Why boxes fail in the warehouse

A familiar sequence: the corrugated shipper meets its specification, the supplier's box compression test (BCT) report is in the file, and the first shipments go out without trouble. Then, after a humid month in a warehouse, the bottom layers bulge, pallets lean and product is damaged.

The supplier's report is usually correct. The problem is that a BCT value describes a box tested for a few seconds, in a conditioned laboratory, standing on a flat steel plate. A warehouse stores the same box for weeks, in changing humidity, on a pallet, under other boxes. In this article we go through the four factors that close that gap, the research behind each one, and a simple way to build them into a specification.

1. What the lab number actually measures

Compression testing of shipping containers follows methods such as ASTM D642, which calls for paperboard containers to be conditioned before testing, normally in the standard atmosphere of ASTM D4332 or of D685 when box compression strength itself is being quantified.^[1] That standard atmosphere is 23 °C and 50% relative humidity.^[2] So a BCT value describes a box at office-like conditions.

When BCT is estimated rather than tested, the usual tool is the McKee formula, published in 1963. Its simplified form is BCT = 5.87 × ECT × √(caliper × perimeter), using the board's edge crush test (ECT) value, the board caliper and the box perimeter.^[3] In a review of world-wide box compression data, Urbanik and Frank note that McKee and colleagues reported an average difference of 6.1% between predicted and actual strength on their data set, with 97% of results within ±15%.^[4] That is a good estimate of the lab value. It says nothing about the warehouse.

2. Humidity

How it shows up. Failures cluster in summer, in coastal or unheated warehouses, near dock doors, and after sea freight.

What the research says. Paper fibres absorb moisture, and the board loses stiffness as they do. A University of Pretoria thesis on corrugated board summarises the effect plainly: a box loses half or more of its short-term compression strength when relative humidity rises from 50% to 90%.^[5] Commonly used industry multipliers estimate a loss of about 20% at 70% relative humidity, 32% at 80% and 52% at 90%, and note that cycling humidity increases the loss further.^[6] Early work at the USDA Forest Products Laboratory found that higher moisture content also shortened the time a box could carry a dead load.^[7]

3. Time under load

How it shows up. The pallet looks fine when it is built and fails weeks later, with nothing new added to it.

What the research says. Corrugated board creeps under constant load. The same industry multipliers estimate a compression loss of about 37% after 10 days under load, 40% after 30 days, 45% after 90 days and 50% after 180 days.^[6] Research at the Forest Products Laboratory showed that the conventional test at a fixed load level does not adequately predict a safe load when the storage time is fixed, and proposed testing at several load levels with cyclic humidity to estimate the load that gives a 5% probability of failure for a given storage time.^[8]

4. The pallet and the stacking pattern

How it shows up. The same box performs well in one warehouse and fails in another. The difference is how the pallet is built: interlocked for stability, boxes hanging over the pallet edge, or a pallet with wide gaps between deck boards.

What the research says.

• Stacking pattern. A 1963 study by Kellicutt observed a 55% decrease in strength for stacks in a block configuration and 18% for a column configuration; a 1975 study by Ievens reported 45% lower compression strength for a blocking arrangement than for a column arrangement.^[9]
• Overhang. A study by Virginia Tech's Center for Packaging and Unit Load Design for the Fibre Box Association found that effective box compression strength decreases as overhang increases. Overhang on two adjacent sides had an extra effect, most likely because the box loses additional corner support.^[10] An earlier Michigan State University thesis gives an example: a single box overhanging by one inch on two adjacent edges loses about 40% of its compression strength compared with a box on a solid surface.^[11]
• Pallet stiffness. Another Virginia Tech thesis found a 27% to 37% increase in compression strength for asymmetrically supported boxes on high-stiffness pallets compared with low-stiffness pallets.^[12]

Putting it together: from lab value to required BCT

A practical way to use these findings is to work backwards from the load the bottom box must carry, then divide by the combined effect of the conditions it will actually see. The combined multiplier is the product of the individual factors, and the safety factor is one divided by that product.

Illustrative example using the industry multipliers above:^[6]

• Storage at about 80% relative humidity: multiplier 0.68
• About 30 days under load: multiplier 0.60
• Combined: 0.68 × 0.60 ≈ 0.41, so a safety factor of about 2.5
• Add an interlocked pattern (best case multiplier 0.8): 0.41 × 0.8 ≈ 0.33, so a safety factor of about 3

In this example the box needs roughly three times the lab strength that the static load alone would suggest. The multipliers are rules of thumb, not data for your board, your product or your warehouse. Use them to see which factors matter, then test conditioned samples in the conditions you expect.

What to put in the specification

• The storage and transit humidity the box must survive, and for how long.
• The pallet type, pallet pattern, maximum overhang and stacking height.
• The required BCT after applying those factors, not just a board grade or ECT value.
• Whether the product supports part of the load or the box carries it all.
• A test at the expected humidity, not only at the standard 23 °C and 50%.

Box strength under real conditions is one of the risks in our free Packaging Project Risk Register, with a trigger and a response ready to adapt.