Learning from earthquakes and the US plan for coordinating post-earthquake investigations

Thomas Holzer¹

Careful documentation of earthquake effects is essential to improve prediction and mapping of seismic hazards, to design safer engineered structures, to refine loss modeling, and to formulate better public policy. Nevertheless, post-earthquake investigations typically are conducted with little coordination among a diverse range of professional specialists because damaging earthquakes are infrequent. Recently the U.S. National Earthquake Hazards Program (NEHRP) developed a Plan to coordinate technical post-earthquake investigations. The Plan requires that the NEHRP agencies and their partners convene immediately after a significant earthquake and decide whether or not to formally implement the Plan. Upon implementation, a physical technical information clearinghouse and web site are established to coordinate the immediate post-earthquake reconnaissance (Phase I). All field investigators regardless of affiliation are encouraged to work within the clearinghouse. The Plan requires that a NEHRP Investigations Coordinator (NIC) be appointed within 24 hours to oversee the coordination, to establish long-term research priorities, and to ensure appropriate liaison with emergency managers. The NIC also has the responsibility to convene a meeting at the conclusion of the reconnaissance to establish priorities for substantive collection of perishable data by NEHRP funded investigators (Phase II). In about one month after the event, the Plan calls for a workshop to establish priorities for long-term research to be funded by NEHRP agencies (Phase III). The Plan also recognized that more systematic collection and archiving of observations from post-earthquake investigations are needed in the United States, particularly with regards to damage and loss data.

The Plan recognizes that a fundamental tenet of emergency management is that the execution of a plan is as important as the plan itself. Thus, upon completion of the Plan, three exercises were conducted with “surprise” earthquake scenarios to familiarize the NEHRP agencies with the Plan. In the likely absence of frequent opportunities to implement the Plan, an annual review of the Plan was also recommended to maintain institutional familiarity with it and to ensure it is up to date.

¹ U.S. Geological Survey, Menlo Park, CA

Paper 01

Implementing the Building Act 2004 – Wellington City Approach

Claire Stevens¹ and Katharine Wheeler¹

As part of the implementation of the Building Act 2004, Wellington City Council (WCC) was required to develop and adopt a policy on earthquake prone buildings within its district. Implementation of this new policy was started in July 2006.

Initially WCC undertook some code comparisons for the Wellington region to identify a date after which buildings were unlikely to be earthquake prone and created a database to identify the buildings that would meet the selection criteria for further assessment.

The policy identifies the assessment tool as the Initial Evaluation Procedure (IEP) prescribed by Assessment and Improvement of the Structural Performance of Buildings in Earthquakes, a guideline developed by New Zealand Society for Earthquake Engineering (NZSEE) and the process that WCC will embark on once a building has been assessed as having an IEP score of less than 34.

This paper discusses the approach adopted by WCC, some of the challenges that have arisen and knowledge that has been gained in the process.

¹ Wellington City Council

Paper 70

Reflections on aspects of New Zealand’s seismic resilience: comparisons with Californian practice

Andrew Charleson¹

Observing and reflecting on the earthquake practices of another country is one means of assessing the relative progress being made in one’s own country towards improving its seismic resilience. During a three month stay in California, personal observations broadly linked to structural engineering issues were made of a range of earthquake engineering endeavours. The areas discussed in the paper comprise seismic hazard awareness, construction of residential buildings, seismic retrofitting of earthquake-prone buildings, new construction, introduction of new technologies and architectural detailing for seismic movements. Overall, it appears New Zealand practice is in step with that of California. Even though both regions appear to have some similar strengths and weaknesses there are a number ways New Zealand can learn from current Californian practice.

¹ Victoria University of Wellington

Paper 59

Resilience Urgently Required for a Brittle System Producing Brittle Buildings

John Scarry¹

A serious decline in professional structural engineering and construction standards has been occurring in English speaking countries of New Zealand’s type for many years now, but, for a large number of reasons has been the worst affected. One paradoxical situation causing major problems is the fact that, whereas all heavily populated urban areas in New Zealand can be subjected to a devastating seismic event at any time, the actual return period of such events is far greater than in other states that take earthquake resistant design seriously, such as California, Japan and Chile. Hence, highly suspect NZ design and construction practices are not exposed and ‘weeded out,’ but tend to accumulate.

With particular reference to the well documented Reason Error Prevention Model developed by Professor James Reason, the author shows how, in New Zealand at this time, all too frequently the numerous parties involved in the design, review, detailing, fabrication, construction and inspection of a particular structure are not only committing the most basic, obvious and very serious mistakes and bad practices, they are not noticing and preventing those made by others.

Within the actual design process itself, the author shows that all too often, the common ‘piece meal’ approach to design leads not to a resilient, robust, redundant ductile design with positive, dependable load paths, but instead to a fundamentally flawed seismic resistant structure, with weakest links where the links should be strongest.

¹ John Scarry Engineering, Auckland

Paper 07

The 20 December 2007 Gisborne Earthquake and GeoNet observations of it

Graeme McVerry¹ and Sergey Samsonov¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ GNS Science

Poster 1

The built environment of Gisborne, prior to the earthquake, and immediately after it

Ian Petty¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ Gisborne District Council

Poster 2

The rapid building safety evaluation (triaging) of CBD buildings

David Brunsdon¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ NZSEE Working Party on Integrated Planning for Earthquake Response

Poster 3

The earthquake’s impacts on buildings and lifelines

Noel Evans¹ and John Wells¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ OPUS

Poster 4

The earthquake's impacts on particular buildings

Peter Smith¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ Spencer Holmes

Poster 5

The EQC response and the earthquake's impacts on residential properties

Lance Dixon¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ EQC

Poster 6

Key matters arising from the earthquake experience

David Hopkins¹

This presentation may be viewed at www.nzsee.org.nz after the conference.

¹ Department of Building and Housing

Poster 7

Seismic Loss Estimation for Efficient Decision Making

Brendon Bradley¹, Rajesh Dhakal¹, Misko Cubrinovski¹, Greg MacRae¹ and Dominic Lee²

In order to incorporate seismic risk of facilities into a decision making framework, procedures are needed to quantify such risk for stakeholders. Seismic loss estimation methods combine seismic hazard, structural response, damage fragility, and damage consequences to allow quantification of seismic risk. This paper presents a loss estimation methodology which allows various means of quantifying seismic risk of a specific facility. The methodology is component-based and can therefore distinguish between different structural configurations or different facility contents and is consistent with state-of-the-art loss assessment procedures. Loss is measured in the forms of direct structural and non-structural repair costs, and although not considered in the example, business disruption and occupant casualties can also be considered. This framework has been packaged in a computer code available for future dissemination in the public domain so that users need only to have a basic understanding of the methodology and the input data that is required. Discussion is given to the flexibility of the framework in terms of the rigour which can be employed at each of the main steps in the procedure. Via a case study of a high-rise office building, the use of the methodology in decision-making is illustrated. Methodological requirements and further research directions are discussed.

¹ Department of Civil and Natural Resources Engineering, University of Canterbury
² Department of Mathematics and Statistics, University of Canterbury

Paper 32

The Challenges of Reconstruction after the October 2005 Kashmir Earthquake

Hamid Mumtaz¹, Habib Mughal¹, Maggie Stephenson¹ and Jitendra Bothara²

The 8th October 2005 Kashmir Earthquake was one the largest earthquakes in Northern Pakistan in its recorded history. It caused an unprecedented level of damage and destruction in Pakistan Administered Kashmir (PAK) and the North Western Frontier Province (NWFP). It damaged or collapsed more than 0.6 million buildings - leaving 3.5 million people shelter less as winter approached. A large part of the earthquake-affected area is difficult to access and highly snow-prone, with rugged terrain and scattered settlements. It posed unique challenges and efforts on a massive-scale for reconstruction. For residential buildings, the Pakistan government adopted a house-owner driven approach. The reconstruction policy stated that the government and other agencies would provide equal technical assistance and subsidy to each family, without differentiating between who lost what. To increase capacity in earthquake-resistant construction, there was large-scale training of artisans, technicians, engineers, and community mobilisers. Campaigns to “build back better” have raised awareness in the communities. Local Housing Reconstruction Centres have been established for training, advice, and dissemination of earthquake-resistant technology. This decentralised approach has helped in achieving reconstruction smoothly. This paper will present the authors’ first-hand experience in the reconstruction effort, and the opportunities and unique challenges faced.

