Vessel on legs: Moss Chapter 4 checks

Wind and seismic arrive as a base shear V and an overturning moment. They are entered as resultants from the site calculation, or worked here from a simple pressure on the projected area or a horizontal g-factor. The engineer says whether they are at strength level, taken as 0.6W and 0.7E, or already at allowable-stress level. No one-third increase is taken for occasional loads, as ASME Section VIII Division 2 Table 5.3 does not take one.

Each case (empty, operating, test, with wind or seismic) is run for each load direction. The overturning moment is resolved into leg loads at the top of the leg attachment.

The tool goes further than the one formula. The vertical share of each leg follows Moss Table 4-11, exact for any leg count and either load direction. The horizontal shear goes to each leg by its stiffness across the load. Legs are taken as pinned at the base and fixed at the vessel, so a leg bends under its eccentric load and under its share of the shear times its free length.

Each leg carries axial load and bending together. The allowable axial stress Fa comes from AISC E2 for the leg's slenderness KL/r; Fb is 0.66·F_y for a compact section and 0.60·F_y otherwise. The interaction is AISC H1, with Cm = 0.85 and the Euler term where fa/Fa is above 0.15.

• K is 1.5 to 2.0 for unbraced legs, as Moss gives, and 1.0 for braced legs. A KL/r above 200 is flagged: brace the legs or use a stiffer section.
• A leg in uplift is checked in tension at 0.6·F_y.
• The lateral deflection y = V·ℓ³/(3·E·ΣI) is held to ℓ/200, Moss's 6 in per 100 ft, and gives the period of the vessel on its legs.

With cross bracing (Moss 4-5) the shear goes through the braces, shared between the panels by their direction, and the legs carry no shear-times-length bending. Each brace is checked in tension at 0.6·F_y; Moss allows a 1.2 increase for wind or seismic, and it is not taken. With two rods per panel, the compression rod is checked as a column over half its length if the rods are pinned where they cross, or the whole length if not.

The base is pinned, so the base plate sees the axial load only (Moss 4-12): bearing P/(B·D) against the concrete allowable, and the plate thickness from its cantilever beyond the section. Anchor bolts (Moss 4-16) are needed when the weight is less than 4M/d. Their area follows from (4M/d − W)/Sb, and their shear from V over the bolt area when there is no uplift.

Above each leg the shell is in compression. The tool checks fc = f1/(L1·t), with L1 the leg attachment width plus 2√(R·t), against the external-pressure factor B, and the weld shear f1/(2h). Then the local stress: for a leg on the cylindrical shell, the governing leg's loads are passed to a WRC 537 clip check at the junction. Legs coped at a cone junction, legs wholly on a head, and top platform legs go to the shell model.