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Case Study #9 — Fastener Failure Workshop

Transmission Towers
 

Industry:

Electric Power

Transmission Towers

Problem:

The heads broke off many of the fasteners that were used to bolt together steel power transmission towers. Most of the towers had collapsed over a six-mile long area.

Injuries:

None.

Damages:

Crumpled towers that were scattered throughout the countryside gave an appearance of severe earthquake damage. Estimates provided on the towers alone were in excess of US$400,000. The man-hours needed to repair these structures would be significant.

Observations:

Not all of the fasteners were broken, but enough had failed by cracking at the head-shank juncture to allow the metal structures to fold. Some of the failed bolts looked as if they were pulled apart by "necking out" the threads.

Investigation:

The type of bolts selected by the power company for this application was from the ASTM A 394 Standard, Type 0. They opted for a square head design rather than a hex, and these products were dip zinc coated. A quantity of bolt samples from the same lot as the failed bolts was submitted to an independent lab for testing. The hardnesses were found normal, but the tensile test results were erratic. Some of the heads were breaking off during the test. It was suspected that the required stress relief annealing operation had not been effective. As a test, the lab performed a stress anneal to a sampling of pieces. All of those pieces passed the tensile test, and very consistently.

 

A visit to the fastener company's heat treatment subcontractor found that they had used a very large metal bin to hold and process the fasteners. This bin was placed in a huge box style furnace. The furnace was then brought up to the operating temperature, and held at that point for a period of time.

Conclusion:

The heads of the bolts in transmission tower applications are subject to a variety of harsh loads. The stress relief annealing operation for this product is critical to having the heads stay on the bolts when forces act upon them. Complicating this application, a square head is more susceptible to cracking, because the larger across corner dimensions provide a greater leverage opportunity on prying loads. The annealing process was an attempt to reduce costs without the controls in place to assure proper results. The huge bin held so many pieces that the time at temperature was insufficient for the many products in the center area of the bin to be properly annealed. This could have been monitored with a temperature sensor.

Recommendations:

The ASTM specification has a sampling plan for mechanical tests; however, for the testing to be effective, a true random sampling of the test specimens must be performed. Audit your suppliers to make sure you aren't just buying fasteners with possible misleading documentation.

 

As appeared in Fastener Technology International magazine.

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