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. 2/2018