FYI...one of the better discussions on the I35W bridge.

Note that so far it is still just guess work.

TMT


Metal plates examined in bridge collapse By MARTIGA LOHN, Associated
Press Writer

The metal plates that held the girders together on a failed 1960s-era
interstate bridge were originally attached with rivets, old technology
that is more likely to slip than the bolts used in bridges today.

Some of the plates, or gussets, also may have been weakened by welding
work over the years, and some may have been too thin or too small,
engineering experts said Thursday.

The National Transportation Safety Board, in a brief Wednesday
advisory to states to check such plates in bridges nationwide, cited a
"design issue" with the bridge's gussets. Engineers say that the
plates are an obvious place to start looking, but that a number of
other factors might have contributed to the Aug. 1 collapse that
killed at least seven people and left at least six still missing.

On Thursday, NTSB officials said "people have run maybe a little bit
too far" with the statement on the gussets.

"Simply by finding a piece of metal that's been sheared or twisted
doesn't necessarily mean it's a critical piece of the puzzle," said
Bruce Magladry, director of the NTSB's Office of Highway Safety. "We
see a lot of steel that's damaged because of the bridge collapse. What
we need to ferret out is what's an initial cause of damage vs. what's
a secondary cause."

Engineering experts said failure of the plates, which usually sandwich
the bridge's steel beams where they intersect, in a critical spot
could have brought down the whole bridge, although no one has
pinpointed a gusset as the cause of the failure.

"What they'll be looking for is to see whether one of the gusset
plates may have fractured," said W. Gene Corley, a forensic engineer
with the Skokie, Ill.-based engineering firm CTL Group. "If one of
those gusset plates breaks, then you have lost half the strength at
that location, and most likely the other one can't carry the load
then."

The bridge's builders in the mid-1960s riveted the plates together,
which required many more holes than bolts would have. More holes
weaken steel, said Abolhassan Astaneh-Asl, a professor of structural
engineering at the University of California-Berkeley, who compared
them to Swiss cheese.

The rivets also tend to slip more than bolts and can lead to more
cracking, Corley said. Bolts are preferred in modern bridge
construction, and were used in more recent repairs.

Welding work on some gussets - at temperatures of 2,600 degrees or
more - could also have caused tiny cracks to form as superheated steel
cooled, which may have developed fatigue cracks.

Astaneh-Asl reviewed 1965 construction drawings of the bridge that
showed varying thicknesses of the gussets. Some in key spots over the
Mississippi River were only a half-inch thick, he said, and his rough
calculation of the pressure they could withstand suggested they were
weaker than the beams they connected. A cracked gusset is visible in
photographs taken after the collapse, he said, but it's unclear what
role that might have played in the bridge's failure.

State transportation officials say damage seen on the bridge's gussets
might have been caused by the collapse.

Various problems in the bridge may simply have added up over the years
and created stresses that the designers never contemplated, Astaneh-
Asl said. For instance, at least one expansion joint locked up,
possibly pulling one of the bridge's piers out of alignment and
leading to undetermined pressures on other parts of the bridge. Such
things could have made fatigue cracks worse, he said.

Inspectors who completed the bridge's last full inspection in June
2006 noted problems - "section loss, pitting, heavy flaking rust" - on
several of the plates. They also reported loose bolts on another
gusset.

Corley, who has been invited to be part of a private investigation
into the collapse, said he saw "lots of rust" on the gussets.

"It brings the issue of load and brings the issue of fatigue there as
well as corrosion," he said.

But something could have gone wrong in design, too, Corley said. Each
plate is individually designed, and someone could have miscalculated
the load or weight-bearing capacity of an individual gusset plate, he
said.

"In design there's always the chance for a blunder," Corley said. "One
of the most common causes of collapse of any type of structure is the
blunder."

Investigators are looking closely at the weight that was on the bridge
when it fell.

Construction crews had piled up sand and gravel on the bridge as they
prepared to pave a 520-foot stretch of two southbound lanes of the
freeway, said Liz Benjamin, a construction engineer with MnDOT.
Equipment on the bridge included a cement truck, a concrete mobile
mixer, buggies to haul the concrete and personal vehicles of the
workers. Workers also were using 45-pound jackhammers to remove the
top layer of pavement.

The bridge was one of Minnesota's busiest, carrying 140,000 vehicles a
day. Mark Hallenbeck, director of the Washington State Transportation
Center at the University of Washington, said that's dramatically
higher than designers would have considered in 1965.

The traffic would have contributed to fatigue over the years, Corley
said. But the weight of truck and vehicle traffic is "pretty
insignificant" next to the weight of the bridge itself, he said.

___

Associated Press writers Patrick Condon in Minneapolis and Seth
Borenstein in Washington contributed to this report.