Rosales Gottemoeller Schlaich Bergmann

The City of Chicago's plans to construct five new pedestrian bridges that reconnect several Chicago neighborhoods with Lake Michigan offers the opportunity to significantly change the image of the city, particularly as seen from the lake. Each of the five bridges will be set in prominent locations bordering the lake, visible not only to nearby residents and workers but also to innumerable daily travelers along Lake Shore Drive and other nearby streets. These important links have the potential to become new landmarks for Chicago while providing new connections across Lake Shore Drive, the Metra railroad tracks, and the Chicago River. They should also exemplify Chicago's tradition of excellence in the design of public works and join that group of signature structures, such as the Hancock and Sears Towers and the movable bridges along Wacker Drive, by which Chicago understands itself and that have come to represent Chicago's image to the rest of the world.

The most beloved bridges in America, such as the Golden Gate Bridge and the Brooklyn Bridge, are symbols of their eras and the cities in which they are located. Like the best bridges of their time, they create their aesthetic impact through the shaping and proportioning of their structural components, using the best materials available to their builders. Their designs are the result of pursuing efficiency, economy, and elegance, all at the same time. Today we have high-strength steel, high-performance concrete, and computer-based design tools to create cable-supported bridges of surpassing transparency and grace.

For Chicago's five new pedestrian bridges we are proposing five cable-supported bridges, including a cable-supported movable bridge, united by the theme of achieving elegance by the efficient and economical use of contemporary structural materials. This family of bridges will, as they are seen in succession by travelers and pedestrians, create a memorable and coherent family of structures easily recognizable from a distance. Three bridges on the south side and one on the north will be orientated to face the downtown, serving as book ends to the movable bridge over the Chicago River, while emphasizing views toward city landmarks and the city itself. This set of unique and elegant bridges will reinforce each other and add to Chicago's reputation for pursuing the best of modern design while maintaining its wonderful architectural traditions.

CONTEXT AND ISSUES TO BE ADDRESSED
Lake Shore Drive and the Metra railroad tracks effectively cut the Chicago South Side neighborhoods off, physically and visually, from the lake and waterfront. The tree lines bordering Metra completely obscure street level views. The view looking from the Burnham Park back toward the neighborhoods is similarly cut off, and the traffic on Lake Shore Drive enforces a further sense of separation. The steps and narrow decks of the existing bridges seem more like obstacles than connections. Finally, people traveling Lake Shore Drive have little indication that there are neighborhoods beyond the tracks. The existing bridges are too small within the overall scene to register as connectors to much of anything and visually get lost when seen from a distance. However, once on the existing bridges impressive views to the skylines in both directions as well as views of Burnham Park and the lake become available to pedestrians and bicyclists. It is important to maintain and enhance those views.

APPROACH
For both new pedestrian bridges at 41st and 43rd Streets we recommend curved cable stayed bridges supported by single masts 125' (38.1 m) high placed between Metra and Lake Shore Drive. The curvature will face north toward downtown Chicago, emphasizing views to city landmarks. The curved alignment allows for the placement of cables only on the side of the deck away from the downtown, making the bridges appear to float over the landscape. The bridges will provide two clear spans of 240' (73.2 m) from the tower to each abutment; no other piers or supports are required. Users of the curved bridges will experience a variety of views as they move along the curve and their axis of focus changes, creating a dynamic walking experience not available with a straight bridge. The curved deck facing toward downtown will create a kind of aerial amphitheater focusing views toward the city and parks, with the cables and tower behind the viewer. Curved cables parallel to the deck and a few feet below it resist the torsion in the deck created by the cable stays

The curved shape of the bridges and the opposing pull of the cable systems inherently stabilize the bridges against dynamic loads without the use of dampers, avoiding the problems recently seen in other long-span pedestrian bridges.

Two backstays will stabilize each tower. Elegantly inclined, it will have a height of approximately 125' (38.1 m) and will be visible from the adjacent neighborhoods and transportation corridors, becoming a recognizable marker in the landscape that will help provide a sense of place and orientation to this area. Placed off to one side of Lake Shore Drive, the towers will not interfere with travelers' views to the skyline. Each bridge will be dramatically lit at night. The towers, deck and cables will be emphasized. The towers in particular will have an accent color to clearly delineate their shape and inclination against the cables.

A cable-stayed structural type is proposed to reduce the size of the abutment anchorages. At the ends of each bridge the deck will flow down smoothly and transparently to grade at the abutments with no change in deck thickness and no heavy end element that would block views to the lake or park. On the lakeside the bridges will span the park drive and bike path, bringing the touchdowns close to the revetment to establish a visual and tactile connection with the lake. On the city side each bridge will look like an extension of the 41st or 43rd Street sidewalk, drawing users smoothly onto the bridge without intervening steps or switchbacks.

The superstructure of the bridges will consist of a thin concrete deck supported by elegantly shaped curvilinear steel ribs that together with the attached ring cables will create a three-dimensional truss-type structure that will be very transparent. The final configuration of this truss type structure will be determined during the final design of the bridge. The pedestrian railings have been shaped to complement the steel ribs and will also be curved in shape. They will be fabricated of stainless steel for durability and visual appeal. A transparent stainless steel mesh will be used to provide safety for users and visually enhance the bridges creating a very light appearance. This mesh will be hung from the railing and will follow the curvature of the bridge. The top rail will be shaped to allow for the integration of LED light strips, which will illuminate the deck surface at night. Since we are proposing the same type of bridge for all three sites on the South Side we will distinguish them from each other by the use of color, particularly at night.

