Contributed by Fibrelite Composites Ltd.
What do a multi-million-dollar America’s Cup sailboat and a lowly manhole cover have in common? Answer: They are both examples of load-bearing structures that have been improved through the use of composite materials.
In the case of America’s Cup sailboats, their hulls must be designed to be as light as possible while still being capable of handling the stresses created by high winds and pounding ocean seas. This strength-to-weight analysis must take into consideration the tremendous loads experienced when sailing offshore. Every time a sailboat hull slams into a wave, the forward sections of the hull are pummelled by walls of dense salt water. Likewise, every time a sailboat plunges down off the crest of a wave and hits the trough at the bottom, the spars and rigging continue to move forward, putting tremendous strain on the hull. The combined loading caused by the weight of the water and the rigging is equivalent to the loads experienced by a suspension bridge that must support not only the roadway loading but also the pull of the suspension cables.
In the 1960s, sailboat designers started to experiment with composite materials such as fiber-reinforced plastic (commonly known as FRP or fiberglass) as a material for building hulls because fiberglass offered a much lighter alternative to traditional materials such as wood and aluminum. However, while the lighter boats would be faster, the fiberglass hulls would need to hold up to the tremendous forces and loads involved in offshore racing.
While early designs were prone to cracks and stress fractures, by 1986 a leading sailboat designer predicted that by the end of the decade “all of the syndicates involved in the America’s Cup will have FRP boats” (Orlando Sentinel, 1986). During the last 30 years, sailboat designers have made composite materials essentially the only type of material that is used for offshore racing hulls. Any early concerns with the strength or durability of composite hulls have been overcome and composite materials are now undeniably the most prevalent material used in boatbuilding.
Just like that multi-million-dollar America’s Cup sailboat, composite materials have been used to make the “lowly” manhole cover lighter, safer, and essentially a high-tech version of its former self while still retaining the capacity to bear the wheel weight of a fully loaded city bus.
Typically, manhole covers installed in roads, sidewalks, and parking lots are made of cast iron. Cast iron is simply iron which has been heated until it liquefies and then poured into a mold and allowed to solidify into a useable product. In particular, ductile cast iron (cast iron with added magnesium) has been used for manhole covers and frames since the mid 20th century due to its durability and high compressive strength. The engineering community, and in particular engineers designing underground infrastructure, rarely considered any reason to use an alternative material than cast iron for manhole covers or related products.
High costs of occupational injuries
This view began to change in the 1980s as companies began to be concerned about the high costs of occupational injuries. In addition to their social costs, workplace injuries have a major impact on an employer’s bottom line. It has been estimated that employers pay more than $1 billion per week for direct workers’ compensation costs alone (ASSE, 2002). As manhole covers and trench panels are used to access underground systems and piping, they are a key component in the infrastructure of many municipalities and numerous industries including electrical utilities, telecommunications, energy companies, wastewater treatment, and water distribution. Given their prevalence in underground infrastructure, it is obvious that access covers are handled frequently by individuals involved in maintaining such systems.
While cast iron manhole covers provide for a highly durable, strong product capable of standing up to heavy loads and severe impact, the weight of cast iron creates its own problems. The weight of a cast iron manhole cover varies but is typically around 450 pounds per cubic foot. As a result, even a small 32-inch-diameter cast iron manhole cover can weigh as much as 250 pounds.
When a manhole cover weighing that much is removed using traditional tools such as a crowbar or pick, it has the potential to cause a multitude of injuries to the individual or individuals attempting to remove it. The types of injuries caused by heavy municipal castings are numerous and can be quite severe – lacerations, amputations of fingers and toes, broken bones, repeated muscle strains and sprain, wrenched backs, and repetitive trauma caused by improper lifting techniques.
One of the first industries to explore alternatives to cast iron manhole covers was major oil companies operating gasoline service stations. In an effort to reduce employee injuries, these companies became interested in replacing their cast iron covers installed above underground gasoline storage tanks with lighter composite covers. The covers used at a typical gasoline station or fueling facility are removed often to fill the tanks with fuel and for inspection and maintenance. Because of the frequency of removal and replacement, employees and contractors at the gasoline stations experienced a significant number of occupational injuries caused by handling the heavy cast iron covers. These injuries typically included hand and foot trauma, back pain, and muscle and ligament strains.
Fibrelite Composites was one of the first companies to respond to the needs of the retail petroleum industry with a lightweight composite cover designed to be removed and replaced with a lifting handle that allowed the cover to be pulled out of the frame. Working with structural and composites engineers from one of the leading universities in the United Kingdom, Fibrelite designed a fiberglass cover that could support the same wheel loads as a 32-inch cast iron manhole cover while weighing one third of the weight or less. The lightweight cover and ergonomic method of removal and replacement eliminated the possibility of injury to the employees or individuals handling the covers. As Fibrelite’s composite covers offered a superior technical design that provided the best strength-to-weight ratio in the industry, Fibrelite quickly became the leading provider of composite manhole covers to oil companies and gasoline service stations.
The limitations of cast iron extend beyond the heavy weight of a manhole cover. One of the primary issues challenging engineers regarding cast iron is its ability to conduct electricity and heat. The electrical and thermal conductivity of metals is high and, as manhole covers are likely to come into contact with humans, this ability to conduct electricity and heat is a negative attribute.
