The trend towards taller, more slender buildings on smaller footprints with the implicit challenges of making these buildings stable and serviceable has necessitated a rapid development in the technologies used in the design of such structures and the development of sophisticated and innovative structural solutions that meet all of the governing relevant building codes with even more rigorous wind-resistance, earthquake-resistence and robustness requirements.
All optimal building solutions require compromise to balance competing factors such as floor efficiency – usable/lettable versus gross area – and structural efficiency. This is even more of a challenge for tall buildings on small sites where the size of the core is constrained. To make these slender buildings stable and serviceable requires the development of innovative structural solutions, which must also deal with the generic tall building complexities such as transfer structures and column shortening as well less common challenges like the need to bridge and/or isolate acoustic or vibration-sensitive obstructions, for instance when buildings are located over rail tunnels.
With increased slenderness and optimization of the structural stability system, the need to meet occupant comfort requirements in terms of perceived movements (horizontal accelerations induced by wind loads) becomes a significant and often governing criterion. Wind accelerations are predicted from wind tunnel testing and then compared with world’s best practice guidelines to determine the need for added damping.
Meinhardt has engaged with two leading wind consultants to develop solutions to incorporate added damping on two towers currently being designed in Melbourne. In both cases, tuned liquid dampers were selected as the preferred means of adding damping partially due to the fact that a water tank is typically required near the top of the building as part of the fire suppression system. In these cases, the tank can serve dual purposes – fire safety and damping.
For each of the case study buildings, optimization of the overall structural solution was sought by precluding or minimizing the need for transfer beam structures which are expensive, slow to build and take up valuable height in the building. On Prima (Pearl) the architectural planning constraints up the building were particularly challenging as they typically coincided with where floor uses changed, as seen as at level 10, which transitioned from apartment to car park. Accordingly, transfer structures had to be utilized at four discrete levels involving up to 1.8 metre deep beams at Level 10 and a maximum of 0.45 metre deep transfer bands at Levels 53, 55 and 60.
At 568 Collins Street, a solution has been negotiated whereby only three of the main tower columns need to be transferred over the carpark circulation ramp. Some other smaller raking tower columns at the corners of the floorplate are also transferred at the top of the podium.
An increased focus on serviceability considerations and more stringent acceleration criteria has led to a need for added damping becoming more commonplace, particularly in slender residential towers.
Reserving space for these dampers is challenging if not identified early. We suggest the following top tips:
- Use code-based estimates for acceleration in the early concept phase to determine whether accelerations may be a governing factor,
- Highlight the potential need for added damping for each structural solution and consider the opportunity for potential savings by using added damping to address acceleration issues rather than increasing stiffness,
- Engage a wind consultant early and spend a little more to allow them to run multiple analyses for a number of potential structural solutions that consider increased stiffness and added damping to address acceleration issues,
- If added damping is predicted to be needed as a contingency, work with all stakeholders to develop a solution to reserve space for it.
By Mark Hennessy Sector Leader, Property & Buildings (VIC) and Doug Wallace is Associate at Meinhardt Australia
Figure 1. Damper Option A – 3 Level U-shaped column (Source: Meinhardt)
Figure 2. Damper Option B - Two sets of stacked (2 high) rectangular (Source: Meinhardt)
Figure 3. Damper Option C – One set of stacked 4 high tanks (Source: Meinhardt)
Figure 4. Damper Option D – Two sets of stacked (2 high) rectangular (alternative) (Source: Meinhardt)
Figure 5. Prima (Pearl) Tower, Melbourne, Australia (Source : PDG Corporation / Schiavello / Disegno Australia)
Figure 6. 568 Collins Street, Melbourne, Australia (Source : Stamoulis Property Group / Bruce Henderson Architects)