquired from laboratory tests that purport to represent actual building behaviour. In the process of performance assessments, consideration should be given to accounting for the various sources of uncertainties and randomness related to earthquake demand and structural capacity. The New Zealand Building code insists that these factors are duly considered within design methods and they are addressed in some form within the current design procedures. For example, loadings standards use load factors (which are usually greater than 1) and materials standards use partial safety factors (which are usually less than 1). These factors are presumably based on statistical data and reliability analyses [Ellingwood, et al., 1980]. It is therefore believed that buildings designed using such factors will achieve the target performance levels as intended, however it is not guaranteed. However, it is possible to assess these performance measures in the form of performance functions. Performance functions are probability distributions that indicate the probability that losses of specified or larger magnitude will be incurred as a result of future earthquakes. To effectively consider the sources of uncertainties within performance-assessment methods, a probabilistic approach is more suitable than a deterministic approach. Also, the performance outcomes or performance measures should be expressed in probabilistic terms and those measures should be useful to the building owners and stakeholders.

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4.0 FACTORS AFFECTING PERFORMANCE OUTCOMES

Sources of uncertainties related to ‘earthquake demand’ and ‘structural capacity’ could contribute to deviations between actual and expected performance outcomes of a building. Buildings are designed for chosen levels of intensity to satisfy the limit states. Very often, building design practices adopt acceptable solution methods. As mentioned before, there is no guarantee of achieving expected performance outcomes as a result of adopting acceptable solutions. This discrepancy is exacerbated by other gaps identified in design practices. Verification of acceptable solutions and their relationship with expected performance levels happens when a real earthquake event occurs. A real event can be at any level and may or may not match the design intensity. It may only be possible to ‘interpolate’ or ‘extrapolate’ the performance outcomes at that intensity experienced.

4.1 Barriers to Performance-based Design

There are many barriers to further development and implementation of performance-based solutions in building designs as given by [Spekkink, 2005], as follows: • The suspicion felt by building designers that the application of performance-based

design will undermine the design profession; • The conviction of most people active in the design process that the most important

quality aspects of buildings cannot possibly be translated into performance specifications and further into quantifiable performance indicators;

• The conviction of some people that the responsibility for the functional and architectural design on the one side cannot be separated from the responsibility for the technical design on the other;

• The segregation and fragmentation of design, engineering and construction; and • The low level of research and development investment in the construction industry. In practice designers often start to develop solutions immediately, without proper understanding the intended performance of a building in-use from the building owner’s point of view. Performance-based design is all about integral design. Someone has to integrate the contributions of all parties involved, and the architect is best positioned for that. In many countries the architect has lost his integrating role in the building process, because he is not able to cope with all the technical systems. However, it is timely to bring back the position and responsibilities of architects. One of the main problems in performance-based design is how to predict the performance of a building on the basis of a design. For many quality aspects the total building performance depends on a complex interaction of many influences. On the one hand there are no validated, standardized assessment methods available to predict the total building performance, but on the other hand this performance will determine the client’s perception of the quality delivered to a great extent. Well-developed procedures for design and assessment as in FEMA 356 are providing only discrete measures of acceptance criteria at a component level. Further, they are not often supported by quantitative or rational evaluation of achievable performance levels. Therefore, it is possible that even though being too conservative, the procedures might not adequately provide the performance capability expected by the decision makers.

