The Effect of Pallet Stiffness on the Compression Strength of
Asymmetrically Loaded Boxes
Chandler Quesenberry, Laszlo Horvath, John Bouldin and Marshall S. White
1
1.1
Introduction
Pallet Stiffness
Most packaged goods are transported in the form of a unit load. A unit load consists of
restrained units either a single item, multiple items or a bulk material (White and Hammer
2005) which allows for safe warehousing, transporting and handling of products using a
pallet or slipsheet. Unit loads are popular because they allow companies to move multiple
products with only one material handling movement. Forklifts or pallet jacks can pick up
all of these products through interfacing with a pallet, the most common method for
transporting unit loads, rather than the products. It is estimated that 508 million new and
341 million repaired wooden pallets were produced in 2016 to transport unit loads (Gerber
2018). ANSI’s (American National Standards Institute) Material Handling committee
defines a pallet as “a portable, horizontal, rigid, composite platform used as a base for
assembling, storing, stacking, handling, and transporting goods as a unit load” (MH1
Committee 2016). Pallets are assumed to be a rigid platform, meaning they have an
inability to bend. Inherently, due to the weight placed in top of pallets, bending will occur,
resulting in a deformable platform rather than a rigid platform. The amount of bending
depends on the design of the pallet thus can be controlled.
Modern Material Handling subscribers who use pallets reflected that the purchase price is
one of the most important factors when selecting a pallet (Bond 2018). Reducing the
thickness of top deckboards reduces the price of the pallet since less lumber is being used.
Yet, altering deckboard thickness also changes the pallet’s mechanical behavior. Having
thinner deckboards reduces the bending stiffness of the pallet and results in an increased
bending of the deckboards under load.
1
A system-based design of unit loads requires the understanding of how packaged products,
pallets and material handling equipment mechanically interact (White and Hammer 2005).
Since pallets carry packaged products, the performance of these packages could be affected
by different levels of pallet bending.
1.2
Box Compression Strength
Corrugated boxes are commonly stacked during warehousing and transportation; therefore,
the compression strength of the box is important. Boxes should provide enough vertical
compression strength to support the weight of stacking in distribution or package failure
will occur. Factors such as relative humidity (Kellicutt & Landt 1952, Stott 1959, Whitsitt
& Mckee 1972), storage duration (Kellicutt & Landt 1952, Whitsitt et al. 1967) and
stacking pattern (Kellicutt 1963, Ievans 1975, Singh & Singh 2011) affect the strength of
a box. Once palletized, the compression strength of a box is further reduced due to
deckboard gaps and unsupported box corners (Ievans 1975, Monaghan and Marcondes
1992, Disalvo 1999, Baker 2016).
1.3
Pallet Stiffness Effect on Box Compression Strength
Recently, deckboard stiffness has been investigated in relation to box compression
strength. It was concluded that the compression strength does not change due to stiffness
when there was a pallet gap (Baker 2016, Phanthanousy 2017), but for solid deck pallets,
Baker found a 26% difference in compression strength from a rigid surface to the lowest
stiffness surface tested. All the investigated corrugated boxes by Baker and Phanthanousy
were investigated using a symmetrically supported box placed in the middle of the
deckboard centered between two stringers.
However, corrugated boxes are not just shipped on the pallet top deck centered between
the stringers; boxes are placed everywhere on a pallet to effectively use a pallet’s area. Box
loading symmetry refers to how the corners of a box are supported. Symmetrical loading
is when all four corners are supported in a similar manner (Figure 1 – Left)). Asymmetric
box loading means at least one of the four corners is supported differently from the rest
2
(Figure 1 - Right). Two of the box corners are over the deflecting span of the top deckboard,
while the other two corners are supported by the stringer where minimal deflection will
occur. Unequal support conditions are present. The level of deckboard deflection is
dependent on the pallet stiffness where a flexible pallet bends more and creates more
unequal stresses than a stiff pallet.
Asymmetric
Symmetric
Figure 1 Illustration of symmetric vs. asymmetrically supported boxes. Left, all box
corners are similarly supported. Right, box corners are supported differently
Baker (2016) also investigated the effect of box location and the resulting box symmetry
on box compression strength (Figure 2). Three locations were investigated, and it was
found that an asymmetrically loaded box (B and C) was 15% weaker than other locations
on a low stiffness pallet with no deckboard gaps. The box with two corners supported by
the stringer and two corners being supported by the deckboards (B) was the weakest in
compression strength in relation to the other two investigated locations. This difference in
compression strength can be attributed to how a box is stressed on a low stiffness pallet.
