Free span
Free span is the unsupported distance a pallet must bridge between its supports, such as the clear gap between two rack beams. A 48 in long pallet racked across its length typically has about a 44 in free span; the longer the span, the lower the pallet's capacity and the greater its deflection.
Also called unsupported span, clear span, beam span
Reviewed
Why free span drives capacity
A pallet over a free span behaves like a simple beam. For a uniform load, the capacity is roughly proportional to 1 / span, and deflection grows much faster than the span. Small changes in span therefore change the rating noticeably.
| Pallet | Racked across | Typical free span |
|---|---|---|
| 48x40 | length (on stringers) | about 44 in |
| 48x40 | width (on outer stringers) | about 36 in |
palletdesign.ai's indicative rack check assumes 2 in of bearing on each beam, so the span is the pallet dimension minus 4 in.
What the Uniform Standard asks for
When stating the conditions of use, the Uniform Standard asks for the maximum unsupported free span along the length and along the width, the maximum stack height, and the bearing areas between the pallet and its supports.
Why it matters
Two racks that both "take 48x40 pallets" can have different beam spacing. Give the actual clear span in the spec so the rack storage rating and deflection check use real numbers. See racking pallets.
Questions
- What free span should I use for a 48x40 pallet in a rack?
- About 44 in when racked across the length and about 36 in across the width, unless your rack's actual clear span is known.
Related
- Rack storageRack storage means a pallet spans between two rack beams with nothing underneath. It gives the lowest rating, e.g. 1,558 lb racked vs 5,223 lb on the floor.
- DeflectionDeflection is how far a pallet sags under load. Typical limits are about 2% of span in racking (about 0.75 in on a 44 in span) and 0.5 in for automated systems.
- Racking pallets: rack capacity, orientation and spanHow pallets carry load in racks: across-length vs across-width racking, beam span, bearing, deflection limits, and how to size stringers for a racked load.
- Pallet load capacity: safe load, support conditions and ratingsPallet load capacity depends on support: floor, stacked, forklift or racked. How safe load differs from breaking load, and how ratings are set.
- GMA palletA GMA pallet is the 48x40 in stringer pallet from the grocery industry spec: 3 notched stringers, 7 top and 5 bottom boards, 5-1/2 in lead boards, 4.75 in tall.
- Green lumberGreen lumber is wood that has not been dried, with moisture content above about 28%. Green mixed hardwood pallets weigh about 4.37 lb per board foot.
Sources
- Uniform Standard for Wood Pallets (NWPCA, now Woodpack Global), 2012 edition
- PDS example specification, drawings and analysis (v5.2)