As documented in “ATLANTA BRIDGE COLLAPSE: What burned under the I-85 overpass?” by Richard Elliot and Rikki Klaus of wsbtv, the material stored under the I-85 bridge for 10 years was High Density Polyethylene. As I discuss what that means fire-safety-wise, I’ll be generally talking about polyethylene.
Polyethylene is constructed from the combination of ethylene monomers into a solid material. “Ethylene monomer is a gas with molecular weight of 28. At 6 mer units the product is a liquid, at 35 mer units the material is a grease, at 140 mer units a wax, and at 430 mer units it has a molecular weight of 12,000 and is in the form of a hard resin. Further increases in the number of mer units result in higher molecular weights, which affect the processability, physical and mechanical properties, and the chemical resistance”.
Such notable disasters as Bhopal, India in 1984, Phillips Petroleum Company in Pasadena, TX in 1989, BASF in Cincinnati, OH in 1990, and IMC in Sterlington, LA in 1991 illustrate the dangers of this material in its more basic forms.
Polyethylene is a tough thermoplastic with “excellent chemical resistance and electrical insulating quality; near zero moisture absorption; used for packaging, molded housewares, and toys”.
Thermoplastics present a fire hazard by acting and burning like flammable liquids when melted and/or depolymerized. Depolymerization is one of the by-products of decomposition. “When containers of these materials are subject to the heat of a fire, the decomposition or depolymerization becomes rapid, and the lower molecular weight oligomer fractions act like flammable liquids. With these materials, storage areas with high sprinkler discharge densities over relatively large areas are necessary to protect these types of plastics…Plastics should be stored, handled, and piled according to their fire characteristics.”
High Density Polyethylene is around 95 kg/m^3 in density, has an upper service temperature of 240 degrees Fahrenheit (115 deg C) under no-load conditions, and a combustibility index near 25 (combustibility ratio: “the rate of heat release is proportional to the ratio of its heat of combustion to its heat of vaporization”).
Based on the properties described above, polyethylene is classified as a Group A plastic. Depending on its method for storage, materials with this classification fall among a variety of Commodity Classes. “A Class III commodity shall be permitted to contain a limited amount (5 percent by weight or volume or less of Group A or Group B plastics…A Class IV commodity shall be defined as a product with or without pallets, that meets one of the following criteria:…(2) Consists of free-flowing Group A plastic materials, (3) Contains within itself or its packaging an appreciable amount (5 percent to 15 percent by weight or 5 percent to 25 percent by volume of Group A plastics.”
So given that polyethylene is a Group A plastic whose Commodity Class could be either III or IV, the specific storage conditions found under the I-85 bridge determine how they should be protected.
 NFPA Fire Protection Handbook
 NFPA 13 Standard for the Installation of Sprinkler Systems