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Virginia Tech: Analysis Improves Performance, Trims Costs of Block Stacked Unit Loads
By understanding how pallets interact with product packaging and the materials handling environment, pallet manufacturers can better design pallets to reduce costs and improve overall unit load performance.
By Peter Hamner and Marshall White
Date Posted: 3/1/2008
Raw material is the largest cost of pallets, packaging, and materials handling equipment. Consequently, cost reductions typically involve reducing raw material requirements by redesigning some or all of these components. Because these three components mechanically and physically interact during product storage and shipping, the redesign of one component of the system will potentially affect the performance of one or both of the other components.
By understanding how pallets interact with product packaging and the materials handling environment, pallet manufacturers can better design pallets to reduce costs and improve overall unit load performance. For example, products packaged in rigid type containers, such as corrugated boxes and plastic pails, are often unitized on pallets and stored and distributed in unit loads that are block stacked two or more high. Under these support conditions, the performance and cost of the product packaging is often limited by its own compression strength.
Depending on the stiffness of the pallet deck and the amount of load bearing area available to the product packaging (both above and below the unit load), stress concentrations throughout the load can vary significantly. Areas of high stress are often associated with small bearing areas between the pallet and the packaged product on the pallet. To prevent container and potential load failures when unit loads are block stacked, it is necessary that packaging be over designed to withstand areas of high stress. These added costs can be avoided.
Research at Virginia Tech has shown that compressive stress on rigid packaging may be significantly reduced by increasing the effective load bearing areas between the rigid packaging (pail, box, etc), and the pallet. The effective bearing areas depend on the pallet top and bottom deck coverage, deck board spacing and deck board location.
To illustrate this point, Figure 1 shows two pallets with different top deck designs and the stacking pattern of pails that could be loaded onto them. The effective load bearing area of the pallet on top is 26.1 square inches compared to 31.6 square inches for the pallet on the bottom. It is also important to note that the stiffness of pallet decks affects the bearing area and load concentrations by an order of magnitude. Once deck boards begin to deflect under load, the effective bearing area diminishes, and stress concentrations begin to multiply.
Most often, the best ways to increase effective load bearing area and decrease stress concentrations in block stacked unit loads are by stiffening the pallet top deck and using reusable, portable, unit load top caps. Top caps allow the load from the pallets stacked above to be distributed evenly into the unit load below.
These design changes can minimize or eliminate the constraints caused by stress concentrations and can allow for significant reductions in packaging costs. This will not only maximize unit load performance but also provide a significant net savings for the packaging in the unit load. Something as simple as redesigning the pallet could allow customers to save as much as $10 or more in packaging costs per unit load.
The systematic approach to evaluate and improve unit load design for block stacked pallets loaded with products packaged in rigid containers are described below.
1) Use field audits to document the mechanical stresses imposed on unit loads as they move through the supply chain.
For customers that use pallets to unitize products packaged in plastic pails or corrugated boxes, field audits must be conducted at every location that palletizes, stores, and distributes these specific product-packaging combinations. A comprehensive field audit of the customer’s supply chain is important because it helps identify potential constraints where high stress concentrations may occur. High stress concentrations limit the performance and cost effectiveness of rigid packaging materials. The field audit documents all pallet, packaging, and material handling equipment specifications used in the customer’s supply chain, including:
• Pallet designs
• Packaging designs
• The stacking patterns of packaged products on pallets
• The weight of loaded pallets
• Load stabilizers used
• All support conditions used — fork lift support, conveyors, warehouse racks, block stacking heights, etc.
• Modes of transportation — tractor trailer, rail, air freight, ocean freight, etc.
2) Measure the mechanical resistance of components in the unit load and compare them to the stresses imposed on unit loads as they move through the supply chain.