¹ UN-HABITAT, Pakistan
² Beca Carter Hollings Ferner Ltd, Wellington

Paper 34

Appropriateness of Seismic Strengthening Interventions in Heritage Buildings: A Framework for Appraisal

Alistair Cattanach¹, Gareth Alley¹ and Adam Thornton¹

New Zealand is entering a critical era in the preservation of its heritage buildings, resulting from the Earthquake-Prone Building provisions of the Building Act 2004. Structural Engineers are often given the task of designing what they believe to be the most appropriate intervention for the building, often without necessarily having the “heritage” skills or external review specialists to aid their decisions.

The following paper suggests a framework under which the appropriateness of structural interventions may be measured. The level of performance of primary and secondary elements are discussed, as well as criteria to judge the appropriateness of each intervention into the historic fabric.

The recent upgrade of Shed 13 on Wellington Waterfront, recipient of a NZIA (Heritage) Architecture Award, is discussed as a case in point. The strengthening utilises a combination of un-bonded post-tensioning of the existing brickwork and hysteretic energy dissipaters as primary load-resisting elements.

¹ Dunning Thornton Consultants Ltd.

Paper 30

Performance Assessment of Existing Buildings in New Zealand

S.R. Uma¹, Jitendra Bothara², Rob Jury² and Andrew King¹

Earthquake risk assessment of the New Zealand building inventory requires assessing the physical vulnerability of building assets to the required level of hazard. The lateral strength and deformation capacity of building categories differ due to various reasons including the construction vintage, material of construction and other structural characteristics. One of the biggest challenges in deriving the physical vulnerability model is to acquire an appropriate building inventory model. In this paper, the types of building structures in New Zealand are reviewed and a suitable classification scheme is proposed. The implications of code recommendations on the existing building stock are discussed to underline the inherent building characteristics with respect to the building system and the vintage of the code. Further, to obtain the lateral strength and deformation profiles of the buildings, a rational procedure is developed based on the lines suggested for Initial and Detailed Assessment Procedures recommended by a study group of the New Zealand Society for Earthquake Engineering. In this paper, fragility functions are developed for typical wooden houses, reinforced-concrete moment resisting frames and shear-wall structures, which predominantly comprise the New Zealand inventory.

¹ Institute of Geological and Nuclear Sciences, Lower Hutt
² Beca Carter Holdings and Ferner Ltd., Wellington

Paper 45

Seismic design of bridge structures with allowance for large relative girder movements to avoid pounding

Nawawi Chouw¹ and Hong Hao²

Pounding between bridge girders has been observed in almost all previous major earthquakes. This is because conventional bridge expansion joint gaps are usually only a few centimetres, which is insufficient to avoid pounding. There are often large relative displacements between bridge girders, caused by differing vibration properties of adjacent bridge spans, varying ground motions at bridge supports and varying soil-structure interaction (SSI). A new bridge expansion joint design method is introduced in this paper. Instead of tolerating pounding and providing possible mitigating measures, the proposed design approach provides for large movement between bridge girders, which entirely avoids pounding. The new expansion joint is called a Modular Expansion Joint (MEJ). Large movements are obtained by providing a number of small gaps within the joint. This study investigates the MEJ gap size required to completely avoid girder pounding. The most significant influence factors, namely the varying vibration properties of adjacent bridge spans, the effect of SSI and ground motion spatial variation on expansion joint size required to preclude pounding, are calculated. The relative importances of the various structural and ground motion properties that generate relative displacements of adjacent bridge girders are discussed.

¹ University of Auckland
² University of Western Australia

Paper 10

Effect of bearing characteristics on the response of friction pendulum base-isolated buildings under three components of earthquake excitation

Montazar Rabiei¹

This investigation has been conducted to examine the effect of bearing characteristics on the response of friction pendulum base-isolated buildings under three components of earthquake excitation. The structure is idealized as a three-dimensional single-story building resting on a friction pendulum bearing. The coupled differential equations of motion for the isolated system are derived and solved in the incremental form using Newmark’s step-by-step method of integration. The response of this system subjected to three components (including vertical component), two components (excluding vertical component) and single component (excluding vertical component and no interaction between orthogonal directions) of Tabas 1940 and two records of Northridge 1994 earthquakes are investigated. The effect that the vertical component of the earthquake has on the peak values of both the bearing displacement and the base shear of the isolated structure is investigated. This includes variation of the bearing characteristics, such as the isolation period and friction coefficient of the sliding surface. It is demonstrated that bearing characteristics could significantly influence the response of friction pendulum base-isolated structure subjected to three components of earthquake excitation.

¹ AmirKabir university of technology, Tehran, Iran

Paper 17

A Ground Shaking Amplification Map for New Zealand

Umut Destegul¹, Grant Dellow¹ and David Heron¹

A ground shaking amplification map of New Zealand has been compiled from data held by GNS Science. The resulting map is being used in RiskScape, a tool for comparing risks at a given site from a variety of hazards by estimating potential losses.

A GIS-based geological map with national coverage has been compiled from several sources, and is used as the base data. Geological maps at a scale of 1:250,000 from the QMAP project which is the geological mapping of New Zealand have been used where available supplemented with detailed geological maps at scales ranging from 1:25,000 to 1:50,000 for the larger urban areas. The gaps have been filled by the 1:1,000,000 ‘Geological Map of New Zealand’.

Every geological polygon in the composite geological map has been assigned one of the ground shaking amplification classes from the New Zealand Standard for Structural Design Actions NZS1170.5 to produce the product map. The classes conform to the site class definitions in NZS1170.5, which describes five classes with respect to ground shaking amplification. Assignment of these classes was straightforward for rock sites but more involved for soils where, for example, at boundaries between weak rock and deep soil sites a buffer zone of shallow soil was applied.

The product map was checked by comparing it against 687 sites in a database of accelerograph locations where the ground class had been determined from site-specific information. Currently, for 72% of the sites the NZS1170.5 site class is the same for both the site specific data and the site class assigned to the geological polygon.

¹ GNS Science

Paper 41

Design of Retaining Walls for Outward Displacement in Earthquakes

John Wood¹

It is often not practical to design major retaining walls and bridge abutment walls to resist the peak ground accelerations expected in the design level earthquake in regions of high seismicity. The current Transit NZ Bridge Manual requires structures on important routes to be designed for a 2,500 year return period event and this leads to high design level accelerations even in areas of moderate seismicity. A satisfactory design approach is to design for a resistance level less than the peak ground acceleration and accept some outward movement. Outward movements can be computed using the Newmark sliding block method. However, published procedures for applying the Newmark method to walls have been numerous and there are variations in analysis complexities including whether or not to allow for vertical acceleration effects. It is not clear which approach is best for design applications and what values of peak ground velocity, required as one of the input parameters, should be used to be consistent with the provisions of NZS 1170.5 and the Transit NZ Bridge Manual.

The paper compares the most widely used of the numerical and empirical methods of applying the Newmark method including Matthewson et al (1980), Ambraseys and Menu (1988), Cai and Bathurst (1995), Bray and Travasarou (2007), and Jibson(2007). Recommendations are made as to the best approach for wall design applications and charts are presented that enable outward displacements to be rapidly estimated when using the Transit Bridge Manual Provisions and NZS 1170.5.

¹ John Wood Consulting

Paper 12

Nonlinear Foundation Response of Liquid Storage Tanks under Seismic Loading

Michael Chung¹ and Tam Larkin²

This paper presents a brief review of liquid storage tank performance under earthquake loading. The dynamic properties of a tank system are also addressed. Emphasis is placed on the development of a nonlinear analytical model in the time domain. The model captures the important aspects of foundation compliance, including both foundation uplift and soil yielding beneath the tank base. Experimental and analytical work performed by Bartlett (1976) provides the framework for this work. Results predict that the response of tanks under lateral loading may be significantly influenced by soil structure interaction effects. These effects may increase or decrease seismic loading depending on the individual circumstances of the tank and foundation.

