A cover for in-ground meter enclosures used in traffic loading conditions employs a relatively thin plate member formed of a ductile iron alloy and is dimensioned to extend laterally across a top opening of an in-ground meter enclosure. A plurality of relatively thick rib members of the ductile iron alloy are formed with and extend generally normal from the bottom side of the plate member. The plate member and the rib members together have sufficient strength and ductility to withstand total loads on the order of at least about 20,000 pounds on the top side of the plate member.

CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and incorporates by reference application Ser. No. 09/376,135, filed Aug. 17, 1999, now U.S. Pat. No. 6,116,813 which is commonly owned with the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to enclosures for in-ground meters for both residential and commercial use. In particular, this invention relates to covers for such meter enclosures that are designed for use under traffic loading conditions, and a method for making those covers. 
     2. Description of the Prior Art 
     On-site water meters are commonly used to permit a utility to determine the amount of water usage by a customer. Typically, the water meter is located in a meter enclosure extending into the ground adjacent the customer's residence or commercial building, to permit easy access by the utility meter-reader. The in-ground meter enclosure may be located in an environment not subject to traffic loading conditions (i.e., where automobiles, trucks and other traffic are not likely to pass across the top of the meter enclosure) or may in fact be exposed to potential traffic loading conditions. In the past, it has been customary for the water utility to maintain two inventories of covers for meter enclosures, one for use in non-traffic conditions and the other for situations where the meter enclosure may be exposed to traffic. Meter enclosure covers for non-traffic use are generally inexpensive, lightweight and not capable of sustaining traffic loads. On the other hand, meter enclosure covers for use in traffic loading conditions are typically made of high-grade steel and are generally substantially more expensive than the first type described previously. But maintaining two different inventories for such meter covers can be quite expensive. Further, while a meter installation may initially be perceived to be in a non-traffic environment, circumstances may change and the meter cover later subjected to traffic loading. In that situation, the meter cover may be broken, creating possible personal injury hazards. Thus, there is a need for a meter enclosure cover which is relatively low in cost, but is capable of withstanding significant traffic loads. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an in-ground meter enclosure cover for use in potential traffic loading conditions, and comprises a relatively thin plate member having a pattern of interconnecting rib members extending from the bottom side and including a peripheral rib member. The plate member and the interconnecting and peripheral rib members are dimensioned and fabricated from a suitable material, preferably a ductile iron alloy, such that the composite construction has sufficient structural strength to withstand total loads of at least about 20,000 pounds on the top side of the plate member. 
     In the preferred embodiment, the plate member and the rib members are cast together as a unitary cover, with unitary cast joints between the intersections of the rib members. 
     The cover further comprises a meter access opening extending through the plate member between the top and bottom sides and intermediate between adjacent interconnecting rib members, with a meter access lid pivotally positioned in the meter access opening. 
     Preferably, the cover is elongated in one direction, with the rib members including interior ribs comprising at least two longitudinal ribs extending parallel to the direction of cover elongation and at least three lateral ribs extending generally normal to, and intersecting with, the longitudinal ribs at spaced unitary cast joints. The rib members all have a dimension extending normal from the bottom side which is substantially greater than the dimension of the plate member between the top and bottom sides. By way of example, the peripheral rib has a dimension from the bottom side of the plate member which is on the order of at least six times the thickness of the plate member; similarly, the interior ribs have a dimension extending from the bottom side of the plate member which is on the order of at least four times the thickness of the plate member. Preferably, a portion of one of the elongated ribs is substantially thicker adjacent to and generally parallel with the bottom side of the plate member along the meter access opening, and includes a recess adapted to support the meter access lid when closed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other important features of the present invention are described below and are illustrated in the accompanying drawing, in which: 
     FIG. 1 is a top plan view of a meter enclosure cover in accordance with the present invention, in which a portion of the top side is cut away to illustrate details of the meter access lid and opening. 
     FIG. 2 is a bottom view of the enclosure cover shown in FIG. 1, with the meter access lid removed. 
     FIG. 3 is a cross section of a portion of the cover shown in FIG. 2, taken along the lines  3 — 3 . 
     FIG. 4 is a cross section of a portion of the cover shown in FIG. 2, taken along the lines  4 — 4 . 
     FIG. 5 is a top plan view of the meter access lid shown in FIG.  1 . 
     FIG. 6 is a cross sectional view of an alternate embodiment of the enclosure cover of FIGS. 1 and 2, with the cover shown installed in a conventional in-ground meter enclosure. 
    
    
     DETAILED DESCRIPTION 
     A detailed description of covers in accordance with the present invention useful with in-ground meter enclosures in potential traffic loading situations will now be described with reference to FIGS. 1-6. 
     First noting FIGS. 1-5, a first embodiment of the cover  10  includes a plate member  12  having a top surface  14  and a bottom surface  16 . The plate member is defined by opposing elongated and parallel major sides  18 ,  20  and opposing parallel minor sides  22 ,  24 . 
     Referring now to FIG. 2, the cover  10  further includes a system of rib members extending generally perpendicular from the bottom side  16 , including a peripheral rib  25  which extends completely about the sides  18 ,  20 ,  22  and  24 . The system of rib members also includes plural intersecting rib members interiorly of the peripheral rib  25 , including at least two spaced longitudinal ribs  26 ,  28  extending generally parallel with the major sides  18 ,  20  between the minor sides  22 ,  24  and at least three lateral ribs  30 ,  32  and  34  extending generally parallel to the minor sides  22 ,  24  and between the major sides  18 ,  20 . Each of the interior ribs  26 ,  28 ,  30 ,  32  and  34  form unitary joints  36  at each point of intersection with other interior ribs and a unitary joint  38  at each point of intersection with the peripheral rib  25 . 
