CA2805429A1 - Metallic pallet with improved corner construction - Google Patents

Abstract

A metallic pallet (20) having generally planar top (22) and bottom (40) sides. Corner pieces (56) join the top (22) and bottom (40) sides and provide controlled deformation in the event of corner impacts. Spacer block (78, 80) function as risers to establish a space between top (22) and bottom (40) sides for receiving forklift tines (24). The top side (22) is formed in a rectangular formation with long (28) and short (30) rails establishing a perimeter for a plurality of decking members (32) which are welded at their ends to one another and to the corner pieces (56). Each corner piece (56) includes a separately formed plastic cover (64). The inside of the cover (64) may include vertical notches (92) for engaging vertical ribs (70) formed by structural webbing inside the corner piece (56). The outer edge (66) of the cover may include an outer corner section configured to directly engage a chamfer (94) in the metallic body of the corner piece (56). An RFID tag (58) may be located in a hollow corner piece (56) for tracking purposes.

Description

METALLIC PALLET WITH IMPROVED CORNER CONSTRUCTION

CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Provisional Patent Application No.
61/365,409 filed July 19, 2010, the entire disclosure of which is hereby incorporated by reference and relied upon.
BACKGROUND OF THE INVENTION
Field of the Invention [0002] The invention relates generally to horizontally supported planar surfaces such as industrial platforms having a support surface provided with supporting means whereby a load may be supported relatively close to a floor and having means to receive handling means beneath said surface or load and wherein the support surfaces are made of metal.
Related Art [0003] Pallets are commonly used to support and transport bundled objects or loads in applications like that shown in Figure 1. Traditionally, pallets have been constructed from wood members assembled together. However, wooden pallets are subject to many drawbacks, including a relatively short lifespan, chemical contamination, insect infestation, and fire hazard, to name a few. More recently, pallets have been constructed from plastic materials which can be more durable than wood and are typically insect resistant, however are still subject to chemical contamination, fire hazards and other shortcomings. While additives can be used to make a plastic pallet more fire retardant, such additives are costly and generally disfavored in situations where food handling is required. There is a desire within the material handling industry to identify new materials suitable for manufacturing cargo carrying pallets which will provide superior functionality at a cost competitive price with other options.

[0004] Some examples of pallet constructions representative of prior art teachings may be found in US Patent No. 5,367,960 to Schleicher granted November 29, 1994, US
Patent No.

5,522,325 to Knauer granted June 4, 1996, US 2002/0078863 to Marr et al.
published June 27, 2002 and US 2005/0178299 to Rasmussen et al. published August 18, 2005. These examples of prior art illustrate various approaches in mixed forms of construction which include at least some metallic components. In these examples, the pallets include a top deck or load engaging surface which may be composed of parallel top slats or decking attached to a surrounding frame made from plastic or wood.
[0005] One drawback found in prior art constructions relates to the general lack of strength of the pallet in terms of resisting damage due to rough handling. Excessive damage most typically manifests when a pallet experiences a laterally applied force to one of its corners. This may occur during transport such as when a forklift is moving a pallet, or when the pallet is stationary but some object collides with it. It will be appreciated that metallic pallets are substantially more expensive to manufacture than wooden pallets. The cost proposition for a metallic pallet therefore only becomes favorable when the metallic pallet can enjoy a substantially longer service life than the typical wooden pallet or other available alternatives.
Accordingly, constructing a metallic pallet that has the ability to withstand rough handling and other stressful conditions is imperative to the business proposition for such objects.

[0006] Accordingly, metallic pallets have inherent benefits and are being increasingly accepted within an industry accustomed to utilizing cheap wooden pallets. Nevertheless, there remains opportunity to further improve pallet designs and constructions with the objectives to reduce severe damage resulting from corner impacts, and thus to enhance service life.
SUMMARY OF THE INVENTION

