THE STAR, DARLING HOTEL AND SPA PYRMONT, NSW. Associate Director, Taylor Thomson Whitting Ltd, Sydney, Australia

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1 THE STAR, DARLING HOTEL AND SPA PYRMONT, NSW H Nguyen BE (Hons), CPEng MIEAUST NPER Associate Director, Taylor Thomson Whitting Ltd, Sydney, Australia ABSTRACT The redevelopment at The Star in Sydney includes the brand new five star hotel and spa, The Darling, located at the southern end of the Casino on the site of the former Pyrmont Switching Station. The new hotel building structure includes an eleven storey hotel tower over three podium levels comprising retail, casino and hotel facilities and four basement levels incorporating loading dock, plant and parking. The building structure has been designed to allow for future Sydney Metro tunnels directly under the building. This paper discusses the structural design aspects of the hotel complex with regards to how the structural framing and design was improved from conventional design methods. It discusses current and future design practices for structural design trends that are tailored to achieve an architectural design vision. In this case, The Darling hotel presents an iconic, highly distinctive raked form tower, nine metre tall free standing feature sandstone walls and glass awnings with up to twenty metre cantilevers. The paper also outlines the processes involved in the design to integrate the new hotel with the existing Casino complex and to assess the effect of the future Sydney Metro tunnels on the building structure, where the proposed tunnel depth is close to the building foundation level. Figure 1: Eastern view with the raked form Tower, free standing stone wall and cantilevered awning Figure 2: Raked form tower

2 Keywords: Concrete structure, Raked Column, Vibration, Pool Isolation, Metro tunnel INTRODUCTION The redevelopment at The Star in Sydney includes the brand new five star Hotel and Spa, The Darling, located at the southern end of the Casino on the site of the former Pyrmont Switching Station. The new Hotel building structure is a conventional concrete frame structure with glass façade to the eleven storey tower and sandstone clad façade to the three storey podium. The three storey podium structure along Union Street is linked with the main tower structure with lightweight composite steel pedestrian bridges. The area between the two building structures is covered with a three storey high glass atrium that forms part of the Hotel entrance foyer. This paper focuses on the design aspects of the structure with regards to improving the structural grids, the raked form tower, floor vibration, cantilevered sandstone walls, cantilevered awning, the integration with the existing Casino complex and the allowance for future Sydney Metro tunnels. New Hotel Site New Hotel Site Figure 3: Site Plan Figure 4: N-S Cross section Figure 5: As Built Aerial View STRUCTURAL DESIGN The adopted floor system on the typical Hotel floor is a conventional flat plate system comprising 210 thick post tensioned slabs spanning up to 9 metres with 3 meters cantilevers. Slab thickness in the wet areas is reduced to 170 thick to allow for a 40mm set down, ensuring a flat soffit to the slab reducing formwork cost and achieving maximum ceiling heights. The podium and basement levels are comprised of post tensioned banded slab systems. Lateral stability of the building is provided by two main services cores at each end of the hotel floor plate. Although the hotel services lift starts at ground floor and loading dock level, the structural core walls are continued down to the foundation level. The major driver behind the structural grid adopted, was a desire to eliminate transfer structures throughout the building whilst accommodating the architectural layouts for the hotel, podium, loading dock, porte cochere and carpark levels. Structural Grids Historically, structural column grids were generally adopted to suit the function of each building use, ie hotel, casino, retail, loading dock, parking etc. This typically results in the requirement for transfer structures within the building. The existing Casino Hotel is one example of a building with numerous transfer levels. This design approach is quite common because the structural engineer tends to get involved at a later stage, following the completion of architectural scheme design and building layout.

3 Within the new hotel, the proposed structural grid has been developed to make the car park efficient within an irregularly shaped site, whilst also working at all hotel floor layouts, gambling floor, hotel foyer, retail level, porte cochere and loading dock without transfer beams. Column sizes and shapes were selected to accommodate the function of each architectural area, ie long blade columns that fit within the hotel partition walls, exposed round columns in the podium and dock level and rectangular columns for parking levels. Column transitions and column offsets as shown in figure 6 play a major role in the effort to eliminate transfers. The design floor to floor heights of the new hotel were to match the existing Casino complex levels which are typically 2850mm for hotel and 2600mm for carpark levels. The elimination of deep transfer structures in the new hotel enabled these tight heights to be met whilst minimising the structural zone, leading to an improved, more flexible building for services interaction. Raked Form Tower At the eastern end of the hotel tower, the building floor plate increases in length gradually towards the east at each floor level and forms a distinctive raked form tower. A pair of sloping columns follows the raked façade to support vertical building loads. A full 3D finite element Strand7 model of the building structure was set up to determine structural member forces, building sway and stability for 36 load combination cases. Tension and compression forces at each end of the columns caused by its angle are resolved into the floor system then are globally resolved in the main core system. Interaction of these internal forces for strength, serviceability and global stability was carefully reviewed through the creation of the 3D finite element model. Figure 7 shows the internal stress in the floor for one load case. Stress distribution in the floor Fig 6: Typical column transition Main core Raked column Figure 7: Strand7 model Figure 8: Raked tower during construction Floor Vibration In recent years, the review of floor vibrations due to dynamic loads is commonly considered as part of the serviceability design, where in the past, floor vibration was usually ignored in the structural design of general floor plates, particularly in concrete structures. However, floor vibrations in thin long span post tensioned floors can be a particular issue, which could affect human comfort in occupied spaces. In the case of a hotel, it is critical for the hotel levels that floor vibration from either walking along the corridor or from using the swimming pool does not affect the occupants of the hotel rooms.

