A vessel on legs: the leg-to-shell check by WRC 537

The Moss check gives each leg an axial load and a shear for every case. The vertical share follows Moss Table 4-11, exact for any leg count and either load direction; the largest is f1 = Fv/n + 4·M/(n·d). The horizontal shear goes to the legs by their stiffness across the load. The legs are taken as pinned at the base and fixed at the vessel.

Two lengths turn those into moments at the weld. The eccentricity e is the radial offset of the leg's centroid from the shell's mid-surface, so the axial load bends the shell by Fn·e. The free length ℓ runs from the base plate to the bottom of the attachment, so a leg that is fixed at the vessel and pinned at its foot carries a moment V·ℓ at its top.

• The most loaded leg, in line with the lateral load: its axial load as the longitudinal shear VL, the shear in line with the load as the radial load P, and ML = Fn·e + V·ℓ.
• The leg at 90° to the lateral load: its dead-load share as VL, the shear it takes round the shell as VC, and MC = V·ℓ, with ML from its own eccentricity.
• Torsion is zero in both rows. Loads are magnitudes at the junction, from the case with the largest leg load.

With braced legs the lateral shear goes through the braces, and the legs carry no shear-times-length moment. The two rows then carry only the axial loads and the moments from eccentricity.

The leg is entered as a rectangular clip: its contact width on the shell round the circumference, and its welded length along the shell. A pad under the leg is the second rectangle, checked at its own edge on the shell thickness alone. A tubular leg touches the shell along a narrow strip, so its footprint can be long and thin. Check that its sides stay within the 1/4 to 4 range of the clip curves; outside it the calculator reads the curves at the limit and says so.

Beside the WRC 537 rows the hand check gives the shell's compression above the most loaded leg, fc = f1/(L1·t), with L1 the leg's width on the wall plus 2√(R·t), and the general compression from weight and moment at the attachment. Both are compared with the allowable compressive stress, factor B, which the engineer reads from the material chart in their own copy of ASME Section II Part D.

WRC 537 takes a clip on a cylindrical shell only. If any part of the attachment is on the bottom head or cone, the tool gives no WRC 537 rows and says so: the shell model gives the local stress there. That covers legs coped across the shell-to-head or shell-to-cone junction and legs wholly under the head.