Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007) / Chapter Skim
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Pages 69-80
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From page 69... ...
NCHRP 24-25 Page 69 Phase II Final Report 6. SCOUR MANAGEMENT CASE STUDIES This section offers a few examples of how the scour guidelines in the previous section may be used to evaluate bridges with unknown foundations and select a management plan.
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From page 70... ...
NCHRP 24-25 Page 70 Phase II Final Report Table 28 Summary of Required Data Required Data Value Source Required Data Value Source Detour length (miles) NBI item 19 Truck running cost ($/mi)
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NCHRP 24-25 Page 71 Phase II Final Report Table 29 Annual Probability of Failure, Example 1 Data/Parameter (source) Value Interpretation NBI item 26 (bridge survey)
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NCHRP 24-25 Page 72 Phase II Final Report 3. If scour analysis indicates that countermeasures are warranted, countermeasures should be designed using FHWA HEC-23 (8)
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NCHRP 24-25 Page 73 Phase II Final Report more years. This example will be evaluated as an unknown foundation even thought the foundation is known to be scour-critical and therefore unstable.
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NCHRP 24-25 Page 74 Phase II Final Report 668,799,5$ )
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NCHRP 24-25 Page 75 Phase II Final Report Next, scour countermeasures are considered warranted if the lifetime risk of failure is greater than the estimated cost of scour countermeasures, which TXDOT estimates to be about $50,000. In this case, scour countermeasures are probably warranted because the lifetime risk of failure ($131,500)
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From page 76... ...
NCHRP 24-25 Page 76 Phase II Final Report assumption. Spread footing depths are easily discovered and an assumption should not be necessary for this type of foundation.
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NCHRP 24-25 Page 77 Phase II Final Report 70% of the high priority case studies) , whereas about 4 percent of the bridges nationwide with unknown foundations support principal arterials.
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NCHRP 24-25 Page 78 Phase II Final Report guidelines may not have recommended a sufficiently aggressive management plan for three of the case studies – specifically: #89S42900017 in Tennessee, #0670091 in North Carolina, and #091100041802028 in Texas – when the NBI item 113 code was scour critical and therefore unstable. Alternatively, there was one bridge – #160062 in Florida – for which the scour guidelines recommended foundation reconnaissance when the NBI item 113 code indicated that the foundation is stable with respect to scour.
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From page 79... ...
NCHRP 24-25 Page 79 Phase II Final Report Table 32 Summary of Bridge Case Studies Mitigating Action Decisions State Structure No. (NBI item 8)
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From page 80... ...
NCHRP 24-25 Page 80 Phase II Final Report Mitigating Action Decisions State Structure No. (NBI item 8)
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