Slender art structures require efficient dampers

Melbourne, Australia. The Red  Sticks are 25-metre-tall, slender art structures that – after 27 years of exposure to the elements – required structural refurbishment. The challenge of the retrofit was to design efficient TMDs with tuneable natural frequency that also fit into the compact dimensions of the Red  Sticks.  MAURER solved this demanding task with TMDs with adaptive friction plate damping and springs for the on-site tuning of their natural frequency.

The Red Sticks, together with a yellow beam affectionately known as the  Cheese Stick, are an art installation that has been a landmark of Melbourne since 1999. The 39 Red Sticks are 25 m tall and positioned over a length of 420 m close to the motorway that connects Melbourne’s city centre to the city’s  airport. There are 21 Red Sticks to the north of the road, and 18 Red Sticks to the south.

Due to their square cross-section and the harsh weather conditions, the Red Sticks are prone to wind-induced vibrations. After 25 years, issues began to arise at their base. The main problem was that the existing TMDs were primarily activated at greater wind forces, but provided barely any supplemental damping to the Red Sticks at lower but more frequent wind loads.

The retrofit consisted of the following two measures:

• The section between the concrete foundation and the steel structure of each  Red Stick was reinforced via concrete confinment to reduce bending stresses.
• New TMDs were installed that generated supplemental damping over a wide  range of wind loads.

The specific requirement in this project was the need for the TMDs’ natural frequency to be tuneable on site, in order to retune this to the first bending mode eigenfrequency of the Red Sticks following the confinement at their base.  MAURER met this requirement using TMDs with a pendulum mass, combined with tuning springs with variable quantity and angle.

Pendulum mass with damping

MAURER friction plate TMDs are robust and provide adaptive friction plate damping. Their natural frequency can be optimised to the structural eigenfrequency via tuning springs. Their pendulum mass is designed to provide the required supplemental damping for the structure, while the computation of the TMDs’ natural frequency also takes into consideration the friction plate dynamics.

The MAURER TMDs with adaptive friction plate damping and tuning springs mitigate the Red Sticks’ vibrations over a wide range of amplitudes. Four friction plates, each with different mass and geometry, are coupled to the pendulum mass in such a way that the supplemental damping provided to the Red Sticks hardly depends on the vibration amplitude. In addition, tuning springs installed between the rack and the pendulum mass allow the TMDs’ natural frequency to be retuned to the changed eigenfrequency of the Red Stick. The special shape of the TMD rack was developed to allow the TMDs to be installed horizontally in the Red Sticks, which are inclined by approximately 20 degrees.

Tuning the TMDs’ natural frequency was challenging, as the eigenfrequencies of the Red Sticks changed when the old TMDs were replaced by the new  MAURER TMDs, as well as when the confinement was made at their base. The confinement also took place at the same time as the TMD installation. The optimum design value of the TMDs’ natural frequency was derived via non-linear simulation, taking into consideration all the aforementioned issues. The natural frequencies of the new TMDs were checked by  Factory  Production  Control (FPC) in Munich before they were shipped to Melbourne.

Optimising the natural frequency

After the  MAURER TMDs were installed at the top of the Red Sticks, the damping of all 39 Red Sticks with working TMDs was experimentally verified in April 2026. Here, each Red Stick was manually excited by two people up to its maximum vibration amplitude. The total damping (supplemental plus inherent damping) was then derived from the subsequent free vibration response. Where necessary, the natural frequency of the TMD was fine tuned to the actual Red Stick eigenfrequency.

Seven damping tests had to be performed overnight, as seven of the Red Sticks lean over the motorway, requiring a road closure between 10 pm and 5 am. The damping measurements in April 2026 confirmed that the  MAURER TMDs led to a total structural damping of 2 % to 3.5 % and that the MAURER TMDs were  activated at the Red Sticks’ top amplitude of ±5 mm.