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5.0 TOWARDS ACHIEVING TOLERABLE PERFORMANCE LEVELS

The performance-based approach is an internationally accepted approach in which the design is focussed on satisfying the performance expectations of building owners and stakeholders. The potential of this approach has been recognised at various stages of its development, and on-going research efforts are contributing to its improved benefits and outcomes. Some such efforts are discussed below:

5.1 Advanced Methods of Reliability Analyses

Structural failures or failures to achieve adequate performance as expected by prescriptive code design procedures encouraged engineering professionals to identify knowledge gaps and areas for further improvement. In this regard, uncertainties associated with several factors in design were recognised as making significant contribution to unsatisfactory performance of buildings. It was also recognised that the objectives of performance-based design could also be achieved by adopting reliability-based design formats in which large uncertainties could be handled in a systematic manner so as to achieve ‘target reliability’. Note that certain performance-based codes [e.g. Euro Code 0, 2001] provide ‘target reliability’ measures associated with specific performance goals/levels. Note however that the target reliability is often based on expert judgement. In current seismic design procedures, only design-level earthquakes are expressed in probabilistic terms; the engineering demand parameters such as inter-storey drift are given in deterministic form corresponding to the limit states chosen for design. To strictly enforce reliability performance goals, the target probabilities need to be set directly for the limit states rather than for the design earthquake [Wen et al., 1994; Wen, 2001]. In evaluation, the performance of the structure is satisfactory if the limit state probabilities are below the target values. In developing reliability-based design formats, one starts from these target reliability goals corresponding to physical limit states such as incipient damage and incipient collapse and develops the required design format, which then will yield a design that satisfies the goals. Reliability procedures can adopt target reliabilities involving cost parameters so that economic design can also be achieved. Some studies [e.g. Faber and Sorensen, 2002] have suggested methods for code calibration using reliability principles to increase the confidence in achieving target performance levels. By means of structural reliability methods the safety formats (load factors and partial safety factors) of the design codes may be chosen such that the level of reliability of all structures designed according to the design codes is homogeneous and independent of the choice of material and the prevailing loading, operational and environmental conditions. This process including the choice of the desired level of reliability or “target reliability” is commonly understood as “code calibration”. Reliability-based code calibration has been formulated by several researchers, [e.g. Ellingwood et al, 1980; Ravindara and Galambos, 1978] and has also been implemented in several codes, e.g. OHBDC, 1983; NBCC, 1980; and more recently Eurocode 0, 2001. In the following sections, developments from abroad are reviewed and potential relevance to New Zealand is highlighted.

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5.2 FEMA 349

In year 2000, FEMA 349 developed an action plan to implement a performance-based approach to design and to resolve several issues that were considered as obstacles to its implementation. FEMA 349 indicated the need for a comprehensive effort to bring the various interested parties to a consensus. Some of the challenges identified as action plans are given in Table 4. Table 4 Action plans to develop and implement performance-based seismic design

Actions Description

1 Increasing the current knowledge base of building behaviour, particularly to understand and collect information on structural and non-structural performances

2 Raising awareness among stakeholders about how performance-based design can address many of the problems they already perceive with current design practice

3 Developing performance-based designs to be compatible with the stakeholder’s economic interests

4 Communicating the complex concepts and information in a way that is understandable to all stakeholders

5 Reducing uncertainty about how performance-based seismic design will effect stakeholders, in terms of cost and possible changes in liability exposures

6 Implementing incremental changes in the current standards, to create a continuum of design improvement rather than a perceived radical change.

Specific tasks were required to be developed that were in alignment with the above action plans. The tasks were supposed to focus on developing a cohesive set of products and guidelines that would meet challenges. The products were expected to include creating education and implementation programs to bring all stakeholders on board. One of the products was to be a set of guidelines for seismic performance assessment of buildings.

5.3 ATC 58

The Applied Technology Council has undertaken a project “ATC 58” to develop “Guidelines for Seismic Performance Assessment of Buildings” in alignment with Next-generation Performance-based Seismic Design Criteria [FEMA 445, 2006]. The guidelines document summarises the methodology, procedures and criteria needed to predict the probable earthquake performance of individual buildings based on their unique structural, non-structural and occupancy characteristics, and the seismic hazard exposure at a given site. The Guidelines address the performance assessment process which includes determining the characteristics of the building, evaluating its response to earthquake shaking, and based on this response, projecting the amount of damage that might occur and the consequences of this damage. However, they do not address the selection of appropriate performance objectives or procedures to develop preliminary designs that are likely to meet those desired performance objectives.

