Running with Elk

May 11, 2009

During the late 1990s, the Oregon Department of Transportation noted deterioration of conventionally reinforced concrete bridge structures throughout the state. Historically, this bridge type has been widely used on Oregon highways dating to the earliest days of reinforced concrete technology, with heavy application from the 1930s through the 1950s, after which prestressed concrete became the more prevalent choice.

During the late 1990s, the Oregon Department of Transportation noted deterioration of conventionally reinforced concrete bridge structures throughout the state. Historically, this bridge type has been widely used on Oregon highways dating to the earliest days of reinforced concrete technology, with heavy application from the 1930s through the 1950s, after which prestressed concrete became the more prevalent choice.

The deterioration identified on these structures coupled with an extensive load rating effort being conducted statewide led to heavy load restrictions being imposed on many of these structures.

These load restrictions underscored the widespread negative economic effect that restricted freight transport has on a state’s economy. Studies commissioned at the time estimated the consequences of not repairing the Oregon structures at $123 billion in lost production and 88,000 in lost jobs over a 25-year span.

In response to this study, the Oregon Legislature in 2003 enacted the third Oregon Transportation Investment Act (OTIA III), which included $1.3 billion for bridge work on the state highway system. The goal of ODOT’s OTIA III Program was and is to repair or replace hundreds of aging bridges on major corridors throughout Oregon by 2013 in a timely and cost-efficient manner.

Numerous projects have been completed since the start of this program, with contracts let in a variety of manners. While the majority of projects have been contracted via the traditional design-bid-build (DBB) method, many projects have been contracted using the more contemporary design-build (DB) contracting method. This has been the method of choice when design innovation or a quick construction schedule was required to deliver a project. In the case of the Elk Creek Bridges project at the Tunnel through, achieving a design to maintain traffic movement throughout the duration of the project construction was the primary goal.

This level of investment, contracting method and collaborative climate for innovation resulted in an ideal environment to explore all available methods for delivering the best bridge construction avenue for the project.

For the project, the use of rapid replacement techniques greatly reduced interference with traffic mobility at this rural location; a very critical issue to the affected communities. And the team of engineers and builders for these bridges, with the approval of the agency, developed a highly successful replacement plan that was implemented in only two days.

Double crossing

The two bridge structures identified for replacement were Elk Creek Crossing No. 3 and Elk Creek Crossing No. 4, which had the unusual configuration of an extremely close proximity to the west and east portals of the historic Elk Creek (Hancock Mountain) Tunnel. Crossing No. 3 is approximately 150 ft from the west tunnel portal and Crossing 4 is less than 50 ft from the east portal.

In both cases, the existing bridge configuration included a Howe deck-truss main span and a narrow width of only 24 ft of roadway from curb to curb. Both of these factors precluded the use of staged construction by preventing any partial removal of the existing bridge.

The limited room between Crossing 4 and the east tunnel portal made the geometry of a detour alignment around the structure essentially impossible, because it would have required modification to the tunnel portal and use of single-lane geometry at a very low level of service.

A detour at Crossing 3 might have been possible; however, the roadway approaching the west end of the bridge crosses a very steep side hill resulting in very difficult topography to modify approaching such a detour. Meeting the challenge of detour alignments outside the structures was further complicated given the environmental expectations for the project.

The OTIA III replacement program has been successfully accomplished through the use of an environmental performance standard (EPS) document that is the key document used to permit work under this statewide program. Given this broad charge, the document is necessarily somewhat more restrictive on what practices are allowed. Individual permitting is an option for difficult sites; however, such a process cannot be achieved within the time frames allowed by a design-build project.

Solution strategy

To meet this challenge, the design team, led by T.Y. Lin International (TYLI), crafted a rapid replacement solution. The contract, as let by the state and executed by Slayden Construction, Stayton, Ore., allowed for a limited full closure of the highway to facilitate construction and traffic handling modifications. Working within this criterion, TYLI engineers set about to produce bridge replacement designs that could be installed within the highway closure time limits.

The length of the existing structures (Crossing 3: three spans, 320 ft, and Crossing 4: two spans, 222 ft) coupled with the time frame for replacement measured in hours made what should have been a very simple construction—using integral deck elements and rapid assembly of the entire structure—seemingly impossible.

The final strategy employed for both crossings was the construction of a replacement substructure in and around the existing bridge, while it remained in service, and then fully constructing the bridge superstructure, to one side, on temporary support structures.

Upon completion of the preliminary assembly, the highway was fully closed to traffic, the existing bridge was demolished, and within hours the new superstructure was moved into place and lowered onto bearings atop the new substructure.

Technical merit

To achieve success with this strategy, a number of technical challenges needed to be fully met.

Demolition progress needed to be enhanced to speed the completion of that phase. Substructure designs needed to be designed around the existing bridge in order to ensure that the existing structure would remain stable throughout installation. Jacking schemes to lift the new bridge onto skids and then lower it onto the new substructure were required to avoid damage and ensure that a uniform load on bridge bearings was achieved. Simple clearances during moves needed to be checked and verified. Detailing to provide adequate seismic connectivity between the superstructure and substructure needed to be considered.

During the demolition process, the skidding strategy was expanded to include removal of the existing main truss spans by skidding them to the side. This required the installation of lifting points at the first panel point on the truss spans. This is not a natural support location and therefore complex engineering was required to design strengthening of the truss at those locations to support the dead load of the main span.

The installation of a new substructure in and around the existing substructure was particularly a challenge at Crossing 3 because the skews of the existing and new structures differed. The different skews resulted in the east abutment being constructed under the cap beam of the existing structure at the extreme southeast corner of the bridge. A significant portion of the existing Bent 3 of the old bridge needed to be removed to allow the new cap beam to support the new structure.

For Crossing 4, the structure consisted of four prestressed concrete beams in the short span and five beams in the long span, all with individual elastomeric bearing supporting pads at either end. The stiffness of the support diaphragm created a potential for uneven bearing on the pads with even a very slight variation in as-constructed pad elevation.

To resolve this issue, the structure was lowered onto these pads with a wet grout layer under the bearings. At the point where the grout had been compressed, the structure was held on the jacks until the grout set and then the full weight of the bridge was applied to the bearings.

Dressed for success

This project provided a unique opportunity to use a method that is becoming more prevalent throughout the country: accelerated bridge construction (ABC).

The communities affected by this project (Drain and Elkton, Ore.) voiced a very strong positive feedback to ODOT, thereby supporting the concept that a heavy impact, for a very short duration, is much more palatable to the traveling public and has a measurable reduction in economic effect to a given corridor.

Given the success of this method, it is the expectation that ABC will continue to expand as a viable option for bridge replacement projects. As such, today’s design engineers will have the opportunity to re-think some of the traditional methods by which a bridge is designed, and with creativity, produce designs that have superior performance compared with those built with conventional methods.

For this project, T.Y. Lin engineers were able to design Crossing 3 in a manner that allowed for establishment of a compressive force in the deck once it was placed into its final location. This was done by elastically deforming the three-span, continuous steel structure by constructing camber in such a way as the end abutments would settle onto their final support prior to the interior bents being supported. This created a prestressing effect through the use of imposed deformation on the structure.

Through the use of creative solutions and a unique design approach, coupled with an outstanding relationship between the engineer, owner and contractor, the rapid replacement structures at Elk Creek Tunnel are currently in service and standing as a testament to the engineering innovation that can be used to overcome what might be considered impossible at first glance.

About The Author: Nettleton is with T.Y. Lin Internatioanl, Salem, Ore..

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