Info

^demand > ^capacity

Retaining wall

Corrosion

^demand < ^capacity

Wall bows

No spill on highway

Causes accidents

Causes fatalities

No action

$200,000

$450,000

$1,200,000

$3,600,000

FIGURE 36.9 Event tree that describes the failure consequences and costs associated with the failure of a retaining wall.

The annual probability of failure that would justify the support beam replacement is $5,841/ $645,000 = 0.0091. By continuing the analysis in Table 36.3, the best-fit values for H(28) = 0.0074 and H(30) = 0.0103 indicate that the replacement should be justified by the 30th year of service.

36.7 Inspection

The reliability of a structure that is forecasted over several decades is only as valid as the input data and models that support it. Despite the best intentions, a deterioration model taken from the literature or from the behavior of a similar structure under similar conditions could be grossly inaccurate. A systematic inspection program is needed to verify that the structure is behaving as predicted and to provide new data to modify the prediction models if it is not. Inspections can be expensive, so it becomes important to schedule them to optimize the benefit of the information gathered through inspections.

36.7.1 Nondestructive Evaluation Inspection

A nondestructive evaluation (NDE) inspection that is targeted for specific information and does not damage the structure is usually the preferred type of inspection. The information obtained from these inspections is not readily observable and often correlates with a specific defect such as fatigue cracks in members, corrosion of girders, or deterioration of concrete. These techniques (AASHTO 1994) can include radiography, acoustics travel tomography (Bond et al. 2000; Kepler et al. 2000), ultrasonics, thermography, or electric potential, among others. In some cases, such as the thickness of corrosion on exposed steel, a direct measurement is made. In other cases where a defect is hidden, such as scaling of concrete, acoustics travel tomography, thermography, or radar are useful to infer damage. Special methods are used when there is a high degree of correlation between a test result such as electric potential or an acoustical sounding and the defect being sought such as the presence of corrosion in the steel reinforcement of a concrete structure. Sometimes several techniques may be used in combination to improve the confidence in the structural condition. For example, magnetic particle tests, dye penetrants, and ultrasonics are all useful methods to detect fatigue cracks, and if similar conclusions are reached by all three methods, the credibility of the results is enhanced.

Future trends include adaptive computer networks, innovative sensors and wireless technology that will be able to offer continuous monitoring and automated diagnostics on critical structures. In all cases, a probabilistic analysis must account for the ability of the equipment to detect a specific level of damage or distress. Similarly, false positives, where the equipment indicates damage where none exists, must be considered.

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