Major gasoline spills and fires often occur when top-heavy gasoline tankers roll over. The average gasoline tanker holds about 9,000 gallons, almost as much as the jets that slammed into the World Trade Center and the Pentagon.
For this reason, manholes and other fittings along the top of gasoline tankers must by law be protected from rollover damage. The regulations say that each protector must be able to support the weight of the loaded tanker. They also say that tanker weight may be distributed along the protector, even though most rollovers do not create a distributed load.
Manufacturers may prove that their guardrails meet this requirement using tests or calculations. Most have chosen to do calculations, even though the results vary widely depending on the assumptions. Therefore, we decided to test a typical guardrail; the expert who designed it admitted that it had never been tested, even though it had been in use for 20 years.
Our vertical tests showed that in the upside-down position, the guardrail is much stronger than the regulation requires. The guardrail supported more than 900 pounds of vertical weight per linear inch before it began to fail. Target strength fixed by the regulation depends on tanker weight and guardrail length; most target strengths per linear inch are less than about 180 pounds.
For the horizontal test, the results weren't so good. The guardrail met the distributed load requirement, but it folded over under a slightly greater weight.
Therefore, it is not difficult to imagine a situation like the one here, where the guardrail folded over as the tanker rolled because weight on the guardrail was not evenly distributed. When this happens, the manholes and other fragile fittings on top of the tanker are susceptible to damage, and major gasoline loss and fire can occur.