Interim Report Section 1: Summary of Recommendations
Introduction
Uncertainty is inherent in the prediction of earthquakes, particularly in terms of locations, magnitude and timing of events. GNS, New Zealand’s leading research organisation into seismic hazards, is aware of the active faults within New Zealand, but is unable to identify many of the below-ground ‘blind’ faults, such as those in the recent Canterbury earthquake sequence, which show no evidence of a fault on the ground surface.
GNS has built, and maintains, a National Seismic Hazard Model (NSHM) that aims to predict likely magnitudes and frequencies of occurrence of significant earthquakes for use in engineering design. The NSHM estimates future earthquake activity for New Zealand using the locations, estimated magnitudes from geological studies, recurrence intervals and types of ‘characteristic’ earthquakes for fault sources that have been recognised from detailed geological and geophysical studies.
It is not practical for GNS to identify all active faults in a region because many have no surface expression.
Key issues and recommendations
The Royal Commission is, at this stage of its Inquiry, able to make firm recommendations on a handful of matters for which information has been gathered and assessments completed. For most of the Inquiry issues, the Royal Commission is either awaiting further advice and/or yet to undertake analysis of advice recently received. In addition, hearings on the Inquiry issues have not commenced.
The Royal Commission has, however, been able to form conclusions and make recommendations on several matters that will, in its view, “inform early decision-making on rebuilding and repair work that forms part of the recovery from the Canterbury earthquakes” as required of it in its Terms of Reference, and some which have national significance.
This Interim Report presents the following conclusions and recommendations:
1. Seismicity
The Royal Commission recommends that parties with relevant expertise appearing before it in forthcoming hearings address the following issues in the Inquiry:
1. The seismicity model, which is reflected in the relevant New Zealand standard NZS 1170.5 Structural Design Actions, Part 5: Earthquake Actions-New Zealand (“NZS 1170.5:2004”), assumes that the ground motion associated with hidden faults is represented by a magnitude 6.5 earthquake located at a distance of 20km. In the new seismic model developed by GNS this has been replaced by a magnitude 7.2 earthquake at the same distance.
2. Changes were recently made for the new model in the way in which the design ground motions were derived from observed earthquakes. The Royal Commission understands that the peak ground accelerations for different building periods in NZS 1170.5 were deduced from the most critical of the two horizontal ground motions, which were measured at right angles to each other, suitably modified to allow for terrain, attenuation and other effects. However, it appears from a GNS report that in the new seismic model the ground motion has been based on the geometric mean of the observed shaking in the two horizontal directions. This assumption reduces the design seismic ground motions.
3. In the new seismic model a different magnitude weighting factor has been used from that applied for the model in NZS 1170.5.
4. The shape of the design response spectra (which defines how the effective acceleration of a structure in design varies with the period of vibration) is a poor fit to the observed spectra derived from the Canterbury earthquakes for the deep alluvial soils in the Christchurch locality. Some revision of the spectral shapes for these soils would appear to be justified.
5. There is some indication from the damage sustained in the Canterbury earthquakes that vertical ground motion may have contributed to the damage. There is a poor correlation between the calculated spectral shapes from the earthquakes and the specified shape in NZS 1170.5 for actions induced by vertical ground motion. Consequently, the method of defining design actions for vertical ground motion needs to be reviewed.
6. The implications of the points made above should be addressed by structural and geotechnical engineers in addition to seismologists.
The Royal Commission also wishes to receive further information addressing the effect of the high vertical ground motions in the 22 February 2011 earthquake.
2. Geotechnical Considerations
It is necessary to understand the behaviour of the soil-structure system during strong ground shaking and the contribution to this behaviour made by the foundation soils and the foundations themselves. Best practice internationally is for the issues of foundations on deep alluvial soils to be addressed by either:
a) comprehensive geotechnical investigations of the site and robust design methodology considering the soil-foundation-superstructure system including use of in-depth analysis to scrutinise the performance of the system; or
b) avoiding locations with difficult soil conditions.
