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Articles, Christchurch uncovered

Presenting a selection of the aerated (or soda, if you prefer) water bottles that have surfaced so far on Christchurch archaeological sites. Brace yourselves: there may be water puns (although, honestly, most of the ones we could think of were simply too … Continue reading →

Research papers, University of Canterbury Library

According to TS 1170.5, designing a building to satisfy code-prescribed criteria (e.g., drift limit, member safety, P-Δ stability) at the ultimate limit state and relying on the inherent margins within the design code would lead to an acceptable mean annual frequency of collapse (λ꜀) in the range of 10−⁴ to 10−⁵. Modern performance objectives, such as λ꜀ and expected annual loss (EAL), are not explicitly considered. Although buckling-restrained braced frame (BRBF) buildings were widely adopted as lateral load-resisting systems for office and car park buildings in the Christchurch rebuild following the Canterbury earthquakes in New Zealand, there are currently no official guidelines for their design. The primary focus of this study is to develop a risk-targeted design framework for BRBF buildings that can achieve the performance objectives desired by stakeholders. To this extent, key factors influencing λ꜀ and EAL of BRBF buildings are identified. These factors include gusset plate design, number of storeys, design drift limit, BRBF beam-column connection, brace configuration, brace angle, brace material grade, and analysis method (equivalent lateral force vs. modal response spectrum). A novel 3D BRBF modelling approach capable of simulating out-of-plane buckling failure of buckling-restrained brace (BRB) gusset plates is developed. Prior experimental studies on sub-assemblies conducted elsewhere have demonstrated that gusset plates and end zones may buckle out of plane prematurely, before BRBs reach their maximum axial compression load carrying capacity. Current 2D BRBF macro models, typically used in research, cannot simulate this failure mode. A conventional 2D BRBF model underestimates the λ꜀ of a case-study 4-storey super-X configured steel BRBF building (designed according to NZS-3404) by a factor of two compared to the estimate from the proposed 3D model. These findings suggest that the current NZS-3404 gusset plate design method may undersize gusset plates and that using a 2D BRBF model in this case can significantly underestimate λ꜀. Three improved alternative gusset plate design methods that are easy to implement in practice are identified from the literature. Gusset plates in two case-study 4-storey steel BRBF buildings with super-X and diagonal configurations are designed using both the NZS-3404 method and alternative methods. All three alternative design methods are found to be conservative, resulting in an almost three-fold lower λ꜀ for both case-study BRBF buildings compared to those designed using the NZS-3404 method. Analysis results indicate that (i) bidirectional interaction has no significant effect on gusset plate buckling and (ii) mid-span gusset plates are more susceptible to buckling than corner gusset plates. A framework for seismic loss assessment using incremental dynamic analysis (IDA), called loss-oriented hazard-consistent incremental dynamic analysis (LOHC-IDA), is developed. IDA can be conducted with a generic record set, eliminating the arduous site-specific record selection required to conduct multiple stripe analysis (MSA). Traditional IDA, however, is limited in producing hazard-consistent estimates of engineering demand parameters (EDPs), which LOHC-IDA overcomes. LOHC-IDA improves upon existing methods by: (i) incorporating correlations among engineering demand parameters across intensity levels and (ii) using peak ground acceleration (PGA) to predict peak floor acceleration (PFA). For two case-study steel BRBF buildings, LOHC-IDA estimates the EAL and loss distributions conditioned on the intensity level that closely match the MSA results, with an average absolute error of 5%. The influence of factors beyond gusset plate design on the λ꜀ and EAL of 26 case-study steel BRBF buildings (designed in accordance with TS 1170.5) is examined. Hazard-consistent λ꜀ and EAL for these buildings are estimated using the FEMA P-58 loss and risk assessment framework. Among the 26 case-study buildings, 23 satisfy the maximum code-specified λ꜀ limit of 10−⁴. The EAL, normalised by the total building replacement cost, is highest for 2-storey BRBFs (0.22% on average), followed by 4-storey BRBFs (0.16% on average) and 8-storey BRBFs (0.11% on average). Reducing the design drift limit has the most significant effect on lowering λ꜀ (all BRBF designs were drift governed), followed by transitioning from pinned to moment-resisting beam-column connections, reducing the brace angle, and increasing brace strength. BRBF buildings designed using the equivalent lateral force method, on average, have a lower λ꜀ compared to those designed using the modal response spectrum method. Diagonally configured BRBFs exhibit the lowest λ꜀, followed by super- X and chevron configured BRBFs. Most design variables, apart from drift limit and beam-column connection, have limited influence on EAL. A simple method for EDP-targeted design of steel BRBF buildings is proposed. For this purpose, linear regression and CatBoost machine learning models are developed to predict steel BRBF building EDPs using peak storey drift ratio (PSDR) and PFA estimates from the 26 case-study buildings at intensity levels ranging from 80% to 0.5% probability of exceedance in 50 years. The R²ₐₔⱼ of these models is around 0.98, while the average prediction error is less than 10%. Fundamental period (T₁), total building height (Hₜ), and pseudospectral acceleration at T₁, denoted as Sₐ(T₁), are selected as the features to predict PSDR, while T₁, Hₜ, and PGA are the features selected to predict PFA. The EDP-targeted design has three steps: (i) for a given Hₜ value, the PSDR prediction model is used to identify a suitable T₁ that can achieve a desired PSDR target at the design intensity, (ii) a force-based design is then conducted iteratively to achieve the target T₁ by using an appropriate ductility factor and design drift limit, and (iii) based on the T₁ in the final design iteration, the PFA demand estimated by the PFA prediction models is used as a conservative input for the design of acceleration-sensitive non-structural elements. An equation to predict λ꜀ at the design stage is proposed for collapse risk-targeted seismic design of buildings. This equation comprises three principal components: reserve building strength, a proxy for effective structural stiffness, and reserve building deformation capacity. This equation is calibrated for the collapse risk-targeted design of BRBF buildings in New Zealand using results from 26 case-study BRBF buildings. The validity of this equation is demonstrated with three design verification examples designed to specific λ꜀ targets. Considering λ꜀ from hazard-consistent incremental dynamic analysis as the benchmark, the mean absolute percentage error in the design-stage prediction of λ꜀ of the verification buildings is approximately 10%.