[0007] A metallic pallet is provided of the type for supporting objects for storage and transport.
The pallet comprises a generally planar top side having a substantially rectangular configuration defining four corners. The top side includes four elongated outer top frame members. The top side also includes a plurality of decking members arranged parallel to one another and parallel to two of the outer frame members. A generally planar bottom side is provided having a substantially rectangular configuration defining four corners. The bottom side includes four elongated outer bottom frame members. The bottom side is disposed below the top side so that its corners are vertically aligned with the corners of the top side in corresponding pairs.
Likewise, the outer top and outer bottom frame members are arranged in opposing pairs. Four generally hollow corner pieces are provided. Each corner piece extends between a respective aligned pair of corners of the top and bottom sides. The corner pieces each interrupt the generally planar top and bottom sides such that the top and bottom sides do not overlie the corner pieces. Each the corner piece comprises a pair of adjoining perpendicular back walls in direct abutting contact with the top and bottom sides. Each corner piece further comprises an upper plate and a lower plate respectively affixed to the back walls. The upper plate is generally planar with the top side, and the lower plate is generally planar with the bottom side. Each corner piece includes a separately formed cover. The cover is configured to simultaneously engage the back walls and the upper and lower plates.

[0008] The subject invention is distinguished from prior art designs by providing a unique corner construction that is robust and easily manufactured. This design, which represents a departure from prior art techniques, increases the ability of a metallic pallet to resist damage caused by undesirable impact forces to the corners, which thereby favorably extends the service life and hence cost benefit of a metallic pallet.
BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:

[0010] Figure 1 is an environmental view showing a pallet constructed in accordance with one embodiment of this invention utilized to support cargo and poised for transport via a common forklift vehicle;

[0011] Figure 2 is a perspective view of the subject pallet;

[0012] Figure 3 is a perspective view showing the bottom of the pallet according to Figure 2;

[0013] Figure 4 is a top view of the subject pallet;

[0014] Figure 5 is a bottom view of the subject pallet;

[0015] Figure 6 is an end view of the subject pallet;

[0016] Figure 7 is a cross-sectional view taken generally along lines 7-7 in Figure 6;

[0017] Figure 8 is a cross-sectional view of an exterior spacer block taken generally along lines 8-8 in Figure 6;

[0018] Figure 9 is a perspective view of a corner piece according to an embodiment of the invention;

[0019] Figure 10 is an exploded view of the corner piece of Figure 9;

[0020] Figure 11 is a cross-sectional view taken generally along lines 11-11 of Figure 9;

[0021] Figure 12 is a perspective view of the cover for the corner piece of Figure 10;

[0022] Figure 13 is a schematic view illustrating tracking capabilities for the subject pallet when configured to include an RFID tag;

[0023] Figure 14 is a perspective view of a first alternative corner piece construction;

[0024] Figure 15 is a fragmentary perspective view showing a second alternative corner piece in partially exploded fashion and including an optional RFID tag in the hollow space therein; and [0025] Figure 16 is a perspective view of a third alternative corner piece design.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Referring to the figures, wherein like numerals indicate like or corresponding parts throughout the several views, a pallet constructed in accordance with embodiments of this invention is generally shown at 20. Standard pallet dimensions are given as 48 inches x 40 inches x 5.6 inches, however the specific dimensional properties of the pallet 20 are subject to alteration to suit any given application.

[0027] As perhaps best shown in Figures 2-7, the pallet 20 includes a generally planar top side, generally indicated at 22. The top side 22 establishes a generally horizontal surface for supporting objects such as cargo for storage and transport. The top side 22 is elevated above the floor or other supporting structure so that the tines of a forklift 24 (Figure 1) can be inserted under the top side 22 in the customary fashion. The top side 22 has a substantially rectangular configuration as suggested by the dimensions suggested above, however this is subject to variation depending upon design choice. The top side 22 defines four corners which, in the preferred embodiment, are represented by notches 26. The top side 22 also includes four elongated outer top frame members comprised of two long rails 28 and two short rails 30. The long 28 and short 30 rails are arranged about the perimeter of the top side 22 and extend each into the notches 26 thus establishing a generally rectangular shape for the top side 22. The long 28 and short 30 rails are preferably made from extruded aluminum having any desirable and suitable cross-sectional shape such as, for example, shown in the cross-section of Figure 7.
[0028] A plurality (seven in the examples shown) of decking members 32 are arranged parallel to one another and parallel to the two short rails 30 with generally equal spacing therebetween.
The decking members 32 are preferably made from an extruded aluminum material and may have a cross-sectional like that shown for example in Figure 7. Each decking member 32 is welded at its ends to the inside edge of the two long rails 28. The top side 22 in one embodiment further includes a backbone 34 disposed directly underneath each of the decking members 32.
The backbone member 34, perhaps best seen in Figures 3 and 7, extends parallel to the two long rails 28 to provide mid-span support for the top side 22 for increased weight bearing capacity for the pallet 20.

