With the object of checking wind tunnel findings on the drag coefficient, cd, a full scale motorway sign gantry was instrumented with strain gauges, anemometers and wind vane. Data were recorded on seven occasions, the wind being normal to the structure on all but one. Strains normalised to a wind speed of 10 m/s and also averaged for each experiment were substituted in expressions which had been evolved for cd. The resulting figure of 1.86 was significantly above the smooth-flow wind tunnel value of 1.6, due probably to the effect of turbulence in atmospheric flow. Strain and wind speed spectral densities were also calculated, showing the natural frequency of the structure to be 2.49 Hz, and used to determine values of aerodynamic admittance. These were found to be similar to the admittances of flat plates and are discussed in relation to the standard deviations of strain, pressure and wind speed. A detailed comparison of the results with the gust factor in BS 5400 indicated that the wind loading clauses in this new code are applicable to sign gantries. Arising from this, a simple rule is proposed for estimating the design wind pressure on a sign gantry in normally directed winds. (Author/publisher).
Samenvatting