Project Case Study: Context-sensitive bridge design

February 2008 » Feature Articles
New structure improves traffic flow and compliments city architecture.
Eric Johnston, P.E. and Jeff Von Handorf, P.E.
New structure improves traffic flow and compliments city architecture

Project
Lincoln Avenue Bridge, Cheboygan, Mich.

Civil engineer
ROWE, Inc., Flint, Mich.

Product application
Precast, 60-foot span arch units from CONTECH Bridge Solutions Inc. meet difficult bridge-replacement restrictions.

The city of Cheboygan, located along Lake Huron, near the northern tip of Michigan’s Lower Peninsula, experiences heavy snowfall and cold winter winds from the lake. The original Lincoln Avenue Bridge, a prestressed concrete, box-beam structure, carried traffic over the Cheboygan River for nearly 40 years and connected two busy downtown areas. But, because of the area’s severe winter weather, de-icing salts were frequently used on the structure from November through April. Once the asphalt overlaying the existing bridge’s beams cracked, salts entered the old structure and began to corrode it.

"The existing bridge was constructed to last 50 to 75 years, but didn’t make it to 40," said James Muschell, mayor of Cheboygan. "Over the years, the salt ate away at and deteriorated the beams to a point where the tops were in very bad condition."

Because of its deteriorated condition, the bridge was posted for a load limit of 16 tons. The only other river crossing, a nearby drawbridge at US-23, was often open to allow boat traffic to pass during the summer season. Large trucks, school buses, and emergency vehicles traveling to and from a local hospital were forced to reroute to the frequently closed drawbridge, often leading to traffic backups and congestion.

The 380-foot-long, six-span, Lincoln Avenue Bridge over the Cheboygan River opened in November 2007.

 

The city of Cheboygan requested state of Michigan Critical Bridge funding to replace the structure. In 2005, the governor of Michigan, along with the Michigan Department of Transportation (MDOT), announced that the state would spend an additional $53 million to repair or rehabilitate 76 local bridges, which included the city-owned Lincoln Avenue bridge. The initiative was part of the administration’s long-standing commitment to increase investment in the preservation of local infrastructure.

Design factors
Utilizing a context-sensitive design process, ROWE, Inc., Flint, Mich., reviewed the project needs with the city of Cheboygan and other community stakeholders. Concept plans were prepared using different structure types, roadway configurations, and aesthetic considerations. Roadway geometrics, bridge height above the river in the channel, width of channel opening, and potential uses of the structure were all considered in preparing design concepts. The concepts were presented to the Cheboygan city council, planning commission, and other local organizations for review and consideration. Using the input gathered from these presentations, ROWE was able to narrow down the structure type, needs of the structure, and other considerations that would be included during the design and construction process. A final concept was presented to the city and MDOT for review and approval prior to start of structure design.

ROWE, assisted by United Design Associates, Inc., Cheboygan, faced several unusual challenges during design, including scheduling, permitting, constructability, and recreational requirements. In addition to the obvious requirement to carry vehicular traffic, pedestrians, snowmobiles, recreational water craft, and utilities all had to be considered during the design and construction of the structure. Also, With Lincoln Avenue being the southern gateway to the Cheboygan community, and thus a focal point of the area for entering tourists, aesthetics were important in reconstruction of the bridge.

Installation time was also a major factor in structure selection, with several limitations imposed on the construction schedule and methods. A gas line, which runs across the structure, serves as a main feed for an adjacent community and could not be shut down from the end of September to the beginning of April.

The city and the U.S. Coast Guard also wanted to limit the impact of construction on boating traffic on the Cheboygan River. Just downstream from the bridge, a dam and locks transport boats to Lake Huron; immediately adjacent to it is a Michigan Department of Natural Resources (MDNR) boat launch. Construction of the new structure had to ensure safe navigation for recreational boaters and cause minimal shut down of the waterway.

"Recreational boating is an important part of the city’s economy," said Scott McNeil, Cheboygan city manager. "It was critical that traffic along the river remain as unaffected as possible during construction of this bridge."

