Christchurch Outpatients Building

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 Opus.

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.

Sustainability

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. 

Value

The Ministry of Health and Canterbury District Health Board are extremely pleased with the Christchurch Outpatients Building, which exceeded their expectations while being delivered 10% under budget. The building is of great importance to the community, providing 10,500m2 of state-of-the-art facilities for more than 20 outpatient services.

Refer to the Elegance & Good Detailing section which explains how the structural solution resulted in cost efficiency.

The building was designed to be appealing and inviting, with many flexible shared clinical and administrative spaces to promote a lean and efficient operational model that maximises the building’s usefulness. More than this, it is the first major building in the new Health Precinct, which focusses on better and wider community well-being in postearthquake Christchurch.

Elegance & Good Detailing Design

The structural solution has enabled elegance in the architectural blend of form and function, and engineering elegance in the dual system of moment resisting steel frame with supplementary dampers.

Designing the steel frame for 70% of the design base shear reduces the size and cost of the frame required to take lateral seismic loads. Increasing the level of damping further reduces foundation loads, which reduces the size and cost of the foundations. The smaller steel sizes also enabled more prefabrication, reducing erection time on site, saving cost, time and improving safety. The dampers also lower floor accelerations in a seismic event, which reduces damage to secondary structure and building contents.

The Dampers were designed into smaller bays to reduce their length and make the brace steeper. This improved the building’s performance, dramatically reduced the size of the brace and improved programme and safety on site, as they could be installed with a simple block and tackle from within the building, rather than a crane.

Beams incorporate reduced beam sections. This ensures any yielding in a major earthquake occurs beyond the connection to the column where it can be more easily repaired.

Circular hollow sections were chosen for the columns as they provided an efficient section to resist the bi-directional loading of a dual seismic resisting system, while minimising visual and space intrusion on the floor plate. The steel columns were also filled with concrete and reinforcement. This increased compression capacity and stiffness and provided a cost-effective fire rating and post-fire lateral stability requirements. Finally, the circular hollow sections allowed for the innovative use of reinforcing ‘starter cages’ to pass through into the columns from the piles to resist shear, which avoided the need for shear keys, which are difficult to construct.

To maximise floor area on the tight triangular site, the floor plan is slightly irregular. Beams running at an angle to the orthogonal frame lines were pinned and the supplementary damping was tuned to minimise torsion.