Brain waves over water waves

Bridges Article August 06, 2008
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The new I-35W St. Anthony Falls Bridge over the Mississippi River in Minneapolis is a modern prestressed concrete segmental bridge designed with multiple levels of redundancy to provide over 100 years of service life. Two parallel structures, with a maximum span of 504 ft, will carry 10 lanes of interstate traffic. The Minnesota Department of Transportation (Mn/DOT) selected the team of Flatiron-Manson Joint Venture (FMJV), with Figg Bridge Engineers Inc. (FIGG) as the bridge designer, based on a best-value process. Notice-to-proceed for the design was given on October 8, 2007.

A unique aspect of the Flatiron/Manson/FIGG team’s proposal was to incorporate innovative sensor-based monitoring systems in the bridge to provide information that supports construction activities, records structural behavior throughout the structure life and enhances bridge security. These sensors combined with the intelligent transportation system (ITS) elements and sensors for the deck automated deicing system provide Mn/DOT with a smart bridge that will increase efficiency of operations and maintenance.

Advanced advice

Sensors were placed at select locations in the bridge to assist in making decisions during construction. These included concrete maturity meters embedded at key locations during concrete pours. Data collected from these sensors helped determine when the concrete had achieved sufficient strength to proceed with the next step in construction, such as the removal of forms. Another use of sensors during construction was thermal monitors embedded in mass-concrete elements such as the foundations, piers, abutments and superstructure diaphragms (in general, those elements with a minimum dimension greater than 4 ft). These sensors reported the internal temperature of the concrete near the surface and core during curing when heat generated by hydrating cement raises temperatures. This information allowed the contractor to manage the temperatures in these elements through the use of insulation and cooling tubes, resulting in higher-quality concrete.

Live wired

An array of sensors in the new I-35W bridge will provide information about structural behavior throughout the service life of the bridge. A data acquisition system collects and stores the information, which is accessible via fiber-optic cables from Mn/DOT’s Operations Center and the University of Minnesota Department of Engineering. The gathered information will be managed in a partnership among Mn/DOT, the Federal Highway Administration (FHWA) and the University of Minnesota. The sensing devices include:

  • Vibrating wire strain gauges embedded or mounted at select locations in the bridge superstructure and in the drilled shafts and pier columns at Pier 2 on the south bank of the river. These strain gauges provide information about the bridge’s response to loads and temperature variations, which can be used to determine moments and stresses at key locations in each of the four box girders in each span;
  • Temperature sensors at various locations in the superstructure measure the temperature of the bridge. This information, in tandem with data collected from the strain gauges, can be used to measure the response of the bridge to thermal conditions;
  • Accelerometers near the center of each span in each box girder record the structure’s response to live loads;
  • Sensors based on fiber-optic cables located in the Span 2 box girder provide a means to collect strains over the longer gage lengths across the span and can be used to determine curvatures and bridge deformations. The data from the fiber-optic sensors will be used to correlate and confirm data collected from the other sensors in the superstructure;
  • Sensors embedded in the top surface of the bridge deck concrete measure the potential for corrosion in sacrificial steel bars at various depths in the concrete below the deck surface. This information will assist Mn/DOT with scheduling concrete wearing surface replacement; and
  • Linear potentiometers at the expansion joints measure longitudinal bridge movements. This data will be compared to expected values to assist in identifying potential future maintenance needs for the bearings and expansion joints.

Information obtained from these sensor systems will complement routine bridge inspections, providing a more in-depth understanding of the structure. This will assist Mn/DOT engineers with documenting conditions and making maintenance decisions. The information also provides an opportunity for the University of Minnesota and FHWA to study structure behavior under various conditions to further advance state-of-the-art bridge design and construction.

Traffic on tape

An integral part of Mn/DOT’s traffic-management strategy for the Minneapolis/St. Paul area is feedback from ITS components that have been incorporated into the new I-35W bridge project. Sensors and cameras collect data on traffic flow and transmit it to the Mn/DOT operations center via fiber-optic cables. The fiber-optic cables cross the I-35W St. Anthony Falls Bridge in conduits inside the box girder cells and there are accommodations for additional ITS elements in the bridge for the future.

Video cameras placed along the corridor are monitored by operators via closed-circuit televisions at the control center. Vehicular loop detectors placed along the corridor also allow operators at the control center to monitor data such as volume and speed of traffic. If traffic incidents should occur, operators have the ability to alert drivers with variable message signs. Data also will be used for operation of the ramp metering system that controls the flow of traffic entering the corridor via traffic-control lights at the on-ramps.

Taking its temperature

The microclimate on the bridge over the Mississippi River can be colder, windier and more humid than on the bridge approaches. To maintain the best possible surface conditions in poor conditions, the I-35W St. Anthony Falls Bridge project features an automated anti-icing system.

Sensors on the main span of the bridge measure conditions including ambient temperature, humidity, wind speed and deck temperature. This data is used to determine when icing conditions are eminent, triggering the system to start applying deicing chemicals to the bridge deck surface from spray disks embedded in the deck.

In addition to the various sensor systems for bridge management and operations, the new I-35W St. Anthony Falls Bridge features a security system. This system consists of various sensors, placed at select locations, to detect activity in and around the bridge. For instance, security contacts on the access doors to the bridge will instantaneously relay a signal to the Mn/DOT operations center when entry into the bridge occurs. Also, infrared-capable video cameras monitor key areas of the project and are observed at the Mn/DOT operations center.

Smart answer

The smart bridge systems installed on the new I-35W St. Anthony Falls Bridge benefited the construction process by monitoring the concrete temperatures and curing to achieve high-quality concrete. Information collected from the sensor systems during the service life of the bridge will assist Mn/DOT in managing operations by enhancing bridge inspections with structure performance data; maintaining efficient and safe traffic flow; and providing the infrastructure security. This information also will be a valuable resource in the sustainable design of bridges for the future.

In order to achieve the design-build program in less than 15 months, close coordination was developed between Mn/DOT, FIGG, Flatiron-Manson JV, system suppliers, FHWA and the University of Minnesota. Numerous coordination meetings among all parties have resulted in a successful program for the smart bridge system. With approximately 250 sensors, the new bridge provides an intelligent system that will be an example for other modern bridges in the future.

About the author: 
Chiglo is the I-35W project manager for Mn/DOT. Phipps is with FIGG.
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