Tall steel monopole towers sometimes appear to lean, putting into question their stability and plumbness. Yet, at different times of day, the same poles may appear vertically straight. Is this a cause for concern?
The best way to determine monopole plumbness is a detailed survey check of the base flange and anchor rods. What may be happening to the steel tower itself could be the effects of thermal expansion, commonly referred to as “sun camber” or “sunflower effect”.
Steel has a thermal expansion coefficient of 6.5×10-6 per inch per degree Fahrenheit. As such, throughout the day, ranging temperatures and effects of shade and direct sunlight can cause steel to grow or shrink due to its elasticity. Leaning or curving of steel monopole structures is more noticeable because the tower will not only grow in height but also the portions exposed to direct sunlight will grow at a different rate than the shaded side. This can temporarily cause an uneven appearance.
Recently, our Survey + Scanning team performed topographic survey of a newly installed transmission line of monopole towers to determine their plumbness top to bottom, as well as the straightness within each segment.
Using a Leica ScanStation P50 long range 3D laser scanner, with a scanning accuracy of 3mm and up to 1 kilometer in range, the team was able to measure plumb deviations from each towers’ centerline.
Although, two of the towers appeared to exceed the plumb tolerance, the survey helped determine that solar camber and wind effects were at play, but the towers were within plumb tolerance.