Comparison of API650 to AWWA D100
This comparison between API650 and AWWA D100 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 API650 8^{th} edition, and AWWA D100a89.
AWWA D100 allows the roof live load to be as little as 15 psf (pounds per square foot) based on temperature conditions. API650 specifies a minimum of 25 psf.
AWWA D100 specifies a wind velocity factor of (V/100)^{2 }for wind speeds greater than 100 mph. API650 applies this velocity factor to all wind speeds. AWWA D100, in the definitions of section 3.5.1, specifically states that the minimum value of (V/100)^{2 }is 1.0.
AWWA D100 differs in the computation for the maximum height of the unstiffened shell. AWWA D100 includes a wind pressure term. API650 does not include a pressure term.
AWWA D100 employs an equation to compute shell thickness similar to the thickness equation in API653 section 2.3.3.1. AWWA D100 references the bottom of the course, while API653 references one foot above the bottom. The minimum shell course thickness requirements of AWWA D100 are the same as API650.
AWWA D100 specifies a maximum unit stress of 15000 psi (pounds per square inch), while API650 provides TABLE 32, where the allowable stress is a function of the material specified.
AWWA D100 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 D100, there are no allowances for these types of tanks in API650.
AWWA D100 provides for seismic design in a manner very similar to the eighth edition of API650, for bottom ground supported tanks. The following comparisons are with reference to seismic design, and the eighth edition of API650.
 The zone coefficient tables in both codes are identical.
 In the equation for overturning moment, AWWA D100 uses the structure coefficient (K) where API650 uses the importance factor (I). The values of (K) are higher than the values of (I).
 In the equation for overturning moment, AWWA D100 uses a multiplier for the structural weight terms of 0.14. API650 uses a value of 0.24.
 In the equation for overturning moment, AWWA D100 uses two multipliers on the sloshing term. AWWA D100 applies the "site amplification" and a multiplier based on the first sloshing mode period. API650 uses a single multiplier, based on the first sloshing mode period. The only common item is the computation for the period.
 In AWWA D100 the limit on "wl" is 1.28 HDG, while in API650 the limit is 1.25 HDG.
 AWWA D100 uses a different equation for the width of the annular ring. Additionally, D100 includes a limit on this width, above which the tank must be anchored.
 The limit for tank stability is higher in API650 (1.57) than in AWWA D100 (1.54).
 The equation for stress when there is uplift is different between the two codes.
AWWA D100 contains an appendix with alternate rules. This appendix recommends API650 for certain H/D ratios.
AWWA D100 specifies (c.3.2.3) a maximum thickness value.
AWWA D100 provides a different table for the minimum thickness of the bottom annular rings.
