A first for New Zealand
The Christchurch Outpatients Building is the first public building in New Zealand to incorporate supplementary viscous dampers. It is classified as an IL3 (Importance Level 3) building due to its high value to the community, meaning that it is designed for 30% more seismic demand than most buildings. This is only the second new building in New Zealand to use this seismic resisting system, the first being an office at 12 Moorhouse in Christchurch, also designed by WSP.
Design and Analysis
We took a displacement-based design approach, given the importance of displacement for the viscous dampers and overall performance, and used non-linear time history analysis to validate performance and derive floor spectra for designing seismic restraints for building components. Holistic Resilience The client’s drive for enhanced resilience was in the brief and it became our mission, as the Structural Engineer to ensure this requirement would be realised.
In the Christchurch Earthquakes, some buildings experienced minor structural damage but building movement damaged glazing, partitions and contents. Damage to fire-rated linings of egress routes also prevented reoccupation of buildings following aftershocks.
We carefully considered these problems for the Outpatients building, and mitigated them by:
- Identifying the demand on components(by the floor spectra) and the lateral movement allowances (via the movement report).
- Including supplementary viscous damping, which reduces floor accelerations.
- Minimising residual drift using the elastic restoring force of the steel frame, and the viscous dampers, which have most effect when the building is in its intended position and least effect when the building is at its maximum drift.
The site has limited laydown area for construction materials, making it challenging to build on safely and efficiently. The structure was designed to be fast to erect, by using full-height columns, which enabled the beams to be simply bolted between column ‘trees’ which included the welded ‘stubs’ protruding from the column collar.
The strength of the primary structural frame was a ‘contractor’s dream’, as described by Mike Pearce of Leighs Construction. Floor slabs could be poured topdown without completing the entire footprint, columns were grouted from the bottom making construction faster, safer and cheaper, as it could be carried out at ground level, not 20m in the air. It also allowed for any final positional adjustments to be incorporated.
The steel moment frame was designed to take 70% of the design base shear, such that viscous dampers did not need to be in place to resist a constructionlevel seismic event. This meant that construction of the frames and building could continue, as the steel frame had sufficient capacity without the long lead time dampers. Viscous dampers and braces were simple to install and were chain blocked into place after the floor slab was complete.
A key Ministry of Health objective for the building is ‘Long Life, Loose Fit’. This means the building envelope is designed to maximise flexibility of use. It is expected that as technology and models of healthcare change, the equipment within the building and its functionality can be changed. The structure has been designed to maintain large open areas, so the interior can be altered without having to be demolished and re-built.
The building is resilient, meaning it is more likely to be available for immediate use and less likely to be damaged in a future earthquake. This in turn reduces downtime and the amount of damaged materials being sent to landfill, resulting in an environmental benefit. The New Zealand Green Building Council Green Star rating tool allows innovation points where this benefit can be demonstrated.
The building is located on a brownfield site and uses locally-produced steel where available. All steel reinforcement used in the building has a recycled content of over 99%. All of the steel used in the building is fully recyclable
A copper beach tree on the site was protected for its aesthetic value and had an influence on the shape of the structure, as seen in the photo. Special paving details were developed to protect the tree roots and incorporate rainwater percolation.
Health and safety was a vital consideration, especially through the early stages when safety in design considerations drove several of the buildability and maintainability initiatives, which reduced time on site and working at height.