Metallurgical Failure Analysis of a Gas Station Canopy Collapse

A gas station canopy collapsed during a heavy rain storm, which resulted in injuries to those below. The subsequent investigation revealed that one of the supporting purlins had corroded to the point where holes could be seen through the steel. This corrosion was also observed on other purlins.

The purlins had been in place for approximately twenty years with only a shop primer coating for protection. Employees had complained about canopy leaks for months prior to the collapse. In addition, three years prior, a company had surveyed the roof fascia for a brand name change but saw no corrosion damage. Two hurricanes subsequently hit the area, all resulting in damage which was repaired. The fascia was replaced 15 months prior to the accident, again with no existing corrosion being observed. The contractor that surveyed the roof 3 years earlier was sued for negligence in failing to address the extensive corrosion damage.

Examination of the purlins after the accident revealed a consistent water line of corrosion attack on the bottom of the purlins. This meant that several inches of standing water had been present in the canopy which was not designed for this type of loading.

The plaintiff provided an expert witness who argued using published atmospheric corrosion rate tables that the corrosion should have been observed three years earlier. They concluded that the contractor should have seen and reported the corrosion damage. Metallurgical Consulting performed DC corrosion tests with locally collected rain water to determine actual corrosion rates. To duplicate the environmental conditions that the purlins experienced, 1) actual samples of the purlins were used, 2) a small amount of salt was added to the water to imitate residual salt left from the hurricane, and 3) the water was heated to 100°F to duplicate probable day time temperatures.

These corrosion tests yielded corrosion rates more than adequate to perforate an undamaged purlin over the time of the reported leaks. The canopy drains had become clogged possibly because of hurricane blown debris, resulting in standing water increasing the corrosion damage, which ultimately caused the purlins to buckle under the weight of the water.

For more information on this project, contact Metallurgical Consulting directly.

Images From The Metallurgical Services Provided For This Structural Collapse Investigation

(click on thumbnail to enlarge)

Figure 1: A line of severe pitting can be seen on an adjacent buckled purlin. The pitting line shows the purlin had been bowed under the weight of standing water prior to failure.

Figure 2: A line of pitting corrosion on the most corroded purlin can be seen on the purlin indicating standing water.

Figure 3: A better picture of perforation of the bottom flange is shown.