Like its famous younger cousin just to the east, the mighty Mackinac Bridge, the U.S.-2 bridge over the Cut River is known for its elegant construction and breathtaking views of Lake Michigan.
The 62-year-old structure became such a tourist attraction that in 1985 it was widened to accommodate sidewalks on each side for pedestrians who had previously braved passing logging trucks to snap photos from the railings. It was this widening project that added complexity to the Michigan Department of Transportation’s (MDOT) planned redecking of the historic bridge in 2008.
Built in 1947, the Cut River Bridge is one of only two cantilevered deck truss bridges in the state. With five spans — two bearing spans connected to sandstone abutments, two cantilevered spans, and a center suspension span — it measures 641 feet end-to-end. Its graceful center arch sits astride the Cut River gorge, a distinct contrast to the trickle of a waterway 149 feet below.
The deck truss design is the similarity the Cut River Bridge shares with the I-35W Mississippi River bridge in Minneapolis, which, as we all know, collapsed in August 2007. This fracture-critical design was an additional concern for MDOT engineers as they prepared to replace the Cut River Bridge’s deck the following construction season.
While the approach spans were repaired in 1975; portions of the structural steel painted in 1979; and the suspended span’s deck joints, bearings, and pins replaced in 1991; the concrete deck remained original since the bridge was constructed.
The 888-ton steel superstructure and sandstone substructure remained in good condition, but the deck put the Cut River Bridge on MDOT’s list for replacement several years prior to the I-35W bridge collapse. However, the tragedy in Minneapolis and its historic nature brought special attention to the Cut River Bridge and the upcoming project. In addition to its own in-house bridge designers and structural engineers, MDOT hired the engineering firm of Alfred Benesch & Co., to consult.
Anlaan Corp., the successful bidder, hired Williams & Works as a consultant for the project. Prior to the start of construction, Anlaan was required to provide a false decking design to provide fall protection during construction, and to catch debris above the trout stream below.
Part of the false decking design included wooden 2-by-12s beneath the outer 16-inch and 10-inch beams, cantilevered to support plywood sidewalks for workers. While the six original 22-inch main beams could easily support corrugated steel false decking, design calculations showed that the outermost fascia beams would fail with the deck removed, possibly leading to failure of the entire bridge.
Delay in finding an approved false decking design extended into July, in a region with weather usually conducive to construction only between July and October each year. With the chance to complete the project in one season as planned slipping away, MDOT and the contractor worked together to find a solution.
The pedestrian extensions on each side of the bridge include a floor beam between the fascia and inner beams every 25 feet. Engineers found that by adding two intermediate floor beams within those spans, the fascia beam could sufficiently resist the torsion forces applied by the false decking design. This added $149,000 in steel reinforcement to the originally awarded contract amount of $3.1 million.
Meanwhile, Anlaan worked to replace a diagonal brace within the superstructure. Accessing the structure through 3-foot-square holes through the bridge deck, the contractor applied a temporary support system to the steel below the strut to distribute loads during replacement. Although the bridge’s historic nature called for the use of rivets in the replacement, engineers obtained a waiver to use modern bolts instead.
When it became clear that completion of the project would not be possible during the 2008 season, it was decided to prepare the bridge for traffic until the following May. Holes drilled to locate beams and the strut-replacement access hole had to be filled, and railings removed in anticipation of the deck removal needed to be replaced. Those tasks, along with patching by maintenance crews, consumed the remainder of the season.
When work resumed in the spring of 2009, the railings were again removed. The deck was cut into slabs, which were then pulled sequentially to balance the distribution of the remaining weight on the structure. After cleaning and coating the beam tops with epoxy-based primer, stud shear developers were applied, incorporating new technology without detracting from the historic appearance of the bridge. As with the removal of the deck, pouring the new reinforced concrete deck was made in four sequential pours to distribute the load.
The new 8-inch-thick deck was cast in early August. After installing the new railings, the Cut River Bridge is expected to reopen to traffic in early October 2009.
Bridge monitoring In the future, traffic on the Cut River Bridge, and the forces traffic subjects it to, will be monitored through MDOT’s Critical Highway Infrastructure Monitoring Project. This project integrates sensors and data collectors on the Cut River and Mackinac Bridges, and communicates that data wirelessly between the bridges and to Lansing (Figure 1).
In partnership with the Michigan Economic Development Council, the Center for Automotive Research, and Motorola, Inc., MDOT installed four vibrating wire gage sensors to demonstrate that a wireless communication system could be used for temporary installation for the annual Labor Day Mackinac Bridge Walk in 2007. Results from this demonstration showed that wireless communications could be used to monitor other MDOT bridges throughout Michigan.
Locating a similar sensor array at the Cut River Bridge — which is 24 miles away from the Mackinac Bridge — presented additional challenges for power sources, data transmission, and weather conditions.
In this project’s second phase, 16 fiber optic strain gages will be installed on the Cut River Bridge to detect the stresses placed on load-bearing members. Environmental sensors to detect moisture, temperature, chloride, and icing conditions will be installed in the bridge deck, and traffic monitoring sensors will be placed on the approaches to measure traffic speed, volume, and occupancy. Two closed-circuit television cameras will help verify data gathered by the sensors. A nearby weigh-in-motion sensor also feeds information to the data stream.
An Environmental Sensor Station, part of MDOT’s pilot Road Weather Information System (RWIS) program, was installed near the bridge, and will complement the data gathered by the bridge sensors.
MDOT’s RWIS program is in its early stages, being tested first in the Upper Peninsula. It includes eight new stations, adding to six existing stations in the region. The U.S.-2 corridor along Lake Michigan can be a particularly treacherous area in winter, and is among the areas most frequently closed because of whiteout conditions. The first stations were funded through a $1 million federal grant, and the next round is slated for funding through the American Recovery and Reinvestment Act of 2009.
A 70-foot tower at the Cut River Bridge holds a five-panel solar array that charges a battery system at the site, providing as many as 16 days of reserved power. The tower also gives the wireless transmitter the necessary height to clear the tree line between it and the south tower of the Mackinac Bridge, which will receive the data.
That data, along with information gathered from eight strain gages on the Mackinac Bridge, will then be gathered at the Mackinac Bridge Authority in St. Ignace, and made available via Internet to bridge engineers in Lansing.
Benefits of this program include the efficient scheduling of maintenance work, asset management planning, early alerts for any structural abnormalities, and security and threat detection, among others. Use of these types of sensors will augment MDOT’s field inspection programs required biennially on all bridges, and better overall management of transportation infrastructure for increased safety, mobility, and security.
John Batchelder, P.E., is the manager of MDOT’s Newberry Transportation Service Center. He can be reached at batchelderj@michigan.gov. Pete Paramski, P.E., is a delivery engineer with MDOT’s Newberry Transportation Service Center. He can be reached at paramskip@michigan.gov.
Get the latest from the RAI NewsBlog in your inbox!
• Screenshot • Subscribe