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Earthquake Engineering is facing an extraordinarily challenging era, the ultimate target being set at increasingly higher levels by the demanding expectations of our modern society. The renewed challenge is to be able to provide low-cost, thus more widely affordable, high-seismic-performance structures capable of sustaining a design level earthquake with limited or negligible damage, minimum disruption of business (downtime) or, in more general terms, controllable socio-economical losses.
The Canterbury earthquakes sequence in 2010-2011 has represented a tough reality check, confirming the current mismatch between societal expectations over the reality of seismic performance of modern buildings. In general, albeit with some unfortunate exceptions, modern multi-storey buildings performed as expected from a technical point of view, in particular when considering the intensity of the shaking (higher than new code design) they were subjected to. As per capacity design principles, plastic hinges formed in discrete regions, allowing the buildings to sway and stand and people to evacuate. Nevertheless, in many cases, these buildings were deemed too expensive to be repaired and were consequently demolished.
Targeting life-safety is arguably not enough for our modern society, at least when dealing with new building construction. A paradigm shift towards damage-control design philosophy and technologies is urgently required. This paper and the associated presentation will discuss motivations, issues and, more importantly, cost-effective engineering solutions to design buildings capable of sustaining low-level of damage and thus limited business interruption after a design level earthquake. Focus will be given to the extensive research and developments in jointed ductile connections based upon controlled rocking & dissipating mechanisms for either reinforced concrete and, more recently, laminated timber structures.
An overview of recent on-site applications of such systems, featuring some of the latest technical solutions developed in the laboratory and including proposals for the rebuild of Christchurch, will be provided as successful examples of practical implementation of performance-based seismic design theory and technology.
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Professional engineers have provided a range of inputs into the responses to the Canterbury Earthquake Sequence and the recovery process that has followed. This earthquake sequence has been unique in many respects, including the intensity of shaking produced in the Christchurch CBD by each of the major aftershocks in February, June and December 2011. For engineers, the heavy workload has been continuous from the response to the original 4 September 2010 Darfield earthquake, and will extend for several years to come.
There have been many post-earthquake challenges for seismologists and geotechnical and structural engineers, commencing with urban search and rescue responses and rapid building evaluations, and extending through the more detailed assessments and repair specifications during the recovery phase. Engineers are required to interface with owners, regulatory authorities and insurers, and face many challenges in meeting the objectives of these different sectors, which are rarely aligned.
Adding to the technical demands has been the requirement for many scientists and engineers to provide input into the Canterbury Earthquakes Royal Commission of Inquiry and other investigations. The Royal Commission was set up to investigate the failure of buildings that led to the loss of 185 lives in the 22 February 2011 aftershock, and has placed close scrutiny on many aspects of engineering activities, particularly those undertaken following the 4 September 2010 earthquake. The prominent public reporting of the Royal Commission hearings has placed additional pressure on many engineers, including those who volunteered their services following the original earthquake into a role for which they had received only limited prior training. Interpreting and communicating ‘safety’ in relation to the re-occupancy (or continued occupancy) of commercial buildings continues to be a challenge in the face of liability concerns.
A more comprehensive understanding of the technical and process guidance required by engineers and authorities has resulted from the work undertaken in response to this earthquake sequence. Much of this guidance has now been produced, and will be of considerable benefit for future major earthquake events.
This paper reflects on the range of work undertaken by scientists and engineers during the response and recovery stages. The scope and implications of the various official inquiries are summarised, and the potential impacts on engineers involved in the response to and recovery from future major earthquakes are briefly discussed.
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The Christchurch earthquakes have highlighted the mismatch in expectations between the engineering profession and society regarding the seismic performance of buildings. While most modern buildings performed as expected, many buildings have been, or are to be, demolished. The ownership, occupancy, and societal costs of only targeting life-safety as the accepted performance standard for building design are now apparent in New Zealand.
While the structural system has a significant effect on the seismic performance of the entire building, including the contents, it is only about 20% of the total building cost. Hence, structural engineers should view the seismic performance in a wider context, looking at all the systems of the building rather than just the damage to structural items and life-safety.