¹ Connell Wagner
² University of Auckland

Paper 43

Revised NZSEE Recommendations for Seismic Design of Storage Tanks

David Whittaker¹ and Dean Saunders¹

The 1986 NZSEE document Recommendations for Seismic Design of Storage Tanks has been updated and will be republished during 2008. The original publication has been widely used and acknowledged internationally. Since 1986 there have been substantial changes to legislation and applicable standards in New Zealand and internationally. The design basis used in the 1986 document assumed no yield or damage being permitted to tanks under the design earthquake loading, and therefore led to some conservatism in the design of large steel storage tanks. The 2008 document bases design seismic loads for tanks on the recently issued national Standard for loads for buildings in New Zealand, NZS 1170.5. Design seismic loads are based on the ductility and damping applicable to tank behaviour. Modest levels of ductility (or force reduction) are permitted for steel tanks on grade, which generally reduces the load demands from those given previously. Benchmarking comparisons of the revised approach against the previous document and other relevant codes suggest that the proposed approach is reasonable. NZSEE is aiming to have this document recognised as a code of practice in New Zealand. The document has been structured in a way that it could also be used with other international codes.

¹ Beca International Ltd

Paper 04

Earthquake performance and permanent displacements of shallow foundations

Jeremy Toh¹ and Michael Pender²

The standard approach to seismic design of shallow foundations is equivalent to ensuring that the bearing strength factor of safety does not fall below a certain value. However, brief instances of bearing failure (yielding) during an earthquake may not necessarily be serious. A more important consideration will be the residual foundation displacements accumulated at the end of the earthquake. Macro-elements provide a simple way of capturing the essential features of soil-foundation interaction, including the residual foundation displacements, and are readily amenable to routine structural analysis.

The shallow foundation macro-element examined in this study is based on existing macro-elements. It accounts for both the elastic (small displacement) and plastic (yielding) phases of dynamic behaviour. Modelling with the macro-element produces important insights into shallow foundation performance. Permanent vertical displacement (settlement) of the foundation is predicted to accumulate only during prolonged yielding, with the magnitude of residual settlement being dependent on the earthquake magnitude and duration, and the static vertical factor of safety against bearing failure. Residual rotation and translation of the foundation is inferred to be dependent on the characteristics of an individual earthquake.

¹ Tonkin and Taylor Ltd
² University of Auckland

Paper 40

Preliminary analysis on critical factors for restoration of water distribution pipelines in the Hutt City after a magnitude 7.5 earthquake from the Wellington fault

John Zhao¹, Jim Cousins¹, Biljana Lukovic¹ and Warwick Smith¹

The water distribution network for Hutt City consists of nearly 700 km of pipes, 6500 valves, 4000 hydrants, and 21 reservoirs located on the surrounding hills. The bulk water is pumped to hillside reservoirs from trunk lines and some water for the city is also taken from artesian wells located on the floor of the Lower Hutt Valley. The Wellington fault, capable of generating an MW 7.5 earthquake with a return period about 600 years, lies at the western side of the Hutt Valley and passes through both residential and commercial areas. The shaking intensity from this earthquake is expected to be MMI 9 (where MMI is Modified Mercalli Intensity) or greater over the entire City. We simulate the damage to the pipelines using fragility curves derived from overseas data, which are functions of MMI, material type and soil condition. We use a Poisson process to generate damage locations along a pipe, given an expected break rate, then repair the network using an algorithm based on a likely restoration sequence. First all valves are closed. Then starting from a reservoir, a valve is opened and pressure is established. We account for pipe repair time (a function of pipe diameter), opening and closing of valves, checking hydrants and travelling from one location to another. Multiple crew repair parties are used. In the present study, we have excluded the repair time required for damages to the bulk supply pipes (trunk lines) that carry water from water treatment plants to the reservoirs of the surrounding hills in the Hutt City, the reservoirs, wells and the customers own pipes that link individual properties to the council-owned distribution system.

¹ Institute of Geological & Nuclear Sciences, Lower Hutt, Avalon

Paper 51

Ground motion records for time-history analysis of URM buildings in New Zealand – The North Island

Claudio Oyarzo-Vera¹, Graeme McVerry² and Jason Ingham³

The New Zealand Standard for Structural Design Actions, NZS 1170.5:2004, defines a criterion to select ground motion records for time-history analysis based on similarity between the seismological signature of earthquakes used for the analysis and those that are expected to be encountered at a given location. However, as most structural designers are not familiar with the specific details of the probabilistic seismic hazard model used to determine the design spectra, further information is currently required before designers can readily select appropriate earthquake records.

The objective of the study reported here was to integrate the seismologist’s and the engineer’s specialist knowledge, and to present a method to select the best set of records for different locations considering the hazard level for each location and the seismological characteristic of the expected ground motions. For this purpose has been divided into several seismological hazard zones considering the mapping of the Hazard Factor presented in NZS 1170.5:2004, and the fault mechanism. Furthermore, a suite of records is proposed for use when conducting time-history analysis of existing New Zealand unreinforced masonry (URM) buildings, satisfying the Standard requirements. Recommendations are presented for selecting the records to be used in each zone. Preliminary results are presented for the North Island of New Zealand. A similar proposal for the South Island is still under study.

¹ Universidad Catolica de la Santisima Concepcion, Chile.
² Institute of Geological and Nuclear Science, Lower Hutt
³ The University of Auckland

Paper 25

Internal forces of concrete floor diaphragms in multi-storey buildings

Debra Gardiner¹, Des Bull¹ and Athol Carr¹

Simplistic design methods are commonly employed by design engineers to determine the approximate magnitude and distribution of inertial forces in reinforced concrete floor diaphragms of multi-storey buildings. Various researchers have identified that the commonly employed simplistic design method, the Equivalent Static Analysis method, in some cases, provides a poor representation of the true structural response. This research investigates the magnitude and trends of forces in concrete floor diaphragms, with an emphasis of transfer forces, under seismic loading. This research considers the following items: inertial forces which develop from the acceleration of the floor mass; transfer forces which develop from the interaction of lateral force resisting elements with different deformation patterns, such as wall and frame elements; and variation of transfer forces due to different strengths and stiffness of the structural elements. The magnitude and trends of forces in the floor diaphragms have been determined using 2-dimensional inelastic time history analysis. Trends have been identified which will aid the improvement of seismic floor diaphragm design methods.

¹ University of Canterbury

Paper 21

Seismic Performance of Hollow-core Flooring: the Significance of Negative Bending Moments

Lisa Woods¹, Richard Fenwick² and Des Bull¹

Hollow-core flooring units, as described in the technical literature, are intended to be used as simply supported members. However, in construction continuity is often established between the units and supporting structure by the addition of insitu topping concrete and reinforcement. This change in structural form can result in negative moments and axial forces being induced in the floor by gravity loads, wind and seismic actions. Vertical seismic ground motion in particular can make a significant contribution to negative moments induced in the floor. This paper focuses on two failure mechanisms which may occur in negative moment regions of hollow-core floors, namely a flexural failure and a shear failure. It is shown that, with the detailing in common use prior to the release of the Structural Concrete Standard, NZS 3101-2006, there is a potential for brittle negative moment failure to occur under seismic conditions. Analytical work indicates that under some conditions a diagonal tension (shear) failure may also occur. As the failure of a floor may lead to progressive collapse it is important that these two aspects are considered along with a number of other potential failure modes in the retrofit or design of buildings. Guidance is given on methods of assessing the negative moment flexural strength and shear strength of hollow-core floors.