In cities, stray voltage issues and hot manhole covers have become a significant concern for utilities. In 2004, a woman was electrocuted to death after stepping on an electrified metal manhole cover while walking her dog in New York City (Ramirez, 2004). There are also numerous incidents of pedestrians being burned by hot manhole covers, including a well-publicized “branding” in which a large utility company’s logo was allegedly burned onto the victim’s back (New York Post, 2004).
In an effort to improve the safety of their systems, utilities began looking for an alternative to cast iron covers for electrical and steam vaults. Composite fiberglass products were given consideration because fiberglass has a conductivity approximately 150 times less than cast iron and is categorized as an insulator (no ability to conduct heat or electricity). Any heat or electrical voltage coming into contact with a composite manhole cover or frame will not be transmitted to a pedestrian or company employee touching the product. A well-designed composite fiberglass cover capable of handling wheel loads is therefore a perfect antidote to the problems of hot or electrified manhole covers.
Composite covers are also extremely resistant to corrosion caused by salts, oils, water, and steam. This is extremely important for municipalities and utilities in colder climates where road salt is routinely applied during the winter months. It is also critical for industries that are transmitting or storing corrosive liquids such as wastewater, steam, or even sulfuric or hydrochloric acid. Certain types of composite materials are highly resistant to these types of corrosive liquids and can therefore extend the working life of a manhole or access cover used in such environments.
EMI and RF signals pass through
There are a number of high-tech applications requiring composite covers that are being driven by the need to transmit data or energy through the surface of a cover. In addition to being far less conductive than metal or cast iron covers, composite manhole covers also allow electromagnetic (EMI) or radio frequency (RF) signals to pass directly through them. Metal or cast iron covers will shield such signals and therefore will prevent any transmission of energy or data. By allowing EMI or RF energy to pass through the cover, composite covers are now allowing engineers to install sensors and other devices inside a manhole that can then communicate to a receiver or network located aboveground.
Composite covers are also being used in wireless electric vehicle charging systems that are designed to be installed underground in parking lots. These systems will transmit power to the car battery parked overhead via an electric coil placed inside a composite manhole cover. This method of transmitting power allows for much faster vehicle charging than traditional cable charging as there is no power lost due to the resistance of the cable.
Metal theft epidemic
One of the biggest potential applications for composite covers is being driven by poverty in both third world countries and in big cities in developed countries such as the United States. Cast iron and steel manhole covers and drainage grates are routinely stolen for the scrap value of the metal cover or grate. This problem is widespread around the globe and generally worsens when the price of scrap metal is high.
Last year, more than 900 manhole covers disappeared in nine months in the city of Birmingham in the U.K. by people brazenly posing as city workers. In the Indian city of Calcutta, the problem of manhole covers disappearing became so severe at one point that the government started making the covers from concrete, according to a report in the Telegraph India (www.telegraphindia.com/1040907/asp/calcutta/story_3724114.asp). But thieves took them anyway, and cashed in on the steel rebar embedded inside. Even in New York City, thieves are willing to risk injury pulling heavy cast iron manhole covers out of the street to convert them into cash at the smelter (McGeehan, 2012).
Based on current commodity prices for iron, a stolen cast iron manhole cover might fetch as much as $30 to $40 at a recycler. But it costs a utility at least $300 to replace each stolen cover and that doesn’t include the cost of labor required to replace the cover (McGeehan, 2012). And while the financial cost of replacement is quite high, the thefts also lead to open manholes which create significant hazards in the roadway for drivers and on the sidewalks for pedestrians. Composite covers can solve this problem; the fiberglass or composite material used in composite covers has no inherent scrap value. As thieves cannot convert composite covers into quick cash at the recycling yard, they will have no desire to steal such covers.
While not every application for manhole covers currently requires a composite alternative, the technical challenges of a safer and higher-tech world are causing the designers of underground infrastructure to begin to consider their use. Companies such as Fibrelite are continuing to develop new composite materials that will make manhole covers lighter, stronger, and safer while end users continue to find new reasons to move away from the traditional cast iron products. As composite manhole covers gain acceptance in the engineering community, they may one day be as commonplace in the sidewalk as a fiberglass sailboat is on the water.
This article was contributed by Fibrelite Composites Ltd. (www.fibrelite.com).
- Orlando Sentinel, 1986, “America’s Cup Has New Controversy – Fiberglass Debate Causing Hard Feelings as Challenger Semifinals Near,” Dec. 21.
- American Society of Safety Engineers (ASSE), 2002, White Paper on the Return on Investment for Safety, Health, and Environmental (SH&E) Management Programs, June 8, page 1.
- Ramirez, Anthony, 2004, "East Village Woman Was Electrocuted on Street With Metal Plate, Medical Examiner Says" (www.nytimes.com/2004/01/19/nyregion/east-village-woman-was-electrocuted-street-with-metal-plate-medical-examiner.html), New York Times, Jan. 19.
- New York Post, 2004, “Skateboarder Falls onto Red-Hot Manhole,” Aug. 16.
- McGeehan, Patrick, 2012, “Manhole Lid Theft Is On the Rise,” New York Times, May 3.