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Understanding Cover Load Ratings

Understanding Cover Load Ratings

March 13, 2023

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The American Association of State Highway and Transportation Officials (AASHTO) serves as a prominent standards-setting organization responsible for the publication of specifications, testing protocols, and guidelines employed in the domain of highway design and construction across the United States. Among its multifaceted contributions, AASHTO produces the Standard Specification for Highway Bridges, which encompasses provisions pertaining to load-bearing criteria for manhole covers. In this context, AASHTO has instituted specific weight categories to govern load limits for ordinary vehicular traffic, which are as follows:

H20/HS20 = 16,000-pound wheel load, 32,000-pound axle
H20/HS20 = 16,000-pound wheel load, 32,000-pound axleH25/HS25 = 20,000-pound wheel load, 40,000-pound axle
H25/HS25 = 20,000-pound wheel load, 40,000-pound axleCovers designed to fulfill the fundamental load requirements necessitate proof load testing to meet these specified minimums. In cases where a specification mandates H20/HS20 loading, it implies that the cover must satisfy the design load criteria of 16,000 pounds per wheel without the inclusion of a safety factor.

The General Services Administration Specification RR-F-621 represented the initial comprehensive specification introduced by the federal government, directly addressing the performance standards for construction castings. This specification originally mandated that a casting sustain a 25,000-pound load applied to the center of the casting via a 9-inch by 9-inch pad (or platen). Subsequently, this specification has been reclassified as Commercial Item Description (CID) A-A-60005. It's essential to note that there is no direct correlation between this proof test and the H20 design load.

AASHTO M306, initially published in 1989 and subsequently extensively revised in 2005, imposes a 2.5 safety factor in the proof load testing of castings. For H20/HS20 loads, the casting is obliged to withstand a 40,000-pound proof load centered on the casting, applied via a 9-inch by 9-inch pad (platen), thereby exceeding the design load of 16,000 pounds by a 2.5-fold safety factor. Conversely, for H25/HS25 loads, the casting is required to endure a 50,000-pound proof load in the center of the casting through a 9-inch by 9-inch pad (plate), offering a 2.5 times safety factor over the design load of 20,000 pounds.

It is important to recognize that the presence of a specific load requirement within a cover specification does not necessarily imply that the product in question has been subjected to testing with a safety factor exceeding the specified load. Rather, it indicates that the product may have been tested to a proof load that aligns with the required load stipulated in the specification, without incorporating an additional safety factor.

For jurisdictions or specifiers seeking assurance that a casting or cover has undergone testing with a minimum 2.5 safety factor over a particular load requirement, the inclusion of a requirement for AASHTO M306 proof load alongside the desired load requirement is a straightforward approach. For instance, one may specify, "Covers must adhere to AASHTO M306 H20/HS20 Traffic Rating."

Alternatively, the Canadian standard governing grease interceptors, CSA B481, offers specific load ratings and test methodologies for covers under B481.0 section 6.1. Grease interceptor covers and top rims are rated in accordance with the table provided.

Table 1: Load ClassificationClause 6.1.4 of this section describes Clause 6.1.4 within this section outlines the protocol for determining load failure, specifying that the load at which failure occurs shall be the lowest of the following:

a) For materials exhibiting brittleness, the load at which the first fracture emerges on any part of the cover or rim.

b) For materials demonstrating ductility, the load at which the maximum deflection of the cover exceeds 5% of its largest transverse dimension while under load.

c) The load at which a permanent set is incurred (at the point of loading) after the load has been removed, exceeding 2% of the largest transverse dimension of the cover.

Clause 6.1.5 delineates the procedure for calculating the maximum safe live load. It prescribes that the safe live load of the cover and rim should be determined by dividing the load at failure established in Clause 6.1.4 by two.

Clause 6.1.6 elucidates the methodology for ascertaining load classifications for cover/rim configurations. The cover/rim load rating is to be classified by matching the safe live load of the cover/rim, as determined in Clause 6.1.5, with the load classification safe live load values listed in Table 1. The assigned rating should correspond to the load classification featuring a safe live load equal to or less than that of the cover/rim. For example, if a cover/rim experiences failure at 3,000 kg (6,600 pounds), resulting in a maximum safe live load of 1,500 kg (3,300 pounds), it will be categorized as medium duty. This classification is based on the next smaller safe live load indicated in Table 1, which stands at 900 kg (2,000 pounds) for the medium duty load category.

What is a Platen?

A "platen" denotes a flat plate designed to accommodate the application of pressure. Notably, covers designated for indoor use must undergo loading tests at room temperature (20oC ± 5o) (68oF ± 9o). In contrast, for outdoor installations, covers must be subjected to testing at both the minimum and maximum ambient air temperatures, in accordance with the manufacturer's specifications. These covers are then subjected to failure testing, with their load ratings determined by dividing the load at failure by two. For instance, if a cover/rim exhibits failure at 3,000 kg (6,600 pounds), resulting in a maximum safe live load of 1,500 kg (3,300 pounds), it would be classified as medium duty. This classification aligns with the next smaller safe live load listed in Table 1, which designates 900 kg (2,000 pounds) as the safe live load for the medium duty category. It's worth noting that this approach provides a minimum two-times safety factor for the safe live load, as opposed to the 2.5 times factor mandated in AASHTO M306.


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