Agenda Item: / 650-633
Title: /

Seismic Bottom Uplift Length

Date: / Dec 29, 2006
Contact: / Name: / Larry Hiner
Company: / ChicagoBridge and Iron co.
Phone: / 815-439-6125
E-mail: /
Purpose: / Modification to API 650 Edition 11 Appendix E
This agenda item clarifies for self anchored tanks subject to seismic accelerations:
  • the bottom annulus thickness used to determine minimum annular plate width
  • the 3.5% D maximum width of bottom uplift zone applies to both tanks with and without annular rings.
  • a minimum seismic annular ring width when an annular plate is not otherwise required by this standard shall be 460 mm (18 in.)
  • adds the term wint into the formula for “J” to account fro uplift on the roof plate for due to internal pressure.

Source: / AST Discussions initiated by Bhana Mistry
Revision: / 1 Dated Jan. 17, 2007
Impact: / This agenda item serves to clarify requirements. Reducing misinterpretations which can lead to overly conservative requirements.
Background: / API 650 10th Edition Addendum 4 contained a complete rewrite of Appendix E. The text is now in unclear what thickness of bottom annulus to use when determining the width of annular plates.
Proposal: / The basic change necessary to implement this clarification is to state that the thickness of bottom/annular plate used to determine the minimum “L” dimension for annular plates and to determine the maximum allowable “Ls” (uplift influence zone).
Set the minimum radial inward distance from the shell to the end of a seismic required an annular ring to 460 mm (18 in.)
Define the minimum bottom thickness required to make the tank stable without anchorage (J1.54) as twa.
Include internal pressure acting on the roof to shell junction in the formula for determining the J ratio in E.6.2.1.1.1
Rationale: / To decide whether a tank is capable of self anchorage, the overturning resistance from product weight (as uplift occurs) must be determined. The maximum product resisting force“wa” that can be considered to resist overturning is based on the bottom thickness at the shell. Although the minimum bottom thickness for carbon steel tanks is ¼ inch, the required thickness for the necessary product resisting force may be significantly less.
If additional bottom thickness is supplied (greater than twa), and assuming the same uplift load, the stress levels in bottom plate will reduce and additional width of annular would therefore not be required.
Notes: / The required modifications to Appendix E to accommodate this change are as follows:
Renumbered Eqns
E.2.2
Add “thickened” and “annular ring” to definition of L
Add definitions for Ls and twa
E.6.2.1.1
Remove the confusing terminology (i.e. ta > tb).
Added note to clarify Ls maximum applies for all self anchored tanks - Added to note 2 “This limit applies whether or not a thickened bottom annular ring is used (see Eqn 38a or 38b).”
E.6.2.1.1.1
Revised Eqn 36 to include wint
E.6.2.1.1.2
Add minimum annular ring projection to note3 such that the new wording is:
the minimum projection of the supplied thicker annular platering inside the tank wall, L, shall be the greater of 450 mm (18 in.)or Ls:
Revise equations 38a and 38b to include variable twa instead of ta, and Ls instead of L
Add formulas 38c and 38d and the following notations:
Where: twa= the minimum bottom thickness required under the shell for J 1.54 per Eqn 38c or 38d
is determined using Equation (37)

In the following, added text is underlined and deleted text is lined out. Formula changes are detailed above.

E.2.2 Notations

LRequired minimum width of thickened bottom annular ring measured from the inside of the shell m (ft)

LsRequired width of annulus (bottom or thickened annular ring) necessary to provide the resisting force for self anchorage, measured from the inside of the shell m (ft)

twaMinimum thickness,excluding corrosion allowance, mm (in) of the bottom plate under the shell required for J = 1.54. The bottom plate for this thickness shall extend radially at least the distance, Ls, from the inside of the shell. Applies for self anchored tanks only.

E.6.2.1.1 Self-anchored

For self-anchored tanks, a portion of the contents may be used to resist overturning. The anchorage provided is dependent on the assumed width of a bottom annulus uplifted by the overturning moment. The resisting annulus may be a portion of the tank bottom (i.e. ta = tb) or a separate butt-welded annular ring (i.e. ta > tb). The resisting force of the annulus that lifts off the foundation shall be determined by Eqn (35)

In SI units:

 196 HDGeEqn (35a)

In customary US units

 1.28 HDGe Eqn (35b)

The tank is self-anchored providing the following conditions are met:

  1. The resisting force is adequate for tank stability (i.e. the anchorage ratio, J 1.54).
  2. The maximum width of annulus (Ls)for determining the resisting force is 3.5% of the tank diameter.This limit applies whether or not a thickened bottom annular ring is used (see Eqn 38a or 38b).
  3. The shell compression satisfies section E.6.2.2.
  4. The required annular plate thickness does not exceed the thickness of the bottom shell course.
  5. Piping flexibility requirements are satisfied.
E.6.2.1.1.1 Anchorage Ratio, J

Eqn (36)

Where:

Eqn (37)

Table E.6-A - Anchorage Ratio Criteria

J
anchorage ratio / Criteria
J 0.785 / No calculated uplift under the design seismic overturning moment. The tank is self anchored.
0.785 J 1.54 / Tank is uplifting, but the tank is stable for the design load providing the shell compression requirements are satisfied. Tank is self-anchored.
J > 1.54 / Tank is not stable and cannot be self-anchored for the design load. Modify the annular plate if L < 0.035D is not controlling or add mechanical anchorage.
E.6.2.1.1.2 Annular Plate Requirements

The thickness of the tank floor plate provided under the shell may be greater than or equal to the thickness of the general tank floor plate (i.e. ta > tb) with the following restrictions. (Note- In thickening the bottom annulus, the intent is not to force a thickening of the lowest shell course, thereby inducing an abrupt thickness change in the shell, but rather to impose a limit on the bottom annulus thickness based on the shell design).

  1. The thickness, ta, used to calculate wa in Equation (35) shall not exceed the first shell course thickness, ts, less the shell corrosion allowance,
  2. Nor shall the thickness, ta, used in Equation (35) exceed the actual thickness of the plate under the shell less the corrosion allowance for tank bottom.
  3. When the bottom plate under the shell is thicker than the remainder of the tank bottom (i.e. ta> tb) the minimum projection of the supplied thicker annular plateringinside the tank wall, L, shall be the greater of 450 mm (18 in.)equal to or greater thanor Ls:however L:

In SI units:

Eqn (38a)

In customary US units

, 0.035D (ft)Eqn (38b)

Where: twa= the minimum bottom thickness required under the shell for J 1.54 per

Eqn 38c or 38d

is determined using Equation(37)

In SI units:

In customary US units

Page 1 of 5