SR 32 Bridge over Stony Creek

A new bridge installed in suburban Indianapolis is part infrastructure project, part research collaboration. And its use of steel traces directly back to a February 2018 NSBA brainstorming workshop aimed at making steel bridges more competitive in the short-span marketplace.

The workshop introduced a bolted-up tub girder concept as a strategy that capitalized on the steel industry’s ability to fabricate components on demand with steel that is already in stock. It more closely mirrors the workflow used by the precast concrete industry, which dominates the short-span bridge market. Indiana Department of Transportation (INDOT) representatives were in attendance and intrigued by the concept.

In January 2019, the University of Notre Dame, in collaboration with HNTB Corporation, pitched the same concept to INDOT. It was funded in the next fiscal year, with a start of January 2020.

Innovation is a core INDOT value and aligns with INDOT’s mission to focus on new, practical ideas and technology to continuously serve customers better and more efficiently. Using built-up press-brake-formed tub girders (PBFTGs) to replace the State Road 32 bridge over Stony Creek in Noblesville offered just the opportunity to take the plunge. It represented a typical bridge in Indiana traditionally delivered using precast prestressed concrete girders. Fostering innovation in short-span steel bridges can enhance competition in that span range and help DOTs reduce project costs and delivery schedules.

Using steel was critical, but readily available steel was the essential component of condensing the project timeline and lowering costs. Leveraging the workflows and equipment that form an integral part of the fabrication of transmission poles (a nearly $3 billion North American market) was a primary goal. The project created a path for new fabricators to enter the bridge market, especially given the unprecedented funding for infrastructure in the Infrastructure Investment and Jobs Act.

The State Road 32 replacement project included a unique and collaborative delivery team of engineers, researchers, steel fabricators, and suppliers: INDOT, the University of Notre Dame, HNTB, Delta Steel, and Nucor Corporation. The team’s rare level of depth and comprehensiveness was critical to implementing a novel design concept in less than three years. Nucor and Delta Steel donated their steel and fabrication time for this project, representing a remarkable commitment from the steel industry to develop new and innovative solutions.

The SR 32 bridge project fits the PBFTG approach in several ways. First and foremost, it extended the span range of applicability. It allowed the team to answer the key implementation questions that go with any new technology. Its more rural setting and water crossing helped the research team gain significant access to the bridge to validate performance before erection, following deck slab completion and under live load.

The project had five key objectives:

1. Develop and implement a new design methodology (HNTB) consistent with AASHTO, but also build on recent research on internal redundancy and folded press-brake tub girders.

2. Conduct research (Notre Dame) to ensure design workflows are appropriate for design and load rating of this new bridge typology, measure the behavior of the built structure as experimental evidence of performance, and ultimately develop a kit-of-parts approach for wider industry adoption (ongoing).

3. Work together with the PBFTG manufacturer (Delta Steel) to develop an implementation strategy that uses their workflows (press brake and bend radius tooling, CNC drilling, and plasma cutting capability).

4. Ensure the steel used for the press-brake bent webs meets or exceeds AASHTO requirements (Nucor).

5. Incorporate PBFTG bridges into INDOT’s established workflows, including shop inspection, shop drawing reviews, and bridge load ratings.

The team’s unwavering collaboration allowed this project to go from a sketch at a workshop to a completed project open to traffic in less than three years, despite the inherent challenges of implementing innovation into practice.

While PBFTG bridges are becoming increasingly common and are an alternative to replacing prestressed concrete vehicular bridges, they are typically limited to spans of less than 90 ft and are not designed as continuous. The approach developed for the State Road 32 bridge used a bolted-up section with press-brake-formed webs bolted to flat bottom and top flange plates. Because this bridge design allows for changing plate sizes in top and bottom flanges and much deeper tubs, spans up to 300 ft are possible.

Additionally, bolted built-up fabrication allows for the internal redundancy design methodology, further optimizing the bridge for efficient and reliable service with tailored inspection protocols intrinsically linked to the damage tolerance and resilience for which this design approach is known.

