For Construction Castings, such as Manholes & Meter Pit Lids, the Meaning for “H-20” is Frequently Misunderstood
The M306 Standard
The American Association of State Highway and Transportation Officials developed a standard, M306 that calls for proof load testing for iron castings. For an H-20 proof load test, that standard requires a 40,000 lb. load be applied to a 9” x 9” square plate for one minute. Any cracks or permanent deformation greater than 1/8” are cause for rejection.
In recent years, polymer lids have been used for a variety of applications, such as:
- Water meter pits and manholes, where existing cast iron lids blocked the radio signal.
- Flow monitoring devices in manholes, where signals of rising flow levels are likewise blocked by cast iron lids.
- Theft, due to the scrap value of iron.
- In industrial and sanitary manholes to reduce finger and back injuries, due to their light weight.
How the M306 Standard Relates to Polymer Lids
Municipalities and engineers wanting a traffic rated polymer lid frequently specify an “H-20” rating in their specifications. There can be confusion since AASHTO published bridge design criteria, known as H-20, calls for truck axle loading of 32,000 lbs, or 16,000 per wheel load. The AASHTO M-306 standard calls for a safety factor of 2.5 to be applied to the 16,000 lb. wheel load. (2.5 x 16,000 = 40,000 lbs). However, for traffic rated lids, a 40,000 proof load test is required.
Explanation of the Confusion
Many water departments began to install radio read meters but couldn’t get a good read through cast iron lids. Frequently, many were only thinking of a radio friendly material such as polymer, without giving much thought to its strength. As a result, many municipalities received thin plastic lids that could only withstand a proof load test of 5,000 to 10,000 lbs. Frequently breakage occurred from lawn tractors and other loads. To be conservative and have an industry standard they can specify, many municipalities require a 40,000 proof load test, even where a lid is not in the roadway. Some suppliers advertise their lids as H-20, but are referring to a 16,000 load and not a 40,000 proof load test. Some furnish lids that cannot even meet the 16,000 load they advertise as “H-20”. To avoid confusion, specify the proof load test you require. Simply specifying “H-20” may not provide the safety factor you need, particularly where traffic loads are involved. To make sure they get what they want, experienced engineers specify a proof load test of 40,000 lbs.
How to Avoid the Confusion
To avoid confusion, your specs should require a 40,000 lb. proof load test, conducted per AASHTO M-306. To be safe, require a report from an independent test lab. The AASHTO M-306 standard requires the load be placed at the center of the lid and not transfer any of the load directly to the frame. For smaller lids, such as 12” diameter, a plate smaller than 9” x 9” must be used. If you would like a proof load test performed on the lids you are using, contact Trumbull.
Trumbull offers a variety of polymer lids rated for a 40,000 lb. proof load. Reports from independent test labs are available upon request.
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