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Wed. Apr. 25th, 2018

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Arbor Rail Line Bridge, Nebraska City, Nebraska Print Email

2001 ACI NE Chapter Award of Excellence 

Tadros Associates’ work on OPPD’s Arbor Rail Line Bridge 5.90 demonstrates the benefits that value engineering can provide to the owner and the significant engineering challenges Tadros Associates faced in providing an alternative concrete bridge design.  In value engineering Bridge 5.90, Tadros Associates not only provided a more economical solution, but also one that could be constructed faster and safer.  In addition, the concrete alternate would be more durable and requires no painting or other maintenance expenses over the life of the structure.

The Arbor Rail Line is one of OPPD’s main coal lines feeding the Nebraska City power plant.  During an inspection, the bridge was found to be deficient and in need of replacement.  The replacement was designed as a steel bridge and let for bid with the construction contract being awarded to L.G. Barcus & Sons, of Kansas City, Missouri.  After the construction drawings were released, Tadros Associates started working with the owner and contractor on possible ways to cut construction time and cost. The contractual agreement specified that the rail line could be down for only 72 hours.  It was believed that if a precast alternative could be provided for this structure, while still preserving the profile of the already shallow and heavy steel system, it would have to be creative. It would have to use the latest in material technology and structural theory/design.  The most challenging aspect of the bridge was that the clearance available was very tight, even for a structural steel solution.  Tadros Associates, working closely with CSR Wilson Concrete (now Rinker Materials) of Omaha, Nebraska, developed an alternative concrete design.  The design was proposed to the contractor, who in turn proposed it to the owner and original designer.  The alternate design was ultimately accepted by all concerned, and Tadros Associates was directed by the contractor to develop the necessary construction documents.

The first challenge faced by Tadros Associates was to provide a concrete alternate that fit within the profile of the steel structure.  Modifications to the profile grade line, vertical clearance limits, bridge width, or substructure would result in an unacceptable solution.  The original steel design consisted of five rolled W40x324 steel beams as the main load carrying girders.  There were also two rolled W35x150 steel beams that were used to support walkways on either side of the tracks.  The W35 beams were not designed to carry any of the design loads.  The walkways were elevated to avoid interference with the tracks.  The presence of the elevated walkways created a non-uniform cross-section, which, while challenging, also provided much needed additional structural depth. 

Two unique precast prestressed concrete solutions to the steel alternative were considered.  One system consisted of five modified NU I girders with a precast integral deck.  The second alternative consisted of a box girder system.  Tadros Associates performed a preliminary design of both systems and ultimately choose the box girder system.  The box girder system was unique because it consisted of different sized boxes that were transversely post-tensioned.  By using fully designed diaphragms and transverse post-tensioning, rather than the nominal values generally used, all five boxes worked together as a single unit to resist the design load.  Additionally, the box girder system could be produced and erected very quickly; the entire superstructure consisted of only five precast pieces. 

The box girder system consisted of two exterior boxes that were 3’-8” wide and 5’-0” deep and three interior boxes that were 4’-4” wide and 3’-4½” deep.  The use of different size boxes allowed the concrete alternate to match the profile of the proposed steel structure.  The taller exterior boxes created the elevated walkways, and the interior boxes were sized to match the depth of the five W40x324 rolled steel beams.  The combined width of the three interior boxes provided the bed for the railroad tracks and ballast.  After transverse post-tensioning, the exterior boxes became an important element of the combined structural system.  Without structurally designed rigid diaphragms and post-tensioning, only the three interior boxes would have carried the majority of the design load, and the significant capacity of the two exterior boxes would have been wasted.  This important understanding of the structural system is what allowed the concrete alternate to compete favorably with the original steel alternate.  The concrete box system had a span-to-depth ratio of approximately 20, which results in perhaps one of the most slender concrete railroad bridge ever built in the U.S. for this span of 65’-10”.

The structural analysis of this system was complicated by the fact that the cross-section was non-uniform and the loading would occur in “phases”.  Prior to transverse post-tensioning, the boxes had to be designed as individual elements.  Loads included longitudinal prestressing and self-weight.    During the second phase of loading, the five boxes acted together as a combined structural unit.  Second phase loads included the weight of the ballast, tracks and live (train) load.  Because of these complexities, a conventional software design package, such as is typically used for bridge design, could not be used.  Instead, Tadros Associates used a combination of design tools and hand analysis in order to design the system.  The individual boxes were designed for the phase 1 conditions.  To recognize the contribution of the transverse diaphragms and of the post-tensioning, an analysis was performed by modeling the bridge system as a three-dimensional space frame.  This modeling was required in order to determine the load distribution between the five boxes.  The rigidity of the transverse system allowed the cross section of the five boxes combined to be considered to act as a single unit.  Because of the unique cross section, fundamental design of precast prestressed concrete had to be completed without aid of commercial software. The flexural capacity of the system was determined using strain-compatibility.  The current code equations for flexural capacity lump all of the prestressing steel together, which does not accurately account for tensioned strands in the compression zone near the top face of the box.  Strain compatibility allowed for multi-layered strands to be accounted for accurately.  Current flexural code equations also limit the ultimate capacity of a member if the section is over-reinforced.  By using strain compatibility, Tadros Associates was able to more accurately determine the ultimate flexural capacity of the system, and assign the proper strength reduction factors.

Bridge 5.90 was on one of OPPD’s main lines.  Tadros Associates recognized that providing a structure that could be constructed within the allotted 72-hour track downtime was critical to OPPD and to the contractor.  With this in mind, Tadros Associates worked closely with the contractor and the precast producer to make the fabrication and the construction sequence as efficient as possible.  The ease of construction of the precast concrete alternate could not be matched by the original steel solution.

Besides efficiency of construction, value engineering of Bridge 5.90 provided immediate benefits to the owner and the contractor in construction cost savings.  The overall reported cost savings were approximately $70,000, which was split between OPPD, L.G. Barcus, and Tadros Associates. Long-term benefits will be realized by the owner in lower maintenance costs over the life of the structure.

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