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The guidelines claim that the methodology can be used for other purposes such as: (i) by engineers to determine probable performance of buildings (e.g. probable maximum

loss) in support of real estate investment transactions; (ii) by building product suppliers to determine the performance of building components and

the effects of these components on overall building performance; and (iii) by building code developers to determine the performance capability of typical buildings

designed using prescriptive code procedures, as means of evaluating the adequacy of these procedures.

6.0 PROPOSED MEASURES TO IMPROVE PERFORMANCE-BASED DESIGN IN NEW ZEALAND

The performance-based approach is adopted in principle in the New Zealand Building Code regulations. Design standards include provisions and recommendations in the form of acceptable solutions and verification methods which are expected to yield performance outcomes satisfying the intents of regulations. As mentioned before, there are no definitive levels of reliability associated within the design processes that will ensure the achievement of the expected outcomes. To improve the current status of ‘performance-based design’ in New Zealand, there are a number of issues to be addressed and initiatives to be undertaken. The qualitative descriptions of performance levels used to achieve performance objectives are yet to be translated to quantitative measures. Performance criteria should be objectively measurable. However, not all attributes that are important for building design can be expressed in quantified criteria as yet. R&D projects should be focussed to develop quantified criteria for up to 75% of the essential building attributes [Spekkink, D 2005]. Clients and end users, who are more and more demanding value for money and fitness for use of the built environment, form the main driver for performance-based design. Besides that, performance-based building regulations have proven to be a key success factor in the implementation of performance-based design [PBD]. Governmental clients should take the lead in further implementation. Other drawbacks include the segregation and fragmentation of design, engineering and construction, the uncertainty about risk and liability, the (lack of) professionalism of clients, and lack of experience. It seems appropriate that actions should be started to increase the awareness of PBD. At this juncture, it is worth considering and revisiting some of the measures undertaken by overseas organisations (through FEMA funded projects) to encourage the development and implementation of performance-based design. The goal of ATC 58 project is development of performance-based seismic design guidelines. Some of the stages considered within the project that could be relevant from New Zealand perspective are discussed below:

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6.1 Communication of Earthquake Risk (ATC 58-1)

The purpose was to obtain preliminary feedback from a cross section of building stakeholders, including real estate developers, building owners, corporate tenants, lenders, insurers and other interested parties as to how performance-based seismic design guidelines could most usefully deal with issues of earthquake risk. A workshop was conducted to deal with three important issues such as: • identification of those aspects of earthquake related risk that are of most concern to the

stakeholders; • appropriate means of communicating the low-probability but potentially highly significant

consequences of earthquakes; and • appropriate means of communicating the considerable uncertainties associated with

predictions of the effects of earthquakes and the performance of individual structures. The potential attendees suggested included: • Attorneys • Building design professionals, including architects and engineers • Building regulators • Corporate facility managers • Commercial real estate developers • Commercial lenders • University facility managers • Development planning consultants • Earthquake engineering researchers • Health care providers • Insurers • Property underwriters • Social scientists The framework has been documented in the ATC 58-1 report. The feedback from the workshop was used to evaluate performance-based design criteria as reported in ATC 58-2. The suggested framework and evaluations can be revisited from a New Zealand perspective. The outcomes of the workshop may lead to better understanding what might be considered tolerable impacts from earthquakes and also derivation of ‘target reliability levels’ that are accepted by various sectors of the public and which can be included in the building code.