[0029] Turning now to Figures 3 and 5, the pallet 20 is shown including a generally planar bottom side 40. The bottom side 40, like the top side 22, also has a substantially rectangular configuration having corners defined by notches 42. The outer dimensional properties of the bottom side 40 are substantially identical to that of the top side 22 so that the notches 26, 42 can be directly aligned one over the other. The bottom side 40 also includes four elongated outer frame members comprised of long 44 and short 46 rails. The short rails 46 of the bottom side 40 are generally congruent with the short rails 30 of the top side 22, and likewise the long rails 44 of the bottom side are generally congruent with the long rails 28 of the top side 22. Unlike the rails 28, 30 of the top side 22, however, in the preferred embodiment shown in Figures 5 and 7, the rails 44, 46 of the bottom side 40 are significantly wider (measured laterally) than their long 28 and short 30 rail counterparts of the top side 22. As a result, the long 44 and short 46 rails of the bottom side 40 may, in one embodiment, directly connect to one another at their terminal ends via a truncated miter joint 40. This miter joint 48 may be welded to provide direct fixed connection between the long 44 and short 46 rails. The bottom side 40 further includes a first inner bottom frame member 50 and a second inner bottom frame member 52. The second inner bottom frame member 52 perpendicularly bisects the first inner bottom frame member 50 at a bottom intersection joint 54 which may be welded to secure the frame members 50, 52 to one another. Additionally, the terminal ends of each of the first 50 and second 52 inner bottom frame members may be attached at their respective ends to the inner edges of the long 44 and short 46 rails such as by welding to establish a strong matrix or skeletal construction.

[0030] Four generally identical, hollow corner pieces are generally indicated at 56. The corner pieces 56 each extend between a respective aligned pair of corners of the top 22 and bottom 40 sides as represented by their respective notches 26, 42. In other words, the corner pieces 56 fit into each of the notches 26, 42 and are there securely attached such as by welding so as to rigidly join the top side 22 to the bottom side 40 via the corner pieces 56. Thus, because the corner pieces 56 are set into the notches 26, 42, it may be seen that the corner pieces 56 each interrupt the generally planar top 22 and bottom 40 sides so that the top 22 and bottom 40 sides do not overlie the corner pieces 56. As a result, corner impacts to the pallet 20 may be absorbed entirely by deformation of the hollow corner pieces 56 without distorting the planar configurations of the top 22 and bottom 40 sides.

[0031] Various configurations for corner pieces 56 may be found by reference to the applicant's co-pending US 2010/0218705, published September 2, 2010, the entire disclosure of which is hereby incorporated by reference and relied upon. Figures 9-12 illustrate one preferred embodiment of the corner pieces 56 designed for optimal performance, low cost, ease of construction, as well as the ability to safely contain an RFID tag 58 or other tracking features.
Figure 10 is an exploded view of the corner piece 56. The corner piece 56 shown here comprises a cast aluminum body establishing perpendicular back walls 60 in direct abutting contact with the top 22 and bottom 40 sides. Upper and lower plates 62, connect the upper and lower ends of the back walls 60 into a somewhat box-like structure. As a result, the upper plate 62 is generally planar with the top side 22 and the lower plate 62 is generally planar with the bottom side 40 thereby contributing to the load carrying capacity. A cover 64 is formed separately from the back walls 60, and preferably made from a non-metallic materials such as plastic or hard rubber in an injection molding process. The cover 64 is configured to simultaneously engage the back walls 60 and the upper and lower plates 62 to provide a solid, secure exposed area for the corner piece 56. The cover 64 includes an outer edge 66 establishing the leading or terminal corner for the pallet 20, which is expected to receive the initial and most frequent impacts during rough handling of the pallet 20. The material composition selected for the cover 64 should therefore be impact resistant and durable yet somewhat resilient.

