Stacking limits: static and dynamic

Why there are two numbers

A stack at rest only has to hold its own weight straight down. Move it and the forces change: a forklift tilts the mast and bounces over a dock plate, a trailer brakes and rocks, and the top boxes swing on the ones below. The bottom container takes short spikes well above its resting load, and its walls get pushed sideways as well as down. Makers allow for that by publishing a smaller dynamic number.

Both figures assume the maker's test conditions. ORBIS states that its bulk performance figures are based on uniform static loads and may be affected by load distribution, handling, access doors and temperatures outside 0 to 120 F. In plain terms, the printed limit is for evenly loaded, sound boxes on a flat floor in normal temperatures.

Published limits by model

Notice how makers phrase the figures differently: tiers, pounds on the bottom box, kilograms per unit. Compare like with like, and when a sheet uses a term you don't recognize, ask the maker what test it describes.

Worked example: what the bottom box carries

Take ORBIS's HDR4845-25, rated at 2,000 lb with a 118.4 lb tare. Fully loaded, each unit weighs about 2,000 + 118.4 = 2,118 lb.

Stacked six high at rest, the bottom box carries the five above it: 5 x 2,118 = about 10,590 lb, on a footprint of 48 x 45 inches. The stack stands 6 x 25 = 150 inches, or 12.5 feet. Lifted four high, the bottom box carries 3 x 2,118 = about 6,355 lb, plus whatever the mast and the dock add in motion, and the stack is 100 inches tall.

The same check on ORBIS's GP4048-34 gives a six-high static stack of 6 x 34.1 = about 205 inches, over 17 feet, which may meet sprinkler heads or a low beam before it meets the container's limit. Three high dynamic is 102 inches, which fits under the roof of a 53-foot van; trailer counts are on the trailer loads guide.

What lowers the real limit

• Uneven floors. A stack on a sloped or broken slab leans, and the load shifts to one edge of the bottom box.
• Point and off-center loads. One heavy casting in a corner is not a uniform load, however light the total.
• Heat. Plastic softens as it warms; ORBIS ties its figures to 0 to 120 F, and Buckhorn lists -20 to 120 F for several hand-held lines.
• Time. A stack left loaded for months can slowly deform where a short stay would not. Plastic engineers call it creep.
• Damage. A cracked runner or bowed wall is no longer the tested box. We don't stack grade C units at sheet figures; see used container grades.
• Mixed models. Feet and rims from different makers may not register, and an unregistered stack can slide.
• Doors and lids. Open doors weaken a wall; missing lids leave the next box bearing on rims it wasn't designed to sit on.

Hand-held totes on a pallet

Small totes follow the same logic in miniature. IPL Schoeller rates its VDA R-KLT boxes at 20 kg each, about 44 lb, and a pallet column of them puts every kilogram above onto the bottom tote's four walls. Straight columns keep the load on the walls; a layer turned crosswise, which Buckhorn's optional cross-stack bottom allows, ties the columns together so the pallet holds its shape on a bumpy ride, at some cost in evenness of support.

Stretch wrap or a strap around the pallet helps a moving load stay square, but it doesn't raise a tote's rating. Count the weight per tote, multiply by the layers above the bottom one, and compare it to the maker's figure before building tall pallets for a trailer.

Racks, lids and folded stacks

Stacking replaces racking in many plants. ORBIS says its 70x48 BulkPak replaces metal racking and can be stacked without it, upright or collapsed. That only holds when the containers stay inside their published limits, so a floor-stacked area needs its height marked and kept.

Lids matter more than they look. A lid spreads the load of the box above across the walls below and keeps the stack square. Folded units have limits too: IPL Schoeller gives its Magnum Optimum figures for folded transport in standard and mega trucks. Folding mechanics are covered in the folding guide.

Questions before you stack

• What is the maker's static and dynamic figure for this exact model and door option?
• What is the real weight per box, and is it spread evenly?
• How long will the stack stand, and in what temperatures?
• Is the floor level, and what is the clear height to sprinklers or beams?
• Are all boxes in the stack the same make and model, with lids?
• For used units, what grade, and has anything been repaired?