Structural designs for weather conditions means preparing for the unpredictable
Designing structures can be challenging. Preliminary steps include selecting an appropriate site, deciding on the type and configuration needed, determining the correct materials and components and designing the structure within budget. Then there’s the issue of weather. How do engineers design structures based on unpredictable factors?
Ulteig Senior Engineer Sriram Kalaga has studied structural design for more than 30 years. Kalaga explained the factors to consider when designing a structure. For transmission and distribution lines, each structure is a small piece of a large puzzle. “The location of each structure will depend on right-of-way acquisition, easements involving landowners, the type of soil, type and strength of structure and voltage levels of the transmission line,” Kalaga said.
The load weighing on the structure must also be taken into account. The American Society of Civil Engineers (ASCE) describes structural load as forces, deformations or accelerations applied to a structure or its components. In addition to the weight of the structure and wire tensions, weather issues, such as ice and wind, are factored into the design equation. To meet design standards determined by the National Electric Safety Code (NESC) and Rural Utilities Service of the United States Department of Agriculture (RUS/USDA), winds from 40-120 mph and excess ice ranging from 0.5-1.5-inch thickness must be planned for. Kalaga explained high winds can cause galloping or dancing conductors that often damage structures; excess ice can cause wires to sag, pulling down structures with them.
So, why don’t engineers design structures that can resist winds of more than 200 mph and 3 inches of ice? Simply put: the cost.
In general, loading standards usually refer to a 50-year lifecycle of a structure. Because 3 inches of ice may only occur once every 100-200 years, it isn’t economical to design a pole for such an uncertain event, Kalaga said.
“Years ago, structures were built with very high safety factors because companies and engineers did not have a full understanding of weather patterns,” Kalaga described. Designs today use a more rational and economical approach to match the intensity of weather loads with the actual strength of a structure. An increased understanding of weather and weather measurements, combined with technological improvements like high-strength carbon steel and enhanced protection coatings for wood poles, have led to better results, he added.
Another option that includes higher safety features is an underground transmission line. Underground lines are already used near railroad crossings and in areas with clearance restrictions. The issue with these lines is cost; underground lines are four to six times more expensive than traditional overhead structures, Kalaga said. Another issue with underground lines is location and detection of faults. Unlike overhead lines, where visual observations can reveal problem areas, it is difficult to do so with underground lines.
Regardless of design technology and weather knowledge, unpredictable weather still remains a variable. “No matter what materials we use and how well we understand the weather, Mother Nature will always have a say,” Kalaga explained.
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