We arrived on site that evening and immediately began our forensic investigation. What a mess! When the tank collapsed, the ensuing flood had strewn the wooden staves in a jumbled heap, like so many pickup sticks; some of the staves had been hurled through the wall of a building across the yard from the tank, while others had been carried as much as a couple of hundred feet, coming to rest against other nearby buildings.

The tank was built in about 1971 and had been 34 feet in diameter and about three stories tall, with the wood staves held in place by a series of external, 7/8-inch diameter, circumferential steel tie rods. The rods had been arranged in 75 rows, with each row consisting of four rods held together end-to-end by cast metal clevises.

 

Clevis and rod configuration

The tank, which had contained a mixture of white water and 2-percent pulp solids, had been nearly full when it ruptured. Fortunately, no one was next to the tank when it failed, although several workers had passed by it just minutes before the failure. No sign of leakage or other problems were noted prior to the failure.

Picking through the rubble, we found several fractured clevises and many that were significantly corroded. We also found a number of rods that were badly corroded, but only two that had fractured.

Corroded and fractured clevis

Over the course of the next two days, we continued examining the broken pieces as workers gradually cleared and sorted the rubble. We found that about a half dozen clevises were so severely corroded that probably the only thing that had kept them from falling off the tank was a little corrosion oxide! Another dozen or so were so corroded that they probably could only support a small fraction of their original service load.

Continuing our investigation, we studied the pattern of dispersal of the debris, noting where the roof pieces and the staves from the top of the tank had ended up and where the staves from the bottom of the tank had ended up. Gradually, we put together a picture of what had happened .

Fundamentally, the tank rupture was caused by a simultaneous or very rapid sequential fracture of several tie rod clevises that had been severely thinned as the result of long-term corrosion. The first clevises to fail were at ground level. Because one side of the tank was partially supported by the building framework (see left photo, top), the collapse initiated on the other side of the tank, at the bottom end of the staves, where they were notched into the tank floor.

Driven by the wall of water and pulp solids from the tank, the bottom staves from this side of the tank were thrown through the wall of the building across the yard from the tank (photo below left). After losing the lower support on this side, the tank tilted outward away from the building, then collapsed down and outward upon itself.

Of considerable concern to the client—and, for that matter, to those of us standing there at "ground zero"—was the status of another wood stave tank positioned next to the collapsed tank (see photos, top). Of similar size, vintage, and construction, this tank had been struck by some of the debris from the failed tank but did not appear to have been seriously damaged. On the other hand, given its similarity to the failed tank, it was obvious that a close inspection would be prudent. (Oh, and to be on the safe side, it also seemed like a real good idea to lower the level of the contents until we could complete our inspection!)

The inspection revealed several seriously corroded clevises and rods on the second tank, particularly in certain areas. Interestingly, the worst corrosion was not outside in the weather, but rather, inside the adjacent building, where a portion of the tank was shielded from the elements.

Investigating further, we found that inside the building, the clevises had been covered with a layer of pulp, held moist and warm by the high humidity and process steam in the building. Outside, in the weather, the pulp layer had been periodically washed off by the winter rains, allowing the clevises to dry off in good weather, thereby temporarily halting the corrosion process.

Pulp deposits on rods and clevises of second tank

At the conclusion of our site inspection, we recommended that the second tank not be operated at any more than half its capacity until the critically thinned clevises were replaced.

After returning to Portland, we examined a few of the clevises and rods in our laboratory. Our examination showed that in addition to the corrosion damage, several fractured clevises contained preexisting cracks at the time of the failure. These cracks were covered with a thick layer of corrosion, indicating they were quite old, probably months or years, rather than only days or weeks.

Although it was possible that the tank failure could have initiated in one of the cracks, the cracks weren't the primary cause for the failure; rather, it was the severe corrosion, which had gradually reduced the loadcarrying capacity of the clevises until they could no longer support the service loading. Nevertheless, the presence of the cracks was of considerable concern to us, leading to an additional recommendation that all the clevises on the second tank be inspected for cracks.

Unfortunately, this was not going to be a simple task, as the cracks were occurring on the inside of the clevises, which meant it would be necessary to remove them from the tank in order to adequately inspect them. Given the expense of doing this, another option we suggested was to simply consider replacing them rather than trying to remove them, inspect them for cracks and thinning, evaluate which ones were only thinned an "acceptable" amount, and reinstall them.

To establish the load-carrying capacity of the clevis and rods, we tested a few undamaged ones in our laboratory. We found the relatively intact clevises had a pull strength of about 25,000 pounds, while the rods had a pull strength of about 43,000 pounds, both of which were considered to be within acceptable limits.

 

Cross section through corroded clevis	Malleable iron microstructure of clevis

Our laboratory analysis showed the clevises were heat-treated, malleable iron castings and were of commercially acceptable quality.

Interestingly, we found the insides of the clevises had been packed with grease when they were installed. After nearly 30 years of service, some of the grease was still present, and where it was, it had completely protected the inside from corrosion, even though the outside had corroded nearly all the way through on some clevises.

Fiberglass Consulting

Recently, we established a collaborative relationship with FRP1 Consulting Services, a fiberglass consulting company specializing in the design, analysis, and construction quality assurance of fiber reinforced plastic structures, such as tanks, pressure vessels, piping, stacks, etc.

FRP1 is headed by Frank Krmpotich (pronounced Krum-po-tich), who was educated as a mechanical engineer at the University of Zagreb in Croatia. Frank has also studied arctic engineering at the University of Anchorage and has been active on ASME committees on design, fabrication, and accreditation.

Teaming with FRP1 will allow us to combine our laboratory services with Frank's extensive experience in fiber reinforced plastics, thereby providing a more comprehensive service to both his and our clients in the field of fiber reinforced plastics.

MEI-People

Dr. Chakrapani, President, gave a presentation titled "Investigative Metallurgy" at Portland State University on December 4 to the scientific research honor society, Sigma Xi and the PSU physics department.

Dr. Chakrapani's presentation discussed how metallurgical investigations resolved questions regarding the authenticity of a Chinese family heirloom and a silver Peace Medal bearing Abraham Lincoln's image that was given to Native Americans.