Climatic wind tunnel testing of cables
The cold weather and ice we are experiencing these days has been known to cause damages on cars and people when crossing icy bridges. These accidents can be prevented by performing wind tunnel tests.
“Our climatic wind tunnel allows us to test the effects of different weather scenarios ranging from dry conditions with light wind to severe subzero weather,” says Emanuele Mattiello, Project Engineer at FORCE Technology.
A unique wind tunnel facilityThe climatic wind tunnel is unique in the sense that it is designed specifically to test structural cables, and that it has enough space to fit a section model of a bridge cable in a 1:1 scale. “As soon as you add weather scenarios like rain or ice, it is extremely difficult to obtain valuable results with a scaled model. Thanks to the capabilities of our climatic wind tunnel, we test full-size models of bridge cables. This way the designer receives results that are directly applicable and accurately represent reality,” explains Emanuele.
Rain might cause the bridge cables to vibrateRain affects the surface of the bridge cables in a way that could cause the cables to vibrate. Over time, these vibrations might cause fatigue-related problems such as failure of the anchor point between the cable and the deck.
The tests we perform are carried out by simulating scenarios that could provoke these vibrations, i.e., exposing the cables to a combination of rain and wind. We examine if the cables are able to mitigate or suppress the vibrations caused by water and wind.
If this is not the case, we will advise the customer on how to reduce the vibrations either by changing the surface of the cables or by adding a mechanical damper that will reduce the movements of the cables.
Ice accretion could cause severe damagesWhen it comes to ice accretion on bridge cables, it is important to understand the process of how the ice is built up on the cables as well as how it melts off them. To ensure that the processes in the wind tunnel are comparable to what happens in nature, we normally let the ice build up for an hour.
Different wind intensities and cloud types with various water content create different types of ice. Therefore, we measure the performance of the cables when covered with these types of ice. This way we are able to illustrate the potential risk of ice-induced instabilities of the cable to the customer.
During the de-icing process, a layer of liquid water typically forms between the surface of the cable and the ice itself, thereby releasing the cohesion between the two. Our test objective is to identify how this process takes place, and how large the pieces of falling ice would be.To document the de-icing process, we film it using time-lapse. The falling pieces of ice are gathered in a soft net so they can be measured weighed and photographed.
By comparing different cables and orientations of the cables in combination with the wind and different types of ice, it is possible to identify the safe and the critical scenarios.
“Whether it comes to retrofitting an existing bridge or designing a new one, we are able to provide our customers with essential information about how to create a safe, efficient and cost-effective bridge,” says Emanuele.