Major Interstate Bridges Require Updated Design Profiles
Extending 306 miles across the state, Interstate 80 is Iowa’s longest interstate highway. It’s also one of Iowa’s busiest roadways, carrying the vast majority of the truck traffic through and around the state. This heavy traffic also results in the premature aging of highway infrastructure, particularly bridges and roadway surfaces. In a coordinated effort to replace two Interstate 80 bridges over the West Nishnabotna River, the Iowa DOT partnered with Snyder & Associates and their affiliate Shuck-Britson to design and oversee construction on this project.
Each of the original 300′-long x 30′-wide continuous welded girder bridges carry two lanes of interstate traffic — one bridge for westbound and one for eastbound traffic. Not only were these bridges structurally deficient, their narrow design also meant they were functionally obsolete. Current design guidelines for new interstate bridges require the design have full-width shoulders in addition to the two 12′-wide travel lanes. The replacement design called for two 319′-long x 40′-wide continuous welded girder bridges to meet the required guidelines.
Additionally, the current roadway approaches leading to the bridges did not have the necessary “clear zone” geometry required by the current Federal Highway Administration (FHWA) minimum standards. A clear zone is an unobstructed area adjacent to a roadway that allows a driver to stop safely, or regain control of a vehicle that has left the roadway. The necessary addition of this feature provides an enhanced level of safety to all motorists.
Multiple Services Provided for Interstate Bridge Project
The Shuck-Britson and Snyder & Associates design team provided numerous services over the course of this project, including:
- Structural bridge design
- Roadway design
- Hydrology & bridge hydraulics
- Environmental study
- Topographic survey
- Wetland delineation
Time & Design Challenges to Meet Budgetary Constraints
From the outset of the project, the design team was presented with numerous challenges. The first was the need for an accelerated design to meet the funding distribution requirements. Because of this, the team was faced with a short window to complete the design. The second challenge was the need to maintain traffic flow along the interstate during the entire construction period. Finally, the state desired a low-maintenance bridge to keep future costs down.
The professionals at Shuck-Britson and Snyder & Associates coordinated with multiple offices of the Iowa DOT to prepare preliminary plans with hydraulic analysis and documentation, and final bridge and roadway design plans that met the shortened timeframe. To maintain full interstate functionality during the project, one bridge at a time was removed and replaced. This allowed for the use of traffic crossovers to shift traffic away from the bridge under construction. Portable message boards placed several miles in advance of the construction zone alerted drivers to the approaching crossover and bridge construction.
Interstate Bridge Construction & Maintenance Considerations
To keep future maintenance expenses in-check, the replacement bridges feature weathering steel girders, also known as beams. Weathering steel girders develop a protective rust barrier that doesn’t require periodic painting. The girders were also designed to specific depth constraints to achieve the required freeboard clearance above the river’s design flood, minimize transverse stiffeners, and minimize steel fabrication costs. Freeboard is the clearance between the bottom of the bridge girders and the high water surface elevation. An appropriate amount of freeboard allows for the safe passage of ice and debris under the bridge.
Our team designed the bridge so that no joints were necessary for the bridge deck. The entire bridge is tied together as one unit, where some bridges are designed in such a manner that allows for independent movement between the top portions of the bridge (known as superstructure) and the bottom segments (known as substructure). The absence of deck joints extends the life of the pavement by preventing salt-laden runoff from leaking down to the supporting structural elements below. Salt and moisture accelerate deterioration and can require costly maintenance or create the potential for expensive repairs.
The plan also called for T-piers supported on subsurface foundations that increase durability over their service life. Finally, to maximize economy and ensure quick construction, standard materials and construction details were used where appropriate in the bridge designs to further decrease expenses.
This interstate bridge replacement project is a prime example of how the Shuck-Britson and Snyder & Associates team works in conjunction with state agencies to create comprehensive roadway and bridge designs quickly and efficiently.