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. |
