Teng
Architect and Engineer: Teng, Chicago—John Hillman (principal bridge designer); Thomas Hoepf (lead architect); Astrid Haryati (landscape architect)
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•WINNER• 35th STREET |
41st /43rd STREETS | CHICAGO RIVER |
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Evaluation Committee comments
Simple but pleasing.
Ramp integration with park is good and two-directional.
Good, simple idea.
Architect's Statement
It is easy to design a bridge that will link
contrasting termination points and provide a safe and even interesting
traveling experience for pedestrians and bicyclists as they cross. It
is also easy to dress a bridge up to improve the aesthetic quality of
the environment in which it is imposed. It is also easy to make a dramatic
and daring statement with a bridge that draws attention to the structure
itself. One can even push the boundaries of structural form and material
properties to defy intuitive logic in how a structure resists the applied
loads. Where the difficulty lies, is in satisfying all these desirable
qualities in a bridge structure in an elegant and timeless manner.
In its purist form the proposed crossing for 35th Street exploits the
virtues of one of the most simplistic structural forms ever devised for
longer span structures. Since the 19th century, engineers recognized that
by draping a long suspension cable in a pure catenary profile, vertical
hangers could be employed to support a very slender deck. Thus the suspension
bridge was conceived, offering the antithesis to the arch bridge by taking
advantage of a new material with higher tensile strength. The proposed
crossing for 35th Street uses these same classical principles. However,
due to the smaller scale and the desire to enhance the pedestrian experience,
the proposed crossing for 35th Street provides an opportunity to literally
put a twist in conventional suspension bridge technology, while respecting
the laws of physics and sensible structural design.
To keep the scale of the bridge in proportion to the site and avoid any
massive visual obstructions, the proposed suspension bridge is of a self-anchored
type. In this configuration, the suspension cables are actually anchored
at the ends of the deck rather then in massive anchor piers at the abutments.
Other benefits of self-anchoring become evident in analyzing the horizontal
forces imposed on the bridge as a result of the alignment selected.
In order to make the bridge more enticing to the pedestrians to the west
side of the crossing, the western approach is strategically aligned with
35th Street to make the bridge visible from a distance. As the bridge
crosses over the railroad tracks and Lake Shore Drive, a reverse horizontal
curve has been added to the alignment to provide for expanded panoramic
views of the park, Lake Michigan and the Chicago skyline for pedestrians
crossing the bridge. The curvature induced in the bridge from this alignment
not only provides for an interesting visual effect, but also offers an
opportunity to exploit some unique structural features to accommodate
the unusual alignment.
The most evident structural illusion is created from the use of a single
suspension cable extending to each end of the bridge rather than the more
typical arrangement of a symmetrical pair of suspension cables. These
“mono-cables” are intentionally aligned and support the outsides
of the curves on the respective spans. The reason for this is two-fold.
On the one hand supporting the deck on the outside of the curve offsets
the inherent torsional tendencies of curved bridge girders. On another
plane, the horizontal component of force in the hangers applied at discrete
points along the deck equilibrates the large horizontal components of
force induced in the ends of the deck due to the eccentric location of
the suspension cable anchorages. The net result is a self-equilibrating
suspension structure.
Other structural features of the bridge also lend elegant simplicity to
the resolution of internal forces in the structure. One of these features
is evident in the typical cross-section of the bridge. For simplicity
in detailing and constructability, the deck is comprised of a monolithic
concrete section approximately 42 inches deep. This shallow profile provides
for a slenderness ratio of approximately 70 to 1 for the 250-foot clear
spans on either side of the central pylon. The typical cross-sections
demonstrate the triangular shape of the bottom soffit. The apex of the
triangle at the bottom soffit varies with position along the length of
the bridge. At the side piers, the peak is located adjacent to the side
of the cross-section where the suspension cable is anchored. From this
location, the peak follows the same plan profile of the suspension cables
and is centered at the pylon. This plan deviation of the apex not only
reflects the asymmetrical configuration of the suspension cables, but
also results in a more economical design of the cross-section by providing
additional torsional rigidity where it is most effective in resisting
twisting moments induced by live loads.
