Replacing a central bridge in the heart of downtown, in the state capital was going to be a high visibility project for the Idaho Transportation Department (IDT). A gateway to the city, Broadway Avenue (U.S. 20/26) carries traffic into Boise from the I-84 to the south. The average daily traffic count is 24,500 vehicles. With Boise State University's Albertsons stadium at the south end of the bridge, and a park, trails and major amenities just to the north, there is also heavy pedestrian and bicycle traffic.

Protecting the Historic Areas

The $21 million dollar Broadway Bridge project in Boise, Idaho, required complete removal of the old bridge, construction of a new one, widening and improvements to approaching roads and pathways for pedestrian and bicycle traffic, and minimizing impacts to the Boise River, and adjacent parks, historical and archaeological sites. These included Christ Chapel, one of the oldest church buildings in the state. IDT funded a mitigation project to protect the stained glass windows in the Gothic Revival building, which was constructed in 1866 and is listed on the National Register of Historic Places.

The existing bridge, a cantilevered concrete stringer built in 1956, was itself considered historic, and the keystone was preserved and put on display in a park. That bridge had replaced a truss bridge originally built for railway use in 1893.

"The bridge had provided service beyond its service life and was starting to deteriorate," said Daris Bruce, Resident Engineer, Residency 1 of ITD's District 3. The deck was delaminating and parapets and girders were showing signs of decay. "The primary reason for the bridge's replacement was in essence maintenance," Bruce said.

But the repairs needed to maintain and extend the 1956 bridge's service life would have been costly and only added 15 to 20 years to the bridge's service potential. "In the end the pubic would have had a bridge identical to that preceding the project with no improvements," said Bruce. He noted that the 1956 bridge had substandard shoulders and very narrow substandard sidewalks.

Increasing Capacity for Public Use 

After a consultative process with the community, ITD developed a design for the replacement bridge worthy of its status as a centerpiece of public infrastructure. One change reflecting the popularity of the Boise River for recreational use by floaters and others was the reduction of spans along the 472-foot length - from five to four, to accommodate public use of the river. To increase traffic capacity, the 108-foot width of the deck would accommodate six lanes (three in each direction) of motorized traffic, up from the four lanes on the previous bridge. Full bicycle lanes and 10-foot-wide sidewalks also became part of the design. The deck, which is about 13 feet above the river, features six 18-foot-wide belvederes, or viewing platforms, for pedestrians to pause and take in the scenery. The new features also include bicycle and pedestrian ramps and a boat launch for water rescue units.

The entire project was modeled in 3-D using computer-aided design and drafting (CADD) systems, Bruce said. That facilitated the work, but also the public outreach, as ITD was able to post an animated video fly through of the design. 

A "˜Compressed' Project

It was decided to completely close the bridge and approaches to facilitate the work so that it could be completed in one construction season. ITD managed the Broadway Bridge project through a design-bid-build process. The Department designed the project and contractor Knife River won the contract and carried out the construction.

 "A compressed project like this, compressed in schedule, location, and context sensitivity, requires all parties to bring their "A" game," said Bruce. Utilities and their potential conflicts needed to be extensively investigated and verified during the development process, he said, and then collaborated closely during actual construction. The utilities involved included electric power, water, gas, fiber optic and legacy copper telephone wire. "Everything but an oil pipeline," said Bruce.

Bruce said the Broadway Bridge replacement was a successful example of a collaborative process, with good coordination among all the entities involved. A few tweaks and corrections to the design along the way did not throw anyone off their game. "The contractor stepped up and did a very good job," Bruce said, "They took a grin and bear it attitude and worked through the sticking points."

Demo and Construction 

The bridge was closed just after the New Year, on January 4, 2016, and crews began removing the structure.

"The 1956 bridge was demolished in place using conventional methods," said Bruce, including excavators with shears and thumbs, front-end loaders, and cranes. Fortunately, the riverbed surrounding the bridge is shallow enough so that trucks and over the road equipment could drive into place for the work. 

When the demolition was finished, piles were driven, and pier caps installed. Once the piers had cured out crews placed 30, 135-foot weathered steel haunched girders. Above that came roughly 50,976 square feet of bridge decking. About 2,500 cubic yards of concrete were poured in the project, including 0.7 miles of replaced sidewalk. 

Workers made extensive use of GPS equipment during the project, Bruce said, saving time and improving accuracy.

Paving work on the road improvements included 0.3 miles of new asphalt on Broadway Avenue (six lanes wide with shoulder), Bruce said.  

While some finishing work remains, the new Broadway Bridge opened for traffic on September 9. Just in time from the perspective of Boise State football fans - the opening came one day before Broncos hosted their first home football game of the season on their famous blue turf. 

Bruce said the increase in foot traffic on the bridge has been impressive. He also noted as a skeptical, pragmatic engineer, he had not been excited about some of the design aspects, such as the belvederes, or the curved railing with integrated night lighting, and surface treatments on the retaining walls and path. "At first I didn't see the value," he said, but now sees how well they all work, and how the elements "make it pop."