The next generation of performance-based seismic design procedures, outlined in the FEMA P-58 document, provide engineers with the tools to express the seismic performance of the entire building in terms of the future life loss, facility repair cost and repair time. This paper will outline the FEMA P-58 procedure and present the results of a comparative study of six different structural systems for a three storey commercial and laboratory building: moment frame; buckling restrained braced frame; viscously damped moment frame; Pres-Lam timber coupled-walls; cast-in-place reinforced concrete shear wall; and base isolated braced frame. Each system was analysed as a fully non-linear structure and the calculated drifts and floor accelerations were input into the FEMA P-58 PACT tool to evaluate the overall building performance. The PACT tool performs loss calculations for the expected casualties, repair cost, and repair time from which a QuakeStar or SEAONC rating for the building can be obtained.
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Many New Zealand bridge decks consist of simply supported spans, which are interconnected with steel linkage bars. The main purpose of the bars is to restrain and prevent the bridge spans falling in an earthquake. The prediction of the forces imposed on the linkages is quite indeterminate because of the many variables that affect the response of adjacent bridge spans during strong earthquake motions. For economic reasons it is also usually not practical to make the linkages so strong that they will never fail under the strongest likely shaking. Linkage bars are therefore designed for a reasonable and practical strength, and are then detailed to yield and have large plastic extensions before failing in tension.
The paper presents the results of laboratory tensile testing of a range of linkage bar types and conclusions are made regarding the most suitable bar assemblies, taking into account tensile ductility and cold temperature fracture toughness.
Recommendations are made regarding methods of predicting earthquake loads in linkage system and on design detailing for linkage assemblies.
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Reinforced Earth bridge abutment walls were subjected to strong ground shaking in one or more of the earthquakes in the Canterbury earthquake sequence of September 2010 to December 2011. Although the walls at three sites were subjected to ground motions of intensity greater than the design level none of the walls were damaged by the earthquakes.
The paper describes the earthquake design procedure used for the Reinforced Earth abutment walls and back-analyses carried out after the earthquakes to investigate their performance. Calculations based on probable material strengths rather than the dependable design values, and assuming no strip corrosion, gave critical accelerations to initiate sliding movements of the walls that were about 20% greater than predictions based on the design parameters. No significant outward movements of the walls were observed following the earthquakes. This was consistent with the predicted critical acceleration levels for the walls in their condition at the time of the earthquakes.
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The Sliding Hinge Joint is a low damage beam-column connection used in steel moment resisting frames. It dissipates energy through sliding in Asymmetric Friction Connections (AFCs) in the bottom web and bottom flange bolt groups. The AFC confines earthquake induced damage to bolts that can be retightened or replaced following a major earthquake. The other joint components sustain negligible damage and would be kept in service and may thus be subjected to further earthquake shaking during the lifetime of the building. The bottom and top flange plates are also subject to inelastic action about their minor axis under joint rotation. This study evaluates the behaviour of the bottom and top flange plates to determine the weld and plate susceptibility to low-cycle fatigue failure. The basic flange plate deformation was approximated by an arc, with the effects of shear slip considered to obtain estimates of likely strain demands. It was shown that even in the most critical case the fatigue life is more than six times the demand expected in a design level earthquake. As a result, it is concluded that properly designed, detailed and connected flange plates are not prone to low-cycle fatigue failure.
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Understanding seismic hazard and the potential impacts of an earthquake on a population allows better planning of response and recovery. It also allows a better understanding of how to mitigate against the effects of earthquakes.
The Wellington Lifelines Group (WeLG) and the various Wellington lifeline utility organisations over the past five years have synthesised information on the consequences of a major earthquake, drawing upon hazard information (including from the GNS Science-led ‘It’s Our Fault’ studies), learning from civil defence emergency management exercises and from overseas earthquakes, and specialist studies commissioned by individual utilities. During 2012, WeLG facilitated specific discussions in order to summarise the time taken to restore water, transport, power (electricity) and telecommunications services following a rupture of the Wellington Fault, and therefore the effects on the population.