Another benefit of the cross-section is derived from the aerodynamic profile
in regards to dynamic behavior from wind loads. The mass of the concrete
will also result in additional geometric stiffness that will not only
assist in damping wind induced oscillations but also vibrations due to
pedestrian foot-falls.
Every detail of the bridge has been designed to compliment the unique
structural form. The pylons are constructed using an A-shape to resist
the lateral forces induced from wind loads and other lateral force components.
The two legs of the pylons are designed with slow graceful curves to blend
in with the natural curvature of the suspension cables.
The piers at the ends of the main spans have been detailed to be consistent
with the eccentric theme of the bridge while providing an efficient transmission
of the forces in the structure to the foundations. The vertical breaks
in the planes of the piers have been intentionally detailed to align with
the apex in the bottom soffit of the superstructure to complete the lines
of the bridge all the way to their resting points on the ground. A similar
eccentric pier design has been carried through on the approach ramps,
not only for consistency, but also to provide the maximum amount of visibility
below the bridge.
It is a commonly held belief that structures which visually convey a simplistic
internal load path to resist the applied forces, generally result in solutions
offering pleasant aesthetic qualities. The suspension bridge offered for
the 35th Street Pedestrian Bridge satisfies this simple physical principle.
One of the more alluring qualities of this bridge is that it challenges
the observer to look deeper into their own intuitive knowledge of physics
to ascertain all of the nuances of the structural behavior. Yet at the
same time, the bridge can be appreciated for its simplicity and elegant
grace without demanding the viewer to question why.
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Architect's Statement
Sometimes a bridge cannot be in two places at one time. The bridges at 41st Street and 43rd Street epitomize this very dilemma. In many ways these crossings are almost identical in every aspect. Yet at the same time, the question arises as to what criterion ultimately establishes a mandate for the bridges’ appearance. From a purely utilitarian viewpoint the bridges could be the same in every respect. However the subject arises as to whether or not their close proximity makes this a redundant statement. Similarly, by virtue of trying to create world-class icons along Chicago’s lakefront, the question arises as to whether or not one should duplicate icons within throwing distance of each other.
One logical solution to this dilemma is to provide two distinct bridges that represent variations on a theme. It is this very philosophy that has been adapted in developing the alternatives for 41st and 43rd Streets. The proposed solution for these crossings results in two contrasting, yet very similar bridge embodiments that by the very definition of their form, become conversation pieces.
One of the first questions to arise in viewing the bridges is to ascertain what type of structure they represent. It is virtually impossible to invent a new structural form in this day and age. In fact these bridges do not represent a new form of structure, but rather a combination of structural forms. One might argue that they are cable-stayed structures. One might also argue that they are really arch structures, although not in the classical sense. In fact they are both at the same time. The inclined ribs that support the hangers truly function as arch ribs. However, rather than completing the arch ribs and taking the thrust of the arches either to the foundations or to a tie at deck level, the arch ribs are interrupted and the thrust equilibrated by cable-stay elements. In a sense these bridges become cable-stayed arches.
In any event, the bridges make a dramatic statement and become sculptural elements in their own right. The bridges share many common elements and themes. For example, in both cases the main spans crossing the highway and railroad are supported from hangers attached to inclined, steel arch ribs that meet at an apex high above the deck. Both structures have similar deck framing systems. Both structures share common alignments that are aligned parallel to 41st Street and 43rd Street respectively. The alignments were selected to pique the curiosity of the residents to the west of the crossing and draw them to the bridges. Despite these similarities, by providing two variations of this structural form, it becomes evident that comparisons and contrasts can be drawn at many different levels, hence validating their status as conversation pieces.
The bridge at 41st Street is characterized by two identical main spans representing mirrored images of each other. The cable-stay ties for this bridge extend from the tips of the arch ribs to anchorages cast in the concrete abutments at the back spans. The framing on the main spans is comprised of two continuous longitudinal edge girders that support transverse steel beams that are spaced in line with the hanger locations at approximately 15-foot centers. Due to the unsymmetrical span arrangement, the back spans of these two structures are comprised of a similar system, but with concrete transverse beams to provide additional ballast to balance out the longer main spans. The back span is also supported from arch ribs. However the supports for the back span are comprised of circular concrete columns oriented vertically in elevation. The arch ribs for the back span are inclined in the same plane as the main span, but again are constructed of concrete instead of steel to provide additional ballast.