[0032] The corner pieces 56 may further include internal webbing 70 formed integrally with the back walls 60 and plates 62 during the casting process for both strength and controlling deformation. The cover 64 may include notches 92 for receiving or engaging with the outer edges of the webbing 70 which, as shown in Figure 10, is formed as three vertical ribs and one horizontal rib. The upper and lower plates 62 are also formed with chamfers 94. The outer edge 66 of the cover 64 is configured with short extensions to directly engage the chamfers 94 as perhaps best shown in Figure 11. Through this design, the cover 64 and cast aluminum components become mechanically interlocked so that the corner pieces 56 may absorb and partially collapse in response to impact forces without stressing the top 22 or bottom 40 sides and thereby helping to maintain their planar nature for an extended service life of the pallet 20. The hollow corner pieces 56 resist infiltration by liquids or other unwanted substances.

[0033] As mentioned and shown in Figure 15, a small RFID tag 58 may be sealed inside one or more of the corner pieces 56 to provide pallet 20 locating and tracking capabilities. An adhesive may be used to anchor and seal the RFID tag 58 inside the corner piece 56.
Alternatively, the RFID tag 58 may be securely stationed in a pocket or receptacle formed integrally with or subsequently attached to the inside of the corner piece 56. In this way, the RFID tag 58 can be contained in a protected enclosure so as to provide long term service life and functionality for purposes of tracking the pallet 20. If the RFID tag 58 is installed into a corner piece 56 prior to assembly of that corner piece 56 with the top 22 and bottom 40 sides, the RFID
tag 58 is preferably retained in a position spaced as far as possible from each of the back walls 60 where welding occurs during attachment to the top 22 and bottom 40 sides. This will help protect the RFID tag 58 from damage due to excessive heat that occurs during welding operations.
Conversely, the RFID tag 58 is also located as far as possible from the outer edge 66 where the majority of deformation is expected to occur in use. This will help protect the RFID tag 58 from damage for a longer service life.

[0034] Figure 13 shows an exemplary, highly schematic method by which the RFID
tag 58 may be used to triangulate the position of a pallet 20 which has been lost or possibly stolen.
Using known RFID technology, the exact position of a pallet 20 with respect to a digital map 76 can be determined. Of course, the map 76 may be specific to a particular warehouse or manufacturing facility, or used more generally to track pallets 20 which may be stolen or misplaced in a much wider geographic area. Additional functionality may be incorporated into the RFID tag 58 according to known techniques.

[0035] Returning to Figures 2-8, the pallet 20 is shown including a plurality of extruded aluminum spacer blocks 78, 80. Each spacer block 78, 80 extends perpendicularly between the top 22 and bottom 40 sides for holding the top 22 and bottom 40 sides in generally parallel spaced and aligned relation to one another so that the notched corners 26, 42 remain vertically aligned and so that the corresponding pairs of long 28, 44 and short 30, 46 rails remain aligned.
Furthermore, the spacer blocks 78, 80 increase the weight force loading with which the pallet 20 is capable of supporting. Each spacer block 78, 80 may include at least one internal web. Four spacer blocks 78 comprise exterior spacer blocks 70 arranged about the outer periphery of the pallet 20. Two exterior spacer blocks 78 extend between the respective pairs of long rails 28, 44, whereas another two exterior spacer blocks 78 extend between opposing short rails 30, 46.
These exterior spacer blocks 78 are preferably made from extruded aluminum materials and welded in place.

[0036] As perhaps best shown in Figure 8, the exterior spacer blocks 78 have a generally D-shaped profile when viewed in cross-section. This D-shaped profile includes an outer convex portion 82 that is oriented outwardly from the pallet 20. In other words, as perhaps best shown in Figures 2 and 6, the convex portions 82 of the exterior spacer blocks 78 face outwardly and so that if contacted by the tines of a forklift 24, they will gently direct the tines to the side usually with little to no damage. Preferably, although not necessarily, at least one spacer block 80 comprises an interior spacer block 80 disposed for direct contact between the backbone 34 and bottom inner section joint 54 to provide column strength in the center of the pallet 20. The interior spacer block 80 may have a generally rectangular profile in cross-section. Like the exterior spacer block 78, the interior spacer block 80 is also preferably made from extruded aluminum material and securely welded in place at each end.

