Effect of Stringer Repair Methods and Repair Frequency on GMA Performance
By John Clarke, Marshall White, Phil Araman
Date Posted: 10/1/2007
Editor’s Note: The information presented here came from a study conducted by John Clarke, Dr. Marshall White, and Philip Araman of Virginia Tech and the USDA Forest Service, Southern Station. You can obtain a copy of this report by contacting the Center for Unit Load Design at 540/231-7105.
Research Overview
Notched stringers are one of the most commonly damaged pallet components. Metal plates, half companion stringers, and full companion stringers are repair methods described in the U.S. industry standard published by the American Society of Mechanical Engineers. This study evaluated the effect of these three stringer repair methods on the bending strength and stiffness of 48x40 GMA-style pallets spanning the pallet stringers.
Pallets with up to two stringers repaired with full-companion stringers were stronger and stiffer than new, undamaged pallets. In general, pallet with one or two repaired stringers using up to two metal plates or half-companion stringer repairs were as strong or stronger, but less stiff, than the original pallets without stringer repairs.
On the other hand, on average pallets with two stringers repaired with metal plates or half-companions were less strong and less stiff than pallets with only one such repair. These results indicate that all three repair practices (metal plates, half-companions, and full companions), when properly applied, will restore pallet strength when used to repair one notch on one or two stringers of a 48x40 partial 4-way, three stringer pallet.
There may be an additional species effect on repair performance that warrants further evaluation. Current repaired pallet grades group metal plate repairs in upper grades, while companion member repairs are placed in lower grades. Based on functionality with material handling equipment, these grades do not reflect the racking performance of these repair methods.
According to ASME MH1 standards, there are four standard repair methods for damaged pallet stringers: replacement, metal connector plates, full-length companion stringers, and half-length companion stringers. Replacement is the process of removing a damaged stringer and nailing a similar new or used stringer in its place. Stringer replacement is rarely used to repair GMA pallets and was not included in the study. Metal connector plates are applied to each side of a stringer fracture using specialized equipment. Only horizontal or diagonal stringer splits may be repaired. Plates must be 20-gauge steel, galvanized, a minimum of 2.75 inches in length and 11 square inches in area, and have at least four teeth per square inch. A pair of plates is used for each repair, one per stringer side over the fracture.
A full-length companion stringer (full-companions) can be placed adjacent to the damaged stringer and nailed to all top and bottom deckboards. Full companion stringers should be equal in length and slightly less in height to the damaged stringer to be supported. Half-length companion stingers (half-companions) are cut one inch shorter than half of the stringer length, and are slightly less in height than the damaged stringer being repaired. A half-companion is placed adjacent to the damaged portion of a stringer and nailed to applicable top and bottom deckboards.
Full and half-companion stringers should be notched for forklift tine entry similar to the damaged stringer.
The effect of metal connector plate repair on the performance of individual stringers has been studied. These plates, when properly applied, were found to restore the strength of stringers fractured between stringer notches at the notch fillet. Stiffness, however, was only restored by using an L-shaped plate that extended over the stringer notch. Most plates in use are rectangular, not L-shaped.
Because of the level variation of repair methods observed between each grade of repaired pallets, the researchers were not able to determine the relative impacts of various stringer repair procedures on pallet performance.
Most pallet recyclers have established proprietary grades that resemble, but are not identical, to the standard ASTM grades. In general, used pallets with no repairs or metal plate repairs are higher in grade than pallets with half or full-companion stringer repairs. Companion stringers reduce the opening sizes for materials handling equipment, decreasing the functionality with material handling equipment. Therefore, the lower grade ranking given to pallets with companion stringer repairs is due more to this loss of functionality than to any documented loss of strength and stiffness.
In general, the effectiveness of metal plate and half companion stringer repair declines by 15-29% as the number of stringers requiring repair increases from one to two. However, two full companion stringer repairs are an average of 7% and 22% stronger and stiffer than one stringer repair, respectively. Full companion stringers use more wood volume and nails than the other repair methods, however, and therefore are a relatively expensive repair method.
Summary of Research Conclusions
These research conclusions are summarized below:
- The bending strength of GMA-style 48x40 notched stringer pallets with one or two damaged stringers can be restored using metal plates, half-length companion stringers, or full-length companion stringers as described in ASME MH1 Part 3.
- Metal plate and half-length companion repairs will restore an average of 65% to 85% of the stiffness of GMA-style 48x40 pallets, depending on the number of stringers repaired.
- GMA-style pallets with two stringers repaired using metal plates or half-length companion stringers are 15-27% weaker and 25-29% less stiff than the same pallets with only one stringer repaired.
- GMA-style pallets with full-length companion stringers used to repair one or two stringers are stronger and stiffer in bending than the original pallets before damage.
