Denver Millennium Bridge
In the mid 1990’s a focused effort was made to reconnect the Denver City Center with the west Denver neighborhoods. This connection was disrupted in the early 20th Century with massive rail and industrial uses. The reclamation involved the consolidation of the rail lines, reducing a half mile wide zone to a 100 foot wide right of way. The concept was to extend the 16th Street Mall through this reclaimed land and into the adjacent neighborhoods.
The Denver Millennium Bridge was a response to crossing the rail lines while developing a landmark structure that would become a focal point for future development in the area.
The type and geometry of the bridge was developed responding to existing site constraints and proposed urban conditions identified in the Mall Extension Master Plan. The completed structure, initially standing alone within the 16th Street alignment, was designed to eventually become part of an ensemble of buildings and spaces. The bridge was not intended to be a pure architectural form, but a series of elements and episodes that encourage its use, especially the challenge of rising from street level to 30+ feet in the air to cross above a series of rail right of way and then descend back to that street.
The scale shifts from urban design to architecture. The design concept was to develop detailing at all scales that supported an understanding of the structure and how all of the parts worked together by the user. Structure and connections were exposed where possible and steel assemblies and components were artfully composed and detailed. Lighting and landscaping were integrated into the design adding interest and detail both day and night.
The Bridge deck spans 130 feet and is an average of 80 feet wide, matching the width of the 16th Street right of way. A 200 foot tall inclined and tapered steel pylon rises up on the south side of the bridge. This iconic form can be seen from the south end of the 16th Street Mall, almost one mile away. High-tensile steel cables attached to the mast form the back-stays which are anchored at the ground via deep foundations. The deck is hung by a grid of cables along and across the surface. The deck, anchored at the abutment structure, cantilevers across the tracks and hovers over the urban plaza that terminates the 16th Street Mall.
To achieve the proper stiffness and the visual lightness, the deck was designed as a super thin composite structure. The concept of a post tensioned steel frame was developed. Stiffness was derived by installing the deck structure with negative camber. The pylon, with deck cables attached, was pulled into its final position pulling the deck into positive camber. The concrete slab was poured then bringing the deck into a tensioned neutral condition. Stabilizing this condition are two tension rods holding down the cantilevered end of the deck. When completed, the crossing was the first composite post tensioned cable stay bridge in the world.
Structural and Bridge Engineering: Arup
Site Plan of the bridgeand the relationship to the railroad ROW, approach from the city (top) and plaza space (bottom)
The Crossing in the City Context
with the pylon defining the image and scaleof the structure while the deck and its connections weaves itself into the context of the structures surrounding the bridge.
The Architectural Detail was carefully composed to reinforce the shift from the urban to the human scale. Structure was exposed and connections celebrated to describe how the structure performs. Specific code mandated elements such as the guardrail enclosures over the tracks were treated as sculptural elements adding to the overall compositional ensemble