ACSE AWARDS Large Project Parliament Square, Hobart

Transcription

1 ACSE AWARDS 2018 Large Project Parliament Square, Hobart

2 Project Credit Client Citta Property Group Architect FJMT Structural Engineer Taylor Thomson Whitting Contractor Hansen Yuncken Other Consultants Arup, GHD Project Value $110 million 1.0 Project Overview Parliament Square delivers an updated take on a town square for central Hobart and merges heritage elements with modern architecture. Situated in Hobart s Salamanca precinct directly beside the Parliament House building, the new office development by Citta Property Group comprises three basement levels and eight levels above ground. The building is designed as a reinforced concreteframed structure with post-tensioned concrete slabs and precast concrete cores. The lowest level houses carparking and building services plantrooms. The remainder of the levels are Goverment office space with several retail tenancies at Lower Ground and Ground Floor on the Salamanca Place elevation. A steel-framed and glazed atrium structure links the new building with the existing Parliament House. A central architectural feature of the office building is a series of large scale Y columns in the main entry foyer. These columns also serve an important structural purpose, supporting seven overhead levels. On the western elevation, a heritage six-metre sandstone retaining wall (St Marys Wall), runs north to south through the site, with the design calling for this to become a key feature within the new double height entry space on the ground floor.

3 2.0 Response to the Project Brief The project s aim was the seamless integration of heritage and contemporary architecture with new buildings sympathetically designed to blend in and create a vibrant new town square within the heart of Hobart. The precinct will be delivered in multiple stages, with the first stage of new office space recently completed. Stage 1A of Parliament Square had the following key objectives: Unite the city s past with a new contemporary community hub Provide a modern, premium grade office space for Hobart Basement car parking to conserve space in the centre of the city Maximise space available while working around the existing heritage fabric. TTW worked closely with architects FJMT and the builders Hansen Yunken to ensure the Client s brief was realised. Special consideration was taken to ensure the structural elements aligned with the architectural vision. The Y columns are a perfect illustration of functional structures that also work as an architectural feature. 3.0 Design and Construction Process TTW were engaged by Citta Property Group to take the structural design from preliminary stage to full documentation and construction. Design started in 2013 with FJMT as the project architects. After the Tender was awarded, TTW was novated to the builder Hansen Yunken to complete the design documentation and carry out the construction phase services through to completion in late TTW produced the detailed design and documentation of all structural elements including piled shoring walls, site retention, foundation piles, precast core walls, post-tensioned slabs and steelwork. Sections 4 and 7 detail the complexities and solutions involved during the design process. This included additional level being added to the design after the foundations had been constructed, structural solutions to minimise the impact on heritage structures such as the Y columns, and the high level of innovation required to meet the architectural vision. 4.0 Creativity and Innovation A historically significant sandstone retaining wall known as St Mary s wall runs through the west side of the site. To maximise the office space while limiting the visual and structural impacts on the heritage wall a Y column shape was adopted to cantilever the building out over the sandstone. This allowed the new building to sit clear of the wall, and created a dynamic space between old and new that could be used for circulation. Use of the reinforced concrete Y columns proposed to support the eight suspended levels above, while tapering in size and changing shape throughout their height produced complexities and challenges for the structural design. Traditionally, such columns when used to carry high structural loads are often simplistic in shape and without need for high-quality aesthetic finishes. Conversely, those that are more slender or complex in shape tend to support smaller structural loads. In the case of the Parliament Square Y columns, they were required to: serve a significant structural purpose; be slender in shape; involve a variable profile top to bottom; have a high-quality concrete finish to serve as an important architectural design feature of the project. This made them a highly unique and complicated design.