¹ University of Canterbury and Holmes Consulting Group
² University of Canterbury, Visitor, Christchurch

Paper 24

Experimental study on the seismic performance of RC moment resisting frames with precast-prestressed floor units

Brian Peng¹, Richard Fenwick¹, Rajesh Dhakal¹ and Des Bull¹

A three dimensional approximately half scale experimental sub-assemblage is currently being tested at the University of Canterbury to investigate the effect of precast-prestressed floor units, which do not span past the internal columns, on the seismic performance of reinforced concrete moment resisting frames. This paper reports the preliminary results from the test, with the focus on elongation within the plastic hinges and strength enhancement in the frames. The preliminary results have shown that elongation between the external and internal plastic hinges vary by more than two fold. With the addition of the prestressed floor units, the strength of the moment resisting frame used in the test was found to be 25% higher than the current code specified value. In other situations, particularly where there are more than 2 bays in a moment resisting frame, greater strength enhancement may be expected. Any under-estimation of beam strength is undesirable as it may result in the development of non-ductile failure modes in a major earthquake.

¹ University of Canterbury

Paper 37

The Damage Avoidance Design of tall steel frame buildings - Fairlie Terrace Student Accommodation Project, Victoria University of Wellington.

Sean Gledhill¹, Geoff Sidwell¹ and Darrin Bell¹

Recent advances in Seismic Engineering have focused on a “Damage Avoidance Design” philosophy, whereby a structure is designed to withstand a major seismic event with minimal and repairable damage. This typically involves incorporating mechanisms in the structure that can control loads and sustain large deformations without causing damage.

At the request of Victoria University of Wellington, Connell Wagner was tasked to undertake the design of the new high-rise student accommodation Buildings at 74-87 Fairlie Terrace, to incorporate a “Damage Avoidance” philosophy.

The challenge to create damage avoidance design features was complicated by the building form. Typically short, stiff heavy buildings with low periods are suited to base isolation, but few options are available for tall, relatively light, flexible steel buildings. Damage avoidance features available on the market are often viewed as expensive and complicated and have not been widely utilised.

To meet the challenge we developed a cost effective new system for damage avoidance applicable to high-rise steel framed buildings. The system utilised research conducted by HERA and the University of Auckland.

The Damage Avoidance system featured coupled concentrically braced frames with prestressed Ringfeeder Springs and sliding hinge joints between columns and foundation. The system also incorporates steel beams with Sliding Hinge Joints which we refined in conjunction with HERA.

This paper outlines the possible damage avoidance design solutions considered for this project and reviews and outlines the concepts and application of the chosen system. The paper does not address the methods of analysis or detailed design techniques used to refine the system.

¹ Connell Wagner, Wellington

Paper 63

Plastic shear strength of continuous reinforced beams

Col Gurley¹

This paper addresses the theoretical, rigid-plastic yield-line collapse-mechanism analysis of continuous, reinforced concrete beams considered as two-dimensional, plane-stress problems including mixed shear/bending mechanisms. It provides a range of collapse mechanisms that may be sufficient for ‘exact’ analysis using simple calculations of yield segments as free-body static equilibrium problems across the range of ductile-frame beams from long-span, gravity-dominated examples to deep coupling-beams. The analysis assumes that beams are continuous with stronger columns or walls of equal width/thickness thereby avoiding issues related to bearing details. The methods proposed do need to be calibrated to experimental data and, perhaps, they will also assist experimental researchers.

A related issue is that plastic design of steel structures has long been restricted to elements appropriately detailed to ensure ductile failure, for example, by limiting members to sections that are ‘compact’ enough to prevent premature local buckling of webs or flanges under plastic rotations. Perhaps the plastic design of concrete structures should be more explicitly restricted to elements appropriately detailed for ductility despite the tensile weakness of concrete. This suggests stronger rules for minimum reinforcement content in both directions in the web/mid-depth of concrete beams. This issue is discussed in Appendix A, and Appendix B summarises the relevant suggestions of M.P.Nielsen 1999. Appendix C describes an extension to elasto-plastic analysis where there do seem to be some problems.

¹ TAFENSW Sydney Institute

Paper 19

Finite Element Analysis of Old Steel Buildings in NZ

Majid Naderi¹

In the early 20th century, steel frame buildings were built to different standards from those used in modern construction. Riveted, built-up members were used instead of rolled sections, with joints and members encased in concrete for fire protection. These early steel buildings were designed based upon observations of past building performance rather than through detailed calculations and predictions of structural behaviour. The walls were infill masonry and floors were typically reinforced concrete. The strength and stiffness of the semi-rigid connections and masonry infill as well as the effect of floor slabs integral with their supporting beams are not well documented.

Although riveted stiffened seat angle connections are not designed to resist moments, they can develop a considerable moment capacity and exhibit a relatively ductile hysteretic behaviour which could be beneficially considered when evaluating frames built of these connections and subjected to small and moderate earthquakes. Structural engineers have found it challenging to make realistic predictions of the seismic performance of these buildings, many which are quite prestigious, in full service and often enjoying heritage protection. Examples include Auckland’s Britomart Station and Guardian Trust Building, and Wellington’s Tower Corporation, Prudential Assurance and Hope-Gibbons Buildings.

To predict the complicated behaviour of riveted connections, 3D nonlinear finite element models of a sample clip-angle connection taken from drawings of the now-demolished Jean-Batten building in Auckland and a sample T-stub connection taken from drawings of the Hope-Gibbons building have been generated using the ABAQUS finite element software package. The joint models were used to investigate load-displacement, failure mode and energy dissipation under axial loads, shear loads, and combined axial loads plus shear loads.

¹ University of Auckland

Paper 06

Assessment of material strain limits for defining plastic regions in concrete structures

Adam Walker¹ and Rajesh Dhakal¹

The New Zealand Structural Loadings Standard, until its latest revision, used the structural ductility factor as a measure of the deformation demand of all potential plastic hinges in a structure. In the new version of New Zealand Standard for Earthquake Actions (NZS 1170.5:2004) the detailing of potential plastic regions is determined according to the local deformation demand in these regions. The change has been prompted by evidence that the structural ductility factor gives a poor indication of the demand on individual plastic regions. This new approach has also been adopted by the revised New Zealand Concrete Structures Standard (NZS 3101:2006) which classifies potential plastic regions into three categories (namely ductile, limited ductile and nominally ductile) based upon their inelastic deformation demand specified in terms of material strain limits. The material strain limits currently set in NZS 3101:2006 for the three categories of plastic regions are based on limited experimental evidence and need a closer revision. This paper tries to obtain more justifiable values of material strain limits based on experimental data. In this research, reversed cyclic loading tests of beams are conducted to compensate for a lack of data in the nominally ductile range of detailing. Based on the results of the tests conducted, curvature limits for nominally ductile plastic hinges are derived. Combining the experimental results collected from literature and the tests conducted in this project, updated material strain limits for the three categories of plastic regions are proposed. To unify the design process for all types of plastic regions, curvature limits for nominally ductile plastic hinges are also proposed as the multiple of first yield curvature (similar to the existing approach for the other two categories of plastic regions) rather than the existing approach of specifying allowable compressive (concrete) and tensile (rebar) strain limits for nominally ductile plastic regions. To further simplify the process, the representative value of first yield curvature is approximated as two times the yielding strain to the beam height ratio, thereby relieving the designers from having to conduct section analysis to estimate neutral axis depth.

¹ University of Canterbury

Paper 09

Seismic Performance Assessment of Inadequately Detailed Reinforced Concrete Columns

Al Boys¹, Des Bull¹ and Stefano Pampanin¹

Existing New Zealand building stock contains a significant number of structures designed prior to 1995 with inadequate detailing of the internal or ‘gravity’ reinforced concrete (RC) columns. Typically these columns have insufficient transverse reinforcement; lap-splices in the plastic hinge region; and longitudinal bars that are ‘cranked’ at the end of the lap-splice. Columns with such details have been shown to perform poorly when subjected to seismic demand, losing axial load carrying capacity at drift levels less than the building is expected to be subjected to during a design level earthquake.

This paper outlines an investigative program to determine the susceptibility of these gravity columns to axial collapse. A drift based backbone capacity model for shear and subsequent axial failure is presented which has been verified by experimental testing performed to date. Such experimental tests have highlighted the susceptibility of these inadequately detailed columns to lose axial load capacity at drift levels significantly below the seismic demand on such structures due to a design level earthquake.

