FEXLA

Clips and platform supports: entering a footprint

The rectangle WRC 537 needs, how a rolled section becomes one, and what the clip check leaves out.

FEXLA engineering · · 4 min read

Spherical vessel with a stair, platform and handrail brackets welded to it
Fig. 1 · Spherical vessel with a stair, platform and handrail brackets welded to it

WRC 537 checks a clip as a rigid rectangle on a cylinder, given by its half-width round the shell, C1, and its half-length along it, C2. A plain plate clip is entered by its own size. A tee, I, channel, box or angle is entered as the equivalent rectangle from Moss, Procedure 7-3, which the calculator works out from the section's width and length.

This post covers what goes into those two numbers, and what the clip check does not see.

The rectangle the curves need

The coefficients for a rectangular attachment are read against γ = Rm/T and a pair of ratios formed from C1 and C2. The curves cover C1/C2 from 1/4 to 4. A clip outside that range is read at the limit, and the result says so and suggests a shell model.

The fillet weld round the clip is part of the footprint: the clip half-lengths include the weld leg. With a pad, the pad edge is a second rectangle, the pad's own size plus its weld, checked on the shell thickness alone.

On a clip with a pad, the allowable at the clip edge is the lower of the shell's and the pad's. At the pad edge only shell metal is present, so only the shell's allowable applies there, and the shear is spread over the pad's sides on the shell thickness alone. Either edge can govern, and the report shows both.

The WRC 537 calculator's model of a rectangular clip on a cylinder with six load arrows
Figure 2. A clip on the cylinder in the calculator, with the six loads at its centre.
Photo: distillation columns at night
Figure 3. Photo: distillation columns at night. Tall columns carry platforms, ladders and other supports on clips, and their skirts carry the wind. (Free-licence photo, see the image credits.)

Rolled and built-up sections

A structural section does not load the shell evenly over its outline: the webs and flanges carry more than the space between them. Moss, Procedure 7-3, Fig. 7-10 gives factors that turn a section of width b (round the shell) and length h (along it) into an equivalent rectangle, C1 = c1·b and C2 = c2·h.

Sectionc1c2
Single plate0.50.4
Two plates close together0.30.4
Plate on a base plate (tee)0.250.4
Two plates on a base plate0.30.4
I section0.40.5
Box section0.50.5
Channel0.30.5
Angle0.20.4
The factors as coded from Moss, Fig. 7-10. A round solid attachment is taken as a square of half-width 0.875 times its radius.

Moss gives these factors for the placement he draws: b round the shell, h along it. A member turned the other way has no rule, so the calculator offers none. Use the shell model, or enter the two lengths yourself on your own basis.

Loads on a clip

The six loads act at the clip centre on the shell surface. The radial load and the two moments give membrane and bending at the eight points round the clip edge. The shears give shear stress spread over the clip's sides, VC/(4·T·C1) and VL/(4·T·C2).

Torsion is collected and printed, but it adds no stress on a rectangular attachment: the bulletin states that no accepted method is available for it. A clip with a large torsion needs the shell model.

The loads at an attachmentA section through a curved shell with a nozzle standing on it. Arrows show a radial force along the nozzle, a shear across its end, a bending moment as a curved arrow, and torsion as an arrow circling the nozzle. The patch of shell round the nozzle base is shaded.Radial force (push or pull)Sheartwo directionsBendingmomenttwo directionsTorsionAll of it enters the shell through the shaded patch
Figure 4. The six loads. On a rectangular clip the torsion is recorded but contributes no stress in WRC 537.

When to go to the shell model

  • Clips on heads and cones: WRC 537 takes clips on the cylinder only.
  • Offset clips, members turned against Moss's placement, and large torsion.
  • An equivalent-size clip whose stress ratio reaches 0.7. The size is a rule of thumb, so the calculator suggests the model well short of 1.0.
  • Clips close together. Moss's rule of thumb treats clips nearer than √(R·t) round the shell, or 6 in (152.4 mm) along it, as one attachment.

The shell model builds the actual section from its plates, standing on the shell with shared nodes, so the load spreads into the shell the way the section carries it.

Where it stops

  1. 01The shell at the clip edge is checked. The clip itself, its welds and the platform steel are not.
  2. 02The equivalent rectangle is an approximation from Moss, not a derivation.
  3. 03WRC 537 takes clips on a cylinder only; heads and cones go to the shell model, which is early access and under verification.
B-06

Try it in FEXLA

The Clips, brackets and platform supports card checks plate, tee, I, channel, box and angle clips by their footprint, with the shell model for heads, cones and offset clips.

Open the card

References

  • D. R. Moss and M. Basic, Pressure Vessel Design Manual, 4th ed., Procedure 7-3 (Fig. 7-9, Fig. 7-10, notes 1 and 2) and Procedure 7-4 note 3.
  • WRC Bulletin 537.

Method notes this draws on

Next post: Agitator nozzles: loads, gussets and fatigue

Educational article. It explains principles and is not design guidance; follow your governing code and edition.