Permitting for construction of the project was not only required from the U.S. Coast Guard, but also from the Michigan Department of Environmental Quality (MDEQ) and the U.S. Army Corps of Engineers. When the MDNR determined the presence of two protected and one special-concern fish species in this stretch of the river—the Channel Darter, Lake Sturgeon, and Pugnose Shiner—the project was almost brought to an immediate end. Because of the fish’s spawning periods, construction could not occur from April 1 to Aug. 31.

The gas main issue, however, required that the bridge be in place by Oct. 1. Combined with the fish concerns, only one month was available for construction—an impossible time constraint for a bridge project of this size. Understanding the necessity for the bridge and the time required to construct it, ROWE and the MDEQ worked together to devise a construction method that would limit any impact on the protected fish and allow for a longer construction time period. Using steel sheeting cofferdams, floating barges, turbidity barriers, and other methods to minimize the impact to the river bottom, ROWE was able to come up with a method acceptable to the MDEQ that would eliminate construction shutdowns during the spawning periods. The contractor, Milbocker & Sons, Inc., also conducted a field review with the MDEQ at the start of the project to verify that their work plan was in line with the MDEQ’s requirements.

After reviewing bridge options and considering the construction schedule required for each, it became evident that the best solution would be a precast structure. ROWE called upon CONTECH Bridge Solutions Inc. to discuss the precast options available.

The solution became a CON/SPAN precast arch structure, selected primarily for functionality and aesthetics. The CON/SPAN Bridge System, a patented precast modular system, could promise a complete bridge installation within the tight time constraints.

A precast unit is being loaded onto a launching frame, ready to be sent out over the Cheboygan River.

 

"Without utilizing the precast process, the single construction season project would have turned into a two-season project at a minimum," said Patrick Conner, P.E., project engineer with ROWE.

Because of navigational requirements of the Cheboygan River, a minimum span of 60 feet was needed. Precast, three-sided structures are typically limited to maximum spans around 40 feet because of shipping and handling constraints. However, the arch shape and relatively slender section of the CON/SPAN geometry made construction, shipping, and handling of a single-piece, 60-foot span possible. This design is currently the largest single-piece, three-sided structure available.

With the high span-to-rise ratio, piers and abutments needed to be designed to resist large lateral loads. The construction sequence only allowed one span to be set at a time, resulting in unbalanced lateral loads at the piers when only one precast unit leg was present. CONTECH and ROWE worked together to develop a plan to minimize the loads applied to the piers during construction. In five of the six spans, high-strength threaded rods were used to control deflection and resist lateral loads under self weight of the precast units until all cells were installed and loads were balanced.

The bridge’s vertical alignment also posed a challenge. The design had to match the existing elevation of an intersection approximately 40 feet from the end of the bridge, while meeting the U.S. Coast Guard’s height requirements. The Coast Guard, which governs navigable waterways, verified in advance that the rise of the bridge would be high enough for boats to pass under it.

Fabricating the precast members (arches, headwalls, and wingwalls) offsite while the contractor constructed the foundations saved a tremendous amount of time, considering the overall project schedule. Precasting the bridge members in the plant was a four-month process, and the contractor was able to set the precast members in six weeks.

Production of the 60-foot span precast arch units began in July 2007. Installation began in August and continued through early September. ROWE worked with general contractor and the Coast Guard to limit the channel shutdown to two days, which was necessary to set one of the spans over the main river channel. Milbocker also devised construction sequencing that met the regulatory agencies’ requirements.

Construction sequencing for setting the arches was developed during design to limit the amount of time the contractor would need to shut down the channel during the placement of the arches. The contractor was able to set four of the spans using its 110-ton crane with the channel open to recreational boating. However, because of reach and regulatory agency restrictions, the contractor brought in a 550-ton crane to set the remaining two spans. The larger equipment allowed the last two spans to be set in three days, limiting the impact to recreational boaters.

A barge crane removes the second precast bridge unit from the launching frame.

 

Temporary lighting on the substructures, contractor’s barges, and channel location markers assisted boaters in navigating the channel and kept them out of the construction work area. A U.S. Coast Guard-approved work plan was also in place and followed during construction.

