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Skirt supports are the standard support method for tall vertical pressure vessels, distillation columns, and reactors. A cylindrical or conical shell (the skirt) is welded to the vessel bottom head and extends to a base ring anchored to the concrete foundation. Skirts efficiently carry axial compressive loads, bending moments from wind and seismic forces, and thermal growth.

Vertical vessels

Skirts are used exclusively with vertical cylindrical vessels and columns. The skirt provides a large, stable base that resists overturning moments.

Anchor bolt ring

Anchor bolts are distributed evenly around a bolt circle at the base ring, transferring tension and shear into the concrete pedestal.

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Parameters

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Skirt Geometry

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skirtHeight
number
required
Total height of the skirt from the base ring to the attachment weld at the vessel bottom head. Taller skirts provide more access space below the vessel for piping and maintenance.
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skirtThickness
number
required
Shell thickness of the skirt. The skirt must be checked for combined axial compression and bending under operating and erection (empty + wind) load combinations.
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skirtBaseDiameter
number
required
Outside diameter of the skirt at the base ring level. For cylindrical skirts this equals the vessel shell OD. For conical skirts it is the larger (bottom) diameter.
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skirtGeometry
select
required
Shape of the skirt shell. Options: Cylindrical (Straight) or Conical (Tapered).
The skirt has a constant diameter equal to skirtBaseDiameter from the base ring to the vessel attachment weld. This is the most common configuration and is preferred when the vessel shell and skirt are the same diameter.

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Anchor Bolt Pattern

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skirtBoltCircleDiameter
number
required
Diameter of the bolt circle (Dbc) — the pitch circle on which anchor bolt centrelines lie. The bolt circle is typically slightly smaller than the base ring OD.
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boltQuantity
number
required
Number of anchor bolts equally spaced around the bolt circle. Also stored as skirtAnchorBoltCount. Even numbers are required for symmetric loading. Typical values are 4, 8, 12, or 16.
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boltDiameter
number
required
Nominal diameter of each anchor bolt. Also stored as skirtAnchorBoltDiameter. Bolt size is selected based on the maximum tensile force per bolt under the governing overturning load combination.

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Anchor Chairs

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skirtAnchorChairs
select
required
Whether anchor chairs are included at each bolt location. Options: Yes or No. Anchor chairs are welded gusset assemblies that reinforce the skirt shell locally at the anchor bolt penetration and provide a bearing surface for the bolt nut.
Anchor chairs are strongly recommended for vessels in seismic zones or where the anchor bolt tensile load exceeds the skirt shell capacity. Chairs distribute the bolt pull-out force over a larger area of the skirt shell, preventing local yielding.
When Use Anchor Chairs is set to Yes, the following additional parameters appear:
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skirtChairHeight
number
Height of the anchor chair gusset plates, measured from the base ring top surface. Taller chairs provide a longer weld length and reduce peak stress in the skirt shell.
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skirtChairTopPlateWidth
number
Width of the chair top plate, which bears against the anchor bolt nut or washer. Must accommodate the bolt nut flat-to-flat dimension plus edge clearance.
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skirtChairTopPlateThickness
number
Thickness of the chair top plate. The top plate is checked for bending under the bolt pre-tension and operating tensile load.

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