A New Leaf

Arkitektur, Stålbroar
At one end of a pedestrian bridge in suburban Denver, a towering steel leaf plays the dual roles of artistic beacon and important structural element.
The design team refined the bridge’s pylon leaf design while retaining structural integrity. From the leaf, six pairs of cables extend to the north to support the bridge. Soaring 100 ft above Lincoln Avenue, the bridge’s unique form is seen from many vantage points throughout the city of Lone Tree, Colo., and creates a memorable image for those traveling on and over the busy thoroughfare

Lincoln Avenue, an east-west corridor in Lone Tree, Colo., sees a traffic load of 90,000 cars per day. The busiest thoroughfare in this southern suburb of Denver, it divides Lone Tree Elementary School and residential communities on the north from local retail, workplaces, and parkland on the south. Given the area’s growth and Lincoln Avenue’s significant traffic load, it became increasingly treacherous for pedestrians and cyclists to cross at streel level.

But a new pedestrian bridge at a strategic crossing point—near vibrant commercial strips and the school—now allows children from the elementary school to avoid Lincoln Avenue’s traffic and safely reach open space parkland for scientific and ecology education. It also lets workers on the north side easily get to eateries on the south side and provides runners and cyclists with unhindered access to a network of recreational trails that the road interrupts.

The pylon is a 3D lattice truss constructed of steel members, with a twist in the geometry to create a sculptural form.

Symbolic Design

Completed last year, the new 170-ft-long, steel-framed Lone Tree Pedestrian Bridge achieves these goals rather stylishly. In the preliminary stages of the project, the city challenged Fentress Architects to design not only a pedestrian bridge, but one with flair, an icon that would reference the city itself. Lone Tree’s logo is a tree and the Lone Tree Art Center’s (near the bridge on the south side of Lincoln) is a leaf, and the city wanted this foliage theme incorporated in the design.

Fentress and structural engineer Thornton Tomasetti delivered. The steel main pylon, in the shape of a leaf, rises 100 ft above the road, creating a highly visible landmark. From the leaf mast, six pairs of cables extend down to support the bridge deck. The deck is defined by an in-plane steel truss created by longitudinal edge beams, cross beams, and diagonal bracing—all using conventional rolled steel members, mostly W18s (fabricated by King Fabrication)—below the slab to provide lateral stiffness and stability to the span. The bridge deck consists of a 3-in.-thick reinforced concrete topping slab over 3.5-in.-thick precast concrete panels. The topping slab was poured onto the steel beam between the panels, providing a structural connection between the slab and the beams. In addition, a steel “knuckle” at the base of the leaf pylon incorporates a 10-in.-diameter by 4-ft-long stainless steel pin inside of a steel sleeve, resulting in a true pinned base with no rotational restraint at the base of the pylon.

The bridge was erected in only one weekend closure of Lincoln Avenue.

While Fentress has worked on pedestrian bridges that connect offices, laboratories, and airport buildings, this was the firm’s first major pedestrian bridge over a busy roadway. The design team studied a number of structural systems, initially presenting a box truss option and a suspension option along with the chosen cable-stay option. The cable-stay format was eventually chosen as it was a lighter than a deep-girder bridge while also being stiffer than a suspension bridge. It also allowed much of the structure to be located at the pylon rather than within the span, creating a thinner profile for the span itself—minimizing the impact on views to the Colorado mountains and downtown skyline and also meeting the clearance requirement of 17 ft, 6 in. for pedestrian bridges that cross major arteries—and reducing the structure’s weight as well as material costs.

Light as a Leaf

The bridge uses roughly 100 tons of structural steel in all, coated with a zinc-rich epoxy primer topped with fluoropolymer urethane. The leaf is formed from thick-walled pipe, 24-in.-diameter for the primary and 18-in.-diameter for the secondary members, with 10-in.-diameter “veins” in between. The thick walls allowed the sections to have a thinner profile while still achieving the required structural properties.

As a critical connection for local pedestrian traffic, the new bridge spans across the rapidly growing area’s busiest east-west road.

The leaf was delivered to Lone Tree in one piece, and the spans were delivered in two pieces, all from King’s shop in Houston. Thanks to the bridge essentially arriving at the job site in only three major components, it was erected with only one weekend closure of Lincoln Avenue. Once the road closure began (8:00 p.m. on a Friday), the leaf, including the pin at the base, was driven to the road beneath its pedestal. Two cranes were used to lift it, one at the base (300-ton) and one at the tip (500-ton). Once lifted horizontally, the base crane slowly lowered the bottom, then released it. With the installers guiding it, the leaf was lowered onto its pedestal, and adjustments were made to rotate it so that it would align with the anchor bolts. Once the leaf pylon was in place, the backstay cables were connected while the other crane was still holding it erect.

A steel knuckle at the base of the leaf pylon incorporates a 10-in.-diameter by 4-ft-long stainless steel pin inside a steel sleeve.

While the leaf was going up, another crew was installing a shoring tower at mid-span. As work continued on the leaf pylon, the two spans were installed and joined at mid-span. Next came the 1½-in.-diameter forestay cables, which connected to the leaf and the bridge spans. The forestays are a fixed length, so the 3-in.-diameter backstays were jacked in order to tension them. The cables are connected to the pylon at their tops and to the slab at their bottoms via large tension-cable sockets, providing field adjustability during installation and allowing for rotation of the cables during construction. The backstay cables are connected with similar sockets to long steel rod anchors that transfer the bridge cable forces into the earth.

The roof of the bridge consists of a thin ethylene tetrafluoroethylene (ETFE) membrane stretched between pretensioned steel cables anchored at each end. The ETFE system keeps the bridge roof light yet still stiff enough to support snow loads of the roof without significant sagging, while also allowing light through into the covered span. A simple portal frame supported on the main span deck, featuring mesh panels down either side, provides the infrastructure for the enclosure. Ramps on each end of the bridge facilitate accessible design, multimodal access and connect students, residents, and workers to the amenities on both sides.

Stainless steel mesh on the sides and an ETFE roof on top protect bridge users from severe weather while enabling them to enjoy the elements on pleasant days.

The new Lone Tree Pedestrian Bridge celebrates the unification, both socially and physically, of Lone Tree’s north- and south-side communities. The structure’s memorable form creates a major landmark for the city and establishes a model of its vision for the future: a more easily accessible community with safe passage for pedestrians.   

Artikeln är hämtad från Modern Steel Construction: www.modernsteel.com

Owner: City of Lone Tree, Colo.
General Contractor: Hamon Infrastructure, Denver
Architect: Fentress Architects, Denver
Structural Engineer: Thornton Tomasetti, Denver
Steel Fabricators:
King Fabrication, LLC, Houston
FabriTec Structures, Dallas
Bender-Roller: Bendco, Pasadena, Texas

Författare: Curtis Fentress, FAIA