Strapping sawn timber bundles: spacing, edge contact and settling

A package of sawn timber looks simple: boards stacked in layers, a few straps around them, perhaps a wrap on top. In practice the strapping does real work. It turns a stack of loose pieces into a unit that can be lifted, stacked, transported and stowed without the boards shifting. How well it does that depends on where the straps go, what they touch and what happens to the timber after they are tensioned.

Why the package must behave as a unit

Timber shippers deal with this question at sea as well as in the yard. The 2011 IMO Code of Safe Practice for Ships Carrying Timber Deck Cargoes treats the rigidity of sawn wood packages as important to the stability of the deck cargo. It defines a racking strength, the sideways force a package can withstand before it collapses or deforms beyond a set limit, and states that measurements are for the shipper to carry out and provide to the master. For the purposes of that Code, the racking strength should not be less than 3.5 kN per metre of package length.

The Code also asks for sawn wood packages to be marked clearly with their volume and approximate weight, and notes that uncovered timber can gain or lose weight through water during a voyage.

Not every package goes on a ship’s deck, but the principle carries over to road, rail and yard handling. A package that racks, sags or loses end pieces is harder and less safe to handle.

Spacing: keep the ends captive and the forks clear

There is no single correct strap spacing for every package. It depends on package length, board dimensions, the number of layers, how the package will be lifted and how far it may overhang supports. Some principles apply widely:

• Strap near each end. The outermost straps should sit close enough to the ends that short or loose pieces cannot slide out, while leaving enough timber beyond the strap for it to grip.
• Add intermediate straps by length. Longer packages need more straps along their length to keep layers aligned and stop the middle from bowing.
• Align with bearers. Placing straps over or near bearer positions helps the package stay square on its supports.
• Keep clear of fork entry. Straps sitting where forklift tines enter are easily cut or dragged.
• Be consistent. The same package type should be strapped the same way every time, so handlers know what to expect.

Record the agreed positions in the packing specification, ideally on a simple sketch with distances, rather than leaving it to habit.

Edge contact: protect the timber and the strap

Sawn timber edges are rough and can be sharp, especially at the top corners where the strap turns through a right angle. Under tension and vibration, the strap is pressed and rubbed against that corner. Two kinds of damage follow: the strap is abraded or cut, and the timber edge is crushed or marked. For planed or visible-grade products, the mark itself can be a quality complaint.

Edge protectors at the corners spread the load over a larger area and give the strap a smoother path. They are especially worth using where the package will travel far, be handled several times or carry wrapping that the strap could tear. The ASTM D4675-14a guide on strapping selection lists geometry, shear planes and frictional characteristics among the factors to consider, and stresses that users should test their own applications.

Settling and shrinkage: why tension drops

Wood is hygroscopic. The USDA Forest Products Laboratory’s Wood Handbook explains that wood is dimensionally stable above its fibre saturation point, on average about 30% moisture content, but shrinks as it dries below that point and swells as it takes up moisture. Shrinkage is greatest along the growth rings, about half as much across them, and only slight along the grain.

For a strapped package, this means the cross-section can become smaller after strapping if the timber continues to dry. Layers also bed down under their own weight and through vibration in transport. The straps go round the cross-section, so any reduction there reduces strap tension.

Two consequences follow. First, packages of timber that is still drying are more likely to arrive with slack straps than kiln-dried stock. Second, the strap’s ability to follow movement matters. A stiff strap holds the package very tightly at first but has little stretch left when the package shrinks. A strap with more elongation can follow some of that movement before it slackens. Which works better for a particular package is a question for trials, not assumption.

A packing specification checklist

Item	What to record
Package	Species, dimensions, board sizes, drying state and approximate weight
Strap	Material, width and grade, with the closure and tool it is paired with
Positions	Number of straps and distances from ends, on a sketch
Edge protection	Where protectors are fitted and what type
Tension	How tension is applied and checked, and by whom
Inspection	When straps are checked again before dispatch or after storage
Marking	Volume and approximate weight shown on the package

Building the system

Timber packing uses steel strap with seals, plastic strap such as PET, or woven strap with wire buckles, depending on the product and route. If you are reviewing a system, compare woven strap and galvanized wire buckles as a pair, look at PET strap with the joining method your line uses, or check the steel strapping seals for an existing steel line. Send package details and handling conditions through the enquiry page; performance information for a defined combination is available on request.