¹ University of Canterbury

Paper 29

Forced vibration testing of a thirteen storey concrete building

Faisal Shabbir¹ and Piotr Omenzetter¹

Testing of structures to understand their behaviour under seismic conditions can provide an important source of information for safe and economical design. The need for testing the behaviour of full scale structures under dynamic loads stems from the fact that laboratory scale structures cannot account for all complexities involved. This paper describes forced vibration testing of a 13 storey reinforced concrete building having central lift core, shear walls and flat slabs, to find out its dynamic characteristics. Experiments have been carried out using the shakers and sensors within the NZNEES@Auckland Mobile Field Laboratory. Different positions of the shakers and sensors has been tried to determine optimal response of the structure. The field observations have been compared with finite element computer model. An effort has been made to synchronize a computer model with the field observations. It is emphasized that response of complex structural systems may be understood better by using the presented experimental and analytical tools.

¹ Department of Civil and Environmental Engineering, University of Auckland, Auckland

Paper 08

Visualisation and classification of dynamic structural health monitoring data for assessment of structural condition

O.R. de Lautour¹ and Piotr Omenzetter¹

The application of time series analysis methods to Structural Health Monitoring (SHM) is a relatively new and emerging technique. Time series methods are inherently suited to SHM where data is sampled regularly over a long period of time. This study focuses on detecting damage by classification of time series model coefficients. Autoregressive (AR) models were used to analyse acceleration time histories from a 3-storey bookshelf laboratory structure and the ASCE Phase II SHM Benchmark structure in both healthy and damaged states. Preliminary inspection of the AR coefficient data to check the presence of clusters corresponding to different damage severities was achieved using two-dimensional projections obtained from either Principal Component Analysis (PCA) or Sammon mapping. Two classification techniques, Nearest Neighbour (NN) and Learning Vector Quantization (LVQ) were used to classify damage into states based on analysis of the AR coefficients reduced in dimensionality using PCA. The results showed that NN classifiers performed well however, small gains could be made using LVQ.

¹ University of Auckland

Paper 03

Exploring the Feasibility of a Floor System Detached From Seismic Beams in Moment Resisting Frame Buildings

Rajesh Dhakal¹

In current practice, floor slab and beams in the perimeter seismic frames are monolithically constructed and rigidly connected to each other through starter bars. This rigid connection ensures that the shear friction between the floor and the seismic beam transfers the inertial force resulting from the response acceleration of the floor mass and any superimposed dead loads to the moment resisting seismic frame. But, this rigid connection between the floor and perimeter seismic beams leads to several complications such as: (i) possibility of stronger beam (than columns) because of the slab contribution on the negative moment capacity of seismic beams; (ii) possibility of (unidirectional) plastic hinge forming away from column face; and (iii) the floor-beam compatibility requirement leading to severe damage in the slab as the seismic beams deflect in double curvature and grow in length due to elongation of the plastic hinges at the same time. In a quest to avoid these complications, this paper investigates the feasibility of a floor slab that is detached completely from the perimeter seismic beams. In this system, the slab is rigidly connected to the intermediate beams, which are designed to transfer the inertial force to the columns through shear friction and/or torsional resistance. This idea is conceptually discussed and its validity is scrutinized in the paper.

¹ University of Canterbury, Christchurch

Paper 18

Experimental Investigation on a Hybrid Jointed Precast Frame with Non-tearing Floor Connections

Alejandro Amaris¹, Stefano Pampanin¹, Desmond Bull¹ and Athol Carr¹

The effects of beam elongation in precast frame systems have demonstrated a potential source of un-expected damage to precast floor systems, unless adequate detailing is provided to account for displacement incompatibilities between the lateral resisting systems and the floor.

The Precast Concrete Seismic Structural Systems (PRESSS) research program has shown the efficiency of dry-jointed ductile connections for moment resisting frames in order to reduce damage while sustaining high lateral loads. However, damage to precast floor systems, resulting from a geometric elongation of the beam, has yet to be addressed in detail.

In this contribution an overview of alternative solutions developed for post-tensioned “hybrid” connections is first provided. A series of quasi-static cyclic tests on a major 2-D, 2/3 scale, two story, single bay, precast concrete frame system with an innovative “non-tearing floor” connection is presented. The proposed solution consists of an efficient floor to lateral load resisting connection system which can mitigate the effects of geometric (and material) beam elongation. The response of the “non-tearing floor” solution is compared to alternative more traditional solutions.

Both numerical and experimental results confirm the unique flexibility of the proposed solution and highlight the superior performance under seismic loading sustaining only minor damage to the frame, wall and floor systems under major earthquake events.

¹ University of Canterbury

Paper 26

Architectural characterisation and prevalence of New Zealand's unreinforced masonry building stock

Alistair Russell¹ and Jason Ingham¹

Unreinforced masonry (URM) buildings remain New Zealand's most earthquake-prone structures. As part of an effort to develop retrofit solutions for upgrading the seismic performance of these structures, research into characterising the national building stock of URM buildings has been conducted.

This research classifies New Zealand URM buildings into typologies, based on their general structural configuration. Seven typologies are presented, and their relative prevalence, age and locations are identified. Based on these classifications, seismic vulnerability assessments will need to be conducted on a regional and national basis.

An estimate of the population and distribution, as well as the construction date, of existing URM structures in New Zealand is also presented. Effort is being developed to further improve the quality of data, in conjunction with exercises currently being conducted by local territorial authorities. The preponderance of these structures were constructed between 1920 and 1930.

¹ The University of Auckland

Paper 36

Design Procedure and Behaviour of the Advanced Flag-Shape (AFS) Systems for Moment-Resisting Frame Structures

Weng Y. Kam¹, Stefano Pampanin¹, Alessandro Palermo² and Athol Carr¹

The concept of Advanced Flag-Shaped (AFS) systems, in which alternative forms of energy dissipations (yielding, friction or viscous/visco-elastic damping) are combined in series and/or in parallel together with re-centering elements (un-bonded post-tensioning tendons or Smart memory alloy(SMA) elements), has been previously introduced by the authors. Based on numerical analyses on SDOF-systems, the unique combination of friction or hysteretic dampers in series with viscous dampers, further combined in parallel with re-centering and hysteretic dissipation elements, has been shown to be very effective in controlling both force and displacement responses for either far-field and near-fault ground motions. Experimental validation of the effectiveness of the systems based on shake-table testing on wall systems is presented in a companion paper.

In this contribution, the concept of AFS systems is extended to MDOF systems. Preliminary suggestions for a simplified design procedure for AFS connection systems are given within the framework of a Direct Displacement-Based Design (DDBD) approach. Using case-study prototypes of five-storey moment-resisting frame, incorporating four different connection systems, a comparative MDOF study is carried out by the means of non-linear time-history analyses using suites of far-field and near-fault earthquake excitations. The non-linear time history analysis results for both far-field and near-fault earthquakes provided satisfactory validation of the design procedure, though being, as expected, on the conservative side when dealing with velocity-dependent dissipating systems.

As per the results of SDOF systems, AFS systems appear to be capable of providing beneficial attribute to the response of a MDOF system, particularly when dealing with velocity-pulse earthquake record, typical of a near-field event. In addition to providing reduction of peak displacement/drift response and a negligible residual deformation, floor accelerations and column shears due to the higher mode effects are also lessened. In the global performance matrix, AFS systems would achieve a much higher performance level in comparison to the conventional systems. There is however, less than expected contribution from the excitation velocity on dampers’ energy dissipation up the building heights. Based on these results, an approximation for the velocity-dependent devices’ velocities at a given storey is proposed. In conclusion, a brief discussion on limits and potentials for the practical implementation of AFS systems is given, along with anticipation of ongoing and further investigations.

¹ University of Canterbury
² University of Canterbury and Technical University of Milan, Italy

Paper 38

Dynamic Testing of Precast, Post-Tensioned Rocking Wall Systems With Alternative Dissipating Solutions

Dion Marriott¹, Stefano Pampanin¹, Alessandro Palermo² and Desmond Bull¹

During the past two decades, the focus has been on the need to provide communities with structures that undergo minimal damage after an earthquake event while still being cost competitive. This has led to the development of high performance seismic resisting systems, and advances in design methodologies, in order respect this demand efficiently.

