Smart Choices for Composite Manhole Covers
Posted / Last update: 01-02-2000
The technology of composite manhole covers has advanced so far that customers now have legitimate alternatives to steel manhole covers in certain applications.
Since 1984, thirty composite manhole cover products have been introduced to the petroleum market. The Young Group’s James E.
Composite manhole covers were introduced into the marketplace for petroleum service applications in 1984. Since then, total worldwide sales of composite manhole covers have not exceeded the number of steel manhole covers sold each year, but that is about to change. This article provides an analytical framework for evaluating steel and composite manhole covers and deciding on the best alternatives for given uses.
Uncle Sam’s research pays off
Research on composite materials by, or for, the US military dates back to the 1960s and has led to today’s use of composite materials in the transportation and recreational vehicle industries, as well as in the construction of highway bridges and other structures. To replace metal materials for these applications, the composite material has to be lighter and at least as strong.
The petroleum industry has benefited from the development of composite materials, using them in pipes, valves, fittings, underground tanks, manhole covers and other components. The plastics companies that created the original market for plastic moldings have given composite manhole cover manufacturers access to the resin-transfer-molding technology.
Since 1984, approximately 30 different designs of composite manhole covers have been introduced to the petroleum marketing industry. At least half were not successful, and others had to be redesigned and changed. Today, there are a number of different composite manhole covers on the market that have stood the test of time. How do you tell the good ones from the not-so-good ones? How do you select the one that best meets your needs at the lowest cost? Would a steel manhole cover still be the best choice for your situation?
Safety and efficiency
Two important considerations in assessing manhole covers are their safety and efficiency. The safety and efficiency of manhole covers in the petroleum marketing industry are controlled by three critical design characteristics:
Strength—Stresses resulting from vehicle traffic loading should not exceed the design strength of the manhole cover.
Flatness—Every time the vehicle loading weight approaches or exceeds the manhole cover’s design strength, the material “yields.” This can cause the center of the manhole cover to be permanently “dished,” or to incur what engineers call “permanent set.” Each instance of similar loading results in further permanent center dishing, which can exceed safe levels. I discuss how to determine an unsafe condition later in this article.
Weight—If the cover is too heavy for one person to remove and reposition easily, maintenance and other tasks requiring access to the manhole can become inefficient and unsafe.
Finding out the weight of a manhole cover and deciding what weight would be most suitable for your purposes is relatively easy. But what standards or criteria are there for evaluating whether a given manhole cover is strong enough to hold up under the vehicle loading weight (see Figure 1) without failing or permanently dishing?
|Figure 1: Manhole covers must hold up under weight exerted by each set of dual-tire wheels that run over them. Courtesy of EBW, Inc.||
Searching out the standards
There are no clearly established rules, regulations or standards in the US governing the manufacture of manhole covers used in petroleum marketing as such. But, as discussed in detail in a previous article I wrote for PE&T, there are some general standards in the US and some more specific standards in Europe that provide guidance on the performance and safety of manhole covers (see “What Makes Steel Manhole Covers Unsafe?,” Feb. 1999, p. 27). In my opinion, these standards should be used carefully as a guide in the design of manhole covers. By the same token, the standards provide sound criteria for deciding which manhole covers to purchase.
In the US, Standards Specifications for Highway Bridges, published by the American Association of State Highway and Transportation Officials, details the criteria for highway traffic loading situations. That publication cites another document which should also be consulted: United States Federal Specification RR-F-621 (frames, covers, gratings, steps, sumps and catch-basin manholes). Together, these two documents provide loading criteria that should be used in the design of manhole covers and in evaluating whether the criteria have been met.
Under that criteria, the maximum legal axle load on US highways is 32,000 pounds, or 16,000 pounds for each set of dual-tire wheels. This means that every manhole cover subject to truck traffic must be designed to withstand up to 16,000 pounds of loading. The US criteria do not include an allowance for permanent center deflection from such loading. This is because the US standard was written with only cast-iron manhole covers in mind. Cast-iron manhole covers crack and fail once the loading exceeds their load design limit, and as a consequence, permanent center deflection is not an issue. In the case of steel and composite manhole covers, permanent center deflection can occur even though the cover does not crack or fail under loading. As discussed in the following paragraph, the European standard includes criteria not only for total loading weight, but also for maximum permanent center deflection.
In Europe, standards for manhole covers are more clearly established than in the US, but still require careful application. European standard BS EN 124:1994 covers design requirements, type testing, marking and quality control for gully tops and manhole tops for vehicular and pedestrian areas. This standard recognizes that different designs and materials are used to make manhole covers. The standard defines six classes from which to choose the weight loading requirement; it is the responsibility of the designer to select the appropriate class for manhole covers used in the petroleum marketing industry.
According to European officials, classification “B125” is the appropriate classification under the standard for manhole covers at petroleum service stations. B125 calls for test loading of 18,827 pounds, five different times for 30 seconds each. After each such loading, the permanent deflection of the geometric center of the manhole cover is measured (see Diagram A). The European standard limits acceptable permanent center deflection to 1/100 of the cover’s diameter. For a 39.5-inch diameter manhole cover, this would be 0.395 inches.
The European standard is now under review, but no date is set for revisions. Even though some manufacturer literature indicates that the standard has been modified, a British Standards Institution official advised me by letter about two months ago that this was not the case. Although some composite manhole covers are being designed to a specification under which the loading weight is twice the 18,827 pounds called for under classification B125 of the European standard, British officials—as stated above—advise that B125 is the appropriate classification for service station applications.
I do not believe that future changes in the 1994 standard will include changes in loading requirements for manhole covers. Vehicle weight limits allowed on roadways probably will not change. I believe that the testing procedure in the 1994 standard for classification B125 is fair and should continue to be used. Among other benefits, conformance to the standard improves the safety of manhole covers.
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