There are higher box stress concentrations located near the ends of deckboards (Yoo 2011),
creating a surface with uneven stresses for boxes and ultimately stressing one side of the
box more than the other. Baker also found when pallet stiffness of pallets with no
deckboard gaps was increased from “low” to “medium”, there was an increase of 15% in
compression strength.
3
Figure 2 Note. Reprinted from Baker et al 2016. Possible box locations on a pallet
Packaging engineers efficiently utilize the entire pallet area during shipping, resulting in
boxes being located in the asymmetric position. The relevance of the asymmetric location
can be seen in Figure 3. In addition to the frequency of the location, the location results in
the lowest compression strength of a corrugated box within the unit load especially when
low stiffness pallets are used. To avoid failure, corrugated boxes are designed for the worst
conditions. In a unit load, the bottom box carries more load than the box above it therefore,
boxes are all designed assuming that they will be on the bottom. Based on Baker’s research,
boxes supported asymmetrically on flexible pallets are weaker than supported
symmetrically (Baker 2016). However, the effect of asymmetric support is not built into
any box safety factors or design methods. Therefore, more of an understanding of the effect
of pallet stiffness on this location is needed to design safer, sustainable and cost-effective
unit loads.
4
Figure 3 Illustration of asymmetrically loaded boxes in common unit load arrangements
Using a system-based approach to designing a unit load with the understanding of how
pallet stiffness effects box compression strength, there is potential to save money on total
unit load cost. Is it cheaper to change the pallet design (1 pallet in unit load) to increase
box performance or change box design (many boxes in unit load) to increase box
performance? A cost analysis can be found in the discussion section.
2
Objective
Investigate the effect of pallet top deck stiffness on the strength of asymmetrically loaded
corrugated boxes.
3
Experimental Design
During the study, two box sizes and four deckboard thicknesses were investigated. Ten
replicate tests were conducted for each variable. The experimental design is presented in
Table 1.
Table 1 Experimental Design of Study.
Deckboard
Thickness (in.)
3/8
4/8
5/8
6/8
Box Size
16” x 9.75” x 10”
10
10
10
10
5
24” x 9.75” x 10”
10
10
10
10
4
4.1
Methods
Box compression strength
Compression strength for asymmetrically loaded boxes (Figure 4) was recorded using a
Lansmont Squeezer fixed platen compression tester equipped with a 5,000 lbs load cell. A
simulated pallet was centered on the bottom platen. A layer of empty corrugated boxes was
loaded on the top of the pallet section and then a layer of weighted corrugated boxes was
placed above. An identical pallet was placed on the top of the boxes to simulate the load
distribution during double stacking. The unit load simulator was compressed following the
guidelines of ASTM D642 (2015) using 0.5 in./minute testing speed until failure was
observed. The applied force to fail the sample was then divided by two to account for the
number of columns in the unit load. To determine the final compression strength for a
single box, the weight of the top box was added to the measured failure load.
Figure 4 Experimental setup for the box compression testing.
5
5.1
Materials
Corrugated Boxes
Two sizes of Regular Slotted Container (RSC) style corrugated boxes were investigated in
the study. The box sizes were 16 in. and 24 in. in length while the width and height of all
of the boxes were 9.75 in. and 10 in., respectively. The boxes were manufactured on a
6
DataTec computerized cutting table using flat corrugated stock produced by Packaging
Corporation of America in Roanoke, Virginia. The corrugated stock was nominal 32 lb/ in.
ECT (edge crush test) B-flute. The boxes were erected using a jig to ensure 90-degree
corners. The bottom and top of the boxes was closed using 1.88 in. packaging tape (Staples,
Inc.). The boxes were preconditioned at 50% relative humidity and 73 °F for 72 hours
according to ASTM D4332 (2015).
The boxes used for the bottom layer of the test setup were empty, while the boxes used for
the top layer in the test setup were loaded with three times the maximum payload to
simulate a 4-layer high unit load. The weight in the weighted box was determined by
Equation 2 found in the Appendix. For the weighted corrugated boxes, a load spreader
made of 0.75 in. Birch plywood was placed on the bottom of the box. Rectangular metal
weights were placed on top of the load spreader.