The outcome of this work was an indication of substantial post-earthquake restoration times, agreed across and within key utility sectors. The time-scales for restoration of lifelines in a major earthquake are in the tens of days for power and water, and some key roads would not be recovered for up to 120 days. Telecommunications systems, particularly cell phone sites, would be recovered earlier, but are critically dependent upon access and fuel supplies for the refuelling of emergency generators.
Given the significance of these likely restoration times for the community, it was decided to publically release the information, with buy-in from all of the lifeline utility organisations involved. The resulting report was released, with appropriate messaging, via the Wellington CDEM Group to the media in mid-November 2012.
This paper provides a summary of the likely restoration times, background to their derivation, and the initial reactions to the release of the information.
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Identified in the Christchurch Lifelines Study as a bridge vulnerable to damage in a major earthquake, the Ferrymead Bridge on the key arterial route connecting the suburbs of Redcliffs and Sumner to the rest of Christchurch has subsequently been under investigation by the Christchurch City Council to increase its traffic capacity and upgrade its earthquake resistance. A contract was let in 2010 to undertake these works.
Surviving the September 2010 Darfield earthquake undamaged, the bridge fell victim to the February 2011 Christchurch earthquake with extensive liquefaction and soil lateral spread occurring at the site, displacing the abutments and piers inwards towards the centre of the river. After extensive investigation into options for recovering the bridge, the decision was finally taken to replace the bridge with a new structure.
This paper outlines the initial design to widen and seismically upgrade the original bridge, the damage sustained by the bridge from the Christchurch earthquake and measures instituted to stabilise the bridge as a result of that damage, and focuses particularly on the design now developed for the replacement structure. The significant issues involved in achieving earthquake resistance at a highly liquefiable site and in constructing in an environment of ongoing earthquake activity are discussed.
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During the 2010/2011 Canterbury earthquakes, several reinforced concrete (RC) walls in multi-storey buildings formed a single crack in the plastic hinge region as opposed to distributed cracking. In several cases the crack width that was required to accommodate the inelastic displacement of the building resulted in fracture of the vertical reinforcing steel. This type of failure is characteristic of RC members with low reinforcement contents, where the area of reinforcing steel is insufficient to develop the tension force required to form secondary cracks in the surrounding concrete. The minimum vertical reinforcement in RC walls was increased in NZS 3101:2006 with the equation for the minimum vertical reinforcement in beams also adopted for walls, despite differences in reinforcement arrangement and loading. A series of moment-curvature analyses were conducted for an example RC wall based on the Gallery Apartments building in Christchurch. The analysis results indicated that even when the NZS 3101:2006 minimum vertical reinforcement limit was satisfied for a known concrete strength, the wall was still susceptible to sudden failure unless a significant axial load was applied. Additionally, current equations for minimum reinforcement based on a sectional analysis approach do not adequately address the issues related to crack control and distribution of inelastic deformations in ductile walls.
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This paper investigates the validity of the soil class dependent spectral shape factors used to calculate seismic design actions in the New Zealand seismic design standard NZS1170.5, which currently specifies seismic design spectra corresponding to five different soil classes. According to the current provisions stipulated in NZS1170.5, for all natural periods, the seismic demand for structures on soft soil is either equal to or greater than that for structures on hard soil. This is opposite to the basic structural dynamics theory which suggests that an increase in stiffness of a system results in an increase in the acceleration response. In this pretext, a numerical parametric study is undertaken using a nonlinear site response analysis tool in order to capture the effect of soil characteristics on structural seismic demand and to scrutinize the validity of the current site specific seismic design spectra. It is identified that the level of input ground motion intensity and shear stiffness of the soil deposit (represented by its shear wave velocity Vs) greatly affect the maximum acceleration and frequency content of the surface motion. The study found some shortfalls in the way the current code defines seismic design demand, in particular the hierarchy of soil stiffness at low structural periods. It was found that stiff soils generally tend to have a higher spectral acceleration response in comparison to soft soils although this trend is less prominent for high intensity bed rock motions. It was also found that for medium to hard soils the spectral acceleration response at short period is grossly underestimated by the current NZS1170.5 provisions. Based on the outcomes of the parametric numerical analyses, a revised strategy to determine structural seismic demand for different soil classes is proposed and its application is demonstrated through an example.
