Understanding Cover-Load Ratings
The American Association of State Highway and Transportation Officials (AASHTO) is a standards’ setting body which publishes specifications, test protocols and guidelines which are used in highway design and construction throughout the United States. Among other things, AASHTO publishes the Standard Specification for Highway Bridges, which addresses load bearing requirements for manhole covers. AASHTO has established the following categories for weight limits for regular vehicular traffic:
H20/HS20 = 16,000-lb wheel load, 32,000-lb axle
H25/HS25 = 20,000-lb wheel load, 40,000-lb axle
Covers designed to meet the basic requirements for these loads need only be tested with a proof load to meet these minimums. If a specification requires H20/HS20 loading it simply requires a cover meet the design load of 16,000 pounds per wheel without a safety factor.
The General Services Administration Specification RR-F-621, was the first widespread specification put out by the federal government that directly addressed the performance of construction castings. It required a casting maintain a 25,000-lb load in the center of the casting by a 9-inch by 9-inch pad (or plate). This specification has been renamed Commercial Item Description (CID) A-A-60005. There is no direct correlation between this proof test and the H20 design load.
AASHTO M306, first published in 1989 and totally revamped in 2005, requires castings have a 2-1/2 times safety factor in a proof load test. For H20/HS20 loads the casting must maintain 40,000-lb proof load in the center of the casting by a 9-inch by 9-inch pad (plate), providing a 2-1/2 times safety factor over the design load of 16,000 pounds. For H25/HS25, the casting must maintain a 50,000-lb proof load in the center of the casting by a 9-inch by 9-inch pad (plate), providing a 2-1/2 times safety factor over the design load of 20,000 pounds.
A specification with a specific load requirement for the cover does not mean that the product provided will have been tested with a safety factor over the specified load. It doesn’t mean that the cover provided has not been tested to a proof load that meets the required loading of the specification, it simply means that the product may not have been tested with a safety factor.
For jurisdictions or specifiers that want to ensure that a casting or cover has been tested to a minimum 2-1/2 times safety factor over a specific load requirement, simply include a requirement for AASHTO M306 proof load with the specific load requirement desired. For example, “Cover’s must meet AASHTO M306 H20/HS20 Traffic Rating.”
Alternatively, the Canadian standard that governs grease interceptors, CSA B481, contains specific load ratings and test methods for covers under B481.0 section 6.1. Grease interceptor covers and top rims are to be rated in accordance with the following table:
A platen is a flat plate upon which pressure may be applied. For covers intended to be installed indoors only, the loading test must be at room temperature (20 oC ± 5o) (68 oF ± 9o). For outdoor installations, covers must be tested at both the minimum and maximum ambient air temperatures (as specified by the manufacturer). Covers are then tested to failure and their load rating assigned based on dividing the load at failure by two. For example, if a cover/rim fails at 3,000 kg (6,600 lb), then its maximum safe live load is 1,500 kg (3,300 lb) and it will be rated medium duty since the next smaller safe live load listed in Table 1 is 900 kg (2,000 lb) for the medium duty classification. This provides a minimum two-times safety factor for the safe live load, instead of 2-1/2 times, which is required in AASHTO M306.