Introduction
Australia has a number of historical infrastructures that have been constructed with a quality engineering design construction process. For example, Glebe Island Bridge, Gladesville Bridge, Eucumbene Dam, and Sydney Tower represent the excellence of the historic building structure construction in Australia. The high-rise building plays a vital role in national business and global heritage competition. The construction mechanism of the skyscraper is upgrading year by year with the incorporation of new technology and equipment. People are now making more high-rise buildings with more sustainability than before. The Sydney Tower is the tallest observation building in Sydney (Kwok K. , 1983), which was constructed in 1981. It is considered as one of the magnificent historic infrastructures in Australia. The Sydney Tower accumulates several revenues each year as it attracts approximately 600,000 visitors annually (Sydney Tower Eye, 2019). The fundamental framework of the building was constructed in steel members and supported by 56 high tensile steel cables (Fletcher, 2019). The main shaft of the tower was constructed by 46 pre-cast barrel units with gantry crane. In this study, the history, construction process, duration, costing and structural elements of the Sydney Tower will be investigated. Finally, the present improvement of the construction techniques used in this tower will be discussed.

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Sydney Tower
The Sydney Tower, also commonly known as the Sydney Tower Eye, is considered as the most high-raised building in the Sydney and second most tall buildings in the Southern Hemisphere region (Sydney Tower Eye, 2019). Until 2019, the Sydney Tower is the 25th tallest building in the whole world. The height of the building is about 309 metres, where the distance of the tip from the Sydney Harbour Sea level is 323 metres (Sydney Tower Fact Sheet, 2007). The Sydney Tower is the first building structure in Sydney, where the visitor can see the sunrise and sunset from the same floor (Sydney Tower Eye, 2019). The tower is located at 450 Market St, Sydney NSW 2000, Australia, and owned by the Westfield, which is maintained by Merlin Entertainments. The three-dimensional view of the Sydney Tower is shown in Figure 1 [Source: Google Map Inc.].

Figure 1 The Three-Dimensional View of Sydney Tower
Basic Features of the Sydney Tower
The main building can occupy 960 persons at once, which contains two restaurants, two telecommunication transmission levels, and one observation deck (Sydney Tower Eye, 2019). Approximately 600,000 peoples visit the Sydney Tower each year, especially to visit the observation deck, which is 250 metres above the ground level (Sydney Tower Eye Education office, 2016).  To reach the deck from the ground, it has three double-deck lifts, which can carry up to 10 persons at a time, and the quickest time to reach the top from the ground is 45 seconds, i.e., the maximum speed of the lift is 7 m/s. The Westfield security surveillance system monitors the elevators of Sydney Tower on a 24/7 basis. As per the fire safety and emergency risk regulation, the whole building has adequate fire exits for an emergency, which are easily visible (Sydney Tower Eye Education office, 2016).
History of Sydney Tower
The Donald Crone and Associates, now named as Crone Architects, designed the structural and architectural drawings of the Sydney Tower in 1968 (Sydney Tower Fact Sheet, 2007). The construction of the office building was started in 1970; however, the central tower was commenced in 1975. The tower launched 52 shops in 1972 and the main office in 1974. The complete construction phase of the building was finished in 1981, the same year when it was open to providing access to public (Sydney Tower Eye, 2019). The very first image of the Sydney Tower after finishing the construction is represented in Figure 2 [Source: http://www.austehc.unimelb.edu.au/tia/324_image.html (Australian Academy of Technological Sciences and Engineering, 2000)].

Figure 2 The Sydney Tower as well as Centrepoint Tower, Sydney in 1981
During the grand opening of this tower, it was the 4th tallest tower in the whole world. The total construction cost of the tower was approximately A$26 million. Initially, the height of the building was 279 metres which were later extended to 305 metres (Sydney Tower Fact Sheet, 2007). In 1998, the height of the tower was extended by 4 metres which involved a lighting rod at the tip of the existing building. During the construction, the name of the tower was “Centrepoint” which was changed to “Sydney Tower” in 2001 when the tower was acquired by the Westfield group. The retail section of the tower was renovated in 2007 by the Westfields and JWA Architects and expanded to an additional commercial tower (Fletcher, 2019). Out of the three elevators, one flopped in July 2008 from 200 metres above the ground while carrying 11 passengers, and all elevator service was discontinued for 90 minutes.  However, the Merlin Entertainments got the authority to operate the tower in 2011 and changed the name of the building to “Sydney Tower Eye”. 
Structural Sustainability of Sydney Tower
The Sydney Tower is considered as the safest structure in Australia due to the excellent capability of the main building to resist heavy earthquake and massive wind blow (Sydney Tower Eye, 2019). The tower can resist a maximum wind speed of 172 kmph with a maximum sway of 1 metre. In this tower, a large scale tuned mass damper (TMD) was used to enhance the damping level during the construction period (Kwok & Macdonald, 1990). To lessen the implication of the wind-induced motions, a 180 tonnes doughnut shape hydraulic tank was installed into TMD design and placed adjacent to the tip of the tower. Details about the deflection diagram due to wind or earthquake and the design of TMD is graphically shown in Figure 3 [Source: (Kwok & Macdonald, 1990)]. Since the construction of Sydney Tower, many researchers have been investigated the sustainability of the structure against wind-induced acceleration (Kwok & Macdonald, 1990) (Kwok K. , 1983) (Ohtake, Mataki, Ohkuma, Kanda, & Kitamura, 1992).

