RESISTING EXTREME EVENTS - Bridges - page 64

In a typical sequence, engineers first balance potential loads against
resistance to ensure that the bridge meets requirements for the strength limit
state : that it will perform well under its dead load and range of normal live
loads expected during its lifetime. Once they ascertain that it will, they can
get on to the next big question, the extreme event limit state. They ask: over
the same lifetime, will this bridge also perform well in the most intense
earthquake shaking (or other dangerous force) likely to impact it? But before
we say more, we should demonstrate that extreme events are indeed the
most important dangers to a well constructed bridge.
BRIDGE DISASTERS
As of this writing, the Minneapolis bridge disaster of 2007 is still on many
peoples' minds. On August 1 of that year, during rush hour, the eight-lane
Interstate 35W bridge suddenly collapsed, killing 13, injuring over 100, and
removing an economically crucial Mississippi River crossing. In April of the
same year, in Oakland, California, on an elevated interchange approaching
the Bay Bridge, a gasoline tanker crashed and overturned, spilling gasoline
that erupted into flames, causing heat so intense that the elevated structure
melted and collapsed. Fortunately, it was the middle of the night and the
only person injured was the driver. More recently, on May 23, 2013, an
oversized truck struck one of the upper truss members of a through-truss
bridge over the Skagit River in Washington State, leading to one span's
collapse. The collapse fortunately caused no fatalities, only injuries, but it
did sever a critical highway link between Seattle and Vancouver, disrupting
traffic on a wide scale.
A bridge disaster during the early morning of September 15, 2001, was
largely ignored by the national media because it came just days after the
9/11 attacks. It occurred in southernmost Texas, near the Mexican border,
at the Queen Isabella Causeway, which joins the mainland to South Padre
Island. Four loaded barges crashed into a support column, causing two spans,
and later a third, to plunge into the channel. The collapse occurred near
the highest point of the bridge; approaching drivers could not see the gap,
and several cars fell into the water, killing eight.
Over a decade earlier, when the Loma Prieta earthquake struck Cali-
fornia on November 17, 1989, Oakland was served on its waterfront by a
two-level highway, each having four lanes. During intense shaking, a mile-
long stretch of the upper level collapsed, falling onto the lower deck and
crushing or trapping cars below, killing 35.
One more sad event will suffice for now. In April 1987, heavy rain
and floodwater, including spring snowmelt, washed away (the precise term
is scoured) the soils under the Schoharie River Bridge under the New York
State Thruway, upstate New York's most heavily travelled highway. After