4 To address these difficulties the columns were modelled in Rhino and Revit. They were analysed using Strand 7 and Space Gass analysis software and TTW s in-house reinforced column and reinforced member capacity software. The columns were 3D printed in-house to help understand and explain the complex geometry. Reinforcement was detailed in 3D using Rhino to solve congestion issues, and a series of 3D annotated images included in our documentation package with colour coded sections to help visualisation. The reinforcement was also documented on a series of 2D cuts at 300mm centres along the length of the columns. To overcome buildability challenges, an iterative design and prototyping process was employed, including the construction of two 1:1 scale mock-ups as well as extensive reviewing, refinement and detailing ahead of construction of the first in-situ column. The final design saw the inclusion of 100MPa concrete into the critical structural node at the top of the Y column arms, which is understood to be a first in a Tasmanian building project. 5.0 Sustainability TTW worked closely with builders and suppliers to attain a 5-Star Green Star Rating for this project. The primary focus of the development has been the integration of existing structural surrounds with the best of modern architecture and construction methods. Parliament Square provides an important link between city and waterfront, and is designed to blend with the open space environment and revitalise the cultural heritage of the area. In order to achieve the structural steel and concrete credit points required for a 5-Star Greenstar rating TTW had to work closely with suppliers in Tasmania. Collaboration enabled TTW and the builder to reduce the quantity of natural aggregate and portland cement used in the concrete, and the reinforcing steel was produced using energy-reducing technologies and fabricated in an environmentally sustainable manner. 6.0 Built Environment and Heritage The new government office space was required to connect seamlessly with the existing Parliament House without structurally or visually impacting the historic building. This greatly impacted the architecture and structure and a light glass and steel atrium which was structurally independent was adopted to provide a clear distinction. St Mary s Wall an important heritage feature at the western elevation was a major consideration, with the architectural intent being to make the 1880s structure a key feature within the double height entry space on the ground floor. This was achieved through the incorporation of the Y columns.

5 7.0 The Challenges and Resolutions Y Columns The feature Y columns required significant attention to ensure best value for money, while retaining the design intent. We worked through an iterative design process in collaboration with the architect, client and builder to determine the most cost-effective solution. The choice of in-situ concrete provided a more economical solution than clad structural steel or precast concrete. Conservation of St Mary s Wall The additional excavation required for the new building created stability issues for St Mary s Wall. A number of options were explored to allow for the proposed excavation, with the final solution being to excavate behind the wall to reduce the overturning moment. This had the added benefit of creating additional floor space for the next stage of the project. Design axial forces and bending moments were determined from analysis of vertical and lateral loads, and checked using normal column design rules but with due consideration to slenderness effects of the tapered section. These were addressed by using linear elastic buckling analysis of the columns to calculate effective length and slenderness ratios, which were used to determine moment amplification factors. A number of cross sections in the columns were then checked for biaxial axial-flexural capacity. A two dimensional model of each building grid was also set up and analysed in SpaceGass to compare against Strand7 results as an independent check. An IFC file for the columns was supplied by the Architect which we then imported into Revit and used in the final Strand7, Rhino and Revit models. The mesh in the nodal areas was refined to investigate the stresses where the arms met the Level 2 slab. Re-Use of Basement Walls Existing reinforced-concrete retaining walls along Salamanca Place were used as the temporary shoring during construction and incorporated into the final structure. Working with the Geotechnical Engineer we designed a series of hit-and-miss excavations and shotcrete panels to allow excavation below the toe of the existing walls without the need for shoring piles. This provided a creative solution that reduced construction costs and increased the useable space for the development. Additional Level added during construction An extra level was added to the building above the Y columns to meet increased floor space requirements part-way during construction. Footings and several levels had already been completed. To accommodate this, we designed the additional level as a lightweight steel structure and re-planned compactus loading locations in order not to increase footing or column loads. This required close coordination with the Client and Consultant Team, particularly in regards to the acoustic treatment of the steel roof.

6 Atrium link between the Heritage and New Building A stair link and atrium was added during construction to connect the new office building with the existing heritage Parliament House. This link had to physically connect the buildings without transferring any forces that may damage the heritage building. A lightweight steel and glass structure stabilised by a concrete core was designed to bridge the buildings. The glass roof was supported by custom steel beams that cantilevered from a central truss, ensuring the buildings were able to move independently of each other. Extensive modelling of the structure was performed in Strand7 and Space Gass to achieve the slenderest possible beams while maintaining tight deflection limits.