Question 4: Dynamic response of structures to wind loads (25 marks) a) A circular steel pole (diameter of 300mm) has a first mode natural frequency of vibration of 5Hz. The pole is to be designed for a maximum 0.2 second duration wind gust of 30m/s. What wind speed is likely to result in high cross-wind displacement as a result of vortex induced vibration? Explain your results in no more than two sentences. (5 marks) b) Is this likely to be problematic? Why or why not? What design changes are available to engineer to reduce the amplitude or occurrence of these vibrations? (4 marks) c) Consider a 40 metre tall circular light pole of diameter 1200mm. A table showing the variation of design gust wind speed (0.2 second gust) and turbulence intensity is shown below. (16 marks) Z(m) U 0.2s(m/s) Tuu