[0037] Figure 14 shows an exploded view of a first alternative embodiment of the corner piece 156, wherein like or corresponding parts are indicated with reference numerals offset by 100. In this example, the back walls 160 are formed of extruded aluminum and the upper and lower plates 162 are made from stamped aluminum sheet stock then subsequently affixed to the back walls 160 by welding or by other suitable fixation technique. Once assembled, the upper plate 162 is generally planar with the top side 22 and the lower plate 162 is generally planar with the bottom side 40. The cover 164 in this example is formed integrally with the back walls 160 during the extrusion process. The cover 164 is configured to simultaneously engage the back walls 160 and the upper and lower plates 162 to provide a solid, secure exposed area for the corner piece 156. The cover 164 includes an outer edge 166 establishing the leading or terminal corner for the pallet 20. Generous deformation grooves 168 may be formed in the cover 164 laterally on either side of the outer edge 166 to provide controlled collapse in the event of impact. The corner pieces 156 may further include internal webbing 170 formed integrally during the extrusion process to join the cover 164 and back walls 160 for both strength and controlling deformation. The corner pieces 156 are designed to absorb and deform when impacted without upsetting the top 22 or bottom 40 sides. The upper and lower plates 162 provide an enclosed space, i.e., hollow, in the corner pieces 56 to resist infiltration by liquids or other unwanted substances. Each plate 162 may include a small hole 172 aligned over an intersection point 174 of the webbing 170. During the assembly process, the plates 162 are positioned over the extruded body section with the holes 172 providing a convenient location through which to place a small tack weld to the webbing 170 and thereby secure the plates 162 in position [0038] Figure 15 shows a second alternative embodiment of the corner piece 256, wherein like or corresponding parts are indicated with reference numerals offset by 200. In this example, a simplified version of the corner piece 256 includes a less sophisticated separate cover 264 which may be secured in position to a cast aluminum back wall 260 and upper/lower plate 262 construction. A single horizontal web 70 and optional intersecting vertical web may be formed therein to improve crush resistance. An RFID tag 58 can be placed inside the hollow corner piece 256 and secured with a suitable adhesive or by other means as described previously.

[0039] Figure 16 shows yet another alternative construction for a corner piece 356, with like or corresponding parts being identified with similar reference numbers offset by 300. In this example, the corner piece 356 is made from cast aluminum and omits the cover altogether so as to present an open cellular hollow corner for the pallet. In all other respects, the corner piece 356 functions similarly to that described above.

[0040] The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention.

Claims (20)

1. A metallic pallet of the type for supporting objects for storage and transport, the pallet comprising:
a generally planar top side having a substantially rectangular configuration defining four corners, said top side including four elongated outer top frame members, said top side including a plurality of decking members arranged parallel to one another and parallel to two of said outer top frame members;
a generally planar bottom side having a substantially rectangular configuration defining four corners, said bottom side including four elongated outer bottom frame members, said bottom side disposed below said top side such that said corners of said top side are vertically aligned over said corners of said bottom side in corresponding pairs and said outer top and outer bottom frame members are arranged in opposing pairs;
four generally hollow corner pieces, each said corner piece extending between a respective aligned pair of said corners of said top and bottom sides, said corner pieces each interrupting the generally planar top and bottom sides such that said top and bottom sides do not overlie said corner pieces; and each said corner piece comprising a pair of adjoining perpendicular back walls in direct abutting contact with said top and bottom sides, each said corner piece further comprising an upper plate and a lower plate respectively affixed to said back walls, said upper plate generally planar with said top side, said lower plate generally planar with said bottom side, each said corner piece including a separately formed cover, said cover configured to simultaneously engage said back walls and said upper and lower plates.

2. The pallet of claim 1, wherein said cover is fabricated from a non-metallic material.

3. The pallet of claim 1, wherein each said corner piece includes at least one vertical rib extending between said upper and lower plates.

4. The pallet of claim 3, wherein said cover includes at least one vertical notch for receiving said at least one vertical rib.

5. The pallet of claim 3, wherein each said corner piece includes at least one horizontal rib extending between said back walls and intersecting said at least one vertical rib.

6. The pallet of claim 1, wherein each said corner piece includes a plurality of vertical ribs extending between said upper and lower plates.

7. The pallet of claim 6, wherein said cover includes a plurality of vertical notches for receiving said plurality of vertical ribs.

8. The pallet of claim 1, wherein said upper and lower plates have respective outer edges, said outer edges of said upper and lower plates including a chamfer.

9. The pallet of claim 9, wherein said cover includes an outer corner section configured to engage said chamfers in said upper and lower plates.