Pallet
A 28 in. x 20 in. stringer class, non-reversible, flush, two-way, quarter size wooden pallet
section was used for the study. Pallet sections represent the quarter section of a full sized
48” x 40” GMA (Grocery Manufacturers Association) pallet with 7 equally spaced top
deckboards and 5 bottom deckboards (Figure 5).
Figure 5 Depiction of 48” x 40” GMA Pallet
Pallet sections (Figure 6) were constructed from Kiln-dried SPF lumber purchased from
Home Depot, in Christiansburg, VA. The stringers of the pallet section were 1.5 in. wide,
3.5 in. tall, and 28 in. long. The top and bottom deckboards were 3.5 in. and 20 in. long
and spaced 3.91 in. apart. The thickness of the top and bottom deckboards varied to
7
simulate four pallet stiffnesses: low (3/8 in.), medium (1/2 in.), medium-high (5/8 in.) and
high (3/4 in.) The deckboard was attached to the stringers using two 1.25 in. long
construction screws per deckboard joint. The location of the top and bottom deckboard
sections are presented in Table 2.
Figure 6 Overall and component sizes of the 28 in. x 20 in. pallet section.
Note: all dimensions are in inches.
Table 2 Summary table of the deckboard locations for the 28 in. x 20 in. pallet sections.
Top
Deckboard
Number
1
2
3
4
Bottom Deckboard
Number
1
2
3
Deckboard location in reference Deckboard
to front of pallet (in.)
(in.)
0
3.5
7.41
3.5
14.82
3.5
22.23
3.5
Deckboard location in reference Deckboard
to front of pallet (in.)
(in.)
0
3.5
14.82
3.5
22.23
3.5
8
Width
Width
6
6.1
Results and Discussion
Box Failure Mode and Location
The failure location for all of the repetitions was in the bottom layer of the setup. The
failure mode was buckling of the length panel. The crease originated from one of the
corners and extended to the opposite corner in a U-shape, see Figure 7.
Figure 7 Typical Failure Mode of Boxes
6.2
The Effect of Pallet Stiffness on Box Compression Strength
Compression strength measurements for boxes placed on varied deckboard thicknesses are
in Table 3 and presented in Figure 8. The Analysis of Variance (ANOVA) test concluded
that pallet stiffness has an effect on asymmetrically supported boxes (p-value < 0.0001).
Tukey’s Honest Significant Difference (HSD) Post-hoc analysis was conducted to
determine differences in the mean compression strength of corrugated boxes supported on
different stiffness deckboards. Tukey’s HSD found that the strength of the corrugated
boxes was significantly different when the boxes were supported on pallets with different
top deckboard stiffness. However, when the 1/2 in. and 5/8 in. deckboards were
investigated, no significant compression strength difference was found.
9
Figure 8 Pallet Stiffness Effect on Compression Strength of Asymmetrically Loaded
Boxes
As deckboard thickness increases, so does box compression strength for asymmetrically
loaded boxes. For the 16 in. x 9.75 in. x 10 in. boxes, there was a 17% increase in box
compression strength when moving from 3/8 in. to 5/8 in. deckboards and when moving
from 3/8 in. to 3/4 in. there was a 37% strength increase. The same trend was observed for
the 24 in. x 9.75 in. x 10 in. boxes. The strength of the boxes was increased by 20% and
30% when the pallet deckboard thickness was increased from 3/8 in. to 5/8 in. and from
3/8 in. to 6/8 in., respectively. Therefore, the effect of pallet stiffness on box compression
strength is prominent regardless of box size. Both sizes of boxes exhibited higher
compression strength when supported by a stiffer pallet.
Table 3 Summary table of the box compression strength results using different pallet
stiffnesses.
Deckboard
Thickness (in.)
Box Compression
Change1
Strength (lbs)
p-value
Tukey’s
16” x 9.75” x 10” Box
3/8
1/2
5/8
263 (8.8%)
301 (4.4%)
307 (12.5%)
14.5%
16.7%
10
<.0001
A
B
B
3/4
360 (9.8%)
36.8%
C
24” x 9.75” x 10” Box
3/8
276 (6.5%)
a
1/2
314 (2.1%)
13.8%
b
<.0001
5/8
330 (7.2%)
19.6%
b
3/4
360 (3.5%)
30.4%
c
*Note: Values in parentheses are Coefficient of Variation for the average treatment effect.