Recommendations
1. The Christchurch City Council should require thorough soils investigations to be carried out as a pre-requisite to foundation design.
2. Relevant land use and building controls in the Christchurch CBD should reflect the need for care in the placement of buildings of different structural types and sizes, so that soils issues are minimised. These issues should also be considered by those proposing and designing new buildings.
3. Designers of new buildings should:
(a) Carry out in-depth analysis of the soil foundation super-structure system so as to ascertain the likely performance of the system.
(b) Consider available local soil improvement techniques where appropriate.
4. CERA and the Christchurch City Council should consider compiling and making available a public database of all bore logs previously recorded in the CBD, in addition to those made for future buildings. In time this would yield valuable information about soil conditions throughout the CBD.
3. General Performance of Buildings – Unreinforced Masonry Buildings
Prior to September 2010, there were estimated to be around 4,000 Unreinforced Masonry (URM) buildings throughout New Zealand (following the Canterbury earthquakes, there may be 500 fewer). The collapse or partial collapse of URM buildings during the 22 February earthquake in Christchurch resulted in 42 deaths.
In their report for the Royal Commission entitled ‘The Performance of Unreinforced Masonry Buildings in the 2010/2011 Canterbury Earthquake Swarm’, Ingham and Griffith1 identified common types of failure of URM buildings in Christchurch as a result of the 4 September 2010 earthquake. They recommend a four-stage improvement process for strengthening URM buildings as follows:
1st stage: ensure public safety by eliminating falling hazards. This is done by securing/strengthening URM building elements that are located at height (eg, chimneys, parapets, ornaments and gable ends).
2nd stage: strengthen masonry walls to prevent out-of-plane failures. This can be done by adding reinforcing materials to the walls and by installing connections between the walls and the roof and floor systems at every level of the building so that walls no longer respond as vertical cantilevers secured only at their base.
3rd stage: ensure adequate connection between all structural elements of the building so that it responds as a cohesive unit rather than individual, isolated building components. In some situations it may be necessary to stiffen the roof and floor diaphragms, flexurally strengthen the masonry walls, and provide strengthening at the intersection between perpendicular walls.
4th stage: if further capacity is required to survive earthquake loading, then the in-plane shear strength of masonry walls can be increased or high-level interventions can be introduced, such as the insertion of steel and/or reinforced concrete frames to supplement or take over the seismic resisting role from the original unreinforced masonry structure.
The Royal Commission will be considering the issues that arise from the Ingham and Griffith report, the peer reviews of it and submissions made by interested persons at public hearings in November. However, considerations of public safety have persuaded it that some actions should be taken as a matter of urgency.
Recommendations
The Royal Commission recommends that:
5. local authorities should ensure that registers of all URM buildings, their locations and characteristics, are compiled or, where they already exist, brought up to date; and
6. throughout New Zealand, URM buildings should be improved by bracing parapets, installing roof ties and securing external falling hazards in the vicinity of public spaces; and
7. in areas where the hazard factor in NZS 1170.5 is 0.15 or higher, additional steps to provide ties at all floors should be implemented, at the same time as the work referred to in recommendation 6; and
8. these recommendations should be implemented as soon as practicable.
4. Design Practice
Two reports (‘Stairs and Access Ramps between Floors in Multi-storey Buildings’ by Des Bull2 and ‘Preliminary Observations from the Christchurch Earthquakes’ by John Hare3, provided to the Royal Commission for the purposes of its Inquiry, establish that there are aspects of structural design and construction which need urgent attention in the context of the imminent rebuilding work in the Christchurch CBD.
Further, the reports have raised issues that have public safety implications for multi-storey buildings wherever they are located in New Zealand which the Royal Commission considers should be addressed immediately. The Royal Commission has consulted with structural engineers about the implications of the reports, in the process described in Section 3.4. In doing so, it identified the proposed actions that are set out in that section. The proposed actions involve recommendations for changes to construction practice and design standards, as well as further research. They are set out in Annexure 1 (page 44).
