Wind engineering, including wind tunnel studies, is conducted most commonly during the design phase of a project. As repurposing, adaptation, and extension of existing buildings are adopted more to meet sustainability goals, wind engineering can play a larger role during construction by demonstrating performance targets are being achieved, while safety and reliability requirements are met. Quay Quarter Tower (QQT) in Sydney, Australia, is a notable example of this and serves as a model of successful integration of structural engineering, wind engineering, and construction practices.

The QQT project has received multiple awards for taking an existing iconic building on the Sydney Harbour waterfront, reconfiguring it, and extending it into a much larger tower of greater height, with contemporary architecture to meet modern needs. The original AMP Centre Tower was opened in 1976 and was a centerpiece of the Sydney skyline, serving as the headquarters of one of Australia’s largest financial service companies. By the early 2010s, the owners realized that the tower configuration was no longer meeting its needs. The tower owners made a farsighted decision to try and preserve the invested construction energy and embodied carbon within the existing tower structure through adaptive reuse of as much of the existing structure as possible. The previous square plan tower footprint was to be extended toward the north incorporating facade articulation to better capture spectacular harbour views, and the structure was to be reclad and internally fitted with modern, energy efficient materials and building systems. While this approach meets a lot of sustainability goals, it also adds complexity to design and construction processes. The team set a goal of re-using two-thirds of the existing structure, and in the end, the project met this goal. CPP Wind Engineering Consultants was engaged from the start of design through commissioning to provide a range of wind engineering and field monitoring services. Quay Quarter Tower is owned by Dexus Wholesale Property Fund, Mirvac Wholesale Office Fund and Rest. QQT is managed by Dexus.

As is normal practice in the City of Sydney, wind engineers are engaged during the planning approval process to ensure any new construction will not result in excessively windy conditions for pedestrians at ground level. Environmental wind tunnel tests were conducted on a scale wind tunnel model of QQT and the surrounding cityscape in several stages. This included comparative tests with the existing AMP Centre at the time and the adjacent city precincts. Initial testing to investigate pedestrian level wind effects continued for four years by the Sydney CPP team*. From the outset, the impact of various basic building massing concepts was investigated to understand the impact upon the public domain wind environment. CPP then tested the architectural excellence competition winning scheme to develop the design, particularly with regard to refinement of canopy designs and windbreaks on outdoor terrace areas.
Once development approval was received, CPP began its first wind tunnel test to determine wind-induced structural load and accelerations. This initial high-frequency balance wind tunnel test was conducted in 2014. Initial structural wind tunnel test results on QQT were used to develop some modest stiffness refinements to the tower core framework and to assess the impacts of future building surrounds upon structural wind loads. Revisions of the predicted tower responses continued up until 2019, taking into account changes in design and advancement of the structural modelling and dynamic properties. Given the reuse of the existing structure, including 95% of the existing core, this was a structurally challenging project (with several structural engineering companies involved through the course of the design) but one that offered multiple design approach opportunities that can’t always be taken advantage of during more typical design projects.

One key area where CPP was able to support BG&E, the structural engineers who completed the project, and Multiplex, the contractor, was in a series of field measurements of as-built structural properties of both the existing tower and the new additions through construction.
The first early set of short duration site measurements of the structural properties of the existing AMP Centre tower were made in 2011 and 2012 led by CPP Director Dr. Graeme Wood. Accelerometers were used to measure the natural frequencies of vibration and to gain some estimates of inherent structural damping at low amplitudes of vibration. A primary use of these measurements was in calibrating the structural models of the existing tower that would be used as the basis of modeling the performance of the new tower and its impact on the existing structure.

CPP was re-mobilized from 2019 through 2021 to instrument the new tower for long duration site measurements to allow comparison of actual and expected tower behavior during construction. Instrumentation, including accelerometers and tilt meters, was installed on multiple levels throughout the tower and monitored remotely to assess natural frequencies, mode shapes, and structural damping under the guidance of CPP Director Dr. Matt Glanville with reference to measurement and analysis techniques developed in earlier research. The tilt measurements in particular ensured the tower did not lean beyond tolerances prescribed by BG&E. Data was logged continuously and periodically downloaded remotely from the CPP Sydney office.