Compression strengths not connected by the same letter are statistically different
determined by Tukey’s HSD test at α=0.05 significance level.
1
The percentage change in compression strength was calculated compared to the low
stiffness (3/8 in.) pallet.
7
Discussion
The differences in compression strength can be attributed to the level of bending of the
pallet’s top deckboard. A thicker deckboard, acting as a simply supported beam, deflects
less than a thin deckboard under the same load (Rammer 2010). Therefore, there was more
movement with the lowest stiffness deckboard in response to loading. This created a
surface with uneven stresses of the box corners shifting more stress towards the corners
supported by the stringer (Yoo 2011). In contrast, the higher stiffness pallet sections didn’t
deflect quite as much resembling more of a rigid surface and ultimately having more equal
stressing to all box corners. A qualitative example of bending difference is provided in
Figure 9.
11
Figure 9 Representative picture of pallet bending at box failure for the Low stiffness pallet
(Left), Medium-High stiffness pallet (Right).
8
Implications
When the 32 ECT B-flute box was placed on a stiffer pallet it resulted in a higher
compression strength. This means that it is possible to achieve better box performance
without spending more money per box. Traditionally, if a box failed on a unit load, the
board grade of the box needed to be increased to obtain higher compression strength
(Conventional approach). Based on the results of this study, higher box compression
strength can be achieved by increasing the stiffness of the pallet (Systems-Based Design
approach).
To compare the cost of the two approaches, three box suppliers and three pallet suppliers
were surveyed to determine the total costs of a hypothetical unit load. The unit load
contained 48 pieces of 16 in. x 9.75 in. x 10 in. RSC type corrugated boxes made of 32
lb/in. ECT B-flute corrugated board arranged in a 4 x 3 array on the pallet top deck. The
boxes were stacked on a 48 in. x 40 in. wooden pallet made of Kiln-Dried SPF lumber with
7/16 in top deckboards.
A company wants to have more warehouse space, so they decide to triple stack their unit
loads on their warehouse floor, but their unit loads are only designed for double stacking.
A redesign is necessary, or their will be box compression failures. The strength required to
support the new stacking weight is achieved by increasing the board grade of their boxes
from 32 lb/in. ECT B to 40 lb/in. ECT B.
Conventional Solution: Increase the board grade of the box to achieve more strength.
Changes box cost.
Systems-Based Design Solution: From the research, switch the pallet from 7/16 in. to
11/16 in. to achieve more strength. Changes the pallet cost.
Table 4 Cost analysis of using a system-based design solution to solve box failures
12
Scenario
Failed Boxes
Conventional
Solution
System
BasedSolution
9
Board
Grade
(ECT)
32 B
$ 0.64
Number of
Boxes in
Unit Load
48
Top
Deckboard
(in.)
7/16
40 B
$ 0.71
48
32 B
$ 0.64
48
Box
Cost
Pallet Cost
Unit Load
Cost
$ 10.75
$ 41.47
7/16
$ 10.75
$ 44.83
11/16
$ 12.81
$ 43.53
Conclusion
Increasing the stiffness of the pallet increases the compression performance of an
asymmetrically supported box.
The box compression strength can be increased up to 30% to 37% by increasing the
stiffness of the pallet.
A scenario was presented where it is a cheaper to increase the deckboard thickness to
achieve higher compression strength rather than change the board grade of a box.
13
10 References
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
ASTM. (2015). D642-15. Standard Test Method for Determining Compressive
Resistance of Shipping Containers, Components, and Unit Loads. West
Conshohocken, PA, https://doi-org.ezproxy.lib.vt.edu/10.1520/D0642-15
ASTM. (2015). D4332-14. Standard Practice for Conditioning Containers,
Packages, or Packaging Components for Testing. West Conshohocken, PA,
https://doi-org.ezproxy.lib.vt.edu/10.1520/D4332-14
Baker, M. (2016). Effect of Pallet Stiffness and Unit Load Factors on Corrugated
Box Compression Strength (Doctoral dissertation). Virginia Polytechnic Institute
and State University, Blacksburg, VA.
Bond, J. (2018). The Pallet Report: Users want more services & Support. Modern
Materials Handling, 73(9), 30-35
Disalvo, M.H. (1999). Interactive Effects of Palletizing Factors on Fiberboard
Packaging Strength (Master’s Thesis) San Jose State University, San Jose, CA.
