Agenda Item 620-323
Title: Soil-Structure Interaction of Appx R & Q Tanks
Date:
October 24, 2011
Revision:
0
Handled By:
Jack Blanchard / Larry Hiner
Chicago Bridge and Iron Company
14105 S. Route 59
Plainfield, IL 60544-8984
Telephone: 815-439-6522
Fax: 815-439-6560
Email: LHiner@cbi.com
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Purpose:
Provide rule to allow SSI for unanchored tanks when Rw values are restricted to the values in Tables L1Q or L-1R.
Source:
J Lieb & J Blanchard correspondence related to Agenda Item 650-743
Impact:
Appendix R and Q self anchored tanks applying SSI.
Discussion:
API650 Appx E Paragraph E.6.1.6 Bullet 2
This paragraph bullet places limitations on the application of Soil-Structure Interaction for self
anchored tanks.
Standard SSI methods require a fixed base structure model. While the API 650 calculation basis
assumes a fixed base, the inelastic, nonlinear response of a self anchored tank is accounted for in
the Rw factors in table E-4. The inaccuracy of standard methods of determining SSI structural
response effects combined with the inaccuracy of accounting for the inelastic tank response of a self
anchored tank is considered by some to be too high and therefore beyond the scope of API650
Appendix E.
However, the Rw values in API620 Appendix L as applicable to API620 Appendix R and Q tanks
represent a smaller inelastic response, considered well within the tank response capacity (at least
represented by the API650 Appendix E values). When combined with the SSI methods, the result is
considered to be within the intended accuracy of the overall tank/foundation response.
Full containment, concrete outer wall tanks typically have a second liquid containing bottom
separated from the foundation by load bearing insulation. Anchoring full containment tanks to satisfy
the API650 E rule creates penetration connections through the secondary bottom which is counter to
the base API625 requirements of full containment.
Therefore, an added provision in API620 Appendix L is proposed to allow SSI for unanchored tanks
when the applied Rw value is limited to Tables L-1Q or L-1R.
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Proposed Changes:
L,3,2,7 Soil Structure Interaction
Soil-Structure Interaction (SSI) per API650 E.6.1.6 may be applied. Application of SSI to self anchored
tanks is allowed if the Rw value applied is limited to those in Tables L-1Q and L-1R as applicable.
620-XXX
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