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Steel Shots: Decision Delta
Posted by Tasha Weiss on December 6, 2013 at 5:36 PM.

shenendoah-river-bridge_sitn_500.jpg

The new Shenandoah River Bridge superstructure consists of a five-girder, four-substringer system supported by five lines of delta legs - one for each girder. Photo: Courtesy of HDR, Keith Philpott, photographer

 

Just an hour’s drive west from Washington, D.C., the new Shenandoah River Bridge stands in aesthetic harmony with its surroundings.

 

The project exists within a unique ecosystem where the scenic Shenandoah River valley boasts steeply rising wooded mountains, a diverse wildlife habitat, rolling farmland and quaint, historic towns. Not surprisingly, the region has evolved into a desirable getaway from the frenzy of urban life.

 

With the subsequent increase in traffic, the West Virginia Department of Transportation - Division of Highways (WVDOT) determined that the winding two-lane road that carried West Virginia Route 9 (WV9) through the valley was no longer sufficient. In September 2009, it revealed the design for a new alignment: a four-lane divided highway using a bridge over the Shenandoah River. At the crossing location, the proposed grade was nearly 200 ft above the river, and the overall bridge length would be nearly 1,800 ft. While there are no navigation requirements for the river, the environmental constraints for the project and the relatively high cost of substructure units located in the valley dictated that the main span be approximately 600 ft in length. To accommodate these constraints, a three-span continuous deck truss configuration (400 ft - 600 ft - 400 ft) with short plate-girder approach units was initially selected during the design phase.

 

In early October 2009, WVDOT modified the procurement from design-bid-build to design-build and instructed contractors that they could bid the as-designed truss or develop and bid a different structure type, providing they addressed the following criteria:

 

  • The chosen substructure locations for the deck truss bridge generally must be used, with very limited latitude.
  • The established horizontal and vertical alignment could not be changed.
  • Alternatives that required increased amounts of disturbance to the gorge slopes would not be considered.
  • The use of a causeway or cofferdams, other than as shown on the plans for the as-designed bridge and/or in the Section 404 (of the Clean Water Act) Permit, would require re-permitting.
  • The design must comply with all previously established environmental commitments.

 

Following concept approval, structural engineer HDR Engineering developed a delta frame design that delivered significant savings compared to proposals for more traditional designs and also resurrected a tried-and-true form that had been largely forgotten since the 1970s.

 

To learn more about this uncommon design resurrected by the new bridge, you can read the article from the December 2013 issue of MSC (available now!).


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