10. The pallet of claim 1, further including an RFID tag disposed in the hollow interior space of at least one of said corner pieces.

11. The pallet of claim 1, wherein each said bottom frame member is directly connected to the next adjacent bottom frame member along a truncated miter joint,.

12. The pallet of claim 1, wherein said top includes a backbone disposed directly underneath each of said decking members.

13. The pallet of claim 1, wherein said bottom side includes a first inner bottom frame member and a second inner bottom frame member, said second inner bottom frame member perpendicularly bisecting said first inner bottom frame member at a bottom intersection joint.

14. The pallet of claim 1, further including a plurality of spacer blocks, each said spacer block extending perpendicularly between said top and bottom sides for holding said top and bottom sides in generally parallel spaced and aligned relation to one another.

15. The pallet of claim 14, wherein each said spacer block includes at least one internal web.

16. The pallet of claim 14, wherein four of said spacer blocks comprise exterior spacer blocks extending directly between two opposing said outer top and outer bottom frame members.

17. The pallet of claim 16, wherein said exterior spacer blocks having a generally D-shaped profile in cross-section, said D-shaped profile of said exterior spacer blocks including a convex portion oriented outwardly from said pallet.

18. The pallet of claim 14, wherein said top includes a backbone disposed directly underneath each of said decking members, wherein said bottom side includes a first inner bottom frame member and a second inner bottom frame member, said second inner bottom frame member perpendicularly bisecting said first inner bottom frame member at a bottom intersection joint, and wherein one of said spacer blocks comprises an interior spacer block, said interior spacer block extending directly between said backbone and bottom intersection joint.

19. The pallet of claim 18, wherein said interior spacer block has a generally rectangular profile in cross-section.

20. A metallic pallet of the type for supporting objects for storage and transport, the pallet comprising:
a generally planar top side having a substantially rectangular configuration defining four corners, said top side including four elongated outer top frame members, said top side including a plurality of decking members arranged parallel to one another and parallel to two of said outer top frame members, said top including a backbone disposed directly underneath each of said decking members;
a generally planar bottom side having a substantially rectangular configuration defining four corners, said bottom side including four elongated outer bottom frame members, each said bottom frame member directly connected to the next adjacent bottom frame member along a truncated miter joint, said bottom side including a first inner bottom frame member and a second inner bottom frame member, said second inner bottom frame member perpendicularly bisecting said first inner bottom frame member at a bottom intersection joint;
a plurality of extruded aluminum spacer blocks, each said spacer block extending perpendicularly between said top and bottom sides for holding said top and bottom sides in generally parallel spaced and aligned relation to one another such that said corners of said top side are vertically aligned over said corners of said bottom side in corresponding pairs and said outer top and outer bottom frame members are arranged in opposing pairs, each said spacer block including at least one internal web, four of said spacer blocks comprising exterior spacer blocks extending directly between two opposing said outer top and outer bottom frame members, said exterior spacer blocks having a generally D-shaped profile in cross-section, said D-shaped profile of said exterior spacer blocks including a convex portion oriented outwardly from said pallet, one of said spacer blocks comprising an interior spacer block, said interior spacer block extending directly between said backbone and bottom intersection joint, said interior spacer block having a generally rectangular profile in cross-section;
four generally hollow corner pieces, each said corner piece extending between a respective aligned pair of said corners of said top and bottom sides, said corner pieces each interrupting the generally planar top and bottom sides such that said top and bottom sides do not overlie said corner pieces, at least one of said corner pieces including an RFID tag in the hollow interior space thereof; and14 each said corner piece comprising a pair of adjoining perpendicular back walls in direct abutting contact with said top and bottom sides, each said corner piece further comprising an upper plate and a lower plate respectively affixed to said back walls, said upper plate generally planar with said top side, said lower plate generally planar with said bottom side, at least one vertical rib disposed in each said corner piece and extending between said upper and lower plates, said upper and lower plates having respective outer edges, said outer edges of said upper and lower plates including a chamfer, each said corner piece including a cover, said cover configured to simultaneously engage said back walls and said upper and lower plates, said cover being fabricated from a non-metallic material, said cover including at least one notch for receiving said at least one vertical rib, said cover including an outer corner section configured to engage said chamfers in said upper and lower plates.