Keywords: wind, wind barrier, wind tunnel, bridge, traffic safety, measurements, computational fluid dynamics method.

In many places bridges are exposed to winds of high velocities. In that case two problems occur. The first one is the influence of the wind on the bridge structure especially during the critical phases of construction, and the second one is the influence of the wind on the traffic safety during the exploitation period. During the construction of the Slovenian highway network some large viaducts have been built. The largest one (length 1065 m, width 27.1 m and with a height of the main structure over 100m above the ground) is now in final stage of construction. Since the viaduct is positioned near the Adriatic coast it is exposed to very strong north-east and south-west winds. They occur very often especially during the weather changes, mostly in the late autumn, winter and early spring period of the year. The velocity of the wind called Bora can reach the speed up to 200 km/h. To assure a safe passing of the traffic over the viaduct a special wind barrier was designed. It should reduce the wind load on the vehicles so that the safe traffic would be possible even at wind speeds of 140km per hour. It should be transparent in order to avoid the feeling of driving through a tunnel and make the passengers enjoy a spectacular view. The elements of the wind barrier should be designed in special shapes to remain stable during the strongest winds from different directions and at extreme temperatures. The barrier should also be easy to clean and maintain. The solution suggested by the bridge designer has therefore undergone many different loading tests, tests in the wind channel, crash tests and computer simulations before it was accepted.

In October 2003 the full scale prototype of one segment of the wind barrier was transferred to Vienna to the RTA Rail Tec Arsenal testing plant. The prototype of the barrier was fixed to the floor of the wind tunnel. Each of the seven acrylic panels was equipped with two strain gauges in the middle of their span. Another two measuring points were prepared at the bottom of one steel column. During the test the wind velocity raised from zero to over 200 km per hour at three temperature levels. The strains, displacements and the wind velocities were simultaneously measured all the time. The extreme stresses calculated from the measured strains remained far below the fracture values even at the highest values obtained of the air velocity. The system of the elastic fastening of the acrylic panels to the supporting columns has proved itself very successful.

The measurements done in the wind channel were a perfect base for the verification of the reliability of the software for the computational fluid dynamics analysis which has been used for the preliminary and further studies of the efficiency and safety of the wind barrier. For that purpose a very precise model of the wind barrier and the wind channel was generated. The generation of such a precise model was necessary also for the later displacements, stress and strain analysis of the acrylic panels, where the correct modelling of conditions of the elastic fastening of panels are of great importance for obtaining high accuracy of calculation results. For the CFD analysis the FloWorks module was used The following step was the detailed stress and strain analysis of the acrylic panels where the pressure distributions obtained in the CFD analysis were taken as the surface loading onto the panels. Therefore the model of the wind barrier was transferred to the program package COSMOSM. To assure the proper boundary conditions each panel was calculated as an assembly of the acrylic element with the rubber connecting element on both sides. The agreement of measured and calculated results was very good as well as in all other cases. Therefore the decision was made to stop further very expensive tests in the wind channel and to simulate the various traffic situations under the winds of various velocities and directions by using the CFD analysis. To establish how efficient the wind barrier is various traffic situations were modelled and calculated. One of the most characteristic cases was the truck positioned on the traffic lane. In the first case the viaduct was without the wind barrier and in the second case it was included in the calculation model. The comparison of the pressure fields of the both systems proved that the wind barrier drastically reduces the wind loads on the vehicle.

After more than a year of investigations, testing and performing the computer simulations it can be concluded that the results obtained certify that the wind barrier will assure a safe and undisturbed traffic over the viaduct. The experimental results confirmed that the accuracy of CFD simulations in some cases is high enough so that they can partly or entirely replace very expensive tests in wind channels.

1

T.A. Wyatt, "Recent British Developments: Windshielding of Bridges for Traffic", Aerodynamics of Large Bridges, Proceedings of the 1st International Symposium, Copenhagen, ed Larsen, A. Balkema, p 149-170, February 1992.

2

T.S. Higgins, "The Second Severn Crossing, Engineering Studies and Specification for Tender", Proceeding Bridges into the 21st Century, Hong Kong, p 425-436, 1995.

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