API-650 Wind Design with I > 1 or Exposure D

API-650 provides the formulas for designing a tank for wind. However, the formulas are all based on a importance factor (I) = 1.0 and an exposure = C. If your requirements are for exposure D or an importance factor greater than 1.0, you have to modify all of the calculations. For the wind loadings you need to calculate a new horizontal load and a new vertical load using ASCE-7. For this you have to determine values for Kz, Kzt, Kd, G, Cf, and GCp from ASCE-7. From these values you can then calculate your new loadings. You also have to calculate a new horizontal loading based on the base wind velocity. To this you have to add the internal drag pressure specified in 3.9.7.1 and compare that to the base value used in the calculations for maximum wind girder spacing as well as for sizing requirements for wind girders.

The effect on your wind calculations is greatly dependent upon the height of your tank. The API-650 formulas are based on Kz = 1.04. For a short tank with exposure C, Kz could be equal to 0.85 which will give a lower horizontal wind load depending upon the importance factor. The vertical uplift from wind might increase depending upon the tank height. If you have a higher tank or a tank with exposure D these values will increase and will probably result in higher loadings on the tank.

The effect on the wind girder requirements for a short tank probably will not change as you cannot use a factor less than 1. The values will change for taller tanks, tanks with higher importance factors, and tanks in exposure D.

Because there is becoming more of a requirement for importance factors greater than 1.0 and the possibility of clients requiring exposure D, we have just released our new version of ITSdesign to include calculations based on ASCE-7 for these conditions. If you enter either a importance factor greater than 1.0 or exposure D, ITSdesign will automatically provide the calculations based on ASCE-7 requirements.