e-mosty June 2018 American Bridges American Bridges | Page 40
Additionally, the inspection of bridges has tended to
concentrate solely on the condition of the bridge
elements without taking into account the overall risk
of failure of an element.
Risk can be defined in many ways, but perhaps the
simplest is to define it as the possibility of loss, injury
or other adverse condition occurring. More
specifically, it is the combination of the probability of
an event occurring and the consequence of the event
occurring.
Another structure failure in the US, at the Mianus
River Bridge in Connecticut in 1983 (see Figure 2),
caused more concern related to fatigue and fracture-
critical bridges. This failure and further research
resulted in fracture-critical inspections being
mandated. In 1987, scour caused failure of the
Schoharie Creek Bridge in New York. This failure
resulted in the initiation of the underwater bridge
inspection program.
The likelihood of occurrence is determined by the
condition of element (as well as other metrics such as
the capacity/demand ratio and fatigue susceptibility).
However, the consideration of the outcome of failure
and overall risk, and a risk ranking of all elements has
not been carried out in a methodical and systematic
way.
Establishing a risk-based inspection regime, that
addresses the specific characteristics of the bridge, is
the most effective method of managing and
mitigating risk on these large complex bridges.
A risk based inspection regime can be as simple or as
complex as the owner requires or made to suit the
condition of a particular bridge. In all cases, a risk
based inspection will have inspection frequencies that
vary from those prescribed by national and state
authorities.
Therefore, consultation and discussion between all
relevant parties involved has to be a first requirement
before adopting a risk based regime.
In fact, the FHWA requirements for the inspection of
fracture critical members is in fact a recognition of
the need for risk based inspection albeit in a partial
form.
When applying risk based assessments to bridges one
of the largest risks may be construed as structural
failure.
2. Background to Risk and Bridge Inspection in the US
In 1967, the Silver Bridge, a pin-connected link
suspension bridge over the Ohio River at Point
Pleasant, West Virginia collapsed suddenly with the
loss of 46 lives.
As a result, a 1968 Federal Act initiated a national
bridge inspection program that recognized the need
for periodic and consistent bridge inspections and
subsequently, the first National Bridge Inspection
Standards (NBIS) were developed in the USA, in 1971.
Figure 2: Mianus River Bridge Failure
The safety risks associated with the 600,000 bridges in
the USA were tragically highlighted when the
Interstate 35W Bridge (I-35W) in Minneapolis,
Minnesota, collapsed on August 1, 2007, killing 13
people.
Following that collapse, the US Federal Highways
Administration (FHWA) asked the
American
Association of State Highway and Transportation
Officials (AASHTO) to determine whether or not the
FHWA’s National Bridge Inspection Program was
delivering the highest level of bridge safety. The
conclusion was that limited progress had been made
in implementing data-driven, risk-based bridge
oversight. 2
In June 2009, the US Congress passed the MAP-21 Bill
which, amongst other things, requires each State to
develop a risk-based asset management plan for the
National Highway System (NHS) to improve or
preserve the condition of the assets and the
performance of the system 3 .
As a consequence of that requirement, State DOTs
have been putting in place Asset Management Plans.
Ohio State DOT (ODOT) has a huge inventory of
structures including two significant cable supported
bridges.
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