     In accordance with the present invention, the system of rib members  25 ,  26 ,  28 ,  30 ,  32  and  34  all have a dimension extending generally perpendicular from the bottom side  16  which is substantially greater than the thickness of the plate member  12 . By way of example, the peripheral rib  25  has a dimension which is at least three times the thickness of the plate member and preferably on the order of at least six times the thickness of the plate member. Likewise, the interior ribs  26 ,  28 ,  30 ,  32  and  34  all have a dimension extending from the bottom side  16  which is on the order of at least two times the thickness of the plate member  12 , and preferably on the order of at least three times the thickness of the plate member. By way of example, the plate member  12  is about one-quarter inch thick between the top side  14  and the bottom side  16 , and all of the ribs  25 ,  26 ,  28 ,  30 ,  32  and  34  have a dimension which is on the order of 1½″-1¾″ as measured from the bottom side  16 . Alternatively, the interior ribs  26 ,  28 ,  30 ,  32  and  34  may be somewhat smaller in dimension than the peripheral rib  25 , as discussed below with reference to the embodiment shown in. FIG.  6 . 
     Further in accordance with the present invention, the construction of the cover  10  just described is particularly suited for use with a relatively inexpensive ductile iron alloy material, which is capable of being cast in a unitary manner to form the plate member  12  and the system of rib members  25 ,  26 ,  28 ,  30 ,  32  and  34  as well as the unitary joints  36  and  38 . By way of example, a suitable ductile iron alloy is designated as Grade 60-40-18 under ASTM Standard A 536-84 (1993). Grade 60-40-18 ductile iron alloy has a tensile strength on the order of 60,000 pounds per square inch minimum, a tensile strength of 414 MPa minimum, a yield strength of about 40,000 pounds per square inch minimum, and approximately 276 MPa minimum, and an elongation of about 18% minimum (elongation is 2 inches, or 50 mm.). This particular ductile iron alloy is exemplary only, and other ductile iron alloys are also suitable. For example, the other ductile iron alloys listed in the previously referenced ASTM Standard A 536-84 (1993) meet the criteria for use with the present invention. 
     Referring again to FIGS. 1 and 2, the cover  10  includes a meter access opening  42  and a corresponding meter access lid  46 . The opening  42  and lid  46  are both preferably elongated in a direction generally parallel to the major sides  18 ,  20 . The lid  46  includes extending trunions  48  (note FIG. 5) which are engaged in corresponding openings in trunion recess blocks  40  (FIGS.  1  and  2 ). Closure  50  (FIG. 1) extends into the access to the trunion  49 . 
     As shown by dotted line in FIG.  2  and in FIG. 4, a recess  44  extends downwardly into a portion  28 A of rib  28  adjacent the opening  42  in order to support the forward edge of the lid  46 . Suitably, the portion  28 A of rib  28  has a thicker cross-section, to account for any loading on the forward edge of the lid  46 . The lid  46  includes an aperture  52  to permit a tool to be inserted for opening. Similarly, access apertures  54  are provided at opposing ends of the plate member  12 . 
     An alternate embodiment of the cover  110  is shown in FIG. 6, where like reference numerals are used to identify the same feature  5  as previously described with reference to FIGS. 1-5. 
     The embodiment of the cover  110  shown in FIG. 6 is essentially identical to the cover  10  shown in FIGS. 1-5, except that the dimensions of the interior ribs  126 ,  128 ,  130 ,  132  and  134  are reduced somewhat relative to the dimension of the peripheral rib  25 , in order to reduce the overall weight of the cover  110  relative to the weight of the cover  10  shown in FIGS. 1-5. It has been found that this reduction and dimension does not unduly restrict or reduce the overall strength characteristics of the cover for total loading. In fact, both the cover  10  of FIGS. 1-5 and the alternate embodiment cover  110  of FIG. 6 when constructed of the ductile iron alloy has a vertical crush capability which exceeds 20,000 pounds total, when the plate member  12  has a thickness dimension of about one quarter inch between the top and bottom sides, and with the lateral dimension of the peripheral rib  25  from the bottom side  16  being on the order of 1¾″, and tapering from about ⅜″ to about ¼″, and with a uniform thickness for the interior ribs being on the order of about one half inch and a length of 1″-1¾″. 
     As shown in FIG. 6, both the cover  10  and the alternate cover  110  are designed for installation in an in-ground meter enclosure  60  having opposing end walls  62 ,  64 , sidewall  66  and an open bottom  68  extending into the ground  69 . All of the enclosure walls including walls  62 ,  64  and  66  are provided with a peripheral cover retention shelf  70 , which is typically designed to retain the cover  10  or  110  generally flush with a top surface  72  of the enclosure  60 . A meter  78  having a cover  80  is positioned in the water line defined by water piping  82 ,  84  extending through the enclosure  60  via openings  74 ,  76 . 
     It will be appreciated by those skilled in the art that the present invention provides a novel construction of a in-ground meter enclosure cover which is relatively low in cost but which is capable of sustaining very high total loading in potential traffic conditions.