Default Settings - Wind/Seismic

Owned by Former user (Deleted)
Dec 11, 2015
4 min read

Several of these settings are global and will affect all designs.


Concrete Strength: The theoretical concrete strength after at least 28 days of setup. The concrete factor multiplies the strength value.

K Value for Leg Supports: This value can range from 0.50 to greater than 2.0, however some references recommend that values less than 1.5 be avoided. Lower values are more likely to result in a passing design and thus are considered less conservative.K=0.5 represents both ends of the leg as being fixed with no lateral or rotational movement. K=1.0 represents both ends of the leg as being pinned (rotational movement with no lateral movement). K=2.0 represents one end of the leg as being fixed and the other end as able to move laterally.

Increase Base Support Allowable: The options here apply only to the structural components (e.g., saddles, support lugs, unbraced legs, skirts) and do not apply to the pressure envelope. The option selected here will be the increase to the allowable stress for the structural support for Occasional loading cases. This has no effect on the Sustained loading cases.

Apply Seismic Force at: This selection affects skirt supported vessels for seismic loading cases. Seismic loadings are typically considered to be triangular in nature, so the equivalent point load would be at 2/3 the segment height. Some argue that the use of the k factor in the newer seismic code calculations handles this already would set this to the “Middle of Segment.”

Calculate Skirt Stress: For the skirt design for skirt and base ring supported vessels, the calculation will appear in the base ring report. This will only affect base ring designs that include chairs or a top ring. It represents the effect on the skirt from bolting reactions, but it is considered to be increasingly conservative as the number of gusset plates increase and become closer together. The calculation does not consider the mitigating effect of the gusset plates.

Use corroded section thickness when calculating weight: When this is selected, the vessel weight is based on the corroded wall thickness. This is typically not a good option: the corrosion allowances are using conservatively high values and would artificially reduce the weight, and the vessel is heavier in the new condition.

For vertical vessels use higher allowable stresses when doing wind/seismic designs: Selecting this box will allow an increase in allowable stress of 20% for the pressure envelope for Occasional loading cases for vertical vessels. This has no effect on the Sustained loading cases or the structural supports. Building codes typically reduce the load that is applied (wind, seismic, dead load) for the load case combinations and some would consider also increasing the allowable stress to be not-conservative if the load is reduced.

For horizontal vessels use higher allowable stresses when doing wind/seismic designs: Select this box to allow an increase in allowable stress of 20% for the pressure envelope for Occasional loading cases for horizontal vessels. This has no effect on the Sustained loading cases or the structural supports. Building codes typically reduce the load that is applied (wind, seismic, dead load) for the load case combinations and some would consider also increasing the allowable stress to be not-conservative if the load is reduced.

Use Zick’s effective shell width for S3 and S5 calculations: Leonard Zick’s original paper used a different stress attenuation range for these calculations than later adopted references. Based on the literature, leaving this box cleared is advised. This only affects the Zick analysis report/calculations.

User higher allowable stress (1.5 S) for S3 calculations: Leonard Zick’s original paper used a higher allowable stress (1.5 S) for this calculation than later adopted references (1.25 S). Based on the literature, leaving this box cleared is advised.

Increase buckling allowable stress when doing wind & seismic designs: Some references do not suggest increasing the buckling allowable stress, even for Occasional loading cases. This option only affects buckling allowable stresses when other options have been selected to allow for an increase in allowable stress. This will have no effect on Sustained loading cases.

Use minimum C/Rw for UBC seismic design of “Nonbuilding Structures”: Section 2338(d) of the 1991 UBC and Section 1632.5 of the 1994 UBC specify a minimum limit on the ratio of C/Rw for “nonbuilding structures.” This limit is 0.5 for the 1991 UBC and 0.4 for the 1994 UBC. When this box is selected, the C/Rw value will not be allowed to go below that minimum limit.

Does sliding saddle support loads: When this box is selected, the sliding saddle and the stationary saddle will both be considered to handle the longitudinal loading. If this is cleared, only the stationary saddle will be considered to handle the longitudinal loading.

Hide no liquid/no attachment warning: A warning message appears when a structural support is run (unbraced legs, Tower Analysis, etc.) when there are no attachments or liquid information in the Attachments/Loadings screens. Selecting this box will suppress that message and is not advised.