All five steps brought challenges and hurdles. How are shop drawings translated into fabrication drawings for CNC cutting and hole drilling? Could holes be drilled before bending? Would they be sufficiently accurate and reproduceable to allow for web-to- flange assembly? The tubs are fabricated in roughly 30- to 45-ft segments and must be piecewise straight between splices--could these be accurately cambered? Would heat straightening be necessary? Could they be hot-dipped in galvanizing tanks without significant distortion?

Those questions and more had to be answered for successful project delivery. The team’s collaborative nature and additional effort--particularly on the part of INDOT--introduced a shop assembly process before and subsequent to hot-dip galvanizing that was instrumental to ensuring project success. Cross-discipline learning and collaboration in a short time frame is the best way to measure value for this project.

The State Road 32 bridge builds on the PBFTG system and could extend its applicability for much longer spans. Further, it introduces internal redundancy as a strategy for enhanced safety and resilience. By eliminating the need for welding, the State Road 32 approach reduces the fabricator’s need for highly skilled labor, minimizes shop setup and handling, and prioritizes automation. Most importantly, it promises to deliver fabricated steel bridges in weeks instead of months.

Tub girders were half the weight of the traditionally used precast prestressed concrete girders. Moving them required a much smaller crane, and multiple girders could be shipped on a single truck. The movability was particularly useful for the State Road 32 bridge, a river crossing, where the lighter pick sizes simplified the complex crane operations from behind the stub abutments.

Further, the bridge confirmed the original research premise that press-brake bent webs could be fabricated using steel transmission pole manufacturing workflows with in-stock steel, and pre-drilled holes to sufficient accuracy can be readily assembled into tub girders. The potential for these webs to be fabricated on demand as a “kit-of-parts” solution in 6-in. member depth increments (36 in. to 96 in.) over a range of two or three plate sizes (½-in., 5∕8-in., and ¾-in) using in-stock steel gives designers, fabricators, contractors, and bridge owners a new solution to meet the schedule needs for rapid bridge replacement projects.

The bridge opened to traffic in July 2023 and has already generated significant interest in the industry at a level typically seen only several years after the launch of a new approach.

Of all the project’s noteworthy aspects, the time from concept to completion stands out. Implementation of a new bridge typology is typically only undertaken after research and laboratory testing. Even with the team taking advantage of 10 years of work completed on PBFTGs, the new built-up press-brake-formed tubs introduced many new design, fabrication, and construction challenges. Most of those challenges involved implementation, not design, making implementation an important project goal.

The project confirmed several aspects of the fabrication. The webs could be reliably and accurately bent without unusual fit-up issues at the splices. Bolt holes could be pre-drilled prior to bending. Flanges and webs could be drilled using CNC equipment with limited hole rework as part of assembly. Webs can be nested for cost-efficient trucking. Hot-dip galvanizing did not introduce significant distortion and did not impact fit-up. Chorded connections achieved accurate camber. Full flange and web splices, bolted diaphragms, and access openings were all incorporated into the workflows with no welding other than shear studs for composite action.

The State Road 32 bridge extends the range and applicability of PBFTG bridges. It introduces the potential for unconventional steel bridge fabrication workflows and capabilities to provide additional fabrication capability to the industry. Using bolted connections provides for internal redundancy, making the PBFTG structural system even more robust and tolerant to damage with tailored risk management throughout the structure’s life.

Project Team

• Owner: Indiana Department of Transportation, Indianapolis

• General contractor: HIS Constructors, Inc., Indianapolis

• Structural engineer: HNTB Corporation, Kansas City, Mo.

• Consultants: University of Notre Dame, South Bend, Ind.; Nucor, Boise, Idaho

• Steel team:

  • Fabrication team: Infra-Metals/Delta Steel, Houston *AISC full member; AISC-Certified fabricator*; Kard Bridge Products, Minster, Ohio *AISC full member, AISC-Certified fabricator*

  • Detailer: Weaver Bridge Corporation, Granville, Ohio *AISC associate member*

  • Galvanizer: V&S Galvanizing, Columbus, Ohio *AISC associate member*