The dynamic investigation of a structure is traditionally conducted in the frequency domain. Frequency domain methods are less time consuming than time domain methods, and a variety of effective approaches have been developed over time (e.g. Holmes 1994). In order for the frequency domain approach to be valid, the structure must behave in a linear elastic fashion, and the phenomenon producing the loading must be statistically stationary. In many regions around the world, the primary cause of design wind events are thunderstorm downbursts. While a degree of uncertainty still surrounds the true nature of downbursts, they are known to be highly non¬stationary.
Guyed masts represent a cost-effective alternative to freestanding towers due to their lighter use of materials, although tend to be susceptible to dynamic loading effects. Designers need to consider these effects when designing such structures. Cable supported structures also behave in a non¬linearly, as cable stiffness is influenced by the cables geometry. A dynamic investigation of a guyed transmission tower, or a structure subjected to a downburst will most accurately be conducted in the time domain.
The research presented in thesis can be summarised by its objectives, which were:
1. 1. The first objective was to compose a suitable technique for simulating correlated wind speeds in the time domain at multiple points in space, which can be used for estimating loading time histories on a transmission line structure. Several options were investigated, including the use of Computational Fluid Dynamics (CFD). A parametric, empirical/analytical model was composed to represent the non-turbulent component of downburst winds, including characteristics such as the translational movement of the downburst. An amplitude modulated Auto-Regressive Moving Average (ARMA) process was used to simulate the turbulent component of the wind speed time history. The two components are combined to form a three dimensional, time varying model of the downburst winds.
2. 2. The second objective was to parametrically investigate the dynamic response of simple (SDOF) structures subjected to downburst winds. This was conducted using a simple time domain approach in which wind loads were simulated using the downburst wind speed time history developed in the first part of the study. Results indicate that for structures of low natural frequency, dynamic excitation is likely to be lower during a downburst than in a boundary layer wind event of similar peak gust intensity and turbulence characteristics.
This is likely due to the short duration over which peak wind conditions occur during a downburst.
1. 3. Another objective was to investigate gust behaviour occurring in the simulated downbursts, and the ways in which this behaviour is contrary to our current assumptions for “design” events. An understanding of the relationship between peak gust speeds and peak non-turbulent speeds during an event is important for determining simulation strength, and also when applying Extreme Value Distributions (EVD) to estimate gust behaviour and peak structural response. Simulations performed indicate that the duration over which peak factors during downburst events are likely to be significantly lower than those for boundary layer events. A Type 1 EVD is used to characterise the gust speeds occurring during a boundary layer event, however the peak gust speeds occurring during simulated downbursts did not converge to this distribution due to the “short” duration over which peak speeds can occur in these non-stationary events. Emperically, a Type III EVD was more suitable for describing the peak gusts occurring during downbursts.
2. 4. The quasi-static loading conditions in downbursts as it relates to transmission line structure design were also investigated. The downburst wind speed model developed earlier in the study was used as the basis for this investigation. The significance of certain parameters used to define the simulated wind field (such as translational speed) as they relate to loading of transmission line structures is discussed. Also, the implications of the relationship between peak gust speed and non-turbulent speed as it relates to transmission line structure loading is discussed, particularly with respect to conductor loading span reduction factors. A basic comparison of loading described in various design standards is provided.
3. 5. The final object was to investigate an example of a guyed transmission line tower to see if expectations developed from the preliminary loading/response investigates are applicable to a complex structure. The structure was investigated under both a quasi-static analysis and a time domain dynamic analysis. The quasi-static analysis revealed that the response of the structure shows varying levels of sensitivity to variation in parameters that define the simulated downburst wind field, such as height to maximum wind speed and translational speed. Response of the tower was sensitive to the assumed design wind speed, which can be different for downbursts and boundary layer winds in various regions. Response of the tower during the time domain dynamic analysis was dependent on the assumed levels of turbulence.