4 Vibration criterion for human comfort is normally expressed in terms of peak acceleration. For the hotel floor design, the peak acceleration is limited to 0.5%g as recommended in Floor Vibration due to Human Activity by Thomas Murray. The damping ratio is assumed to be 2% which is typical for this type of hotel floor. A finite element analysis was undertaken to predict the peak acceleration. Typically floors were modelled and designed using RAM Concept software. For vibration review, this model was then exported to Strand7 software in which the natural frequency of the floor was determined. The Strand7 analysis also showed the different mode shapes of the vibration pattern and indicated the most sensitive parts of the structure. Figure 9 indicates examples of mode shapes showing hot spots within the floor. Walking forces and response test points were located at these hot spots along the hotel corridor and from there the peak accelerations were calculated using the harmonic response solver within Strand7. Figure 9: Mode shapes showing hot spots within the Hotel floor With the assistance of these advanced software tools, the floor design has not only been optimised for strength design but also for serviceability design which covers both floor deflection and floor vibration criterion. Swimming Pool The pool is located in the north east corner of the level 3 outdoor area, directly over the gaming floor area. The pool is 29m long by 5m wide including the balance tank and constructed from reinforced concrete. The main building floor plate that supports the pool has a 25m long clear span in the direction of the pool to create a column free environment to the gaming floor below. Concrete upstand beams adjacent to the sides of the pool were adopted to minimise the impact to the gaming floor ceiling space as shown in figure 10. To prevent vibrations transmitting from the pool to the main structure, the pool structure was isolated by using isolation strips as shown in figure 11. Concrete upstand beam Figure 10: Cross section through the pool Figure 11: Pool shell deflection, isolation strip diagram and applied pressure

5 Due to the relative difference in stiffness between the base structure and the pool structure, the applied pressure from the pool on the isolation strips will vary across the length and width of the pool as the base slab deflects differently to the pool slab. A finite element analysis was undertaken to provide a prediction of deflections for the main floor plate which in turn were used to predict the distribution of pressure on the isolation strips. This is to ensure the isolated system is adequately loaded and the air gap between the main slab and the pool is not bridged. The use of this analysis method is an advance on past practices where isolation springs or strip loads were determined using a tributary area without consideration of the difference in stiffness between the pool and the main structure. Feature Sandstone Wall The 9 metre tall free standing feature sandstone wall is located at the corner of Union Street and Edward Street. The structural framing of the wall is comprised of a structural steel frame with a Y- shaped bracing member at the roof level which was set back behind the stone wall to achieve the full vertical slot aesthetic as required by the architect. A Unistrut and support angle system was fixed to the main steel frame and provided the support to the stone cladding. A 3D analysis model was set up for the structural design and allowed checks on both the vertical and horizontal deflection of the steel frame to ensure they are within the acceptable limits for stone cladding elements. Figure 12: Structural framing Figure 13: Y shaped bracing Glass Awning The glass awning over the east entry is approximately 26 meters long by 7 meters wide. The structural framing of the awning comprises two main rafters and a series of secondary members which are hung from the main beams and support the glazing. There are only three support points for the awning structure. The south east corner of the awning is supported by a cantilevered beam running in the north south direction. Support point Figure 14: Awning framing Figure 15: Buckling analysis

