Kevin Damron, PE, Adam Demargel, John Michael Johnson, And Brad Robson, PE 2024-02-03 13:30:04
Environmental constraints prompted mid-project design changes to a new steel bridge in Kentucky.
THE APPALACHIAN DEVELOPMENT HIGHWAY SYSTEM (ADHS) links an entire region to major thoroughfares and guides drivers through tricky terrain.
It has 33 highway corridors that connect 13 Appalachian states with Interstate highways. Corridor Q, one of its many corridors, spans from U.S. 23 in Kentucky to Interstate 81 in Virginia. Commonly known as U.S. 460, the corridor traverses mountainous terrain in Kentucky, West Virginia, and Virginia and slices through hillsides—sometimes high above valley floors.
Corridor Q’s final Kentucky link is a twin bridge crossing the Russell Fork River, CSX railroad tracks, and Kentucky Route 80. Spanning that valley meant incorporating a bridge into U.S. 460’s curving path more than 220 ft above it. Steel erection is complete, and the Kentucky Transportation Cabinet (KYTC) anticipates all Corridor Q sections will open by 2025. (Progress can be monitored with the Russell Fork Bridge camera found at us460online.com).
KYTC’s District 12 guided the corridor design, from initial planning studies through design and construction.
Three unique issues at the Russell Fork Bridge affected its location and layout: an active railway loading site, an abandoned mine, and a habitat for a threatened species. KYTC’s decades of experience building bridges in mountainous areas and the early involvement of other design partners helped arrive at a practical design that worked around constraints.
In the early planning stages, the preferred alignment went directly over a facility that stockpiled coal and loaded it onto trains in the valley next to the Russell Fork River. KYTC and Palmer evaluated options to span over the facility with aerial easements and strategically located bridge piers that created long spans. Eventually, though, cost comparisons favored purchasing the load-out facility versus building over it. The acquisition had an unanticipated short-term benefit: owning the rail spur’s bridge allowed for its fast conversion to a vehicular bridge over the Russell Fork to provide construction access.
Meanwhile, an abandoned coal mine sat more than 200 ft below the proposed roadway. Coal had been removed via the room and pillar mining method many years before, but concern about potential mine subsidence and how it could affect the twin bridges led the project team to hire a specialist to evaluate the subsidence risk and potential mitigation. In the end, the team decided to change the roadway’s alignment to avoid the area of potential subsidence.
A third unique environmental challenge arrived in 2016 when the United States Fish and Wildlife Service listed the Big Sandy Crayfish as a threatened species, and the Russell Fork was a potential habitat field. Sure enough, Big Sandy Crayfish were discovered in the river directly in the proposed bridge crossing site. Extensive consultations with wildlife and environmental organizations determined that bridge piers could be located on the deeper side of the river but not on the shallow side where the crayfish made their habitat. The construction timeline adhered to seasonal restrictions on river work that protect the crayfish during spawning. KYTC also committed to placing a habitat structure in the river once all work is complete.
Solutions to those three challenges directly contributed to the bridge’s slight S-curve and span lengths. Once a final alignment was fixed, structural engineer Palmer Engineering advanced the design to optimize initial cost, ease of construction, and long-term maintenance. The spans needed to cross the CSX railroad tracks, the river and Kentucky Route 80 on the complex alignment. The bridge needed to be built despite limited workspace below the proposed site, on the hillsides, and across the river and railroad.
Concrete spliced I-girders and steel plate girders were evaluated. The lighter steel girders were chosen to facilitate erection over the railroad and the river, where crane placement locations were limited. The span over the river is the longest, at 285 ft. Other spans were proportioned for efficiency (ranging from 200 ft to 265 ft), and a constant web depth of 100 in. was maintained throughout. The web depth staying under 120 in. allowed for favorable steel prices, because the major U.S. plate mills competitively roll this size plate.
Each twin bridge consists of two six-span structural units with expansion joints at the abutments and between the two units. There are four lines of plate girders in each bridge. Combined, they feature over 6,200 tons of structural steel and 188 total girder segments that, if laid out end to end, would reach over 22,700 linear ft or 4.3 miles.
KYTC knew the twin bridges (2,810 ft and 2,875 ft long, respectively) represented a sizeable investment and invited contractors to submit Alternative Technical Concepts (ATCs). ATCs were submitted, and KYTC reviewed them confidentially. Contractors could include the approved ATCs as part of their bid on the project. Palmer Engineering wanted its design to be as competitive as possible with any ATC and contacted fabricator Stupp Bridge to ensure the layout and details were efficient and economical. Stupp Bridge did an in-depth review of the preliminary design and offered several suggestions, even though it had yet to secure the fabrication bid.
Two of Stupp’s suggestions significantly altered the final design. First, hybrid girders were chosen for the bridge: AASHTO M270 Grade HPS70W weathering steel is in the high-stress regions and AASHTO M270 Grade 50W weathering steel is everywhere else. The 70 KSI steel is slightly more expensive than the 50 KSI material, so it was used only in the most logical places, which were in the top and bottom flanges over the piers and in some of the bottom flanges in the middle of the spans. That resulted in lighter field sections overall, making the bridge erection more manageable for the contractor.
Second, and more importantly, Stupp Bridge suggested that the curves were mild and said it could slash fabrication costs if the girders did not curve with the roadway alignment. After evaluation, Palmer agreed that the difference in straight segments that kink at the field splice locations could accommodate the curves without greatly complicating the design. The plate girder segments ranged in length between 100 and 140 ft and weighed 25 to 45 tons each. Using low-maintenance weathering steel also saved the cost of shop- and field-painting.
The steel design was further optimized by using rolled angle cross frames shop-welded to gusset plates, which were then bolted to connecting stiffeners in the field. There were few unique cross frame piece marks for a bridge of this size, which allowed Stupp to minimize the set-up and fabrication time of these highly repeatable intermediate cross frames. Lateral bracing was used between a single bay of girders in each bridge and only over the piers and at the abutments.
KYTC received five bids for the bridge in November 2020, ranging from $65.6 million to $87.2 million. It awarded the project to the low bidder, Triton Construction. Triton selected Stupp Bridge to fabricate the girders to Palmer’s original design. Although many challenges were encountered along the way, the Russell Fork crossing is a prime example of the versatility of steel and how projects can benefit from fabricator input early in the design phase.
Owner
Kentucky Transportation Cabinet
General Contractor and Erector
Triton Construction
Structural Engineer
Palmer Engineering
Steel Fabricator and Detailer
Stupp Bridge Company
Kevin Damron (kdamron@palmernet.com) is a Senior Project Manager with Palmer Engineering. Adam DeMargel (apdemargel@stupp.com) is Senior Vice President of Sales for Stupp Bridge Company. John Michael Johnson (johnm.johnson@ky.gov) is the Project Manager for the U.S. 460 Project for the Kentucky Transportation Cabinet. Brad Robson (brobson@palmernet.com) is a Principal with Palmer Engineering Company.
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