Understanding Alternate Sustained (SUS) and Occasional (OCC) Load Cases - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II Version
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CAESAR II includes a designation for static load cases called an alternate load case. The B31.3 2014 code edition merged the requirements of the previous Appendix P into the main body of the standard. This appendix related to stresses due to sustained loads. Because of this change, CAESAR II was modified to address these additional sustained conditions using the alternate support condition.

You can also associate a related alternate Sustained (SUS) or alternate occasional (OCC) load case that uses the restraint status from the previous Operating (OPE) load case by selecting Alternate SUS/OCC in the Static Analysis - Load Case Editor. CAESAR II uses the stiffness values from the OPE case to create the alternate SUS or OCC load case. In addition, the software disables all load case options that are not applicable on the alternate SUS or OCC load case.

B31.3 does not reference an alternate SUS or an alternate OCC load case. However, CAESAR II provides both alternate SUS and alternate OCC because SUS and OCC stresses are both force-based and you may find a need for a similar approach for OCC.

Use this functionality in systems where supports are active in some conditions and inactive in others (such as pipes lifting off supports). In these situations, the changing distribution of sustained loads may influence the difference in strains. The software bases the displacement strain range on the algebraic difference between the calculated positions of the pipe that define the range. In addition to the displacement strain, each calculated position includes the sustained loads present in the condition under evaluation.

The load case generates primary stress corresponding to the restraint configuration for the previous operating (OPE) load case. However, the alternate sustained (SUS) or alternate occasional (OCC) load case is not a valid structural representation of the system. When using the alternate SUS or alternate OCC cases, keep the following in mind:

  • These alternate cases determine the stress and the consideration of this stress state in determining the expansion stress range.

  • Do not use the restraint loads from an alternate SUS or alternate OCC load case as it is not a true structural representation of the system. CAESAR II does not generate restraint reports for alternate SUS and alternate OCC load cases.

  • Do not use an alternate SUS or alternate OCC load case when performing dynamic analysis. Instead, use the corresponding operating case to get the support configuration.

  • Use one of the two new load case templates (.tpl files) when recommending load cases. These templates include support for alternate SUS and alternate OCC load cases. for more information, see Load Case Template.

  • Alternate SUS provides a simple method to meet the requirements of equation (1b) for the liberal allowable expansion stress range. If you use equation (1a), sustained stress does not affect the allowable expansion stress range.

Example

While alternate SUS properly sets the expansion stress allowable limit in equation (1b), alternate OCC only affects the SUS+OCC load case. You may want to use the following load case stress types:

  • L1: OPE (Operating case)

  • L2: Alternate SUS (Using the L1 operating support configuration)

  • L3: SUS (Standard sustained case with supports set by sustained loads alone)

  • L4: OPE (Operating case, or consider including OCC occasional loads)

  • L5: Alternate OCC (Using the L4 operating support configuration)

  • L6: EXP (Expansion case, defined as L1-L3)

  • L7: SUS (Maximum of L2 and L3, to sum with OCC using the Max combination method)

  • L8: OCC (L7+L5, using the Scalar combination method)

For more information, see Stress Type and Combination Method.

For more information on loading conditions and support scenarios that result in the greatest sustained load (SL) for each operating condition, see Appendix S, Example S302, in the B31.3 code standard.