How to cope with aerodynamic forces when designing the world’s longest suspension bridges

03.12.2018 / Allan Larsen

The need for longer, durable, stable and economically efficient suspension bridges puts high demand on the investigations of the aerodynamic stability of bridge designs. Up-to-date building codes, wind tunnels and advanced sensor-technology are among the necessary tools at hand. 

A century ago, gravity was the primary natural force considered by bridge engineers of the day, but the demand for longer, yet durable and cost-efficient bridges to provide sufficient infrastructure around expanding cities has changed this

Obviously, a solid knowledge of structural mechanics and materials science remain key disciplines within bridge engineering, facilitating the fundamental purpose of a bridge to support the loads of roads or railways against gravity.

However, with the need for even longer suspension bridges, another natural force has become equally important - the wind. 

Innovation comes with a price tag

Suspension bridges are a testimony to how bridge designs have evolved with our understanding of structural engineering and our ability to continuously improve construction materials and more efficient shapes, yielding even longer bridges.

A generic suspension bridge consists of relatively few elements: a deck carrying the road or railway, which  is suspended from vertical hanger cables in turn carried by sagging main cables. Often, a suspension bridge includes two towers to support the main cables at a sufficient height for the main cables to provide the necessary load carrying capacity.

The simplicity of the suspension bridge is what makes it so economically efficient and gives the bridge its slender, elegant appearance as if the roadway is floating above the water. 

However, there is a price tag to all innovation. For a suspension bridge, the cable-based structural design produces a highly flexible structure, which makes it vulnerable to the wind.

In fact, if you do not consider wind effects when designing and building suspension bridges, the consequences can be fatal. 

Bridge collapse gave new insights into aerodynamic forces

The wind performance of a planned bridge is investigated through meticulous aerodynamic studies. Aerodynamic tests are conducted in large wind tunnels using scale models, which reproduces the geometric shape, stiffness and mass properties of the prototype bridge. Often, the wind tunnel tests are assisted by computer simulations to allow identification of the optimum geometrical deck shape.

The overall goal of the wind tunnel test is to be able to assess if aerodynamic instability may be triggered by a windstorm - a phenomenon where periodic aerodynamic forces created by the wind may synchronise with the natural frequencies of the bridge structure.

At the onset aerodynamic instability energy builds up in the structure resulting in large twisting oscillations of the bridge deck. 

Wind tunnel testing to examine the wind effects at large was introduced in bridge engineering after the 1940 collapse of the ill-fated Tacoma Narrows Bridge. 

Large amplitude wind induced torsion motions drove the main span bridge to collapse in less than one hour.

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. 

To compensate for the flexibility and lightness of the towers, the engineers are looking to the 509 m tall Taipei 101 building in Taiwan. Built in a windy and hyperactive earthquake-zone, this Taiwanese high-rise is equipped with a 660 tonne steel pendulum, which serves as a tuned mass damper to mitigate wind induced motions.

Any motion of the building due to typhoons or earthquakes will set the suspended counterweight into oscillations, thereby extracting energy that otherwise would build to large amplitude vibrations of the building.

The same principle is on the drawing board for the new bridge. By adding a suspended 25 tonnes counterweight inside each tower leg, the steel towers will be able to resist strong winds without encountering wind-induced vibrations. 

SPEEDING UP AERODYNAMIC INVESTIGATIONS

With today’s computer technology, we can calculate the aerodynamic behaviour for bridge components with good accuracy. Based on such calculations, we will select one or two designs for verification by wind tunnel testing.

The benefit is to increase the speed of the design process. Within a few days, you can make a computer model of your bridge design, and within a week, you will have an idea of the aerodynamic performance.  In a wind tunnel, the full process for investigating each design can easily take 4-5 weeks. 

In short, we are making good use of computer technology to reduce turnover time for aerodynamic investigations. This trend will keep developing in the years to come.

GLOBAL WARMING LEADING TO HIGHER WIND SPEEDS

Looking ahead, global warming will lead to a steady increase of the mean temperature of the world oceans. This will in turn feed more energy into the typhoons developing in the South China Sea and hurricanes developing in the Caribbean. 

A projection of this trend yields an increase of the mean wind speed of typhoons in the North Eastern Pacific from current levels of 55 m/s to approximately 63 m/s by the year 2100 with extreme wind speeds reaching as high as 68 m/s.

The predicted increase in mean wind speeds corresponds to 14% over the next 85 years. (Source: Scripps Institution of Oceanography, University of California, San Diego)

Obviously, these projections are important to consider when testing the stability of bridge designs, which are typically designed to last for at least 100 years. 

The process of wind tunnel testing will not change by global warming, but you must know the maximum wind speed that your bridge model is to be exposed to evaluate the adequacy of the design.

In other words, the changing wind conditions needs to be reflected in the national building codes, which defines the premises for the wind tunnel tests and the structural design.

Consequently, it is very likely that the wind tunnel tests will show that designs with higher geometrical sophistication and possibly more structural material are required to ensure a bridge’s aerodynamic stability in a future impacted by climate change.