RISK ASSESSMENT OF 50-YEAR-OLD CONCRETE CAISSON

Location: Halifax, Nova Scotia

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Scope: 

• Determine the risk of failure of the concrete caisson and supporting concrete wall under mooring load and propose load rating to maintain the operational risk at acceptable levels.

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Complexity:

• The caisson, constructed in 1979, experiences active deterioration.

• Field tests indicate substantial variation in concrete strength.

• Active Alkali Silica Reactivity (ASR).

• Complex reinforcement detailing.

• Client needed to determine the risk of structural failure to plan for replacement/retrofit for short and long terms.

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Approach / Value

• Nonlinear finite element (NFE) model of the wall was developed.

• Over 50 patterns of concrete properties and degradation mechanisms were analyzed using a specialized approach to model the spatial variation across the NFE model.

• Reliability analysis was performed using Monte Carlo Simulation to determine the reliability of the structure and the probability of failure.

• Risk based analysis was used to determine the minimum concrete strength required for the wall to function safely.

• Load limits that meet an acceptable risk limit were provided for short-term operation.