During installation, one span needed to remain open at all times for boat traffic. For that span, the foundations were designed to support the arch reactions without the temporary tension rods that were used on the other spans. The construction sequence was set so that this span could be set last, when the surrounding arch units were in place to help resist the lateral loads from the arch self weight. For some spans, a crane was set on a barge to set the bridge units without disturbing the riverbed.

The new bridge was completed in only eight months and features prefabricated concrete arches, wingwalls, and headwalls. Six spans of 60 feet each are composed of 96 precast arch units, 12 wingwall pieces, 30 headwall pieces, and 12 nosecones.

Engineers also designed a small drainage system to collect water and move it away from the precast concrete members. This drainage system, combined with the arch’s natural ability to shed salt leachate, helps prevent damage from roadway salt and will allow the bridge to last its expected 75 to 100 years.

"Utilizing precast construction methods allows you to expedite construction and gives you a better-looking bridge than the conventional cast-in-place structure," Conner said. "Using the precast units allowed us to set it, grout the legs in place, cut the tensioning cables for the arch, and open the channel very soon after we placed the precast sections,."

Special touches help make the new bridge visually attractive. A prefabricated steel railing blends with the color of the surrounding water and features the city’s logo in a cutout section. Decorative street lighting matches the type used in previous downtown improvement projects, which helps to unify the area’s appearance. Greenstreak fractured rib series formliner was used for aesthetic purposes.

"The new bridge is more aesthetically pleasing," said McNeil. "In addition, the geometric configuration better serves traffic."

The bridge also serves as a popular link between two snowmobile trails. Because Michigan law prohibits snowmobilers from riding on sidewalks, bridge designers had to accommodate snowmobile traffic with a wider structure and by adding an epoxy overlay to the new bridge’s shoulders that allows them to stand up to the wear and tear snowmobile studs impose.

"The new bridge improves upon the old one in many ways," McNeil said. "The project was well planned and the process has gone smoothly."

The new Lincoln Avenue Bridge in Cheboygan was opened on Nov. 26, 2007. The $6.1 million bridge is 380 feet long and 60 feet wide and includes 5-foot-wide sidewalks on each side, three traffic lanes, and a concrete curb and gutter. The six-span structure stands ready to transport city residents and visitors over the Cheboygan River, allowing them to take advantage of the year-round recreational opportunities that the area offers.

Eric Johnston, P.E., is project manager, ROWE, Inc., Flint, Mich. He can be contacted at ejohnston@roweincorp.com. Jeff Von Handorf, P.E., is precast engineering manager, CONTECH Bridge Solutions Inc., Dayton, Ohio. He can be contacted at vonhandorfj@contechbridge.com.


Context-sensitive design
The Lincoln Avenue bridge project’s success results from collaboration among planners, engineers, U.S. Coast Guard, Michigan Department of Environmental Quality (MDEQ), Michigan Department of Transportation, the city of Cheboygan and community stakeholders. With a wide range of vested interest in the project, use of a context-sensitive design process incorporated the needs of the community in preserving scenic, aesthetic, historic, and environmental resources while maintaining safety and mobility.

Preserving aesthetics—The pedestrian railing blends with the color of the surrounding water and features the city’s logo in a cutout section. Decorative street lighting matches that used in other downtown improvement projects, which helps to unify the area’s appearance.

Safety—Temporary lighting was placed to assist boaters in navigating the channel during construction and a U.S. Coast Guard-approved work plan was in place.

Mobility—A precast system was chosen to minimize road closure time. Construction sequencing was developed to limit shut down of the channel. Keeping one span open throughout construction lessened disruption to recreational boat traffic.

Pedestrian-friendly—Bridge designers had to accommodate snowmobile traffic on the bridge, adding an epoxy overlay to the new bridge’s shoulders to allow the shoulders to stand up to the wear and tear snowmobile studs impose.

Environmental sensitivity—Working with MDEQ, construction methods were used to minimize impact on protected fish species in the river.


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