Every project has some elements that diverge from the norm and require the designer to develop and use innovative methods and designs to meet the unique challenges and requirements of the project and the client.
When Howard R. Green Co., a consulting engineering firm in Cedar Rapids, Iowa, began designing two bridges for the city of Clinton, Iowa, it quickly became apparent that the vertical and horizontal clearances would dictate the use of an innovative design. While using creative engineering solutions is common, the design used for these bridges were anything but common. Known as Inverted Tee Girder (IT-Girder) bridges, they were the first of this type to be built in Iowa, and only a handful of this type have been constructed in Nebraska.
Additionally, the IT-Girder Bridges used for the Clinton bridge projects are the first bridges of this type in the state to be approved by the Iowa Department of Transportation (IDOT). Over the course of a year, all companies involved in the project had to go through a lengthy iterative process of design and review with IDOT to prove that the connections between piers and the superstructure and between the abutments and superstructure were adequate and met the IDOT standards.
The original IT-Girder bridges were first designed by Dr. Maher Tadros, Ph.D., P.E. and Dr. Mounir Kamel, Ph.D. from the University of Nebraska-Lincoln, in association with the Nebraska Department of Roads (NDOR), and were built in Nebraska in 1996.
It is important to note that the bridges in Nebraska were used as a pattern for the Clinton bridges. As with any new idea, the original design will evolve each time an engineer uses it, improving on the design with the lessons learned from all of those that came before it. In this way, the design of the Clinton Bridges was unique in its own evolutionary step, while maintaining the original design philosophy of the IT-Girder System.
With IDOT being responsible for approximately 4,000 bridges longer than 20 ft, the time it takes to develop and review a new design is worth the effort if it is to provide a good alternative design which is flexible, affordable and fairly easy to construct.
Due to the increase in bridge funding resulting from TEA-21, additional structures will be programmed for replacement in the near future in the state of Iowa. It is reasonable to presume that many of the bridges slated for replacement in Iowa will be over waterways, roadways or railroads where vertical clearance restrictions are imposed.
Background on Clinton project
The Howard R. Green Co. was retained by the city of Clinton to design improvements to a segment of US 30/US 67, a primary highway though the heart of the city. The design is split into two sections and incorporates pavement widening and an addition of an exclusive left turning lane, bridge widening, ramp widening, signalization, intersection improvements, as well as the total replacement of two bridges in the corridor, the Mill Creek Bridge and the bridge to span the Union Pacific Railroad.
The replacement of the two bridges presented many challenges, the most critical of which were:
* Blending architectural features into the bridges, particularly the railings and approaches;
* Match existing roadway profile;
* Satisfying the Union Pacific Railroad requirements for vertical and horizontal clearances;
* Bridge was designed to be stage constructed one half at a time to keep corridor opened to traffic; and
* Completed feasibility study to determine whether to widen or reconstruct the bridges.
The IT-Girder system is a simple one that uses a pretensioned, prestressed concrete beam in the form of an IT Girder with a 6-in. cast-in-place deck. It is intended to provide an alternative to continuous concrete slab bridges because it has an excellent span-to-depth ratio and it does not require extensive forming as do slab bridges.
The IT-Girder system is a choice for structures which have vertical clearance restrictions but where the required span lengths are greater than those typically spanned by the slab bridges. By utilizing specific continuity design and details, an even more efficient span-to-depth ratio could be realized.
After the girders are placed, plywood decking is set on these notches to use as the form for placing the concrete deck slab. Because the plywood decking stays in place for the life of the structure, removal of the forms from beneath the deck will never be required as it is for traditional superstructure systems. Virtually no false work is required for the superstructure of the IT-Girder system, and there is a significant advantage for railroad or roadway overpasses, where strict limitations for vertical clearances are imposed.
Where have they been used?
Since development, at least five IT-Girder bridges have been built in Nebraska. Dr. Tadros and his consulting firm, Tadros Associates, LLC of Omaha, worked closely with the NDOR, area contractors and prestressed girder manufacturers as the details for the IT-Girder system evolved since the first IT-Girder bridge was designed in 1996.
Now, two more bridges can be added to the list of IT-Girder bridges with the inception of the bridges for the city of Clinton on Highway 30/67. The two bridges posed challenges for vertical and horizontal clearances. Using the IT-Girder superstructure solved vertical clearance problems for both bridges by allowing longer spans while using shallower members. By utilizing the longer spans, the existing piers will be "straddled" by the newly constructed piers, enabling half of the structure to remain open to traffic for staged construction.
Due to vertical restrictions imposed by the Union Pacific Railroad, the need for falsework for a CCSB was eliminated, which was essential for construction feasibility of the railroad overpass. The longer span lengths also eliminated the use of costly crash walls around the piers of the railroad overpass. The IT-Girder system provided a solution to the problem of vertical clearances that would have otherwise been very difficult to overcome without adding prohibitive cost to the project.
In the case of the Mill Creek Bridge, eliminating falsework not only reduced the cost of forming, but also allowed staged construction, as with the railroad overpass. Because it was possible to use longer spans than would have been possible for a CCSB, the piers were kept out of the streambed on the Mill Creek Bridge. This is more desirable with respect to hydraulic and constructability aspects.
The IT-Girder bridge on average is more cost efficient. It is used in comparison with slab bridges because the depth of the superstructure is comparable. However, the IT-Girders are advantageous when spanning greater distances where depth of the structure is critical.
In closing, IT-Girder bridges are reliable and practical despite the fact they are a new concept in design among engineers. One would want to use an IT-Girder bridge where vertical/horizontal clearances are critical. This type of bridge is excellent for those projects where the construction areas are limited and where superstructure forming should be minimized.