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EQ Loading Standards 5/2001

Technical Activities : EQ loading Standards 5/2001

BD6/4 Earthquake Actions on Buildings

These notes were prepared following the BD6/4 review committee meeting of 7^th-8^th May 2001. They reflect some of the key issues which arose from the public comments on the draft and will form the basis of a working group activities which is to be formed to recommend resolution of these issues to the review committee.

The Review Meeting 7th & 8^th May 2001

The revision to the Earthquake loading standard has progressed steadily since receipt of the very extensive (174 pages) of comments following the public comment process. In May, the review committee met and reviewed the issues that had arisen within these comments.

An important aspect was whether the BIA requirements for a complete, correct and clear solution could be prepared, necessary for them to call up as a Verification Method, or whether, as industry had expounded, such a document would be too restrictive for design and that engineering judgement must be retained. The latter view prevailed and was accepted by BIA, who are now attempting to create a new form of cited document which, when applied with 'appropriate engineering judgement' will result in buildings which satisfy the B1 provisions of the New Zealand Building Code. This approach removed many obstructions in drafting a progressive standard which includes many of the more 'leading edge' principles of design that commentators were seeking.

Seismic Hazard Assessment (Spectra, C_h(T) & Zonation, Z)

For the Australian sector

• The Australian spectra as presented was agreed as appropriate although a meeting of Mr McCue Wilson Pham Gibson is scheduled for latter in May to discuss and resolve some aspects implicit within the spectra.
• An addition Australian Type E (very soft soil) spectrum was to be developed (McVerry and Love) following the May meeting.
• Commentary was to be added outlining the characteristics of the earthquake being considered as the basis of the 450 yr return period (ULS) and the 2500 year return period Maximum Considered Earthquake (MCE) earthquake. Of particular interest is
• the duration of strong shaking which will be needed for the material groups to provide the necessary supplementary data to enable 'Limit Ductile' detailing necessary to satisfy the Verification Method 2 (VP2) approach.
• The displacement and velocity demands which are expected to accompany the design event.

For the NZ sector

• The NZ spectra as presented have been normalised at 0.5 sec for Soil Class B which results in a reduced peak ground acceleration of 0.63 for rock sites. It was agreed that this would best be normalised to be 1.0 Rock PGA and the spectra be amplified by 5/8 and the Seismic Zones be reduced by this value. The nett result is that the product of Ch(T)*Z remains unaltered but the presentational style is consistent with ISO and the Australian presentation.
• The seismic zonation map for NZ should be modified so that regions with Z<0.2 (0.13 with the above 5/8 multiplier) would be assigned Z=0.2 and that regions with Z>1.0 (0.63) be assigned Z=1 which would be rounded in the modified form to Z=0.6
• A spectra would be provided for very soft soils (Class E) and this would be considered as being applicable to all soils which did not meet other class descriptions. It would be generally conservative with a plateau out to around 1.5 seconds before declining.

Building Functional Categories

This table is located in Appendix C of Part 0 and is intended to differentiate between buildings of differing importance. It was recognised this was a societal issue which would be most appropriately defined by the regulators. At the present the tabulated values are those prescribed in AS1170.4 and include several distinctions from the similar table used in NZS 4203 (Table 2.3.1 and the related commentary description as to building types within each category). It was acknowledged that this issue needed to be resolved by the BD6/0 subcommittee and the BD6/4 recommendations to that subcommittee are:

• Category I 'Structures requiring special attention' are understood to pose an unusually high hazard should they fail and as such the return period of natural phenomena, such as earthquakes, should be assessed on a case by case basis. They are therefore outside the scope of this standard.
• Category II 'Structures with a special post-disaster function' should have hospitals, medical facilities and emergency centres added
• Category III 'Structures which as a whole may pose risks to people in crowds' should be limited to Airport terminals, major railway stations, corrective institutes, schools, colleges and universities and buildings whose contents are highly important to the community. All other groups identified in this section should be reclassified in Category IV.
• Category V 'Structures of low hazard to life or other property' Buildings within this category would be classified as temporary structures and as such would require a limited life to be assigned at the time of consent.

