Wind tunnel measurement techniques have developed immensely over the years. In the early days, we could only measure the wind-induced displacements of the model, which was recorded on paper to document how the bridge would move and become unstable in the wind.
Pen recorders were important tools to document whether aerodynamic instability would occur and at what wind speed. However, the traditional measurement techniques do not provide insight to what we really want to know - how the aerodynamic loads are distributed and interplay with the motions of the bridge deck.
By looking further into the Tacoma Narrows Bridge collapse, we have gained valuable insights into the potentially devastating impact of the wind, which again has created a need for new and more advanced wind tunnel test technology.
Sensor-technology will improve determination of aerodynamic stability
Present day solid-state and computer technology have provided tiny pressure sensors, which we can use for measurement of instantaneous pressures in real time. This gives us a detailed map of how the wind forces fluctuate on the model bridge deck.
Based on this information, we can now change our designs to counteract undesirable pressure maps and minimise the destabilising aerodynamic forces.
This is an enormous advantage and indeed a new technology, which has developed over the last 20 years, paving the way for longer and more stable suspension bridges.
World’s longest suspension bridge tested to withstand a full-scale wind speed of 90 m/s
Recently, we wind tested a full aeroelastic model of a planned world record suspension bridge in a 22 m wide wind tunnel at RCWE, Chengdu, China. With a main span of 2,023 meters, it will become the world’s longest suspension bridge.
The most important result of the tests is that the bridge will sustain winds in excess of 90 m/s due to the twin-girder design of the bridge deck.
Is experimental proof of aerodynamic stability up to 90 m/s adequate you may ask? Indeed it is!
The highest wind speed ever recorded is a 3 second gust of 113 m/s at Barrow Island, Australia in 1996 during tropic cyclone Olivia. A 3 second gust is much too short to excite a large suspension bridge into motion having oscillation periods at the order of 10 to 20 seconds.
A rough but conservative estimate of the corresponding sustained (mean) wind speed would be about 30% less equal to 87 m/s. Thus, it is a fair claim that the world’s longest suspension bridge has been experimentally proven to be able to stand up to any storm wind this world may throw at it.
As the wind engineer for this magnificent bridge, I am proud that we achieved full-scale wind speed of 90 m/s without encountering signs of aerodynamic instability – indeed a triumph for suspension bridge engineering.
Suspension bridge towers are frequently built from reinforced concrete for economic reasons but the 318 m tall towers of this record span will be made from steel to secure sufficient earthquake resistance. Steel works great for earthquake resistant designs, but from an aerodynamic point of view, the bridge towers become very light and flexible.
Without mitigating measures, the tower legs would literally start to swing from side to side in strong winds, causing aerodynamic instability – or "bounce". This is any bridge engineer's worst nightmare.