The main difference between the bridge at 43rd Street and the bridge at 41st Street is that the two main spans are supported from symmetrical ribs springing in both the east and west directions from a central pier located between Lake Shore Drive (LSD) and the railroad crossings. Both bridges clear span LSD and the railroad crossings without intermediate piers. However as a result of the span arrangements and rib pier locations, the bridges, in elevation become somewhat of an antithesis of each other, or perhaps a male and female version of the same concept. From a distant perspective, the relative closeness of the two structures results in a superposition where the contrasting arch arrangements complete each other.
Other aesthetically pleasing features of these bridges result from the cable arrangements and the inclined orientation of the arch ribs. Regardless of the drastic difference in the span arrangements, both bridges result in a perception by the pedestrians of being in a secure enclosed space as you are crossing directly over the railroad lines or the traffic of Lake Shore Drive. Likewise, the cable arrangements allow for the creation of a refuge point in between these two heavily traveled corridors, where for a brief intermission there are again, unobstructed views of the city to the north as well as the Chicago lakefront to the south. The inclines of both the hangers and the cable-stays provide for a nice experience for pedestrians as they exit from the secure environment of the cable-supported spans to the open expanse of the park and Lake Michigan.
The bridges at 41st and 43rd Street do not have to be different from each other. Neither do they have to be the same. Although the bridges proposed for 41st Street and 43rd Street are no less than rivaling siblings, they are by no means identical twins. Regardless they represent a family of bridges, complete with the similarities and differences that both contribute to lasting bonds. They invoke conversation between themselves, and they invoke conversations between the people experiencing them, either as viewers or users. By this virtue alone they exemplify one of the most compelling attributes desirable in a work of art.
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Architect's Statement
There are more movable bridges in the City of Chicago than in any other city in the world. The evolution of movable bridge technology can literally be traced by the numerous innovations developed and implemented for Chicago River crossings for over a century. This rich historical tradition demands that the solution submitted for the pedestrian crossing of the Main Branch of the Chicago River not only respect the character of the adjacent Lake Shore Drive (LSD) Bascule Bridge, but also perpetuate the innovation of movable bridge technology.
To satisfy these objectives, the proposed structure for the Chicago River crossing tries to balance numerous requirements with respect to adjacent structures and park facilities as well as innovations in operational modes and pedestrian experience. To start with, the proposed bridge is respectful of the character of the existing LSD Bridge immediately to the east. To avoid drastic contrast and to comply with site topography, the bridge follows a fairly flat profile that matches the elevation of lower level LSD and maintains the same vertical clearance to low steel in the navigational channel.
The movable span also emulates the parallel chord truss configuration of the bascule span of the LSD Bridge, but with some contemporary modifications. One of the more obvious differences is that the horizontal alignment of the bridge structure follows a long slow circular curve as opposed to the straight alignment of LSD. The use of the curved alignment affords some interesting structural opportunities to further distinguish the bridge.
In many ways, this crossing of the Main Branch Chicago River defines the geographical center of the lakefront parks of Chicago. This is one of the key attributes that inspired the geometry and operational mode of this bridge. Note that rather than functioning as a bascule bridge, lift span or swing span, the proposed navigational span actually opens by dividing the center span into two halves that slide horizontally to provide the clearance for boat traffic. Not only does the mode of operation provide a dynamic contrast from the inclined opening of the adjacent bascule span, but the curved alignment also creates a unique visual sensation as the opening spans are rotating about a point centered in the river where the boats will be queuing to enter.
The unique mode of operation of this bridge offers possibilities in creating strong urban design connections to the existing urban fabric along the lakefront parks. Particularly in the context of DuSable Park and Harbor, the elegant curve alignment of this bridge complements the gently curve alignment of the lakefront's edge which further enhance the overall aesthetic quality of the space as a dramatic foreground to the City skyline.
