Comprehensive risk analysis for structure type selection.

Comprehensive risk analysis for structure type selection.

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    Final; June 1, 2007-Jan. 31, 2010.
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    Optimization of bridge selection and design traditionally has been sought in terms of the finished structure. This study presents a more comprehensive risk-based analysis that includes user costs and accidents during the construction phase. Costs for bridge projects include actual comprehensive costs, both to the funding agency, the Colorado Department of Transportation (CDOT), and to the public at large, including assessments associated with accidents and user delay times during the construction phase. Bridges were selected to represent a cross section of structure types, ADT (average daily traffic volumes), bridge overall length and CDOT region. A total of 43 bridges were targeted for study, and data were able to be compiled on 20 of these. A questionnaire was developed and sent to CDOT bridge personnel, in order to collect general project information as well as detailed information on construction duration, construction traffic control plans (including detour types and mileposts, time of day scheduling and speed reductions), observed congestion and accidents during construction). Graphs compare the delay costs and bridge costs for each structure, and the trends by region, ADT, construction duration and bridge type. Accident rates were also compared during construction with those before and after (examining property damage, injuries and fatalities). Statistical factor analysis was used to isolate the key factors describing the differences among the bridges studied. Structure type and construction duration are the important decision variables with respect to user costs, and these two are related. User cost can be reduced by shortening the construction duration for a particular type of structure, or selecting a type that inherently has shorter construction duration. For those bridges with an ADT value less than 10,000, the user cost was no more than about 10% of the construction cost. Also, for construction durations under 20 months the user cost was no more than 30% of the construction cost. For CDOT, concrete box girder prestressed bridges built with current construction practices exhibited user costs more than twice as large as construction costs. Implementation: The difficulty in obtaining past data suggests that for projects involving high values of ADT, a policy should be implemented of contemporaneous records of construction traffic control, including congestion and accidents. These records should become a permanent part of the closeout documentation for these projects.
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