Structural System

The lateral stability design of Latitude tower uses an outrigger-braced structural sys-tem. The system consists of a centrally located core, comprising reinforced concrete shear walls, in conjunction with outrigger trusses connecting the core to the perime-ter columns at mid-height. The outrigger design increases the effective structural depth of the building, which augments the lateral stiffness of the core and reduces horizontal deflections and moments in the core [146]. Outrigger arrangements are suitable for buildings with a significant flexural component of deformation, as op-posed to shear dominated deformations.

Core and Service Shafts

The centrally located rectangular core is the primary lateral load bearing element and is constructed of reinforced concrete shear walls. Figures 3.8 to 3.14 display core wall plans at levels where changes occur in the wall configuration from the previous level. Walls W1 and W5 form the ends of the lift shafts and are 700 mm thick at level 1, and reduce to 650 mm at level 9, 500 mm at level 20, 400 mm at level 28,

Figure 3.4: General Arrangement for Levels 12 to 16 (Source: Hyder Consulting)

Figure 3.5: General Arrangement for Levels 20 to 51 (Source: Hyder Consulting)

Figure 3.6: General Arrangement for Levels 52 to 53 (Source: Hyder Consulting)

Figure 3.7: General Arrangement for Levels 54 to 55 (Source: Hyder Consulting)

and finally 300 mm at level 37. Walls W7, W10, W13, and W16 form the centre of each lift shaft and have a constant thickness of 250 mm. Walls W8, W9, W11, W12, W14, W15, and W17 form the outer walls of the lift shafts and have a typical thickness of 170 mm for level 25 and above. For levels 1 to 24, these walls have a typical thickness of 175 mm. Wall W17 has a thickness of 300 mm for levels 1 to 8, and reduces to 170 mm for level 9 and above.

The total width of the core remains relatively constant between levels 1 to 19.

The distance between W16 and W17 is increased at level 9 to accommodate a lift shaft, which increases the overall width of the core from 30 125 mm in Figure 3.8 to 30 610 mm in Figure 3.9. Above level 19, the total width of the core decreases at discrete points as core walls terminate. The total width of the core reduces by approximately 60% between its base and top. The total depth of the core is 12 660 mm at level 1, and decreases as the thickness of walls W1 and W5 are reduced.

Latitude tower also includes two other reinforced concrete shafts in addition to the centrally located core. The first is a low-rise lift and services shaft located adjacent to W17 in Figure 3.9, between grids X06 and X07. The walls comprising this additional shaft, W20 and W21, extend between levels 9 to 16 and are supported by columns that transfer the vertical loads directly to the foundations.

The second reinforced concrete shaft is a high-rise services shaft adjacent to W5 in Figure 3.10, between grids Y05 and Y06. This shaft extends between levels 20 to 52, and is supported by trusses that transfer the vertical loads to both wall W5 and the columns situated on the perimeter of the lower levels, between grids Y04 and Y05 of Figure 3.4. The size and location of the high-rise services shaft, relative to the central core, creates a significant eccentricity between the centre of mass and the centre of stiffness.

The concrete strength f_c⁰ used for the core and shafts was typically 50 MPa.

The walls between levels 34 and 36 were constructed with 80 MPa concrete. The increased concrete strength was required because these levels link the core lift shafts together and also accommodate the outrigger trusses.

Outriggers

Outrigger trusses require large depths to be effective, and their configurations in-evitably clash with architectural aims. Plant rooms that occupy multiple consecutive levels are the most structurally effective and architecturally convenient location for such structures [146]. Latitude tower includes two outrigger trusses located in the plant rooms between levels 34 and 36. The depth of each truss spans between the level 34 and level 36 floor slabs. Figure 3.15 displays the location of the outrigger trusses at grids X03 and X06.

The outrigger trusses at the level 34 plant room directly connect only two

Figure 3.8: Level 1 Core Plan (Source: Hyder Consulting)

Figure 3.9: Level 9 Core Plan (Source: Hyder Consulting)

Figure 3.10: Level 20 Core Plan (Source: Hyder Consulting)

Figure 3.11: Level 28 Core Plan (Source: Hyder Consulting)

Figure 3.12: Level 37 Core Plan (Source: Hyder Consulting)

Figure 3.13: Level 48 Core Plan (Source: Hyder Consulting)

Figure 3.14: Level 53 Core Plan (Source: Hyder Consulting)

columns to the core. A two storey high belt truss [167] surrounds the level 34–

35 plant room at the facade, which joins all the perimeter columns together. This design theoretically forces all perimeter columns that are collinear with the line through the outrigger columns to participate in the moment resisting characteris-tics of the outrigger-bracing system. The belt truss also acts as an offset outrigger [122], in conjunction with the level 34 and level 36 floor slabs, that is effective for deflections in either the north-south or east-west directions. Figure 3.15 displays the location of the belt truss along the northern facade, between grids Y08 and Y09.

Floor Plates

A composite steel-concrete floor system is typically used for level 16 and above.

Steel beams and reinforced concrete floor slabs span from the core to the perimeter columns, which are constructed of concrete filled steel circular hollow sections. The steel beams are connected at the core and perimeter columns by bolted connections, which are not designed to support significant moment reactions. Shear studs welded along the top flange of the supporting beams protrude into the floor slab to ensure a composite action between the beam and floor slab is attained. In this configuration, the concrete floor slab serves as a compression flange.

For levels 15 and below, the floor plates are constructed of reinforced concrete.

The floor slabs are supported by reinforced concrete beams that span from the core to the perimeter columns, or an edge beam.

Figure 3.15: Level 34 outrigger truss locations (Source: Hyder Consulting)

At all levels the floor slabs are typically 120 mm thick, and increase to 150 mm for slab sections within the core and service shaft areas. The floor slabs forming the boundaries of plant rooms, such as level 34 and level 36, are 200 mm thick.

Block-work and Infill Walls

The intended use of the structure as an office tower results in mostly uninterrupted floor spaces. The columns are typically located at the facade, and block-work infill is scarce. In contrast, residential towers are characterised by extensive internal par-titions. Depending on the design and construction of internal partitions, they can influence the structural behaviour by acting as braced shear walls capable of sup-porting moment reactions [146]. For the test structure, block-work infill is confined to areas in the main core of the building, mainly for service shafts and stair wells, and is unlikely to have any significant influence on the structural system.

Foundations

The interface between the base of the structure and the ground uses reinforced concrete pad footing foundations. For the reinforced concrete core, a 1300 mm thick reinforced concrete pad footing extends 920 mm to 1450 mm beyond the perimeter of the core walls. Pad footings for the columns are similar in design to the core pad footing. The underlying ground material is high class sandstone with a bearing pressure capacity of 9 MPa to 12 MPa. Very little change to the existing foundations was conducted apart from minor upgrades to some footings. The new structure was designed to suit the existing foundations. For example, the new levels used a composite steel and reinforced concrete floor plate design to reduce the mass of the structure.

Cladding

The cladding consists of prefabricated sections constructed of glass and aluminium.

Steel brackets embedded in the floor slabs provide bolt eyelets as attachment points for the cladding sections. The connections between the facade elements and the brackets, as well as the connections between adjacent facade elements, consist of movement joints that prevent the transfer of inter-storey shear forces to the cladding elements. From a serviceability perspective, this ensures the glass in the facade elements does not act as a compression strut in resisting inter-storey displacements.