Press Coverage

20 Spring 2019 conservative to use category C and more typical of tank sites. Wind pressure is proportional to the square of the wind speed, so any wind pressure must be stated in terms of the wind speed on which it is based. Long ago, when wind speed maps were not available and wind speeds were measured by averaging over the time it took one mile of air to pass an observer (called the fastest mile wind speed), designers often just used a 100 mph wind speed. When wind speed measurements changed to averaging over a 3 sec. time interval, 100 mph became about 120 mph. At 120 mph, the averaging time is the 30 sec. it takes air to travel a mile. The shorter the averaging period, the faster the average speed, so the wind speed measured by averaging over 3 sec. is greater – as it happens, about 20% greater – than the wind speed measured by averaging over 30 sec. Therefore, API 650 uses 120 mph as a reference wind speed V r for which pressures are stated, and factors that pressure for other wind speeds. Using ASCE 7-16's methodology and values for the wind pressure parameters that are typical of API 650 tanks: The horizontal wind pressure on the vertical projected area of tank shells is: P WS = (11 lb/ft 2 ) (V/V r ) 2 The vertical wind pressure acting upward on horizontal projected areas of roofs with a rise to span ratio of 0.05 or less (typical of supported cone roofs) is: P WR = [(18 lb/ft 2 ) – (0.03 lb/ft 2 )D](V/V r ) 2 The vertical wind pressure acting upward on horizontal projected areas of roofs with a rise to span ratio greater than 0.05 is: P WR = [24 lb/ft 2 – (0.03 lb/ft 2 )D](V/V r ) 2 Where V r = 120 mph, V = design wind speed from ASCE 7-16 for risk category III, and D = tank diameter. These wind pressures differ from those currently in API 650 in two ways: the roof pressures for supported cone roofs are less than the roof pressures for all other tank roofs (which have higher profi les than supported cone roofs), and the roof pressures decrease as the tank diameter increases. API 650's 12 th edition and the proposed roof pressures can be compared with an example. API's current design wind speed is 110 mph for Houston, Texas, US; in ASCE 7-16, Houston's wind speed is given as 140 mph. For a 120 ft dia. supported cone roof tank, API 650 currently specifi es a roof uplift pressure of: P WR = (30 lb/ft 2 )(110/120) 2 = 25.2 psf The proposed roof pressure is: P WR = [(18 lb/ft 2 ) – (0.03 lb/ft 2 )(120 ft)](140/120) 2 = 19.6 psf API 650 currently has a shell pressure of: P WS = (18 lb/ft 2 )(110/120) 2 = 15 psf The proposed shell pressure is: P WS = (11 lb/ft 2 )(140/120) 2 = 15 psf For roofs other than supported cone roofs, the proposed roof pressure is: P WR = [(24 lb/ft 2 ) – (0.03 lb/ft 2 )(120 ft)](140/120) 2 = 27.8 psf This shows that the proposed roof uplift is less for supported cone roofs, the roof pressure is slightly greater for other roofs, and the shell pressure is the same. Uplift pressures are compared in Figure 2 for a range of tank diameters. Compressive strength of shells The 6 th edition of the 'Guide to Stability Design Criteria for Metal Structures' published by the Structural Stability Research Council (SSRC) provides the strength of cylinders with and without circumferential (ring) stiffeners. Section 14.3.5, 'Cylindrical Shells Subjected to Uniform External Pressure', provides a criterion for determining whether such cylinders buckle elastically and their elastic buckling strength. When typical API 650 tank dimensions are used in these strength equations, shells are shown to buckle elastically, and the external pressure causing buckling is well-approximated as: P = 2.1E (H/D)(D/t) 2.5 Where E = the modulus of elasticity of the shell, H = the shell height, and t = the shell thickness. This buckling pressure accounts for the reduction caused by API 650's dimensional tolerances on the tank's Figure 2. Roof uplift pressure as a function of tank diamater. Figure 1. Cone roof tank damage.

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