This section discusses periodic and permanent monitoring of bridge decks to assess their stability.
The stability of bridge decks has been of concern to designers and owners ever since the infamous destruction of the Tacoma Narrows Bridge in 1940 (figure 64) under moderate winds. Geophysical instruments, including accelerometers and tilt meters, as well as strain gages, are increasingly used for short and long term monitoring of bridge decks. Issues regarding stability monitoring and methods used will be discussed here. There is additional information on vibration monitoring provided in this web manual under Vibration Measurements.
Figure 64. Collapse of Tacoma Narrows Bridge. (Ed Elliott, The Camera Shop, Tacoma, WA)
The basic problems considered here are summarized briefly as follows:
- The aerodynamic stability of bridge decks when subject to high winds.
- The long-term stability of bridge decks due to fatigue caused by a variety of cyclic stresses including temperature, increased loading from traffic, and heavily loaded trucks. (See http://www.pubs.asce.org/ceonline/1098feat.html and http://www.kinemetrics.com/newprojects.html#newprojects.bangkok)
- The stability of bridge decks during earthquakes. (See http://www.kinemetrics.com/halkis.html)
The primary method used to evaluate stability is to measure the baseline performance characteristics of a structure and then monitor the change in these characteristics with time. This can be done periodically, such as during an inspection cycle, or continuously using a permanent monitoring system.
The geophysical techniques applied to this problem are principally vibration monitoring, including strains, displacements, rotations, and accelerations, but also some environmental monitoring including wind and temperature, since these affect the response.
Initial Monitoring or Structural Verification
The purpose of initial monitoring is first to measure the as-built structural modal characteristics and use these data to evaluate the accuracy of the designer's model, and through this, the adequacy of the design. The second purpose is to measure the baseline performance to monitor stability over time. These data can also be used to assist in modeling older structures for the purpose of better retrofitting and upgrading.
Advantages: Better model accuracy means better predictions of performance under extreme loads and lower cost because structural upgrades can be planned more accurately.
Baseline performance is vital to long-term monitoring.
Limitations: None. This step is highly recommended for significant bridges.
Features: Two important questions are, "What characteristics should be monitored, and at how many locations?" For each bridge, this will depend on which structural elements the design engineer feels are most critical. Typical measurements include the following:
- Temperature and wind conditions during the study. Bridges are non-linear structures, and response characteristics can change under different conditions.
- Acceleration, both global movement, and individual elements. Sufficient channels are needed to measure simultaneous movements, with preferably at least 64 channels.
- Strains in critical structural elements under specific loads.
- Precise baseline locations (position or tilt) should be measured at critical locations.
The recording system should have the following minimum characteristics:
- Data should be recorded digitally with at least 16 bits of dynamic range.
- Sensors must be highly sensitive and capable of recording very small motions (0.005m/sec2 or less).
- Bandwidth will depend on the structure. For example, the fundamental mode of the Golden Gate Bridge has an 18-second period; therefore, sensors with DC (zero Hertz) response are required. Typically, DC to 50 Hz is recommended.
The recording system must have sufficient capacity to record all 64 channels for many minutes. This will provide statistical redundancy when analyzing the data.
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.
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.
Table 6 summarizes the recommendations discussed above.
|Vibratory Motion||Frequency Range||Sensor Type||Motion Measured||Application<|
|Tilt, or Rotations||0-1Hz||Tiltmeter
|Rotation from vertical in degrees, sometimes expressed as the sine of the angle of tilt||Monitoring long term movement of bridge decks|
|Motion due to earthquakes||0-200Hz||Accelerographs
Structural monitoring system
|Absolute acceleration, m/sec2||Monitor the performance of individual structures during earthquakes, such as piers, decks, and foundations
Monitoring the performance of large structural systems
|Strains due to motions||0-100Hz||Strain gages
|Strain, mm/mm or in/in||Monitoring stresses in structural elements at critical location, steel and concrete
Evaluate fatigue in structural elements
|Motion due to ambient vibration to evaluate structural characteristics||1-100Hz||Vibration Monitoring||Acceleration, m/sec2 sometimes velocity, m/sec||Measure modal characteristics to "calibrate" the designers mathematical model|