The system was custom designed by CPP’s David Bourke and assembled specifically for this application. The presence of accelerometers throughout the construction phase enabled continuous acquisition of acceleration data which was advantageous compared to more typical spot check methodologies, ensuring capture of significant wind events, even those occurring overnight. The continuous data was also important in monitoring the evolution of the structural dynamics of the tower through construction phases and providing feedback to the structural engineering team. This data allowed the structural team to update their structural models of the tower and served as a metric for building structural integrity. Due to the novel nature of reusing an existing structure, the condition of the aged structure was somewhat uncertain. The trend of measured natural frequency (analogous to stiffness to mass ratio) over time was used as a metric for assessing structural integrity relative to expectations, where a sudden deviation could be indicative of a failure in the existing structure, triggering a halt in construction until such an event could be adequately investigated to maintain safety for the site.

The quantity of data available also allowed statistical techniques such as Random Decrement analysis to be implemented for assessment of damping without requiring a specific exercise for the measurement of this parameter.

The wind tunnel tests, supplemented by the field measurements, indicated that tower accelerations during strong wind events were likely to exceed perceptible motion design goals in specific prestigious areas of the tower and supplementary damping would be needed to meet occupant comfort performance targets. A range of auxiliary damper concepts was considered, including tuned sloshing dampers (TSDs) and tuned mass dampers (TMDs). Performance of the TSD option was investigated through multi-phase computational fluid dynamics studies, but a tuned mass damper (TMD) was selected as the preferred option based on a combination of price and space requirements.

Structural damping is one of the least predictable parameters in building response to wind excitation; direct measurements from the existing building provided valuable input for the auxiliary damper design. By knowing the existing tower’s inherent structural damping, it was much easier to predict the additional damping needed and to design the auxiliary damper accordingly. In order to measure structural damping accurately, the structure must be excited to serviceability amplitudes of oscillation. In some cases, this excitation can be achieved by controlled crane drops providing an impulse load or by forced vibration tests as described in earlier co-authored publications by CPP Glanville and Wood, who each played management and technical rolls over the duration of the project. For the team designing QQT, forced vibration tests utilized three scissor lifts that were already on site at the top of the tower and these were moved back and forth in time with a metronome to provide a sustained excitation at the natural frequency of vibration of the tower. Once the tower was oscillating, the excitation ceased, and the tower was allowed to vibrate freely to rest allowing a more accurate estimate of damping to be achieved.

To reduce the accelerations identified from the wind tunnel testing phase, a 175-ton TMD, designed and built by GERB, was installed in the tower. The TMD is a dual-axis sliding design, to meet space constraints and allow damping of both the first sway and torsional modes. CPP took part in the commissioning and performance validation of the damper system. Along with the accelerometers present on-site, an additional accelerometer was installed on the TMD itself to measure the motion of the damper mass. Measurements of tower motion were then analysed to confirm the additional damping introduced by the TMD.

In addition to the standard wind tunnel test for components and cladding pressures, CPP conducted a range of other tests and analyses to improve the performance of the building, including a natural ventilation assessment of the car park and atrium, with particular attention on the impact of smoke exhaust from a fire safety perspective. The CPP team also worked with the designers of the rooftop sunshades to minimize the sunshades’ potential to generate tonal noise.

Quay Quarter Tower has set a new global benchmark for the adaptive reuse of tall buildings, demonstrating how innovative structural engineering can contribute to ambitious sustainability goals. Wind engineering played a key role in the success of this project, ensuring maximum structural efficiency while meeting reliability and wind comfort performance targets. The integration of wind tunnel testing, structural engineering, and field verification undoubtedly assisted in the slew of awards garnered by the project, including the 2023 Council on Tall Buildings and Urban Habitat award for Best Tall Building Worldwide. ■

*Wind Engineering consulting for the project was conducted out of the CPP Sydney office including staff members Peter Bourke, Adam van Duijneveldt, Matt Glanville, Graeme Wood, David Bourke, Joe Paetzold, Christian Rohr, Thomas Evans, Kenneth Fung, Joe Sun and Andrew Nguyen.

About the Author

Peter Bourke is Managing Director and Chief Technology Officer for CPP Wind Engineering Consultants, St Peters, NSW, Australia.

Adam van Duijneveldt is Associate Principal for CPP Wind Engineering Consultants, St Peters, NSW, Australia.

Roy Denoon is an independent Wind Engineering Contractor, Fort Collins, CO, USA.