Displacement Ductility Factor, m

This factor is used to reduce the elastic design load derived from the response spectra to accommodate the acceptable inelastic damage which is in intrinsic component of earthquake design. This currently is achieved by equating the displacement of the inelastic system to that of the assumed elastic oscillator over the long period range, by equating the energy absorption of the two systems over the short period range and using a transition function between. This approach has been simplified by using the equal displacement approach across the complete period range. The public comment response was that this simplification was unnecessary and would create further confusion when /if displacement spectra were developed as the basis of displacement design. The Working Group were asked to review and recommend inelastic spectra derivation method.

Verification Procedure Table

Development of a suitable table which can be used to determine the appropriate methods of design. The suggestion to separate housing from other structures and to include both non-ductile and irregular building penalties is to be investigated. (Refer comments from WG20)

Expectation is that the robustness provisions from Part 0.6 will be sufficient for VP 1 buildings which will control Category IV buildings in low and moderate seismicity areas of Australia, but will not be applicable in NZ.

VP2 will cover low seismicity regions of NZ (Auckland north and Coastal Otago) together with high seismic regions of Australia and some Category II buildings in Australia or some of those on very soft soils. These design methods will involve either a direct capacity design approach or a capacity design methodology which is incorporated into the material standards for limited ductile buildings. Guidance will be required for the material standards development to comply.

VP3 will involve full capacity design procedures to be apply. This will be applicable to buildings in the high seismicity regions of NZ and to Category II and III buildings in NZ and Category II buildings in Australia.

Reconsideration of the Structural Performance Factor, Sp

The working party is to calibrate the Structural performance factor against the revised design spectra in an attempt to explain the apparent reduction of design levels apparent particularly in the higher period range.

General Torsional Requirements and Accidental Eccentricity

The suggestions regarding accidental eccentricity proposed by Prof Paulay are to be investigated as the basis for considering this phenomenon. This is to be considered along with the provisions for torsional considerations particularly in the near collapse, post elastic condition. TP is to provide suggestions. Different approaches may be needed for VP2 and VP3 design methods.

P-D second order effects

The committee are concerned that the current provisions were too severe, particularly the point when they are required to be considered. The working group was asked to investigate/verify alternative provisions. It was acknowledged that this may be difficult within the time frame of this standards revision.

Basis for designing building elements (Parts)

An alternative proposal put forward by Dr Restrepo (University of Canterbury) was considered. The working group was asked to assess the implications and background of the approach being proposed and to address the general committee concern that the levels of loading within the draft are too severe. Such an investigation should also include consideration as to whether an alternative basis for design (Velocity or displacement) was practical. Consideration as to the expected performance of building elements was also proposed whereby elements essential to the ongoing performance of the building, yet which themselves to not represent a high hazard may be permitted to fail at design levels between SLS and ULS. High hazard items and those within essential facilities could be required to perform/survive/avoid collapse when subjected to the full ULS design level.

Design for Rocking Foundations

The current provisions within NZS 4203 permitting foundations to rock during the capacity design assessment needs to be reconsidered and either this or an alternative be reintroduced into the proposed revision.

Classification/description of Capacity Design

The definition and application of Capacity Design needs to be reconsidered and reintroduced into the body of the standard. These principles are to apply to both primary structural elements and also to secondary elements such as precast panels etc. Guidance to the material standards writers is needed to ensure these provisions are understood and applied in a consistent manner to all materials.

Reintroduction of Integrated Time History Analysis

The Time History analysis appendix is to be brought into the body of the standard. The working group is to review these provisions, particularly the requirements relating to the selection and scaling of the ground motion. Selected external input by practitioners conversant with these methods of design is expected.

Performance Objectives

Comment on the three performance objectives stated in Section 1 are to be reviewed by the working group and issues relating to the redefinition of these terms within the Standard (cf the BCA or NZBC) need to be addressed.