By increasing the height of the building, the wind load affecting the building becomes one of the most significant load that the building has to be dimensioned to. While in case of simple shapes, the applicable regional standards (EUROCODE, ASCE etc.) provide for the calculation of wind load with great accuracy, using experiments with models in a wind tunnel, the effect of the wind to the building can be accurately determined for complex shapes as well, thereby saving construction costs.
In a boundary layer wind tunnel, the dynamic particulars of the atmospheric boundary layer also correspond to the real environment (the ratio of the strength and length of wind gusts compared to the average wind speed is the same as in the real environment), so apart from the average load on the building, the peak load caused by wind gusts can also be accurately measured in the wind tunnel.
Using the wind tunnel experiment, the bending and torsional torques can be directly measured at the foundation of the building by integrating a force gauge system. Alternatively, the pressure can be simultaneously measured at hundreds of pressure measuring points, and the resultant forces and torques can be calculated from this data by surface integral.
The latter method is also suitable for the determination of the load on various parts of the outer layer (which are much more sensitive to short term peak load than the whole building).
In addition, the dynamic effect, the gustiness of the wind also threatens slim buildings (with a height of several hundred meters), as they may swing due to their low natural frequencies. In case of certain frequencies, even a small amount of swinging has a negative effect on the wellbeing of people in the building, while higher amounts of swinging can obviously damage the building.
There are several possibilities to assess the dynamic behaviour of buildings:
- The pressure distribution (the excitation of the building) recorded for the model in the wind tunnel with high resolution with respect to time and space is entered to a numerical dynamic model, which then calculates the dynamic behaviour of the building (the frequency and amplitude of deflections) based on such data.
- Instead of regular rigid models, a so called aeroelastic model is made. Similarly to the real building, this model is flexible, and it bends and twists due to the wind. During the measurement in the wind tunnel, the shapes, the frequencies and amplitudes of deflections can be recorded using an oscillation measurement system. These particulars of the model can then be converted into the particulars of the real building by applying the modelling principles.
Whichever option is chosen, the wind tunnel experiment has a key role in the determination of the wind load that the buildings are subject to. Moreover, the often critical wind comfort in the vicinity of high buildings can also be determined by the experiment, so designers may take corrective actions in the problematic zones during planning.
Involved researchers and departments
Recent publications of BME on the subject
Dr. Lajos Tamás: Jelentés a Raiffeisen torony szélcsatorna vizsgálatáról [Measurement report on the wind tunnel testing of Raiffeisen Towers]. BME Department of Fluid Mechanics, 2007.