Racking pallets: rack capacity, orientation and span

Racking is where most wood pallets are weakest, and where most over- or under-design happens. The rules are simple once you see the pallet as a beam.

How a pallet carries load in a rack

In a selective pallet rack, the pallet rests on two horizontal beams with open space between them. Whatever spans that gap carries the whole load in bending:

• Racked across the length: the stringers (on a stringer pallet) run from one beam to the other. The stringers are the beams. This is the strong direction.
• Racked across the width: the stringers run parallel to the rack beams, so the top and bottom deckboards bridge the gap. Thin deckboards are weak beams.

For a block pallet, stringerboards and deckboards run both ways, so either orientation can work, but capacity depends on their stiffness and must be confirmed with the manufacturer.

Span and bearing

The span is the clear distance between the beam supports, which is the pallet dimension minus the bearing on each beam.

palletdesign.ai's indicative check uses the same 2 in of bearing on each beam. If your racks leave a longer span, the capacity drops; bending capacity for a uniform load falls in proportion to span, and deflection rises with roughly the cube of the span.

Worked example: GMA-style 48 x 40 in a rack

From a published Pallet Design System analysis of a GMA-style 48 x 40 stringer pallet (1,500 lb design load, dry service):

Two things stand out. Across the width, stiffness governs: the deflection limit cuts the usable load by more than a third. And the critical member tells you where to add material: the center stringer across the length, the bottom deckboards across the width.

Sizing stringers for racking

A stringer racked across the length is a simply supported beam. For a uniform load, its bending capacity is:

capacity = 8 x allowable stress x section modulus / span, with section modulus = width x height squared / 6

What that means in practice:

• Height matters most. Capacity grows with the square of stringer height. Going from 3-1/2 to 3-3/4 in adds about 15%.
• Width and count add linearly. A fourth stringer adds about a third. A 1-1/2 in stringer instead of 1-3/8 in adds about 9%.
• Notches cut it sharply. A 1-1/4 in notch leaves 2-1/4 in of wood, and that section has about 41% of the full strength. See notched stringers.
• Species and grade change the allowable stress. Denser hardwoods and southern yellow pine carry more than SPF.

palletdesign.ai's simple check shows the same trends on the GMA-style pallet: a fourth stringer adds about a third to the notched pallet's racked bending capacity, and removing the notches roughly doubles it. The check is a conservative screen for catching clearly weak designs and is always labeled indicative; ratings come from a PDS analysis or physical testing.

Deflection limits in racks

Sag matters as much as strength in racks, because a sagging pallet can slip off a beam or jam an automated system. Common limits:

• About 1.9 to 2% of the span for racking, which is about 0.84 to 0.88 in on a 44 in span and about 0.7 in on a 36 in span
• 0.5 in or less for automated warehouses

See pallet deflection.

Rack types

A published PDS example rated the same GMA-style pallet at 4,225 lb on a shelf, nearly three times its racked rating. Wire decking is often the cheapest way to rack a heavy load on an ordinary pallet.

Racking checklist for the spec

1. Rack type (selective, drive-in, push-back, flow)
2. Orientation: which pallet dimension spans the beams
3. Beam span, or beam spacing and beam width
4. Number of beams (2 or 3), or deck support
5. Maximum load per pallet and load type
6. Deflection limit, if any
7. Dry or wet environment (freezers and outdoor racks)

When to switch to a block pallet

If the pallet must be racked across its width, or both ways, a stringer pallet's weak direction usually governs. A block pallet or a pallet with a heavier bottom deck is the usual answer. Confirm racked capacity with a PDS analysis or a test to ASTM D1185 or ISO 8611. See rack storage and pallet load capacity.