Civil Engineering Reference
In-Depth Information
enough soil had washed away, the piers progressively collapsed, causing the
entire bridge to fall. Unsuspecting motorists drove into the gap; ten bodies
were recovered.
These are very different kinds of bridge failures. The Minneapolis
collapse differs from the others, because it was attributable—investigators
found—to a design failure. The alleged flaw was in the gusset plates, which
are metal objects that connect truss members to each other. With bolts,
rivets, or welding, the metal plates are meant to firmly secure the joints
between structural members. With added stress from construction equipment
parked on the bridge, the plates reportedly failed, dooming the bridge. The
primary problem was internal to the bridge design itself.
In the other five disasters, however, the collapse is primarily attributed
to an event external to the bridge structure: a tanker crash and resulting
fire, truck impact, barge impact, earthquake, and scour. These are what
engineers refer to as
extreme events.
THE CAUSES OF BRIDGE FAILURE
We have to be careful about bridge “failure” statistics, in part because data
on the subject are not uniformly or comprehensively collected. According
to the National Bridge Inventory data provided in chapter 2, as many as
100,000 bridges are structurally deficient or obsolete. But their inadequa-
cies are well understood and do not pose immediate danger. Some undergo
planned closing after inspectors determine them to be unsafe, but even they
are not immediately dangerous. They have, rather, simply served out their
expected service lives or are being met with far heavier loads than they
were designed for; they have not “failed” in the ordinary sense of the word.
Where actual failure does occur, the causes are often difficult to diag-
nose. Failure occurs under complex interaction between internal factors
(inadequate structural resistance) and external ones (loads exceeding the
bridge's design load).
Sources of internal failure may include faulty engineering design,
flawed detailing documents (submitted by contractors with engineers'
approval), error or malfeasance during construction, or materials deficien-
cies. The internal failure may also be traced to deterioration due to age and
to inadequate inspection and maintenance. Otherwise, failures are externally
caused: by loads greater than ones that the bridge was expected to resist.
When a bridge fails, the judgment about what caused it to do so
should be made through investigation by qualified forensic engineers. They
examine original design and detailing documents, review inspection reports,
and build computer models that replicate the process of collapse. It is the
forensic study that should say whether the failed bridge did or did not
fulfill building codes and expectations of the profession; whether it had