Figure 3 TMD Design and Deflection Shape of the Sydney Tower
Construction Process and Possible Improvement of Sydney Tower
The construction of the Sydney Tower is a symbol of excellence infrastructure creation and quality steel structure engineering knowledge in the history of Australia (Sydney Tower Eye, 2019). The construction steps of this tower are subdivided into the following sections:
Construction of Tower Structure
The tower structure has two major components, which are the shaft and the turret structure. The fundamental structural frame of the Sydney Tower, as well as the shaft, was constructed by 46 individual pre-cast barrel units (Sydney Tower Eye, 2019). The weight of each barrel was about 27 tonnes and transported to the construction site at a batch of seven segments, which were assembled by welding. The placing of each barrel consisted of the elevator, stairs, hydraulic amenities, fire exits, electrical, and plumbing ducts. After the construction of the first three-barrel units as the base of the shaft, a big gantry crane was utilised to place the rest 43 barrels in chronological order from bottom to the top (Sydney Tower Eye, 2019). The construction stage of the tower is shown in Figure 4 [Source: sydneyarchitecture.com (Fletcher, 2019)].

Figure 4 Construction Stage of Sydney Tower
About 56 high tensile steel wires were placed in the exterior of the tower to protect it from earthquake and wind. The steel bars are approximately 182 metres long, which have created a hyperbolic paraboloid  (Sydney Tower Fact Sheet, 2007). Figure 5 represents the steel wires around the tower [Source: sydneytoweroztrek.com.au (Sydney Tower, 2008)].

Figure 5 The Suspending Steel Bar Around the Tower
The diameter of the middle column of the shaft of the tower is about 6.7 metres which support most of the load of the structure. Although the turret structure had different design and construction properties, the turret tower was constructed above the base of the tower, which was initially four-storied and lifted by hydraulic force with the construction of the shaft of the tower (Sydney Tower Eye, 2019). The combined weight of the barrel units and the turret tower is about 4067 tonnes (Sydney Tower Fact Sheet, 2007). The exposed steel bar of the tower is coated by some unique alloying materials which protect the reinforcement from corrosion and rust. Figure 6 shows the major structural components of the Sydney Tower [Source: SydneyArchitecture.com (Fletcher, 2019)]

(a) Main Shaft with turret structure

(b) Turret Tower

(c) Lighting rod above the turret
Figure 6 Major Structural Components of Sydney Tower
Placement of the Spire of the Tower
The most troublesome phase of the construction of the Sydney Tower was the placement of the spire after completing the tower structure (Sydney Tower Eye, 2019). In the 1970s, the crane did not have many capabilities that we have in the 21st century. The cranes used in the construction of Sydney Tower was unable to lift the whole spire at the top of the tower. Therefore, the spire was divided into two segments, and each segment was lifted from the side of the central tower. However, the crane faced much hurtle of maintaining significant balance while placing the spire at the top of the tower (Sydney Tower Eye, 2019).
Improvement of the Construction Process
The Sydney Tower was constructed in the late 1970s with conventional construction equipment. However, the construction technology has been modified in the present era, especially if the mechanism of lifting elements and protection against wind or earthquake. From the details about the construction process of Sydney Tower, it is observed that the engineers faced tremendous difficulties while installing the Spire at the top of the tower (Sydney Tower Eye, 2019). The gantry crane was used in this task, which had some balancing issue; however, in the 21st century, more advanced and robust tower cranes are used in the construction of a skyscraper. For instance, Kroll K-10000 model tower crane is considered as the world’s most giant crane, which is now used in many skyscraper construction projects (Crane Market, 2016). However, the most important available upgrade in the construction process for the Sydney Tower is the wind-induced motion resistance technology.
Advanced Technologies to Construct Wind-resisted Tall Building
In the construction of the Sydney Tower, tuned mass damper (TMD) techniques were used to resist the vibration due to the earthquake and wind blow (Kwok K. , 1983) (Kwok & Macdonald, 1990). However, there are several options available in the present era to optimise the design and construction of tall buildings against the wind and earthquake. For example, the construction mechanism of the world’s tallest skyscraper Burj Khalifa. To diminish the impact of wind force, the shape of Burj Khalifa kept triangular (Baker & Pawlikowski, 2016). Figure 7 shows some examples of the 21st-century world’s tallest structures [Source: (Baker & Pawlikowski, 2016)].

Figure 7 World’s tallest buildings
The latest advanced technology and materials were used in the construction process of Burj Khalifa. The maximum load-bearing columns were constructed as circular reinforced columns, and specialised GPS monitoring system was utilised to maintain the vertical alignment of the tower. An advanced refinement scheme was used by the architect to ease the impact on the perimeter columns of the structure (Baker & Pawlikowski, 2016). Besides, large scale reliability-based (Spence & Gioffrè, 2012) and performance-based (Petrini & Ciampoli, 2012) design optimisation techniques are helpful to minimise wind effect in tall buildings.
Conclusion
Skyscraper plays an essential role in the tourism industry of a country. Australia has several tall buildings which have specific design and construction characteristics. Sydney Tower is one of the historic steel building which was constructed from 1975-1981. The core of the structure was made by 46 pre-constructed individual barrel units. The tuned mass damper (TMD) was incorporated in the tower to minimise the wind gust and seismic effect. The challenging phase of the construction of the central tower was the placement of spire at the top of the building, which was done in two steps and moving the spire from the side of the building. However, the construction technique of the tall structure has been upgraded day by day. Advanced tower crane can be used in lifting elements in construction. Also, the shape modification, reliability, and performance-based design optimisation are widely used to minimise the wind gust impact in a skyscraper. To maintain the vertical alignment of the structure, global positioning system, i.e., GPS, can be used.
5.     References

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