Frank, B. (2014). Corrugated Box Compression—A Literature Survey. Packaging
Technology and Science, 27(2), 105-12. https://doi.org/10.1002/pts.2019
Gerber, N. (2018). Investigation of New and Recovered Wood Shipping Platforms
in the United States (Master’s Thesis). Virginia Polytechnic Institute and State
University, Blacksburg, VA.
Ievans, U. I. (1975). The Effect of Warehouse Mishandling and Stacking Patterns
on The Compression Strength of Corrugated Boxes. TAPPI Journal, vol. 58 (8),
108-111.
Kellicut K.Q., Landt E.F. (1952). Safe stacking life of corrugated boxes. Fibre
Containers. 1952;36(9):28-38
Kellicut, K. Q. (1963). Effect of contents and load bearing surface on compressive
strength and stacking life of corrugated containers. TAPPI, 46(1), 151A–154 A.
MH1 Committee. (2016). MH1-2016 Pallets, Slip Sheets, and Other Bases for Unit
Loads. Charlotte, NC: MH1 Secretariat.
Monaghan, J., & Marcondes, J. (1992). Technical notes: Overhang and pallet gap
effects on the performance of corrugated fiberboard boxes. Transactions of the
ASAE, 35(6), 1945–1947. https://doi.org/10.13031/2013.28820
Phanthanousy, S. (2016). The Effect of the Stiffness of Unit Load Components on
Pallet Deflection and Box Compression Strength (Master’s Thesis). Virginia
Polytechnic Institute and State University, Blacksburg, VA.
Singh, J., & Singh, S. P. (2011). Effect of Palletized Box Offset on Compression
Strength of Unitized and Stacked Empty Corrugated Fiberboard Boxes. Journal of
Applied Packaging Research, 5(3), 157–167
Stott R.A. (1959). Compression and stacking strength of corrugated fiberboard
containers. APPITA.13(2), 84–89
Rammer, D. R. (2010). Wood Handbook, Chapter 09: Structural Analysis
Equations. General Technical Report FPL-GTR-190. Madison, WI: U.S. Dept. of
Agriculture, Forest Service, Forest Products Laboratory: 9-1 – 9-11. Chapter 9.
14
•
•
•
Whitsitt, W.J., Gander, J.W., and McKee, R.C. (1967) Stacking behavior of boxes
and corrugated board, a summary report to the FKBI. IPST Project 1108–30.
http://smartech.gatech.edu/handle/1853/871.
Whitsitt W.J. & McKee R.C. (1972). Effect of relative humidity and temperature
on stacking performance. IPC report project 2695–9, Nov 10, 1972.
White, M. S., & Hamner, P. (2005). Pallets move the world: the case for developing
system-based designs for unit loads. Forest Products Journal, 55(3), 8–16.ANSI
MH1Yoo, J. (2011). Modeling compressive stress distributions at the interface
between a pallet deck and distribution packaging (Doctoral dissertation). Virginia
Polytechnic Institute and State University, Blacksburg, VA
11 Appendix
Maximum Payload in Corrugated Boxes
The maximum weight in each box was calculated using equation 2. when a unit load
contains four layers and is stacked two high. The BCT for the box was calculated using the
simplified McKee equation (Equation 1).
7
𝐵𝐶𝑇 = 5.87 𝑥 𝐸𝐶𝑇 𝑥 (𝐶𝑎𝑙𝑖𝑝𝑒𝑟 𝑥 𝑃𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟)8 ,
Equation 1
ECT: edge crush test value – 32 lbs./in.
Caliper: caliper of the board
Perimeter of the box: (Length + Width)*2
𝑩𝑪𝑻
𝑴𝒂𝒙𝒊𝒎𝒖𝒎 𝑷𝒂𝒚𝒍𝒐𝒂𝒅 𝒑𝒆𝒓 𝑩𝒐𝒙 = H𝑺𝒂𝒇𝒆𝒕𝒚 𝑭𝒂𝒄𝒕𝒐𝒓∗(𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒃𝒐𝒙𝒆𝒔 𝒔𝒕𝒂𝒄𝒌𝒆𝒅U𝟏)W,
Equation 2
BCT: box strength determined using the simplified McKee equation (Equation 1).
Safety Factor: 3, from ISTA 3E
Number of boxes stacked: 8, assuming that the unit load has 4 layers and it is double
stacked.
The pre-calculated weights for each box size are presented in Table 1.
15