Pallet racking and deflection: rack spans, direction and sag limits

Racking is usually the hardest job a pallet does. On the floor, the pallet is supported everywhere. On forks, it is supported for a few minutes. In a rack, it spans an open gap between two beams, loaded, for days or months. That is why a pallet's racking capacity is its lowest rating and why many pallet failures happen in racks. This guide explains how the support direction, span and time change a pallet's behavior, and how to specify a pallet for rack storage.

#How a pallet sits in a rack

In a standard selective pallet rack, the pallet rests on a front and a back rack beam. For 48 × 40 pallets, rack frames are commonly about 42 in deep, so the pallet extends about 3 in past each beam. Forks enter the 40 in face, and the 48 in stringers span from the front beam to the back beam.

The part of the pallet between the beams has nothing under it. The load bends the pallet down into that gap.

#Racked across length vs racked across width

The direction of the span decides which part of the pallet carries the load.

A stringer is a deep beam (typically 3-1/2 in tall), so it is stiff when it spans. A deckboard is only 1/2 to 7/8 in thick, so across-width racking is often limited by the bottom deckboards. One published PDS example for a 48 × 40 wood pallet gave:

The across-width case carried similar strength but deflected far more, so a deflection limit cut its capacity by about 40%.

#Deflection limits

Deflection is how far the pallet sags under load. Too much sag causes rack strikes, load instability, pallets slipping off beams, and failures in automated equipment.

The rack owner, rack designer or automation supplier sets the limit that applies. Put it in the pallet spec.

#Creep: sag grows with time

Creep is the slow increase in deflection under a constant load. Wood creeps, and plastic creeps much more, especially when warm. That is why:

• design software such as PDS reports estimated deflection after a sustained period (30 days in PDS);
• test standards hold the load for a set time before measuring;
• plastic pallet rack ratings depend on temperature and duration, and many rackable plastic pallets use steel reinforcement to control creep.

A pallet that passes at loading can fail weeks later if creep was not considered.

#What changes rack capacity and stiffness

Engineering beam theory explains most of it.

• Span: for a uniform load, deflection grows with the cube of the span. A span 10% longer deflects about 33% more.
• Member height: stiffness grows with the cube of the member's depth. A 4 in stringer is about 49% stiffer than a 3-1/2 in stringer of the same width and species.
• Member width and count: stiffness grows in direct proportion to width and to the number of stringers. Four stringers instead of three add about a third more stiffness across the length.
• Deckboard thickness: for across-width racking, a 3/4 in bottom deckboard is about 73% stiffer than a 5/8 in board.
• Species and moisture: dense, dry wood is stiffer. Wet wood is less stiff and creeps more.
• Load bridging: rigid loads help the pallet; bags and drums do not. See unit load design.
• Notches: notches reduce the stringer depth where they are cut, which matters if the notch sits near the highest bending stress.

#Rack types and the pallet

Pallet supports (cross bars) or wire decking under the pallet turn a spanning condition into a supported one. They are a cheap fix for undersized, recycled or plastic pallets, but they must be rated for the load.

#Rack safety basics

• The rack's beam capacity and the pallet's rack capacity are separate numbers. Both must exceed the load.
• Do not rack damaged pallets: broken bottom boards, cracked stringers or missing blocks.
• Place pallets squarely on both beams with the intended overhang.
• Nestable plastic and presswood pallets are generally not rackable without full support.
• Rack design, installation and load signage in the US follow the ANSI MH16.1 standard published by the Rack Manufacturers Institute.

#Signs a racked pallet is in trouble

Inspect racked pallets during routine rack inspections. Unload the position if you see:

• visible sag at mid-span, or a load that tilts toward the aisle;
• cracked stringers, often starting at a notch;
• bottom deckboards cracked or split between supports;
• pallet ends that have crept back toward the edge of a beam;
• plastic pallets that stay bowed after unloading.

These signs usually mean the pallet was not rated for the rack condition, the load is heavier than planned, or the pallet has been damaged in handling.

#Specifying a pallet for racking

Give the manufacturer:

1. Rack type and the direction the pallet spans (across length or across width).
2. Beam spacing or clear span, and beam width.
3. Maximum load weight and how it is distributed.
4. Deflection limit, if any, and how long pallets sit in the rack.
5. Temperature and moisture conditions.

Ask for the safe load and deflection for that condition, from PDS analysis or a test to ASTM D1185 or ISO 8611. See pallet load capacity and how to spec a custom pallet. In the designer you can set the rack direction and beam span in the requirements; open the 48 × 40 template.