The mechanical resistance of both pallets and packaging materials must be determined in order to compare their maximum performance levels with the stresses levels imposed on them in use (recorded in the audit). A computer program such as PDS© can be used to provide the mechanical resistance (structural analysis) of all wooden pallets used in the supply chain relative to the specific support conditions identified in the field audit. Maximum safe loads provided by the structural analysis can then be compared to actual loads to identify any pallets that might be significantly under or over designed. While pallet strength is critical, it is the combination of pallet strength and stiffness, effective load bearing area, and container strength under load that ultimately determines unit load performance.
In addition to evaluating the pallets, it is also necessary to determine the maximum compression strength of the product packaging. Product container or packaging compression strength must be measured both individually and in unit load form. Where temperature may affect the compression strength of containers — such as plastic pails — it is necessary to determine maximum compression strength for such containers at a range of in-use temperatures.
Effective load bearing area — the area of a pallet top and bottom deck that is available to support unit loads when block stacked — is critical to the compression strength of containers placed on pallets. The effective pallet load bearing area has two components.
• Pallet top deck surface area—surface area of the top deck that is supporting the packaged products sitting on top of the pallet.
• Pallet bottom deck surface area—surface area of the bottom deck as a stacked unit load from above rests on a unit load below.
Figure 2 shows compression failures in palletized plastic pails that are block stacked. This image also depicts the load bearing area of the pallet top deck supporting the pails. It is essential to determine the maximum compression strength of the containers in terms of the percentage of the container surface that is bearing the load. For example, if 25% of the bottom container sitting on a pallet is overhanging or unsupported by a deck board, the effective bearing area for that container is 75%. Tests should be conducted to determine container compression strength for a range of top and bottom load bearing areas.
Figure 3 shows a unit load being tested to determine the compression strength of the entire load. In this case the effective bearing area on the top of the load is 100% while the bottom is supported on a pallet (< 100%).
The measurements obtained from the evaluation of pallet strength and stiffness and the compression strength of the packaging materials used in the unit load will provide valuable insight for cost reducing design changes of the unit loads used in the customer’s supply chain.
3) Using the principles of load and resistance factor design, identify actions that will reduce mechanical stresses on components in the unit load.
To reduce compressive stresses on unit load containers as they are stored and distributed in block stacking scenarios, without changing the product characteristics, consideration should be given to the following proven means to achieve these results:
• Reducing unit load mass
• Reducing unit load stacking heights
• Increasing the effective bearing areas between the containers and pallets during stack storage. The container-pallet effective bearing areas are most efficiently increased by stiffening the pallet top deck and using portable unit load
• Standardizing pallet designs. This could lead to additional packaging cost savings.
4) Evaluate and decide on supply chain systems design changes that reduce supply chain operating costs using cost-benefit analysis.
Through the course of this systematic evaluation of a customer’s supply chain, important design changes are identified to reduce mechanical stresses on components in block stacked unit loads. This information will provide the opportunity to improve the balance between unit load cost and performance. Areas of high stress concentrations have been identified and eliminated.
Because the rigid packaging materials were previously designed to withstand the areas of high stress, the first step to reduce supply chain operating costs is to redesign the packaging. The following examples illustrate this point. If expensive triple wall corrugated containers were previously required to package a particular product that is unitized in block storage, an improved pallet design may now allow less expensive double wall containers to be used. As another example, perhaps inefficiencies in materials handling were determined to be caused by too many different pallet designs being used to transport similar products. This problem could be resolved by standardizing the pallet design and redesigning the packaging accordingly.
When implementing supply chain system design changes, cost-benefit analysis allows a decision maker to evaluate both the performance criteria of design changes and any resulting changes in costs. The systematic approach presented in this discussion will often provide more than one alternative solution. It is recommended that these alternatives be evaluated using field test and cost-benefit analysis to determine the optimum solution.
(Editor’s Note: Peter Hamner is a research associate for the Virginia Tech Center for Unit Load Design; Marshall White is the former director of the center. For more information, contact Peter at (540) 231-3043 or email firstname.lastname@example.org.)