6.2 Methodology Proposed by ATC 58

The methodology for seismic performance assessment of buildings proposed by the ATC 58 project appears to be a suitable tool to for determining the performance that a building design is capable of achieving. The performance outcomes are obtained in terms of economic loss, casualties and downtime. This methodology is applicable to specific building types, but not for a regional building stock. According to the ATC-58 specification, engineers would conduct a series of structural analyses to predict the building’s response when subjected to the earthquake hazards identified as part of the performance objectives and then use the information obtained from

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the analyses to assess the amount of damage that may occur and the probable consequences of this damage. Following the performance assessment, the engineer compares the predicted performance with the desired performance. If the assessed performance matches or exceeds the stated performance objectives, the design is adequate and the project can be completed, assuming that the cost of completion is acceptable. If the assessed performance does not meet the performance objectives, the design team must either revise the design or alter the performance objectives in an iterative process, until the assessed performance meets acceptable objectives. It is reported that the methodology presented can be applied to the performance of any building type and occupancy. However, in order to effectively implement the methodology and procedures, basic data are needed on the damageability of components that comprise the building, and the consequences of this damage in terms of potential casualties, direct economic loss and downtime. The appropriate data to use for a given building are dependent on the type of structural system, the specific details of its construction, the type, location and means of installation of the non-structural components and systems, and the occupancy and use of the building. Sources of such data can include laboratory testing of individual building components, analytical evaluation, statistical information on the actual performance of similar buildings in past earthquakes, and expert judgement. Software, namely PACT (Performance Assessment Calculation Tool) is designed to integrate the various steps involved in the methodology. Procedures are available to develop and incorporate additional data into the methodology, and additional building types with different structural systems or occupancies. This feature makes it promising for us to apply this tool to New Zealand requirements.

6.2.1 Stages of development

A model for implementing the ATC 58 methodology to perform seismic assessment of New Zealand building typologies is proposed. An important feature of the model is to encourage different organisations and sectors to participate and to contribute towards the seismic assessment process, and to give feedback that can be used to update the status of performance-based design in New Zealand. The tasks and the contributors are listed in Table 5.

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Table 5 Proposed tasks for conducting seismic assessment of NZ building types

No Task Contributors

(1) Identification of building typologies to be included with specific details for each of structural system, non-structural fittings, occupancy, year of construction, location

Advisory panel

(2) Design of buildings Design consultants

(3) Identification of suites of ground motion records GNS, Universities

(4) Performance of non-linear analyses to derive structural responses

GNS, University

(5) Compilation of a database of typical non-structural elements including their fitting/ installation information that is used in New Zealand building construction

BRANZ, GNS, Interior product designers, suppliers

(6) Definition of damage states related to structural and non-structural elements, and whole-building performance

GNS, University

(7) Building up of fragility data for various structural and non-structural components

Universities, BRANZ, GNS, Published literature

(8) Obtaining consequence functions related to damage described in (7)

Loss adjusters, Published literature

(9) Tolerable or acceptable impact levels in terms of loss Advisory panel

(10) Building up of PACT models GNS

(11) Iterations of analyses GNS

(12) Update of engineering parameters for improved design within standards

Advisory panel

It is proposed that an advisory committee will be formed to direct and monitor the project. The advisory panel would include representatives from regulatory boards, design engineers, university researchers, and the construction industry. It is believed that a workshop to communicate risk within the community is vital to educate the community and to get opinion on the acceptable or tolerable impact levels for buildings.

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7.0 SUMMARY

This report discusses the present status of the performance-based approach adopted in New Zealand building design practices, identifies gaps in implementing performance-based design and proposes the way forward to achieve tolerable impact levels in buildings under earthquakes. Performance-based approaches described in the current building code are qualitative in nature. The qualitative descriptions are to be translated into quantifiable performance criteria that can be more helpful to design engineers. Currently, design practices do not explicitly relate to performance outcomes that are meaningful to stakeholders and building owners; however they are expected to satisfy performance criteria such as ‘life-safety’ and ‘serviceability’. Such performance criteria are not accompanied by target reliability levels within the building code. Some of the gaps in knowledge about and barriers to uptake of performance-based design are highlighted. Research efforts need to be taken to translate some of the qualitative description into quantifiable engineering design parameters. Cost/consequence functions are to be developed for New Zealand building components in addition to damage descriptions and fragility functions. Improving the current status of performance-based design can be achieved only through the integrated effort and contributions from various organisations responsible for engineering profession. In all these efforts, it is recognised that risk should be effectively communicated to the public to give them a better understanding of the aims and scope of engineering profession in building design under earthquakes.

