OUTDATED TECHNOLOGY
Engineers and bridge experts say there
are no obvious, well-known design flaws
with steel gusset plates from the
mid-1960s, when the I-35W bridge was
built. The gussets simply would have
been designed with cruder instruments,
such as slide rules, without the benefit
of precise computer technology, said UC
Berkeley's Astaneh-Asl. As a result,
they also would have been unable to
pinpoint high-stress areas, he said.
That isn't by itself a problem, he said.
However, if fatigue cracks and corrosion
are added to the mix "then it becomes a
perfect storm."
Without specific knowledge of why the
NTSB is interested in the plates, Gary
Peterson, assistant state bridge
engineer for MnDOT, said the only reason
he could think why NTSB was
investigating gusset plates would be
corrosion.
On the I-35W bridge, the plates are
riveted to beams, a detail no longer
used by bridge designers. Peterson
called the riveting an "old technology,"
though he said rivets are not inherently
problematic.
Corrosion is mentioned throughout
several MnDOT bridge inspection reports,
and rust was visible on gusset plates
and elsewhere on the bridge. Peterson
said engineers can use special equipment
to test corrosion and determine how much
metal remains.
Gusset plates become susceptible to
fracturing when they become corroded,
said Astaneh-Asl: "If you fracture any
gusset plate in that bridge, that's the
end of it."
As the first photographs and television
images of the collapse circulated,
Astaneh-Asl said he immediately noticed
the gusset plates had fractured. That
might have been a result of the collapse
and not a contributing factor, he said.
Some of the gusset plates may also have
been weakened by welding, he said.
Welding heats steel and then quickly
cools it, he said, creating tiny cracks
smaller than 1,000th of an inch. Such
cracks can be problematic in a piece of
steel, such as a gusset, subject to high
tension. The practice was stopped in the
1970s, he said.
There are other examples of structural
problems in bridges involving welds and
gussets. In 2003, a bridge in Pottstown,
Pa., was closed after inspectors found
that problems involving intersecting
welds at a gusset plate led to brittle
fractures in two spots, according to an
article this spring in the Journal of
Bridge Engineering.
The article also described how cracks
detected in December 2000 in all three
girders of an approach span of the Hoan
Bridge in Milwaukee prompted officials
to shut the 1970s-era bridge for three
months. All three fractures involved
gussets and welds. In 1999, a fatal
bridge collapse in southeast China was
attributed in part to poor welds, as was
a 1997 bridge collapse in Israel. -
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