Data Acquisition Systems Provide Valuable Data for Bridge Assessments

As the nation’s infrastructure ages, government agencies are faced with the never-ending burden of maintaining, repairing, and replacing large numbers of bridges and roads. Federal Highway Administration (FHWA) data from 2004 showed that of the 594,101 bridges in their inventory, 77,796 were classified as structurally deficient. According to the FHWA, the term structurally deficient “refers to bridges that have major deterioration, cracks, or other deficiencies in their structural components, including decks, girders, or foundations.” While the classification of structurally deficient doesn’t necessarily mean a bridge is unsafe, it does mean that these bridges require extra monitoring, carefully-watched maximum load ratings, and possibly repair work.

Visual inspection has been used for years as a method for evaluating bridges and determining condition ratings. Organizations responsible for maintaining bridges have individuals dedicated to this and many have extensive experience that allows them to accurately assess a bridge’s condition.

Data acquisition is another nondestructive evaluation method for assessing bridge condition. In its simplest form, a data acquisition system is composed of sensors and a data logger. Sensors react to the physical phenomena they measure (e.g., vibration, loads) and provide corresponding signals to the data logger. The data logger measures the signals from the sensors at the programmed interval and stores the resulting data. Often, a communications peripheral, such as a cell phone or radio, is integrated with the system to transmit the data to an office or base station—facilitating data collection without having to visit the remote site. Display and analysis software provides tools for interpreting the measured data in meaningful ways.

Numerous sensors types are available to meet the needs of bridge testing and monitoring applications. These include accelerometers, strain gauges, piezometers, tiltmeters, crack meters, and load cells. Low power requirements and rugged design allow some data acquisition systems to operate unattended at bridge sites without AC power for extended periods of time, even in adverse weather conditions.

Bridge data acquisition systems can be an important component of maintaining the safety and health of our infrastructure. Their ability to operate continuously without interruption, measure changes that are imperceptible to humans, make hundreds (even thousands) of measurements per second, can provide many advantages to researchers, transportation departments, engineering firms, and other organizations.  A major benefit is that data acquisition systems can detect potential problems before they become visible on the outside of the structure. Continuous bridge performance data can be extremely valuable over the life of a structure. Whether a bridge is old or new, baseline data can be recorded and compared to future data, providing valuable information regarding changes to the bridge’s condition.  In short, data acquisition systems can provide valuable data about the structural integrity, load limits, life expectancy, needed repairs, and general health of a bridge.

A wide variety of bridge testing and monitoring possibilities exist. These possibilities can range from short-term testing to long-term, stand-alone monitoring. Short-term testing often involves dynamic measurements—very high measurement rates that attempt to capture specific events, such as the impact of ice on a pier or the crossing of a truck over the bridge. Long-term monitoring typically uses static measurements—measurement rates slower than 1 Hz (once per second)—though systems can be programmed to increase their measurement rates based on specific conditions. The following section provides a summary of some of the more common bridge testing and monitoring scenarios.

Loads
Load tests can be used to verify existing bridge-load ratings, help determine if load ratings should be changed, decide if heavy traffic should be rerouted, and validate designs. To carry out a load test, strain gauges are placed on girders and load cells are installed on spans, deck surfaces, and cables (for cable-stayed bridges)—essentially making the bridge into one large load cell. The system measures how loads of known weight or even actual traffic affects the behavior of the structure. These tests can be run for just a couple of days at a time for dynamic testing or over a longer period of time for static monitoring.

Fatigue
Strain gauges installed on the girders and structures of the bridge can help determine the stresses or stress cycles affecting the bridge. These tests could include dynamic measurements (100 to 200 Hz) compiled through a rain-flow histogram algorithm or long-term static monitoring. Data from these tests can indicate the overall health or any deterioration of bridge components.

Wind Loads
Wind can have a significant impact on large bridges. Data acquisition systems can help determine the effects of wind loads on the overall behavior of the bridge or structure. They can also help validate designs of fairings or other wind-dispersing structures. For these tests, wind sensors are installed away from any shadowing (e.g., cables or arches) and measure the intensity and direction of the wind. Accelerometers, tiltmeters, and strain gauges measure the lateral wind loads on the bridge itself. These tests are usually dynamic, requiring measurement speeds of 20 to 200 Hz. The goal of these systems is to provide data that help manage traffic safety, prevent failures, and enhance future bridge designs.

Bridge Scour (Erosion)
Bridge scour (erosion of sediment and rock underneath piers and abutments) causes many bridges to fail. Monitoring systems can help determine the health and condition of the river bed, ocean floor, or rock bed, and to alert personnel if bridges need to be closed or if repair measures are needed. This type of application may use time-domain reflectometry (TDR) probes, submersible optical sensors, and underwater cameras. Measurement rates are usually slower (less than 1 Hz) except during flooding, high tides, storms, and other high-water periods.

Impact
Data acquisition systems can monitor the magnitude of the impact of objects, such as ice and debris, on a structure. These applications often use accelerometers and tiltmeters installed along the major axis of the pier. Systems are programmed to trigger high measurement rates (100 to 1000 Hz) on impact. Resulting data can be used to determine if barriers are needed in front of piers, to design stronger piers, and to learn how impacts affect the overall behavior of the structure.

