Strapping pipe bundles and profiles in hexagonal stacks: geometry, bands and joints

Round tubes and bars pack naturally into a hexagon. Each layer nests in the grooves of the one below, the outline is compact, and a bundle can be counted at a glance. But the shape is only as stable as its bands. If one loosens, the outer pieces roll, the hexagon becomes a heap, and the pipes rub against each other all the way to the customer.

This guide sets out the geometry of hexagonal bundles, what it means for band length and closure choice, and what to check before a bundle leaves the mill or stockholder. The arithmetic is simple enough to put on a packing instruction, and it answers questions that otherwise get settled by guesswork on the shop floor.

Piece counts and dimensions from geometry

For a regular hexagonal bundle with n pieces along each side, all with the same outside diameter d:

• pieces in the bundle = 3n(n − 1) + 1
• width across corners = (2n − 1) × d
• width across flats = ((n − 1) × √3 + 1) × d
• band path around the bundle = 6(n − 1) × d + π × d

The band path follows the outer pieces: six straight runs between the corner pieces, plus a total of one full circumference around the six corners. It is the length of strap in contact with the bundle, before adding the overlap at the joint and any tail the tool needs.

For an illustrative tube of 50 mm outside diameter:

Pieces per side (n)	Pieces in bundle	Across corners	Across flats	Band path
2	7	150 mm	137 mm	457 mm
3	19	250 mm	223 mm	757 mm
4	37	350 mm	310 mm	1,057 mm
5	61	450 mm	396 mm	1,357 mm

Two consequences follow. Each extra ring adds 6d to the band path, which is 300 mm for this tube. And because the piece count grows roughly with the square of n while the band path grows in a straight line, a larger bundle puts much more mass behind each band. Going from 19 to 61 pieces multiplies the count by more than three, while the band path grows by less than double. A strap and joint that suit a 19-piece bundle may not suit a 61-piece bundle of the same tube.

Why tightness matters as much as strength

The GDV’s cargo information on steel pipes says that bundles of small-diameter pipes are held with transverse metal strapping or wire, and that the strapping must be very tight so that the pipes cannot rub against each other. Loose strapping lets tubes move and strike each other, and the strapping itself can chafe surfaces and edges.

Tightness is harder to keep than it looks. As a bundle is lifted, set down and stacked, pieces settle into the grooves and the outline shrinks slightly. A band with very little stretch cannot follow that change: the CTU Code notes that steel band elongates only about 0.3% at its maximum securing load, which makes it sensitive to losing pre-tension when cargo settles. Every extra handling step, whether a yard move, a transhipment or a re-stack at a stockholder, gives the bundle another chance to settle. The practical answer is to form the hexagon tightly before strapping, in a cradle or bundling station, rather than relying on the band to pull a loose stack into shape.

Band positions and joints

Set band spacing in the packing specification agreed with the customer, based on length, diameter and handling. Two principles apply to most bundles: bands near each end stop the end pieces from splaying, and bands spaced evenly along the length share the load. Where practical, place the joint on a flat of the hexagon rather than over a corner piece, where the closure sits on a curve and is more easily struck during handling.

For steel strap, the seal must match the strap width, thickness and sealer; the seal compatibility guide explains the checks. For coated or galvanized tubes, consider woven or composite polyester strap with wire buckles, or protection under steel strap, to avoid marking the surface. The GDV notes that surface-coated pipes such as galvanized pipes are more sensitive to corrosion than hot-rolled pipes and should be kept away from moisture at all times, and that pipes with sensitive inner walls, threads or chamfers are fitted with protective end caps.

Profiles and mixed sections

Square and rectangular hollow sections stack in rectangular blocks rather than hexagons, and their corners concentrate band pressure. Use corner protection where the band crosses an edge, as the sharp edges guide describes, and keep the joint off the corner. Open profiles such as angles and channels can be nested in pairs before bundling. Whatever the section, the aim is the same: a block that does not change shape when it is lifted.

If you are specifying a new bundle format, record the section, outside diameter, length, pieces per bundle and band positions on a controlled drawing. The drawings guide explains how to keep those dimensions and revisions clear.

Stowing and receiving bundles

The CTU Code notes that round, long items stacked in the grooves of the layer below create lateral forces on the side walls of a container when friction between them is low. It also treats bundling as an auxiliary measure to be combined with blocking or lashing, not relied on alone. On arrival, check for broken, loose or slipped bands and record any bundle that has lost its shape before discharge continues, because a loose bundle is difficult to lift safely.

Specify bundle strapping with YAKU

Send the section, outside diameter, length, bundle size and the strap and tool you use through the enquiry page. For steel strap closures, see steel strapping seals; for polyester systems, woven strap with galvanized wire buckles. Joint performance for a specific combination is on request.