Another noticeable difference from the LSD Bridge is that the main span has but a single truss on the western edge of the span rather than a pair of vertical trusses. Again, the circular alignment affords the opportunity to create an asymmetrical cross-section where the outside of the span is a truss inclined to the west that counterbalances a cantilevered deck to the east. To reduce weight and provide extended life cycle costs, the deck can be comprised of a shallow fiber reinforced plastic deck supported on stringers. The inside of the curved superstructure is stiffened by a longitudinal, curved, steel box beam that serves as the rolling surfaces to guide and support the bridge on the east. This asymmetrical cross-section offers a contemporary yet complimentary structural form relative to the LSD trusses. The cross-section also serves to provide a beautiful and unobstructed panoramic view of the parks, Navy Pier and the emerald waters of Lake Michigan.
Another variation from the LSD Bridge is the use of light cable elements for the diagonal members of the truss. These cables provide for a more transparent structure to avoid darkening the park land below and obstructing the view of Lake Michigan from Lower Level LSD.
The mechanics of the bridge operation are all designed to provide an economical alternative to a bascule span. For example, the movable section of the bridge is actually comprised of two sliding leaves of 250-foot length. The center break is at the middle of the navigational span. The rear break is at the pier supporting the side spans on either side of the river. As a result the portions comprising the main-span are actually balanced about the main piers under dead load. This configuration eliminates the need for a substantial counterweight as well as the corresponding expensive foundations required for a counterweight pit. Because of the proximity to the LSD Bridge, the intention is to integrate all of the instrumentation and controls for the proposed bridge in to the east tender houses for the LSD Bridge to facilitate further cost savings.
The mechanism for opening the bridge is comprised of a simple rack and pinion drive system, where the rack is mounted to the outside of the lower chord of the truss on the west side. The pinion drive is comprised of a vertical electric motor contained in a machine room on the outside leg of the piers adjacent to the river crossing. Vertical and lateral roller guides are attached to the piers to maintain the alignment of the span during the opening and closing movements.
The piers for the movable span and the adjacent approach spans are designed to perform another interesting function. In order to allow the movable span to retract, the two approach spans immediately adjacent to the truss span are suspended from the overhead cantilevered arms of the piers. Hydraulic cylinders within the cantilever arms lift these side spans approximately one foot so that as the movable span is retracting, it slides directly under the side spans. The cantilevered arms also provide the uplift reaction bearings for the sliding spans. All the mechanics of the bridge system are very simple and can easily be designed with redundant and manually operated back up systems.
To maintain a shallow profile for the approaches, these spans are comprised of a very simple cross-section having two longitudinal edge girders of either post-tensioned concrete or steel box-beams. These longitudinal girders then support a transversely post-tensioned concrete slab with a shallow profile of approximately 12 inches at the centerline of the bridge.
To alleviate the congestion resulting from vehicular and pedestrian traffic on the north side of the river, the pedestrian bridge remains isolated from lower LSD and continues on to cross the inlet at North Pier on an alignment that touches down by the park facilities directly in front of Navy Pier. To facilitate the second water crossing, a very similar asymmetrical truss cross section is used for the long, curved span over the inlet. Although this portion of the structure is comprised of a fixed span, the similar truss configuration provides for a continuity of the structural concept as well as provide for maximum vertical clearance at the inlet while avoiding drastic grade changes along the pedestrian path.
Finally, a landing is provided on the outside of the fixed spans on the north side of the river to facilitate a ramp that doubles back and allows for development of additional park lands on the north side of the river.
In summary, the proposed bridge solution for the crossing at the Main Branch of the Chicago River results in a unique solution for a movable bridge that compliments the existing Lake Shore Drive Bridge, both in terms of operations as well as appearance. The curved alignment and asymmetrical design of the cross-section create a dynamic experience for pedestrians and bicyclists, even in the closed position. Yet at the same time, the bridge invites the pedestrians to take pause at center span and enjoy the way the bridge simultaneously embraces Lake Michigan and establishes a signature gateway for the river access that has helped define the unique character of the City of Chicago.