Recommendations
In view of the support for the proposed actions from the structural engineering community, the Royal Commission recommends:
9. establishment of a small group of structural engineers, which involves suitably qualified practising engineers and one or more engineers who are familiar with structural research, to draw up guidelines for the issues raised in this Interim Report (refer Annexure 1);
10. implementation of the guidelines (drawn up from recommendation 9 above) by CERA and the local authorities in greater Christchurch (as that term is defined in the Canterbury Earthquake Recovery Act 2011).
That Act provides the means by which this can be achieved. Where the guidelines require changes to existing practices adopted in order to comply with the Building Act 2004, an Order in Council could be made implementing the guidelines, on the recommendation of the relevant Minister as provided for in section 71 of the Canterbury Earthquake Recovery Act 2011. In the Royal Commission’s view, such an Order in Council would be within the ambit of section 3(a), (f) and (h) of that Act, in that it would:
(a) provide appropriate measures to ensure that greater Christchurch responds to and recovers from the Canterbury earthquakes;
(b) facilitate and direct the rebuilding and recovery of affected communities, including the repair and rebuilding of “land, infrastructure and other property”; and
(c) provide adequate statutory power for those purposes.
The guidelines would apply in greater Christchurch until such time as national building standards have been revised to ensure the shortcomings in current practices have been adequately corrected.
11. that Standards New Zealand be required to initiate the process of amending current building standards in light of the findings from the Canterbury earthquakes referred to above;
12. that the proposed guidelines for the rebuild of greater Christchurch be referred to practising structural engineers to alert them to issues that should be avoided in new construction in other centres in New Zealand; and
13. that the following issues be referred to building consent authorities in other centres in New Zealand:
- the potential vulnerability of buildings where mesh has been used to transfer critical seismic actions to lateral force resisting elements; and
- the vulnerability of stairs in multi-storey buildings designed to meet the minimum inter-storey drift requirements given in previous and current loadings standards (NZS 4203 and NZS 1170.5). This issue has been addressed in Practice Advisory 13, Egress stairs issued by the Department of Building and Housing when this report was in final draft.
5. New Building Technologies
The Royal Commission considers that structural engineers and architects involved in the rebuild of Christchurch should be aware of the content of a report prepared for it entitled ‘Base Isolation and Damage-Resistant Technologies for Improved Seismic Performance of Buildings’ by Professor Andrew Buchanan4 et al. It is likely that the adoption of one or more of these new technologies would result in improved seismic performance of new buildings in Christchurch.
The report describes the current approach to seismic design together with a number of alternative technologies namely base isolation, Precast Seismic Structural Systems (PRESSS) technology and non-tearing floor systems, that can be used to improve seismic performance.
Recommendations
The Royal Commission recommends that:
14. designers give consideration to the use of new technologies discussed in Section 3.5 of this Interim Report and described in the report ‘Base Isolation and Damage-Resistant Technologies for Improved Seismic Performance of Buildings’ in designing new structures to be erected in the Christchurch CBD; and
15. urgent work should be carried out to enable appropriate provisions to be incorporated in the relevant structural design actions standards (AS/NZS 1170 and NZS 1170.5) together with the material design standards (NZS 3101 Concrete Structures, NZS 3404 Steel Structures and NZS 3603 Timber Structures) so as to facilitate the use of these technologies.
Footnotes
2. Associate Professor Jason Ingham, University of Auckland and Professor Michael C. Griffith, University of Adelaide
3. Professor Des Bull, University of Canterbury
4. John Hare, President of the Structural Engineering Society of New Zealand (SESOC)
5. Andrew Buchanan, Professor of Timber Design, Department of Civil and Natural Resources Engineering, University of Canterbury