This paper presents the experimental response of four pre-cast, post-tensioned rocking wall systems tested on the shake-table at the University of Canterbury. The wall systems were designed as a retrofit solution for an existing frame building, but are equally applicable for use in new design. Design of the wall followed a performance-based retrofit strategy in which structural limit states appropriate to both the post-tensioned wall and the existing building were considered.

Dissipation for each of the four post-tensioned walls was provided via externally mounted devices, located in parallel to post-tensioned tendons for re-centring. This allowed the dissipation devices to be easily replaced or inspected following a major earthquake. Each wall was installed with viscous fluid dampers, tension-compression yielding steel dampers, a combination of both or no devices at all – thus relying on contact damping alone. The effectiveness of both velocity and displacement dependant dissipation are investigated for protection against far-field and velocity-pulse ground motion characteristics.

The experimental results validate the behaviour of ‘Advanced Flag-Shape’ rocking, dissipating solutions which have been recently proposed and numerically tested. Maximum displacements and material strains were well controlled and within acceptable bounds, and residual deformations were minimal due to the re-centring contribution from the post-tensioned tendons. Damage was confined to inelastic yielding (or fluid damping) of the external dampers.

¹ University of Canterbury
² Technical University of Milan, Italy

Paper 39

Qualification of Fibre-Optic Gyroscopes for Civil Engineering Applications

Roberto Franco-Anaya¹, Athol Carr¹ and Ulrich Schreiber²

This paper will outline the feasibility of the use of fibre-optic gyroscopes (FOGs) to measure rotation rates, rotations and displacements of civil engineering structures. FOGs are devices that utilise the Sagnac effect to detect mechanical rotations interferometrically from optical beams. They are compact, easy to install and, unlike conventional potentiometers, do not require a fixed reference frame to operate. In this research, shake table tests were performed on a four-storey one-fifth scale structure equipped with a fibre-optic gyroscope. Four different earthquake ground motions were used in the experimental study. During the seismic testing, the FOG was first attached to one of the first floor columns and then to the third floor of the model structure. Relative displacements at the first floor and rotations at the third floor were calculated from the measurements provided by the FOG. A very good agreement was observed between the measurements obtained with the FOG and those provided by a conventional linear potentiometer. The experimental results validated the accuracy of the measurements recorded by the FOG as well as the dynamic range of the instrument. The FOG was also installed on the Sky Tower in Auckland to evaluate the displacements of the structure. A series of measurements were carried out on the 54th and 60th floors during three days. A sample of the measurements for the 54th floor of the Sky Tower is presented and interpreted.

¹ Department of Civil and Natural Resources Engineering, University of Canterbury
² Department of Physics and Astronomy, University of Canterbury

Paper 42

In-plane stiffness of wooden floor

Anna Brignola¹, Stefano Podest๠and Stefano Pampanin²

TThe seismic response of existing un-reinforced masonry (URM) buildings is strongly dependent on the characteristics of wooden floors and in particular on their in-plane stiffness and on the quality of the connections between the floors and the URM elements. It is generally well-recognized that adequate in plane-stiffness and proper connections improves the three-dimensional response of the whole system and provides better distribution and transfer of forces to the lateral load resisting walls. Extensive damage observed during past earthquakes on URM buildings of different type have however highlighted serious shortcomings of typical retrofit interventions adopted in the past with the intention to stiffen the diaphragm. Recent numerical investigations have also confirmed that stiffening the diaphragm is not necessarily going to lead to an improved response, sometimes actually having detrimental effects on the response.

The evaluation of the in-plane stiffness of timber floors in their as-built and retrofitted configuration is still an open question and delicate issue, with design guidelines and previous research results providing incomplete, when not controversial, suggestions to the practitioner engineers involved in the assessment and/or retrofit of these types of structures.

In this contribution, a summary of the state-of-the-art related to the role of the in-plane stiffness of timber floors in the seismic response of un-reinforced masonry buildings is presented and critical discussed based on the limited available experimental and numerical evidences. A framework for a performance-based assessment and retrofit strategy, capable of accounting for the effects of flexible diaphragm on the response prior and after the retrofit intervention, is then proposed. By controlling the in-plane stiffness of the diaphragm, adopting a specific strengthening (or weakening) intervention, the displacements, accelerations and internal forces demand can be maintained within targeted levels, in order to protect undesired local mechanisms and aim for a more appropriate hierarchy of strength within the whole system.

¹ University of Genoa, Italy
² University of Canterbury

Paper 49

Liquefaction Remediation by Compaction Grouting

Rolando Orense¹

After the 1995 Kobe Earthquake, the use of compaction grouting technology gained prominence in Japan due to subsequent retrofitting works, i.e., raising and re-leveling of multi-story apartment buildings which sank or tilted due to soil liquefaction. Following the success of these reconstruction programs, the technology has been used to solve a number of geotechnical problems, including remediation of liquefiable soils. This paper reviews two case histories of compaction grouting application in Japan for liquefaction remediation – the first one under new runways in a busy airport, while the second is under an existing structure. From the experiences derived from these projects, the lessons learned from the grouting process and the merits of compaction grouting as a practical method of liquefaction remediation are discussed.

¹ University of Auckland

Paper 50

Seismically Induced Landslide Mitigation Using Flexible Slope Stabilization and Protection Systems

Steve Farrand¹ and Anthony Teen²

Following earthquake events multiple soil slope failures are observed throughout the structurally fragile soil and rock in North Island Many of these slopes are located in close proximity to various assets including residential buildings and roads, resulting in damage to the structures. Limited mitigation solutions are available to stabilise both the global integrity and surficial slope-parallel instability, along with surficial erosion of the slope. High-tensile steel mesh, combined with an underlying three-dimensional geotextile, provides an effective, economical alternative with a greater design life than shotcrete and massive retaining structures. The flexible mesh is tensioned across the slope with a dimensioned grid pattern of soil/rock anchors that allows the system to be tailored to the site. Special concepts have been developed and are well proven for the dimensioning of the systems to consider superficial instabilities both slope parallel and in between the nails. The mesh promotes re-vegetation or greening of the slope for an aesthetically pleasing, natural looking finish. Numerous cases in New Zealand and internationally confirm that these measures are effective and practical for this application.

¹ Geovert (NZ) Ltd
² Geobrugg NZ Ltd

Paper 55

Lessons from the 2007 Asia Pacific IDEERS Seismic Resistant Design Competition

Rohann da Silva¹, Dmytro Dizhur¹, Ronald Lumantarna¹, John O'Hagan¹ and Quincy Ma¹

The Asia Pacific IDEERS competition (APEC IDEERS) is an international seismic resistant design competition with the aim of promoting earthquake engineering research to school and university students. The annual competition began in 2001 and is hosted by the National Center for Research on Earthquake Engineering (NCREE) in Taiwan. The competition involved designing and constructing the most efficient multi-storey model structures using the materials provided to resist artificial earthquakes as simulated on the NCREE shake table. In the 2007 competition, the rules for the model were substantially modified to allow for more creative designs and constraints were added to increase the level of difficulty. As a result, the prescribed structure was more complex and design strategies from previous years were no longer applicable

A team of four undergraduate students from the University of Auckland (UoA) represented New Zealand in the 2007 competition. The team competed against 35 other undergraduate teams from universities and polytechnics around Asia and Australia. The UoA team finished first in the competition, the highest placing achieved for a foreign team in the seven years of the competition. The final competition model made use of the conventional weak-beam strong column philosophy and incorporated numerous weight saving features. This paper details the UoA team’s design and testing process prior to the competition and documents the many lessons learnt from the trip.

