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From page 13... ... NCHRP 24-25 Page 13 Phase II Final Report 2. GENERAL APPROACH TO RISK MANAGEMENT This section describes a general approach for using risk to manage bridges with unknown foundations with respect to hazard-induced failure. Read the entire page →
From page 14... ... NCHRP 24-25 Page 14 Phase II Final Report at a particular bridge location. These two factors are usually lumped in any given measure of the likelihood of a hazardous event. Read the entire page →
From page 15... ... NCHRP 24-25 Page 15 Phase II Final Report For example, scour refers to sediment erosion that occurs in a stream or river that flows under a bridge. Scour may cause a bridge to fail when enough sediment erodes to undermine and collapse a pier, footing, or abutment. Read the entire page →
From page 16... ... NCHRP 24-25 Page 16 Phase II Final Report 2.1.3. Correlations with Observed Failures If models already exist for predicting the hazardous potential of a site and the corresponding vulnerability of a bridge to fail, then these two quantities can be correlated with the number of observed hazard-induced bridge failures to generate a model of the probability of failure. Read the entire page →
From page 17... ... NCHRP 24-25 Page 17 Phase II Final Report possible. If multiple failure modes are to be considered, the method for calculating the total probability of hazard-induced failure will depend on whether the individual failure modes are correlated or independent. Read the entire page →
From page 18... ... NCHRP 24-25 Page 18 Phase II Final Report 2.2. Cost of Failure Regardless of methodology, certain general economic assumptions are necessary for computation of risk. Read the entire page →
From page 19... ... NCHRP 24-25 Page 19 Phase II Final Report Table 4 Occupancy per Vehicle Mile by Daily Trip Purpose Trip Purpose Mean Standard Error All personal vehicle trips 1.63 0.012 Work 1.14 0.007 Work-related 1.22 0.020 Family/personal 1.81 0.016 Church/school 1.76 0.084 Social/recreational 2.05 0.028 Other 2.02 0.130 1990 through 2000 and forecasts through 2005 Source: The 2001 National Household Travel Survey, daily trip file, U.S. Department of Transportation (www.bts.gov/publications/national household travel survey, accessed May 26, 2005) Read the entire page →
From page 20... ... NCHRP 24-25 Page 20 Phase II Final Report Alternatively, off-the-clock time savings reflect the results of research examining choice situations (e.g., toll versus free route, speed, or housing location) that require choosing to save time versus money or safety. Read the entire page →
From page 21... ... NCHRP 24-25 Page 21 Phase II Final Report Table 8 Values of Time State Mean Wage* ($/hour) Read the entire page →
From page 22... ... NCHRP 24-25 Page 22 Phase II Final Report Table 9 Cost of Bridge Construction Bridge Superstructure Type, Demolition Total Cost ($/ft2) Reinforced concrete flat slab; simple span 50-65* Read the entire page →
From page 23... ... NCHRP 24-25 Page 23 Phase II Final Report A detailed summary of demand elasticity is given Appendix B However, demand elasticity was not incorporated in this study because research indicates that demand elasticity is very low (on the order of 3%) Read the entire page →
From page 24... ... NCHRP 24-25 Page 24 Phase II Final Report Table 11 Assumed Number of Lives Lost in Bridge Failure Average Daily Traffic (ADT) Number of Lives Lost ADT < 100 0 100 ≤ ADT < 500 1 500 ≤ ADT < 1000 2 1000 ≤ ADT < 5000 2 ADT ≥ 5000 (Not an interstate or arterial) Read the entire page →
From page 25... ... NCHRP 24-25 Page 25 Phase II Final Report A = average daily traffic (ADT) from NBI item 29, d = duration of detour based on ADT from Table 3 (days) Read the entire page →
From page 26... ... NCHRP 24-25 Page 26 Phase II Final Report Estimating the risk of failure over longer periods of time (e.g. $ per decade) Read the entire page →
From page 27... ... NCHRP 24-25 Page 27 Phase II Final Report cost of any other attending analyses that are used to assess the bridge's vulnerability. The cost of different monitoring or protective countermeasures, likewise, should include both installation cost and maintenance cost. Read the entire page →
From page 28... ... NCHRP 24-25 Page 28 Phase II Final Report If the foundation cannot be inferred, then any bridges that are deemed critical for emergency services or national security (i.e. "Is it a high priority structure? Read the entire page →
From page 29... ... NCHRP 24-25 Page 29 Phase II Final Report the risk of failure" in Figure 1) , and establish minimum performance levels (MPL; i.e. Read the entire page →
From page 30... ... NCHRP 24-25 Page 30 Phase II Final Report events – but using methods short of using standard failure analyses to guide the installation – may be safer than relying on automated monitoring to predict imminent failure. Thus, Figure 1 first suggests that the cost of automated monitoring be compared to the risk of death (i.e. Read the entire page →
From page 31... ... NCHRP 24-25 Page 31 Phase II Final Report If monitoring is warranted instead of countermeasures or retrofits, then a bridge closure plan should be developed that involves monitoring the bridge for any signs of degradation or increased vulnerability (i.e. below and beside "Are countermeasures/retrofits warranted? Read the entire page →
From page 32... ... NCHRP 24-25 Page 32 Phase II Final Report No Yes No No No Yes No Are retrofits/ countermeasures warranted? Are foundation reconnaissance and standard analyses warranted? Read the entire page →
From page 33... ... NCHRP 24-25 Page 33 Phase II Final Report A detailed application of these guidelines is presented in Section 5, entitled "Scour Risk Management Guidelines. The next two sections, however, present the supporting analysis for the scour guidelines in a similar manner to the general approach to risk management. Read the entire page →

From page 13...

... NCHRP 24-25 Page 13 Phase II Final Report 2. GENERAL APPROACH TO RISK MANAGEMENT This section describes a general approach for using risk to manage bridges with unknown foundations with respect to hazard-induced failure.