8.0 ACKNOWLEDGEMENTS

The author is thankful to Andrew King and Mostafa Nayyerloo for helpful discussions. Review comments from Mostafa Nayyerloo and Jim Cousins are gratefully acknowledged. The work is supported by Department Core Funding Programme: “Post-earthquake Functioning of Cities”.

9.0 REFERENCES

ATC 58 (75% Draft). 2011. Seismic performance assessment of buildings. Prepared by Applied Technology Council. Prepared for Federal Emergency Management Agency. Washington D.C.

ATC 58-1. 2002. Workshop on communicating earthquake risk, Proceedings. Chicago, Illinois. Prepared by Applied Technology Council. Prepared for Federal Emergency Management Agency. Washington D.C.

Bergeron, D., Bowen, B., Tubbs, B., Rackliffe, T. 2001. Acceptable solutions. CIB World Building Congress, April 2001, Wellington, New Zealand, Paper number: 257.

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Ellingwood, B. Galambos, T.V., MacGregor, J.G., Cornell, C.A. 1980. Development of a probability-based load criterion for American National Standard A58. NBS special publication 577, National Bureau of Standards, Washington, DC.

Eurocode 0. 2001. Basics of structural Eurocodes, Eurocode 0. EN 1990.

Faber, M.H., Sorensen, J.D. 2002. Reliability based code calibration. Paper for the Joint Committee on Structural Safety. Draft. March.

FEMA 273. 1997. NEHRP Guidelines for the seismic rehabilitation of buildings. Federal Emergency Management Agency. Washington D.C.

FEMA 274. 1997. NEHRP Commentary on the guidelines for the seismic rehabilitation of buildings. Federal Emergency Management Agency. Washington D.C.

FEMA 356. 2000. Prestandard and commentary for the seismic rehabilitation of buildings. Federal Emergency Management Agency. Washington D.C.

FEMA 349. 2000. Action plan for performance based seismic design. EERI report. Prepared for Federal Emergency Management Agency. Washington D.C.

FEMA 445. 2006. Next-generation performance based design guidelines: Program plan for new and existing buildings. Prepared by Applied Technology Council. Prepared for Federal Emergency Management Agency. Washington D.C.

Haselton, C.B., Goulet, C.A., Mitrani-Reiser, J., Beck, J.L, Deierlein, G.G., Porter, K.A., Stewart, J.P., Taciroglu, E., (2008). An assessment to benchmark the seismic performance of a code-conforming reinforced concrete moment-frame building. PEER Report 2007/12. Pacific Earthquake Engineering Research Center, College of Engineering, University of Berkeley.

NBCC. 1980. National Building Code of Canada. National Research Council of Canada.

NZS 1170.5:2004. Structural design actions - Earthquake actions. New Zealand Standards.

OHBDC. 1983. Ontario Ministry of Transportation and Communication. Ontario.

Ravindra, M.K., Galambos, T.V. 1978. Load and resistance factors design for steel. ASCE, Journal of the Structural Division, 104 (9). Pp. 1337-1353.

SEAOC Vision 2000. Committee. 1995. ‘Performance based seismic engineering. Structural Engineers Association of California, Sacramento, CA.

Spekkink, D., 2005. Performance based design of buildings. PeBBU Domain 3. Final report. Performance-based Building Network (PeBBu). Funded by EU 5th Framework Research Programme. Netherlands.

Wen, Y.K., Hwang, H., Shinozuka, M. 1994. Development of reliability-based design criteria for buildings under seismic load. Technical Report 94-0023. National Center for Earthquake Engineering. University of Buffalo.

Wen, Y.K. 2001. Reliability and performance-based design. Structural Safety, 23, pp. 407-428.

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