¹ University of Auckland

Paper 61

Free vibration tests of a scale model of the South Rangitikei Railway Bridge

Quincy Ma¹ and M.H. Khan¹

The careful design and construction of a scale model of the South Rangitikei Railway Bridge (SRRB) are reported herein. The model was designed in accordance with stringent similitude requirements and a series of dynamic tests including shake table tests were conducted. This paper presents the preliminary results of the snap back tests only. It was not intended to exactly emulate to the actual SRRB but merely to use the SRRB as a realistic existing example, to investigate the general dynamic behaviour of a rocking structure. Despite this, the snap back tests revealed that the scale model matched previously published natural period of the SRRB prior to pier uplift. Furthermore, experimental results suggest the natural rocking period of the prototype SRRB could vary between 1.73 s and 4.33 s depending on the amplitude of lateral displacement. This reaffirms previous research which hinted the natural rocking period of a rocking structure is amplitude dependent and is a non stationary value.

¹ University of Auckland

Paper 62

Simple Design for Yielding Structures Subject to Torsion

Eu Ving Au¹, Gregory MacRae¹, Didier Pettinga², Bruce Deam¹ and Vinod Sadashiva¹

Building torsion resulting from building plan irregularity influences the demands on critical elements of a structure during earthquake shaking. Since inelastic torsion is not accounted for directly with the plane frame analysis tools commonly used in design, methods to estimate the likely variation in response due to torsion are required.

A significant body of literature exists related to the torsional response of structures during earthquakes. However, much of it is in a form which is complex, difficult to understand, or unsuitable for design. Perhaps the most simple and easily applied method for consideration of torsion in yielding structures was developed by Paulay in the 1990’s. However, this concept has often been criticized because it does not consider the inertial mass effect or out-of-plane walls, and his techniques are not appropriate for structures undergoing only moderate inelasticity.

This paper extends the concept of Paulay to consider the inertial mass effect as part of the dynamic response of classical single story structures which may or may not have out-of-plane walls. In this method, impulse ground motions are used to obtain general and unique solutions for structures which are appropriate for all levels of inelasticity.

A relationship between the increase in demand considering torsion for different earthquake records and different intensities is then statistically developed based on the general solution. It is shown that the torsional response due to impulse is similar to the average torsional response due to earthquake.

This work is easily understandable, applicable to all levels of inelasticity, and it provides a statistical degree of confidence that the inelastic response is not greater than a specified level. It also forms the basis for a design methodology.

¹ University of Canterbury
² Glotman•Simpson Consulting Engineers, Vancouver, Canada

Poster 13

Axial Shortening Effects of Steel Columns in Frames

Christopher Urmson¹, Gregory MacRae², Warren Walpole², Peter Moss², Karissa Hyde³ and Charles Clifton⁴

Steel members subject to axial compression and inelastic cyclic displacements, such as may occur during earthquake excitation, exhibit axial shortening due to material inelastic deformation irrespective of the occurrence of buckling. This column axial shortening can cause undesirable effects in the building, especially if it occurs to a different extent in different columns of a seismic-resisting system. This paper summarizes experimental and finite element studies to quantify the axial shortening of columns with known axial forces pushed to inelastic cyclic displacements. A flexural hinge model for a frame analysis program is developed and calibrated against that from experimental and analytical studies. Then, to quantify the effect of axial shortening on realistic moment and eccentrically-braced frames during earthquakes, inelastic dynamic time history analyses were conducted. While axial shortening of more than to 7% of the column length was obtained during experimental testing, the axial shortening was always less than 1% of the column interstorey height in the steel frames studied.

Steel members subject to constant axial compression and inelastic cyclic displacements exhibit axial shortening even before buckling occurs. Buckling exacerbates this column axial shortening. Such shortening, which may occur during an earthquake, can cause undesirable effects in the building, especially if it occurs to a different extent in different columns of a seismic-resisting system.

This paper describes experimental work, in which axial shortening decreases the length of some columns by more than 7%. Analytical studies using the finite element analysis program ABAQUS were conducted which confirm the experimental observations. A simple equation was developed to estimate the axial shortening as a function of the cumulative inelastic column rotation. This equation is independent of the assumed plastic hinge length of the column and it matches experimental and analytical results well.

In order to evaluate the likely amount of axial shortening on realistic frames during earthquake excitation, a moment frame and an eccentrically braced frame designed according to the NZ code, but which expect column yielding, were analyzed. It was found that the amount of shortening was generally less than 5mm in the most critical columns 2 MRF frames and 2 EBF frames analyses with the Los Angeles records scaled to Christchurch and Wellington design levels.

¹ Alan Reay Consultants and University of Canterbury
² University of Canterbury
³ MWH Global, Christchurch
⁴ University of Auckland

Poster 14

Tsunami Effects on Structures

Michael Hewson¹, Roger Nokes¹ and Gregory MacRae¹

A significant body of literature has been developed in the last few years to enable the maximum likely tsunami size at a particular location to be estimated within a certain time period. Also, recommendations for structural design considering tsunami have recently been released. However, these structural design considerations do not consider some of the significant load effects to which a structure may be subject.

This paper presents design equations which may be relevant for New Zealand. Also, the loading effect on a structure which may be open at the side facing the sea, but closed along either side and at the back is described. In this case, the impact of the sea wave on the back wall of the structure may be significantly greater than that on a rectangular structure because the side walls may cause funnelling of the incoming wave. In some cases the back of the structure may be partially open due to the presence of windows or doors which may be smashed as a result of the wave. These too may affect the forces on the back wall of the structure.

Forces on the back wall of a structure open at the front were investigated experimentally. Walls and the back wall were made of clear perspex and the water was coloured, so that when water behind a dam was suddenly released, the level of water at the structure at any interval of time could be captured by a high speed camera. Different initial dam heights, approach roughness, and sizes of opening at the back of the structure were considered. When the opening between the side walls and the back wall was high, it was as though the water was hitting a rectangular wall with no side walls. In addition to the initial impact of the water causing force on the back wall, splash and backwash effects were observed.

It was found that within the limitations of the study, the size of opening did not have a significant effect on the height of water causing pressure on the back wall.

¹ University of Canterbury

Poster 15

Data Processing of observed damage and reconstruction costs after 2002 Molise Earthquake in Italy

Sonia Giovinazzi¹ and Stefano Podestà²

On the 31st October 2002 the Molise Region in Italy was struck by a relatively moderate magnitude earthquake (Mw=5.7). Nonetheless, the earthquake caused a severe level of damage in several villages and towns and the death of 27 children, due to the collapse of a school.

During the emergency and recovery phase, post-earthquake building safety evaluations were performed for all the damaged buildings. The data collected by visual inspection provide a meaningful source of information about the seismic vulnerability of the buildings in that area and the damage undertaken following 31st October 2002 earthquake.

During the reconstruction phase, specific criteria ruled the allocation of funds for the repair/reconstruction of both public and residential buildings. The preliminary design-drawings and the assessment of the reconstruction costs have been therefore produced and provided to the government authorities for all the buildings repaired/reconstructed using public funds.

Reconstruction-costs data, collected during the reconstruction process, and damage and vulnerability data, surveyed during the post-emergency phase, have been processed and used in this paper for comparison, calibration and integration of models currently implemented, within scenario-based earthquake impact assessment approaches, for the vulnerability assessment, damage estimation, cost evaluation and assessment of the needed resources for the reconstruction.

¹ University of Canterbury
² University of Genoa, Italy

Poster 16

Assessing the resilence of roading organisations to earthquakes

Sonia Giovinazzi¹, Andre Dantas¹, Frederico Ferreira¹ and Erica Seville¹

Recent catastrophic seismic events have demonstrated that the functionality of road transport networks is vital in saving lives, reducing costs and enhancing the resilience of the community to recover from the earthquake event. This is recognised and highlighted by the New Zealand’s Civil Defence Emergency Management Act (2002) enforcing that transport networks need to be able to function to the fullest possible extent during and after an emergency event.

In the framework of a wider collaborative research program between University of Canterbury and Transit NZ, a specific research has been promoted in order to understand the strengths and weaknesses of Transit NZ’s Readiness, Response, Recovery and Reconstruction capabilities in the event of an earthquake.