6 A finite element model was set up to determine the buckling length of the main rafter. The analysis included the stiffeners and lateral restraint from the cantilevered beam. The results from this analysis enabled us to optimise the steel section for both strength and serviceability. DESIGN INTERGRATION WITH THE EXISTING CASINO COMPLEX Part of the redevelopment was the integration with the existing Complex, which included providing a network of continuous pedestrian links from all streets through the site, a new porte cochere for the hotel, an extension of the main gaming floor at level 1, improvements to the Lyric Theatre loading dock and extension of the existing underground carpark. Interface to existing building At the interface between the existing Casino and the new hotel, two existing columns were required to be removed to provide a column free environment in the gaming floor. In order to maintain headroom at the existing level, where the columns have been removed, a transfer beam was installed at the roof level in lieu of lower level with the load from the existing column hung to the new transfer beam. Carpark The existing carpark within the Casino Complex is a split level parking structure which did not work well within the new hotel site due to its irregular shape. As such, the adopted parking structure has maximised the number of parking spaces, integrated well with the existing carpark and removed the requirement for the additional structure typically associated with split level carparks and ramps. Loading dock Improvement to the existing Lyric theatre dock involved a dock extension to enable access for 19m articulated trucks and their entry and exit movements. The new hotel loading dock level was also developed to suit the requirements of the Lyric Theatre dock. Structural column locations were adopted to suit both the new hotel dock and the extension of the existing Lyric Theatre dock. STRUCTURAL DESIGN FOR FUTURE SYDNEY METRO TUNNELS The proposed future Sydney Metro tunnels comprise twin bored tunnels which run across the site in the East-West direction from Pyrmont Street to Edward Street directly underneath the building. Two options for tunnel alignment were proposed by Sydney Metro as they could not confirm the position of the tunnels. Figure 16: Proposed tunnel alignment Figure 17: Tunnel depth from foundation level A detailed engineering assessment was carried out to determine the potential impact on the building structure from the future construction of the underground tunnels, and also the effect of the building foundation loads on the feasibility of constructing the tunnels so close to the buildings footings.

7 Future tunnel excavation will alter the stress in the ground and the building loads above will alter the stresses in the tunnel lining. Both have the potential to cause excessive movement to the hotel super structure and affect the performance of the tunnel. Therefore, any displacements due to excavation of the proposed future tunnels are critical when considering the structural and foundation design of the structure. An independent verification of the impact of the Hotel building loads on the proposed future tunnels was also undertaken by the engineers for Sydney Metro. Following a series of design iterations, a finite element analysis and consultation process with Sydney Metro, it was agreed that the top of the proposed tunnels will run approximately 5m below the bulk excavation at Pyrmont Street down to about 7m at Edward Street. The minimum distance between the footing founding level and the top of the tunnel is approximately 3955mm. In conjunction with Douglas Partners, a numerical analysis in 3D was carried out. Our assessment of the impact on the hotel supper structure was then based on the following outputs: Total settlement of column footing due to building load application and future tunnel excavation Incremental settlement due to tunnel excavation after construction of the building Differential settlement between footings at the building column grid after construction of the building A 3D model was set up for the area where the most joints cluster within the bedrock as identified in the geotechnical investigation. This model considered the worst case footings based on their locations and the magnitude of footing loads relative to the tunnels location. The additional strength of the tunnel lining was not considered in the analysis. The assessment was limited to the zone of influence around the tunnels, generally defined by vertical and horizontal boundaries including Protection Zone, First Reserve and Second Reserve as set by Sydney Metro design guidelines. Figure 18: 3D finite element model of tunnel with building loads above At the hotel site, the material under the foundation level and around the future tunnels is high strength sandstone with regular joint and bedding plane as shown in figure 18. Results from the 3D analysis indicated a maximum total settlement of approximately 11.5mm due to the building loads and excavation of the tunnels. Approximately 8.5mm is due to building loads and 3mm due to tunnel excavation. Maximum total differential settlement between individual footings is approximately 6mm or span/1500.

8 The estimated settlements due to tunnel excavation and the differential settlements between column footings are relatively small. With these settlements, the building structure falls into Risk Category 1 or negligible damage classification, based on CIRIA RP30, 1996 in the table below, which provides a recommended limit of 10mm and a maximum slope of building less than 1/500. Figure 19: Deflection contour plot Table 1: Damage Classification CIRIA RP30, 1996 During excavation and construction of the Hotel foundations, an engineering geological mapping report was carried out to verify the geological conditions. The geological features identified during excavation were judged to be generally consistent with the assumptions in the model. This process also validated the structural and geotechnical design of the Hotel structure. The aid of advanced 3D analysis software and the use of numerical analysis from a geotechnical engineer are an improvement on past practices, where deep transfer options over the tunnels would normally have been adopted as a structural solution. CONCLUSION Current structural design practice with the increased adoption of the use of advanced finite element software tools will assist the structural engineer in improving and optimising the structural design solutions for both strength and serviceability of future structures. Early involvement and a proactive approach from the structural engineer on the project will provide a major opportunity to express innovative and practical engineering solutions and to assist the architect to achieve their design vision. Architectural design trends will evolve with more unusual design shapes, large spans, offset cores, leaning towers, taller towers, vertical garden, etc. Structural design trends will also have to continue to evolve and support these visions. Figure 20: Aerial view at November 2009 Figure 21: Aerial view at September 2010 Acknowledgements Additional acknowledgement goes to the design and project team responsible for the delivery of this project; Michael Henry of Echo Entertainment, Greig Chandler and Thomas Lay of APP, Russell Lee, Joe Agius, Alex Small and John Seligman of Cox Richardson, Daniel Murphy and Mark Ciano of Brookfield Multiplex, Charles Marais of Douglas Partners, Richard Green, Michael King, Malay Sengupta and Dean Genner, of Taylor Thomson Whitting.