Images, UC QuakeStudies

A view across London Street in Lyttelton to the Empire Hotel and the Lyttelton Bakery. The buildings have been cordoned off by a safety fence and the facade of the Empire Hotel has been braced with steel beams.

Images, eqnz.chch.2010

Falling gable of the Anglican Community of the Sacred Name building left a gaping hole in the roof in the magnitude 7.1 earthquake that hit Christchurch on 4 September 2010. The chimney would have toppled as well had it not been braced.

Images, UC QuakeStudies

A view across London Street in Lyttelton to the Empire Hotel and the Lyttelton Bakery. The buildings have been cordoned off by a safety fence. Sections of the side wall of the Empire Hotel have crumbled and its facade has been braced with steel beams.

Images, UC QuakeStudies

A photograph of the Christchurch Arts Centre taken from Rolleston Avenue. A spire has been removed from one of the towers and braced on the footpath in front of the building. Wire fencing and road cones have been used to cordon off one side of the road.

Images, UC QuakeStudies

A photograph of the Cranmer Centre on Armagh Street. Bricks have fallen off one of the gables onto the concrete below. The remaining gables have been braced with wood to limit further damage. A cherry picker can be seen below, and there is fencing around the building.

Images, UC QuakeStudies

The southern side of the Christ Church Cathedral with boarded up windows and damage to the roof above both of the transepts. Damaged masonry has been piled on the ground in front and one of the spires has been removed and braced with steel in the foreground.

Images, UC QuakeStudies

Damage to the wall around a circular window inside the Durham Street Methodist Church. The plaster around the window is badly cracked and has chipped away in some sections, revealing the brickwork underneath it. The window has been weather proofed with plywood and braced with planks of wood.

Images, UC QuakeStudies

A photograph of the north side of the ChristChurch Cathedral with the Citizen's Memorial to the left. The roof of the cathedral has been damaged, and many of the doors and windows boarded up. On the memorial, the angel's waist and neck have been braced to keep her standing.

Images, UC QuakeStudies

A photograph of the earthquake damage to a house on Bealey Avenue near Springfield Road. The walls have crumbled, the bricks spilling onto the footpath in front. the ceiling of the building has been braced with scaffolding. Wire fencing and police tape has been placed around the building as a cordon.

Images, UC QuakeStudies

A photograph of the earthquake damage to a house on Bealey Avenue near Springfield Road. The walls have crumbled, the bricks spilling onto the footpath in front. the ceiling of the building has been braced with scaffolding. Wire fencing and police tape has been placed around the building as a cordon.

Images, UC QuakeStudies

A photograph of the earthquake damage to a house on Bealey Avenue near Springfield Road. The walls have crumbled, the bricks spilling onto the footpath in front. the ceiling of the building has been braced with scaffolding. A red sticker on the door indicates that the building is unsafe to enter.

Images, UC QuakeStudies

A photograph of the earthquake damage to a house on Bealey Avenue near Springfield Road. The walls have crumbled, the bricks spilling onto the footpath in front. The ceiling of the building has been braced with scaffolding. Wire fencing and police tape has been placed around the building as a cordon.

Images, UC QuakeStudies

The west-side of the Arts Centre along Rolleston Avenue near the entrance to the Botanic Gardens. Wire fencing and road cones have been placed around the building as a cordon. To the left, a crane is parked next to the building. The spire has been removed and braced on the footpath to limit damage.

Images, UC QuakeStudies

Damage to the Visitors Centre in Kaiapoi, after the September 4th earthquake. The foundations have lifted at the back of the building, giving it a forward lean. Cables have been attached to the balcony over the walkway and braced to posts cemented into the ground. This is to stop the building from slumping further.

Images, UC QuakeStudies

Damage to the Visitors Centre in Kaiapoi, after the September 4th earthquake. The foundations have lifted at the back of the building, giving it a forward lean. Cables have been attached to the balcony over the walkway and braced to posts cemented into the ground. This is to stop the building from slumping further.

Images, UC QuakeStudies

A sign on the door of a shop reading, "I have been reclosed again by the Council due to buckled retaining wall in basement. My landlord is getting wall braced for all our safety. I hope to reopen in 2/3 weeks. I will try and keep you informed by note on shop glass. Ross".

Images, eqnz.chch.2010

The old Christchurch Railway Station Clock Tower was damaged during the 7.1 magnitude quake on 4 September 2010 and was braced by the wooden sleeve until repaired. However, although the sleeve did its job during the 6.3 magnitude quake on 22 February 2011, the lower part of the tower started to break away.

Images, UC QuakeStudies

A damaged building on Cashel Street near Oxford Terrace. The top of the facade has fallen off the building into the street below, taking the awning with it. Bricks from the building still lie on the footpath where they fell. Above, the windows have been braced with plywood. Wire fencing has been placed around the building as a cordon.