This paper presents a first insight of the research method that has been widely developed to assess the Resilience of roading organisations to different kind of crisis events in New Zealand. The specific implementation of the proposed method to earthquake events will be performed with reference to two different desktop study cases, namely “Exercise Capital Quake ‘06” and “Exercise Pandora ‘07”, simulating, respectively, a 7.6 Richter Magnitude earthquake on the Wellington fault line and an 8.2 Richter Magnitude earthquake along the Main Alpine Fault.

¹ University of Canterbury

Poster 17

Determination of Acceptable Structural Irregularity Limits for the Use of Simplified Seismic Design Methods

Vinod Sadashiva¹, Greg MacRae¹, Bruce Deam¹ and Richard Fenwick¹

Approximations to the exact response of a structure under design level excitation may be obtained by conducting a number of 3-D inelastic dynamic time history analyses considering all relevant effects and using the best information available. Factors that should be considered include foundation effects, floor diaphragm effects, and the likely variation of earthquake demand and structural capacity.

In general, this type of approximate analysis is too complex for design engineers, so simpler, and hence more approximate analysis methods are commonly used. These have been calibrated based on the response of regular structures. When it is felt that a structure is too irregular, some approximate analysis methods are not permitted to be used. The amount of irregularity permitted has generally been selected based on engineering judgment.

This paper describes a quantitative method to determine limits for structural irregularity for structures designed using different analysis procedures. The method considers both the design approach estimations and the response from inelastic dynamic time history analysis. Irregularity limits based on a specified level of confidence are selected and these are then proposed for the use in codes. The method is illustrated by an example.

¹ University of Canterbury

Paper 14

Base Shear Scaling

Barry Davidson¹

The base shear scaling requirements of NZS1170.5 are sometimes difficult to implement for the complex buildings for which they are intended. An alternative design approach, the implementation of the Modal Response Spectrum Method without base shear scaling, is proposed. The seismic response of four structures, designed without base shear scaling, is used to illustrate the “satisfactory” response of this approach. It is concluded that the additional strength required for irregular structures could be more rationally achieved through the incorporation of a modified dynamic magnification factor rather than base shear scaling.

¹ Compusoft Engineering Ltd.

Paper 58

Feasibility and detailing of Prestressed Timber Buildings for Seismic Areas

Tobias Smith¹, Stefano Pampanin¹, Andy Buchanan¹ and Massimo Fragiacomo¹

This paper describes the structural design and selection of construction detailing for low-rise multi-storey timber buildings using a new and exciting structural timber system. This system, originally developed for use with pre-cast concrete, combines the use of un-bonded post-tensioning techniques and additional sources of energy dissipation. This system eliminates residual displacement, while greatly reducing the damage to structural members during a significant seismic event. The paper shows how this new structural system can be used with large size structural timber members manufactured from laminated veneer lumber (LVL) or glulam timber, for use in multi-storey buildings, with lateral load resistance provided by post-tensioned structural timber frames or walls, separately or in combination.

An extensive on-going research program at the University of Canterbury has tested a wide range of beam-to-column, wall-to-foundation and column-to-foundation connections under simulated seismic loading, all giving excellent results.

As part of this contribution, a case study of the design methods, construction options, cost and feasibility of a six storey timber office building in a moderate seismic area is carried out. The structural design of this building allowed investigation of different methods of structural analysis, and the development of many construction and connection details offering feasibility of rapid construction. Total building cost was evaluated and compared to equivalent steel and reinforced concrete options.

¹ University of Canterbury

Paper 53

Experimental Study of Prestressed Timber Columns under Bi-directional Seismic Loading

Asif Iqbal¹, Stefano Pampanin¹ and Andy Buchanan¹

Structural members made of laminated veneer lumber (LVL) in combination with unbonded post-tensioning have recently been proposed, which makes it possible to design moment-resisting frames with longer spans for multi-storey timber buildings. It has been shown that prefabricated and prestressed timber structures can be designed to have excellent seismic resistance, with enhanced re-centring and energy dissipation characteristics. The post-tensioning provides re-centring capacity while energy is dissipated through yielding of mild steel dissipating devices.

This paper summarizes an experimental investigation into the seismic response of LVL columns to bi-directional seismic loading, performed as part of a research programme on timber structures at the University of Canterbury. The experimental investigation includes testing under both quasi-static cyclic and pseudo-dynamic protocols. The results show excellent seismic performance, characterized by negligible damage of the structural members and small residual deformations, even under the combined effect of loading in two directions. Energy is dissipated mostly through yielding of external dissipators connecting the column and the foundation, which can be easily removed and replaced after an earthquake. Since post-tensioning can be economically performed on site, the system can be easily implemented in multi-storey timber buildings.

¹ University of Canterbury

Paper 31

Dynamic performance of timber diaphragms in the 1903 Nathan Building

Aaron Wilson¹, Claudio Oyarzo-Vera¹, Piotr Omenzetter¹, Nawawi Chouw¹ and Jason Ingham¹

To date researchers and practitioners have assessed expected unreinforced masonry (URM) building response and formulated seismic retrofit solutions based on limited laboratory data, and inadequately validated modelling techniques. A problem exists that throughout the world almost no in-field testing has been undertaken to generate data with which to validate the accuracy of models and laboratory based studies of retrofit solutions. It is widely recognised that the behaviour of timber diaphragms in URM buildings is crucial to the response of the structure as a whole but little is known about the actual response and dynamic characteristics of these diaphragms in existing buildings. This includes both the properties of the diaphragm itself, and the dynamic characteristics of the diaphragm-wall connection when cyclically loaded either normal or tangential to the plane of the wall.

A current research programme has been initiated to investigate the dynamic behaviour of the timber diaphragms in the Nathan House, located in Auckland’s Britomart precinct. The timber diaphragm was tested in-situ using a dynamic mass shaker. The forced vibration test enabled determination of dynamic properties such as natural frequencies, damping ratios and modal shapes. These modal parameters were used to calibrate stiffness and boundary conditions of an FEM model of the diaphragm, enabling estimation of the stiffness of the diaphragm and diaphragm-wall connections.

The results of this test will improve the understanding of timber diaphragm response, including stiffness characteristics and connection performance to the URM walls. The data collected from this test will aid the development of effective retrofit solutions and is essential in the accurate finite element modelling of URM structures.

¹ University of Auckland

Paper 05

Post-Disaster Benefits of Upgrading Residential Dwellings Foundations

Geoff Thomas¹ and Jamie Irvine¹

A previous study by Irvine and Thomas completed in 2007 showed that approximately 70% of houses in Wellington City had foundations that would not be adequate to resist the design earthquake expected in Wellington. A significant proportion was so poor than they would be expected to fail in relatively minor earthquakes. This paper will discuss the indirect implications of this problem. With many of these houses being destroyed or damaged to such an extent that they are unsafe or uninhabitable then emergency accommodation must be found for the occupants. The difficulties and costs of providing such accommodation, indirect and other costs are explored and indirect cost savings could be as high as twice as the direct costs savings resulting in very favourable cost/benefit ratios for upgrading house foundations.

¹ School of Architecture, Victoria University of Wellington

Paper 56

Seismic performance of brick veneer houses

Stu Thurston¹ and Graeme Beattie¹

Historically brick veneer houses have not performed well in earthquakes. However, modern construction using better brick-ties which are screwed to studs and the use of bricks with internal holes allowing some mortar mechanical interlock is expected to result in a better performance.

BRANZ cyclically racked two large brick veneer rooms (including ceilings) using a system which allowed the total load carried by the brick veneer to be measured directly. One room incorporated windows and had a door opening, while the other had fully separated brick veneer elements. Brick veneer cracking patterns were identified and rationalised. It was determined that in-plane brick veneer slip was significantly resisted by perpendicular walls and the mortar in brick veneer holes acted as dowels which resisted slip along horizontal mortar cracks.

Elemental tests were used to measured brick veneer tension bond and slip strengths and the in-plane brick-tie load versus deflection relationship. These were used to determine the theoretical veneer lateral load resisting strength and a model was calibrated by comparing with the measured room strengths. A new design philosophy is proposed.

¹ BRANZ, Wellington

Paper 11