Commercial Bay is the largest mixed-use development ever undertaken in Tāmaki Makaurau (Auckland) CBD. The project combines two existing office towers with a new three level high-end retail complex and a new 39 level commercial tower—all above three levels of basement car parking and two train tunnels, with associated major complexities and structural transfers. Holmes was the Lead Structural Engineer on this important project, as well as providing fire and infrastructure engineering.
The architecture of the flagship 39 level Commercial Bay tower in downtown Tāmaki Makaurau celebrates and showcases the structural form of the building. Working closely with the architect and the wider project team, Holmes designed visually striking columns and diagonal braces, which are clad to express and accentuate the structure. The structural system of the tower consists of a diagrid structure, featuring composite concrete filled hollow section steel tube columns and braces and long span beams.
In a large complex project with many stakeholders and challenges, the Commercial Bay tower is an example of a well-coordinated structure, underpinned by great BIM execution and a willingness for project partners to work through challenges collaboratively. This result was achieved through excellent continuous ongoing collaboration and communication between project stakeholders—particularly the architect and the structural engineers. Weekly exchanges of Revit files via a shared FTP site ensured well-coordinated Revit models and delivered a clear accurate picture of progress.
The penetrations through the floor slab were modelled in the structural slab, and penetrations for building services were incorporated into the structural steelwork floor beams. Across the project, regular meetings, ongoing communication and open, positive discussions ensured alignment and synergy through the design process.
With a premier site overlooking the harbour, Commercial Bay is already an iconic landmark on the Tāmaki Makaurau foreshore and skyline.
Holmes provided fire engineering advice for the entire development, including master-planning to allow for future subdivision of the entire block into four discrete separate properties. The fire engineering allows for separate phased evacuation and independent commissioning and ownership of fire protection systems serving the retail centre and the office tower. Holmes was also engaged to provided advice to almost all of the retail and office tenants, to integrate their specific tenancy fitout design with the overall base building fire engineering strategy.
Holmes was also engaged to provide specialist structural fire engineering services to rationalise the passive fire protection requirements of the entire tower. Advanced finite element modelling was undertaken to analyse the response of the whole floor and megaframe in fire. The analysis demonstrated the robustness of the whole structural frame in resisting a full burnout fire. The resulting analysis showed the composite columns and secondary beams did not require passive protection and a reduced FRL could be applied for the primary beams. The analysis was peer reviewed by Professor Jose Torero who is a renowned international expert in structural fire engineering.
This resulted in major savings on site with regard to project timeframes, labour and material costs, and the improved aesthetic and air quality within the space. The costs savings in fire proofing alone was estimated to be approximately NZ $4M.
City Rail Link Tunnels
The City Rail Link (CRL) runs beneath the Commercial Bay site on the Auckland CBD waterfront. Holmes, as part of a Fletcher Construction Design and Build Team, were responsible for the structural engineering design and documentation of the portion through the Commercial Bay site for the owner Precinct Properties and key stakeholder Auckland Transport. This separate CRL tunnel portion ties into the two adjacent enabling C1 contracts, C1 (Britomart Station) and C2 (Albert St).
The two box tunnels are on a curved alignment and merge into a twin tunnel box within the site. Separation of the tunnels from the surrounding development and the transfer of loads from the multi-storey oﬃce tower above were among the considerable challenges involved. The CRL box tunnels are constructed within the Commercial Bay basement area, with the structure of the development forming an enclosure over the top of them. Tension piles resist hydrostatic pressures on the base of the tunnels. The cast insitu tunnel base, walls and roof were formed using moveable formwork. The tunnels were designed for IL3 loads, with a 100-year design life.