Comparison of API-650 to AWWA D-100

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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 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.

  • The zone coefficient tables in both codes are identical.
  • In the equation for overturning moment, AWWA D-100 uses the structure coefficient (K) where API-650 uses the importance factor (I). The values of (K) are higher than the values of (I).
  • In the equation for overturning moment, AWWA D-100 uses a multiplier for the structural weight terms of 0.14. API-650 uses a value of 0.24.
  • In the equation for overturning moment, AWWA D-100 uses two multipliers on the sloshing term. AWWA D-100 applies the "site amplification" and a multiplier based on the first sloshing mode period. API-650 uses a single multiplier, based on the first sloshing mode period. The only common item is the computation for the period.
  • In AWWA D-100 the limit on "wl" is 1.28 HDG, while in API-650 the limit is 1.25 HDG.
  • AWWA D-100 uses a different equation for the width of the annular ring. Additionally, D-100 includes a limit on this width, above which the tank must be anchored.
  • The limit for tank stability is higher in API-650 (1.57) than in AWWA D-100 (1.54).
  • The equation for stress when there is uplift is different between the two codes.

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.

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