Settlement
Settlement monitoring systems can help determine if piers or abutments (or girders) have settled too much and have compromised safety. By monitoring settlement closely, engineers can help prevent costly renovations to bridge sections and even prevent bridge failures. These systems employ linear-variable-differential transformers (LVDTs), string potentiometers, and extensiometers for shorter distance movements. String potentiometers and laser sensors are used for longer distances. These systems are usually installed for long periods of time and have slower measurement rates (less than 1 Hz).

Rotational Displacement
Rotational displacement monitoring systems help measure the amount of twist and rotational movement in upright members, such as piers, arches, and trusses. These systems can alert bridge managers to excessive twist and can validate design and construction methods. Typical sensors used for this type of application include accelerometers, tiltmeters, LVDTs, and rotational-differential transformers (RDTs). Measurement rates are usually in the 50 to 500 Hz range.

Deflection
Measuring deflection usually involves installing crack meters, LVDTs, string potentiometers, accelerometers, tiltmeters, or load cells placed on the object of interest (cables, abutments, girders, piers). These short-term tests require measurement speeds in the range of 50 to 200 Hz, depending on the structure. Resulting data can help determine the amount of displacement or loads placed on adjacent members, determine proper redundancy in cable-load dispersion and girder strength, and validate bridge designs.

Design Research
A wide variety of data acquisition configurations can be used to support bridge-design research. Systems can help validate new-material performance, modify or change design standards, and define new design or test methods.

While some organizations have not yet taken advantage of data acquisition systems, many have benefited from them for structural-health monitoring of old and new bridges—both during and after construction. The following section provides a summary of a few projects that have benefitted from data acquisition systems.

Confederation Bridge
The 13-km Confederation Bridge, one of the world's longest continuous, prestressed-concrete, box-girder bridges, stretches from Borden, on Canada's Prince Edward Island, to Cape Tormentine, New Brunswick. Data acquisition systems were installed from the very beginning of the bridge’s expected 100-year service life. The measurement instrumentation included more than 500 strain-measuring devices, 450 thermal sensors, 28 ice-load panels, 12 tiltmeters, 76 vibration sensors, and underwater sonar equipment. Six Campbell Scientific CR9000 data loggers were installed to measure the sensors. The system has been in operation sine 1997 and provides researchers with data regarding ice flow impacts, thermal stress, short- and long-term deformation, traffic loads, and vibration.

Huey P. Long Bridge
The Huey P. Long Bridge in New Orleans, Louisiana, is a cantilevered, steel, through-truss bridge that opened to traffic in 1935. It extends nearly 2400 ft over the Mississippi River and carries dual rail lines between the trusses and two lanes of vehicular traffic cantilevered to the exterior of each truss. In an ongoing effort to widen the traffic lanes on each side of the bridge without interrupting rail traffic, the Louisiana Department of Transportation and Development (LA DOTD) is adding trusses parallel to the existing truss and modifying the piers with additional concrete encasements and steel frames. A structural-health monitoring program is included in the construction contract as a proactive measure to assess whether the anticipated amount of load is being transferred from the widening truss members to the existing truss members. The monitoring system includes 827 static and dynamic strain gauges, Campbell Scientific CR1000 data loggers, and spread spectrum radios for wireless telemetry.

St. Anthony Falls Bridge
When the I35W St. Anthony Falls Bridge over the Mississippi River in Minneapolis, Minnesota, was rebuilt during the year following its collapse, a structural-health monitoring program was instituted to show how internal instrumentation could be used to increase quality assurance, monitor construction loads, and subsequently show traffic and wind-load effects on pier performance over the long-term. The monitoring project involved three phases: (1) internal concrete curing temperature of the foundation elements, (2) construction loads, and (3) long-term health. In support of this, two types of strain gauges and thermometers were installed on one of the three elevated piers. CR9000 data loggers record data from dynamic measurements, while CR1000 data loggers record the static measurements. Data is transferred to the base station by cellular phone.

Illinois Highway Bridges
The Illinois Department of Transportation ran tests on 12 interstate highway bridges in Illinois along Interstates 55 and 70/270 to validate methods for calculating bearing forces. The bridges were instrumented on their beam webs with strain-gauge rosettes to measure shear stresses caused by loads. Test data were collected by CR5000 data loggers for approximately six months at each bridge to determine loading trends. At most of the test points, the researchers installed three triaxial strain rosettes in a vertical line on one side of the web, and also on the opposite web face on selected beams to determine the effect of torsion. For gauge installation on concrete beams, individual strain gauges were arranged in a three-element triaxial pattern at each rosette location.

Campbell Scientific, manufacturer of the data loggers used in the projects above, has more than 35 years of data acquisition experience, specializing in rugged systems for stand-alone operation in harsh environments. In addition to the projects listed above, Campbell Scientific data loggers have been used to test or monitor bridges, small and large, all over the world, including the Brooklyn, Bronx Whitestone, Throgs Neck, Williamsburg, and Verazano Narrows bridges of New York. Having supported many bridge monitoring and testing applications, worked with researchers and engineers, and designed measurement instrumentation for bridge applications, the company recommends data acquisition as a reliable tool for those who have the responsibility to maintain bridges and other structures.
 
Others who are familiar with the testing and monitoring possibilities provided by current data acquisition technologies understand the important contribution these systems can make to maintaining our infrastructure. Whether the need is for short-term testing or long-term continuous monitoring, data acquisition systems can help facilitate successful bridge design, maintenance, and safety.