I have to Get Something off My Chest. I've managed to work myself up over the issue of maximum wind loading for the design of transmission lines, and the unsettling thought of putting too much reliance on the calculation of return period events. But before we start redesigning lines, we need to know what wind issues we face.

My colleague Roberto Behncke (POWER Engineers Inc.; Hailey, Idaho, U.S.) and I have investigated many, many tower failures over the course of our careers and, discounting design or construction issues, every wind-based trigger event evolved from intense thermal actions, including tornados, microbursts and downbursts. These experiences show that high-wind days don't result in knocked down towers (although some moderate high-intensity winds may lurk within).

It is incredibly difficult, if not impossible, to predict when a short-duration, high-intensity wind will occur. In fact, Behncke and I have sifted through 50 years of weather data to see if we could contrive some mathematical formulation that might enable us to predict the emergence of intense gusts within large-scale or synoptic winds, and the correlations just aren't there.

Even with 50 or 100 years of data, one can never know just where you are in the overall scheme of extreme events, and those are the ones of interest to us as line designers. Records of weather data are useful as guidance in common events, but are of little value in assessing the probability of those events that will result in damage to structures. We've seen that a single extreme event, such as the 1998 ice storm in eastern North America, can significantly shift the prognostics for future events.

So, how do we select design criteria for our poles and towers when we know that our traditional design criteria based on synoptic winds are not appropriately based on wind-based damage to our structures?

Let's examine one utility's experiences with high-wind events. Hydro One (Ontario, Canada), formerly Ontario Hydro, determined that tornados were the major cause of weather-related failures of its transmission lines. The utility pursued studies to determine the costs of increasing its design specifics in an attempt to protect against most such events. An early study found that five out of six weather-related line failures could be attributed to tornados, but the seemingly high cost of increasing wind loads to reduce tornado damage deterred action.

I participated in a follow-up study in 1995 that showed that if Ontario Hydro increased its already high but apparently inadequate extreme wind load on structures from 160 kmph (100 mph) to 250 kmph (155 mph), the increase in weight of the steel of new structures would be under 3%. This study was made for 500-kV guyed-V and double-circuit lattice-type towers, with all heights checked. This suggested increase in design wind speed of 250 kmph is the upper range for F2 (Fujita scale) tornados in the United States and a seemingly reasonable value for Ontario, which could significantly reduce expected extreme wind damage to new towers.

Behncke and I performed similar studies for Eskom in South Africa, demonstrating that the weight of lattice steel masts of 400-kV cross-rope suspension towers would increase by no more than 2% when lines were designed to resist a high-intensity wind storm equivalent to F2 in the Fujita scale.

I would add this caution: the 2% to 3% steel weight increase may not be obtained for all designs and does not apply to modifications to existing in-place structures. Still, we've found that some lattice structures can be modified at surprisingly low cost by merely changing internal bracing members in the body.

When looking at the total costs, the penalty associated with reinforcing the structural components to meet higher design wind loads may well be insignificant in comparison to overall maintenance and construction expenditures. Total structural costs of new lines are usually in the range of 15% to 25% of overall final line expenditures. So, taking into account higher design wind speeds would result in an overall project cost increase of less than 1%. Weigh that cost against the added value in protecting against the most prevalent natural cause of transmission line outages, represented by the proposed F2 tornado loading. This should not be a tough decision to make.

Taking the longer view, the influence of increased costs to accommodate such apparently extravagant load criteria as an F2 wind on structures has been overtaken by the multitude of involvements of regulatory bodies and other factors such as rights-of-way and legal costs that are now a fact of life in the permitting processes.

These countless steps over and above the engineer's design function significantly add to the years and costs of commissioning a new line. This leaves room for design engineers to be a bit liberal with the criteria themselves. Do apply at least an F2 wind load to your structures — and use no more than two or three significant figures in your calculations.

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H. Brian White is an independent consultant and has more than 50 years of experience in engineering line design. whitetrans@symaptico.ca