Next year the scheme to build a new gateway bridge for the USA's capital city is expected to move another step forward with the completion of the environmental impact statement. The owner of the new crossing, District Department of Transportation, will also be moving forward with funding plans for the new movable bridge structure. With the alignment of the new crossing in advanced study, and four alternative designs presented to the public, there is growing excitement about the prospect of the replacement bridge.
Plans for a new movable bridge have been under discussion for some time, and four different designs were developed by HNTB along with Wilkinson Eyre Architects. A poll in the Washington Post revealed that the arch bascule and the cable-stayed swing bridges were the most popular with the public, although it is not certain how much this will influence the final choice.
The need for a new bridge has been recognised for some time, with the existing Frederick Douglass Memorial Bridge, dating from 1949, being in an advanced state of disrepair and rapidly approaching the end of its life. Major rehabilitation of the bridge would not be cost-effective, and even if it were, the existing structure would not be able to incorporate the urban planning improvements that are envisioned in plans for the development of the Anacostia waterfront.
The preferred horizontal alignment for the new bridge is south of the existing bridge as it minimises the impact on the properties and facilities in the area.
Vertical alignments on the approach roadways and bridge were developed to meet the roadway design geometric criteria while providing a minimum 10.6m vertical clearance at the navigation channel. The new bridge is intended to be a landmark structure, to enhance the Anacostia Waterfront and the South Capitol Street corridor. The western end of South Capitol Street has been designated as a cultural development area and is to be revitalised through reconstruction and landscaping which will include the development of a new ballpark district that will house shops, restaurants, entertainment venues and public spaces. Across the river to the southeast, Poplar Point is to be restored as a major riverside amenity. The new bridge will link these two recreational spaces and provide multi-modal access for pedestrian, bicycle, transit, auto, and marine traffic, while improving pedestrian access at the waters edge. Based on functional requirements, only a low level bridge with a movable span at the navigation channel is feasible. Working with the District Department of Transportation, HNTB and Wilkinson Eyre Architects proposed four alternatives; a cable-stayed swing bridge, a stayed bascule bridge, an arched bascule bridge and a retractile bridge.
Sufficient architectural and engineering studies were performed on each alternate to establish the basic aesthetic attributes as well as determine pier and superstructure material components and member sizes. Quantities were established based on the conceptual studies, to produce construction cost estimates for each option. The retractile bridge proved the most expensive at US$342 million, with the other three alternatives coming out considerably cheaper. The cable-stayed swing bridge was estimated at US$254 million, the arched bascule at US$245 million and the stayed bascule at US$235 million. All estimates were at 2006 prices. All four options are expected to take approximately three years to build, and additional project costs for engineering, construction management, bridge demolition and other miscellaneous items amount to approximately US$65 million.
The Washington Post poll established that the cable-stayed swing bridge and the arched bascule bridge were the public's preferred options.
All have been developed to meet a set of minimum functional requirements. In the transverse section these include three 3.35m-wide vehicle lanes in each direction, along with a 600mm-wide emergency lane and gutter line on each side of the travel lanes. A median traffic barrier is required to separate eastbound and westbound traffic on the stayed bascule, arched bascule and retractile bridge alternatives, while the cable-stayed swing bridge does not require this as it has independent eastbound and westbound structures. Two 6m-wide pedestrian walkways are required, which are able to accommodate both bike and pedestrian traffic - they may be outside the carriageways, or centrally-located between them.
The transverse section should also include parapets and railings. The transverse section and roadway profile remains constant across the entire length of the bridge for all alternatives except the cable-stayed swing bridge. In this case it varies along the entire length of the bridge and the eastbound and westbound lanes have separate independent profiles. It has a single 6m-wide centrally-located pedestrian walkway is provided for this design.
Only low bridges with movable spans at the navigation channel were considered feasible for the study. With the clear channel requirement of 76m and the skewed alignment with respect to the channel, a minimum movable span length of approximately 104m will be required. A vertical lift bridge was initially considered but later discarded for technical and aesthetic reasons due to the excessive tower height and lift required to accommodate the tall ships which pass through the bridge.
All four designs provide for the construction of the new bridge while maintaining the operational envelope of the existing swing bridge. During construction of the new bridge, only the west side channel opening of the existing bridge will be maintained. The total bridge length between abutments for all options is 502m.
Of the four options, the cable-stayed swing bridge option is likely to have the biggest visual impact on the local environment. The river crossing is marked by a pair of elegantly tapering masts, and two carriageways separated by a void link the two bridge abutments with a gentle curve. This is intended to break down the apparent mass of the bridge deck and enhances the visual lightness of the structure.
Each road deck is supported along one edge by an array of cables anchored at high level to the masts. When viewed from the end of the crossing the two masts frame axial views towards the Washington Monument.
Each mast rotates about its base so that the central portion of the bridge swings open for larger vessels: this operation will be an exciting spectacle, and should be visible from long distances.
Pedestrians will also enjoy crossing the bridge on the undulating footway between the carriageways. The footway deck is cantilevered from the edge of the adjacent roadway superstructure and is overshadowed by the stay cables above. The overlapping planes of cables from opposing masts visually shield the pedestrian from the busy carriageway and reinforce the experience of passing through the gateway structure.
This alternate consists of independent superstructures for the eastbound and westbound roadways separated by an open space and supported on common substructure units. Each roadway consists of an unsymmetrical cable-stayed swing span with a channel span of 104m and back-span of 52m. During the opening operation, the independent bridges swing in opposite directions requiring the back spans to swing over the adjacent structure. The superstructure is a central box girder with cantilever floorbeam extensions supported by a single plane of cables anchored to an inclined tower. The roadway deck of the river arm is a steel orthotropic deck with asphaltic wearing surface and to provide transverse balance the pedestrian walkway is a high performance lightweight concrete deck. A concrete blister on top of the box girder provide
