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This comparison between API-650 and AWWA D-100 is made from an "analytical" point of view. There may be other differences between these two codes in the areas of: materials, fabrication, inspection, and maintenance. This comparison is based on API-650 8th edition, and AWWA D-100a-89.
AWWA D-100 allows the roof live load to be as little as 15 psf (pounds per square foot) based on temperature conditions. API-650 specifies a minimum of 25 psf.
AWWA D-100 specifies a wind velocity factor of (V/100)2 for wind speeds greater than 100 mph. API-650 applies this velocity factor to all wind speeds. AWWA D-100, in the definitions of section 3.5.1, specifically states that the minimum value of (V/100)2 is 1.0.
AWWA D-100 differs in the computation for the maximum height of the unstiffened shell. AWWA D-100 includes a wind pressure term. API-650 does not include a pressure term.
AWWA D-100 employs an equation to compute shell thickness similar to the thickness equation in API-653 section 220.127.116.11. AWWA D-100 references the bottom of the course, while API-653 references one foot above the bottom. The minimum shell course thickness requirements of AWWA D-100 are the same as API-650.
AWWA D-100 specifies a maximum unit stress of 15000 psi (pounds per square inch), while API-650 provides TABLE 3-2, where the allowable stress is a function of the material specified.
AWWA D-100 provides for three types (classes) of tanks: flat-bottom, ground-supported tanks, pedestal, and cross-braced elevated tanks. The pedestal and cross-braced elevated tanks are specific to AWWA D-100, there are no allowances for these types of tanks in API-650.
AWWA D-100 provides for seismic design in a manner very similar to the eighth edition of API-650, for bottom, ground-supported tanks. The following comparisons are with reference to seismic design, and the eighth edition of API-650.
AWWA D-100 contains an appendix with alternate rules. This appendix recommends API-650 for certain H/D ratios.
AWWA D-100 specifies (c.3.2.3) a maximum thickness value.
AWWA D-100 provides a different table for the minimum thickness of the bottom annular rings.