Permanent Monitoring...

The purpose of a permanent monitoring system is to monitor long-term behavioral and performance changes through continuous measurements.  High-speed acquisition can be triggered by important events, such as an earthquake or high winds.  Low-speed acquisition can be programmed for daily, weekly, or monthly measurements for long-term studies.

Advantages:  The availability of the system is a primary advantage.  The system is always available during peak motions, such as an earthquake or windstorm.

The consistency of the system is a second advantage.  Measurements recorded today can be directly compared with measurements obtained 10 years ago.

Limitations:  A critical limitation is cost.  Long-term monitoring systems are expensive and require maintenance to stay in optimal condition.

As new instruments and methods are developed, the system may need to be upgraded to provide maximum utility. 

Features:  The same two important questions remain:  "What characteristics should be monitored, and at how many locations?"  Again, for each bridge this will depend on which structural elements the design engineer feels are most critical.  Typical measurements include:

• Temperature and wind conditions.  Bridges are non-linear structures, and response characteristics can change under different conditions.
• Strains in critical structural elements under specific loads. 
• Permanent displacements or rotations of piers and decks.  These should be measured at critical locations
• Triggered event recording is an important benefit of a permanent monitoring system.  This is particularly true if the bridge is located in a seismic zone.  The system must be able to turn on automatically and record the event.  The sample rate for these channels must be adequate to record full response, to at least 100Hz..