Periodic Monitoring...

The purpose of periodic monitoring is to evaluate long-term behavioral and performance changes through intermittent measurements, separated by months or years.

Advantages:  The primary advantage is cost. A temporary monitoring system can be mobilized for the survey at any time. Long-term monitoring systems are expensive and require maintenance to stay in optimal condition.

Another advantage is flexibility. New parameters can be added to or removed from the system as needed during each survey. 

The third advantage is the potential for improvement. As new instruments and methods are developed, these can be deployed for better monitoring.

Limitations:  The primary limitation to periodic monitoring is lack of availability. Usually, the system is not available during peak motions, such as an earthquake or windstorm.

Periodic monitoring can lead to inconsistency. It might be difficult to repeat the same measurements using the same instruments.

Features:  Again, two important questions are, "What characteristics should be monitored, and at how many locations?"  As with initial monitoring or structural verification, this will depend on which structural elements the design engineer feels are most critical.  Typical measurements include the following:

• Temperature and wind conditions during each study.  Bridges are non-linear structures, and response characteristics can change under different conditions.
• Strains in critical structural elements under specific loads.  Even though measurements are temporary, permanent mounting locations should be established for periodic mounting of strain gages.
• Permanent displacements or rotations of piers and decks.  These should be measured at critical locations.