Agenda Item 650-499

Agenda Item 650-741Title: Vertical SeismicDate: February 28, 2012Revision: 3Handled By:Jack Blanchard / Larry HinerChicago Bridge and Iron Company

14105 S. Route 59

Plainfield, IL 60544-8984

Telephone: 815-439-6522

Fax: 815-439-6560

Email: LHiner@cbi.com__________________________________________________________________________________________

Purpose: Reduce vertical seismic design conflictsSource: J Blanchard comments to L HinerImpact: API620 L / API 650 E designed tanksDiscussion:

*Changes made subsequent to Fall 2011 balloted version are shown in blue. Changes made in Fall 2011 Ballot are shown in red. E.2.2 Notations

The definition for Av uses coefficient while the body of the Appendix, Paragraph E.6.1.3, uses parameter. This proposal retains parameter.E.6.1.3 Vertical Seismic Effects

The application of vertical seismic is inconsistent, and in places incorporates reduction factors twice.

The first sentence of the second bullet sets the maximum vertical seismic acceleration parameter, Av, as 0.14SDS. Although there is no explanation for the basis for this value in the commentary, it is consistent with the ASCE7 value of 0.2 SDS. The basis appears to be (2/3) x 0.3 x (1/1.4) x SDS = 0.14 SDS. 2/3 is a standard reduction from the horizontal spectral acceleration at short periods. 0.3 represents a load combination reduction for when combining vertical and horizontal seismic forces (100% + 30%). 1/1,4 = 0.7 is the ASCE7 allowable stress adjustment factor. The ASCE 7 basis is then (2/3) x 0.3 x SDS = 0.2 SDS.The first sentence of the first bullet requires combined vertical and lateral acceleration effects to be combined by the SRSS method. Applying the basis for the 0.14SSD parameter discussed above, this load combination would be a second reduction and thus not appropriate.

The last sentence under the first bullet states: Vertical acceleration effects need not be combined concurrently for determining loads, forces, and resistance to overturning in the shell. However, the requirements of E.6.2.1.1 include Ge which is defined as the effective SG including vertical seismic effects (Ge = G x (1-0.4Av)). Thus the equations have included for the reduction of the liquid weight due to vertical seismic effects. The equations also include for a 40% reduction which is already in the Av factor.

Equations E.6.2.1.1.1-1; E.6.2.1.2-1; E.6.2.2.1-1a; E.6.2.2.1-1b; E.6.2.2.1-2a; E.6.2.2.1-2b; E.6.2.2.2-1a; E.6.2.2.2-1b; and E.6.2.3-1 also includes a 40% load combination factor with the Av factor. Thus these equations also double dip.

One solution would be to change the equations containing the duplicate load combination reductions by deleting the 0.4 factor. This would effectively apply the 100% + 30% combination method in place of the 100% + 40% currently used in some of the API650 equations. However, paragraph E.6.1.3, second bullet also allows the purchaser to specify the vertical ground motion. That is almost always in the form of a vertical spectra. Thus the load combination factors currently contained in the equations would be lost.Another solution, applied below, is to back out the load combination reduction from the Av basis. This works well with the purchaser specified value and allows all equations to apply load combination factors clearly. The direct link to ASCE 7 might appear to be removed, but paragraph 15.7.2 c of ASCE 7 only lists added forces to be combined with other forces. Thus, the load combination factor in ASCE 7 is contained within the 0.2 factor. To clarify the basis for the API650 E equation, the notation for Av in section 2.2 is also revised.E.6.1.4 Dynamic Liquid Hoop Forces

With the change discussed above, increasing Av to 0.47 SDS, the vertical seismic component of equation E.6.1.4-6 becomes the largest component for determining dynamic hoop stress. This equation incorporates Rw for the impulsive and convective forces but not for vertical force. Although applying an Rw value greater than 1.0 to vertical seismic applications is not appropriate for any actions that could involve buckling, applying the fully amplified elastic response of the shell hoop tension caused by the breathing response mode is also not correct. Therefore, the vertical component of the hoop stress equation is conservatively divided by a factor of 2.5._________________________________________________________________________

Proposed Changes:

E.2.2 Av Vertical earthquake acceleration parameter coefficient, %g = (2/3) x 0.7 x SDS = 0.47 SDSE.6.1.3 Vertical Seismic Effects When specified (see Line 8 in the Data Sheet), vertical acceleration effects shall be considered as acting in both upward and downward directions and combined with lateral acceleration effects by the SRSS method unless a direct sum combination is required by the applicable regulations. Vertical acceleration effects for hydrodynamic hoop stresses shall be combined as shown in E.6.1.4. Vertical acceleration effects need not be combined concurrently for determining loads, forces, and resistance to overturning in the tank shell except as applied in the equations of this appendix.

The maximum vertical seismic acceleration parameter shall be taken as 0.47SDS 0.14SDS or greater for the ASCE 7 method unless otherwise specified by the Purchaser. Alternatively, the Purchaser may specify the vertical ground motion acceleration parameter, Av.. That acceleration shall be multiplied by 0.7 to obtain the vertical acceleration parameter, Av.

Equation E.6.1.4-6

Change last numerator term to (AvNh/2.5)2

620-741

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