I-91 Interchange 29 Flyover Bridge, Charter Oak Bridge
Originally constructed in 1940, the Charter Oak Bridge is a major steel bridge crossing the Connecticut River connecting the City of Hartford downtown with the Town of East Hartford. Often referred to as “the congestion buster,” the three-year, $240 million Charter Oak Bridge project combined two major projects that involved three heavily traveled roadways and 22 bridges. The route serves as a major connector between I-91, I-84, and Route 2 in East Hartford.
CHA was selected to deliver the final design to address safety issues associated with congestion and operational performance of Interchange 29, where I-91 northbound exits to the Charter Oak Bridge. With over 44,000 vehicles traveling northbound daily, the project required significant modification to seven corridor bridges, including widening the first five spans of the Charter Oak Bridge and the reconfiguration of several interchanges. The centerpiece of this project was the innovative reconstruction of Exit 29, where I-91 northbound connects to Route 5/15 northbound. The original right-hand single-lane ramp was a source of significant congestion that led to daily back-ups, accidents, and delays. The new interchange configuration includes a left-hand high-speed, two-lane diverging ramp connecting I-91 northbound to Route 5/15 northbound. The configuration of the new ramp led to many challenges associated with crossing the two roadways at a very flat intersecting angle, which often requires the use of a straddle bent structure that can span over the lower roadway.
The new exit ramp bridge is a continuous trapezoidal box girder bridge with hammerhead piers and a straddle bent pier intended to decrease traffic congestion. Steel straddle bents have always been considered “fracture critical,” meaning that fracture of the girder might result in the collapse of the entire bridge. The national bridge design specifications allows the use of fracture critical sections, but national bridge inspection standards mandate more frequent and costly inspections to ensure public safety. The CHA team developed a unique solution to this problem involving an innovative design of the straddle bent using the “Load Path Redundant Member” (LPRM) approach. This was accomplished by converting a typical single-cell box girder section into a triple I-girder member. The triple I-girder configuration provides adequate load path redundancy, eliminating the fracture-critical designation and the special long-term inspection requirements. An added benefit is that the design is less expensive to build due to the simplicity of the detailing. The fabricator for the bridge indicated that the fabrication costs are approximately 50% less than an equivalent box girder section. The benefits of this design are substantial, with improved safety, reduced construction costs, and reduced long-term maintenance costs.
Our team also designed significant strengthening measures necessary for the existing Charter Oak Bridge based on our load rating analysis using full finite element modeling of the entire bridge and a strain gauge investigation to verify actual live load stresses. The existing bridge was also widened by approximately 40 feet with curved, splayed girders spanning more than 270 feet.
Awards:
- 2023 ACEC Connecticut Engineering Excellence Awards - Engineering Excellence Award (Transportation Category)
- 2022 National Steel Bridge Alliance Bridge of the Year
- 2022 National Steel Bridge Alliance Prize Bridge Award (Medium Span Bridge)
Read more about the I-91 Interchange 29 Exit Ramp Flyover Bridge and 2022 Prize Bridge Award recipients in the July edition of Modern Steel Construction: https://lsc-pagepro.mydigitalpublication.com/publication/?i=749779&p=28&view=issueViewer.
To read more about the project and National Steel Bridge Alliance Bridge of the Year award, visit https://www.aisc.org/nsba/prize-bridge-awards/prize-bridge-winners/i-91-interchange-29-exit-ramp-flyover/.
Project Services:
- Bridge Design
- Highway Design
- Community Outreach
- Water Resources
- Survey
Ingenuity:
- Unique stability solution for widening curved, splayed structure
- Unconventional materials to mitigate settlement concerns
- Unique pier design to span existing highway
- Complex design/load rating pushing code limits