Issue link: https://geospatial.trimble.com/en/resources/i/1415432
Spring 2019 19 1 D amage to storage tanks from wind is infrequent, but when it occurs, it can be catastrophic, as shown in Figure 1. This is partly because the cylindrical shape of storage tanks is effi cient at resisting internal hydrostatic pressure from stored liquids, but very ineffi cient at resisting external pressure from wind. Also, wind uplift depends on the tank diameter squared, and thus wind is a much more signifi cant factor in the design of large storage tanks. While these large tanks are less familiar to storage tank designers, they are becoming more common as costs and regulations limit the number of new tanks. Finally, the strength of tank components that resist wind loads, such as wind girders, has been inconsistently addressed in tank design standards such as API 650, 'Welded Tanks for Oil Storage'. Consequently, API is considering changes to modernise 650's wind load provisions. Wind speeds As pressures are diffi cult to measure, wind is usually recorded in terms of wind speed. Wind speed measurement, however, is elusive, because speeds vary over measurement time, height, type of terrain, and geographic location. In the US, wind is measured 10 m above the ground and over a 3 sec. averaging period, referred to as the 3 sec. gust wind speed. To further complicate matters, the maximum wind speed for a given location depends on how often this maximum is expected to occur, called the mean recurrence interval (MRI). Design loads for US locations are established in the American Society of Civil Engineers standard ASCE 7, 'Minimum Design Loads and Associated Criteria for Buildings and Other Structures', most recently published in 2016. ASCE 7-16 includes four wind speed maps, one for each risk category type of structure, I through IV, with I being the least risk (such as greenhouses) and therefore the shortest MRI. By ASCE 7's defi nitions, storage tanks are in risk category III, defi ned as "Buildings and other structures not included in Risk Category IV (including, but not limited to, facilities that manufacture, process, handle, store, use or dispose of such substances as hazardous fuels)." The MRI for category III structures is 1700 years, which has a 3% probability of being exceeded in 50 years (50/0.03 = 1700). The 1700 MRI is an increase from the 100 MRI used in ASCE 7 before 2010, but because it is accompanied by a 0.6 factor on wind load when used for the allowable strength design method used in API 650, there is no change in wind loads. Wind pressures The pressure exerted by the wind over a surface at a given wind speed depends on the average height of the surface, surface shape, elevation above sea level, structure symmetry, structure spacing, surrounding terrain, and other factors. While these parameters vary among tanks, simplifying assumptions can be made for their values for typical tanks to establish design wind pressures for tank standards. For example, the terrain may be assumed to be ASCE 7 category C, open terrain with scattered obstructions that have heights generally less than 30 ft. This includes fl at, open country and grasslands. For ASCE 7 category B terrain (urban and suburban areas, wooded areas, or other terrain with numerous, closely spaced obstructions that have the size of single-family dwellings or larger), wind pressures are 25 – 30% less, but it is