The City of Chicago's plans to construct five new pedestrian bridges that reconnect several Chicago neighborhoods with Lake Michigan offers the opportunity to significantly change the image of the city, particularly as seen from the lake. Each of the five bridges will be set in prominent locations bordering the lake, visible not only to nearby residents and workers but also to innumerable daily travelers along Lake Shore Drive and other nearby streets. These important links have the potential to become new landmarks for Chicago while providing new connections across Lake Shore Drive, the Metra railroad tracks, and the Chicago River. They should also exemplify Chicago's tradition of excellence in the design of public works and join that group of signature structures, such as the Hancock and Sears Towers and the movable bridges along Wacker Drive, by which Chicago understands itself and that have come to represent Chicago's image to the rest of the world.

The most beloved bridges in America, such as the Golden Gate Bridge and the Brooklyn Bridge, are symbols of their eras and the cities in which they are located. Like the best bridges of their time, they create their aesthetic impact through the shaping and proportioning of their structural components, using the best materials available to their builders. Their designs are the result of pursuing efficiency, economy, and elegance, all at the same time. Today we have high-strength steel, high-performance concrete, and computer-based design tools to create cable-supported bridges of surpassing transparency and grace.

For Chicago's five new pedestrian bridges we are proposing five cable-supported bridges, including a cable-supported movable bridge, united by the theme of achieving elegance by the efficient and economical use of contemporary structural materials. This family of bridges will, as they are seen in succession by travelers and pedestrians, create a memorable and coherent family of structures easily recognizable from a distance. Three bridges on the south side and one on the north will be orientated to face the downtown, serving as book ends to the movable bridge over the Chicago River, while emphasizing views toward city landmarks and the city itself. This set of unique and elegant bridges will reinforce each other and add to Chicago's reputation for pursuing the best of modern design while maintaining its wonderful architectural traditions.

CONTEXT AND ISSUES TO BE ADDRESSED
The new Lake Shore Drive pedestrian bridge will be located next to the existing Lake Shore Drive movable bridge over the Chicago River. This bridge is a two-level bascule structure with distinctive Art Deco towers at each end. The new pedestrian bridge will be at roughly the lower level of the existing bridge, partly hidden to view from the Loop. From the lake side and Navy Pier, the bridge will be visible with the vehicular bridge providing a backdrop. The appearance of the vehicular bridge in the open position is also a consideration. To the south of the vehicular bridge the Lake Shore Drive viaduct provides an environment dominated by strong transportation forms. The northeast corner of the existing bridge is dominated by Lake Point Tower, an architectural masterpiece. The bike path works its way through several narrow spaces as it approaches the river crossing from the north. Once over the river expansive views to the south and east will open up allowing bike path users to experience both the lake and the city.

APPROACH
In order to establish a visual relationship with the adjacent vehicular bridge and to give the new bridge some visual presence among its very strong neighbors we recommend a movable cable-stayed bridge supported by four towers. The towers are made of high-performance steel that are as slender as possible so as to not block views to the existing bridge. They will be placed in close relationship to the towers of the existing bridge. The towers and their cable arrays will also create a sense of enclosure and security for bridge users. The cable pattern will act as a transparent visual screen for the existing bridge, blending old with new. Similar to the existing vehicular bridge, the new pedestrian bridge will open in the middle by rotating the two bridge halves about pivot points at the tower bases. When both bridges are open the new bridge will provide a complementary geometric shape so that the two bridges together form an attractive ensemble.

The cable-stayed bridge system gives maximum structural efficiency for medium span bridges. It was adopted for the 310' (94.5 m) main span by providing two individual structurally stable systems cantilevering from each side of the riverbank.

The mast height was chosen for maximum mast stability and efficient cable inclination. They also closely match the height of the towers of the existing bridge at approximately 95' (29 m) in height. The masts are detailed to establish a visual connection to the towers of the existing bridge behind them. The two masts are stabilized by a connection to each other via a cross box beam underneath the bridge deck, thereby avoiding visually disturbing cross beams above the deck. A further increase in slenderness is achieved by fixing the masts to the primary structure of the main span, which means that the masts swing back as the main span opens. Since the back span does not need support from the cable system a single backstay cable replaces the traditional multiple backstay cables.

Shortening the back span cable lifts the bridge. At the maximum of 60° rotation, the bridge provides the full required clearance. The single backstay cable resists the forces during the daily use of the bridge (live load/self weight). A multi-cable pulley connected parallel to the main backstay cable shortens the backstay cable by 72' to lift the bridge to the full 60°. During lifting the backstay cable will become slack. This "doubling up" of two systems allows the operational system to be free of tension while the bridge is closed to allow for easy maintenance. Using a multiple cable pulley allows a reduced cable diameter and a reduced winch diameter for the operating cables so that the whole mechanism is similar to those used for cranes. Standard reliable crane technology can be utilized for the cable drums and pulleys to minimize the cost and maintenance of the moving elements. The two tips of the bridge halves are connected in the closed position by a locking device to stop differential movement and transfer live loads. The current design allows openings in strong storms. The bridge structure can be designed (using a few additional cable stays) to accommodate higher wind speed during operation. However, it seems to be out of proportion to open the bridge during higher winds as no vessel (especially masted yachts) would be able to leave or come into the harbor under those conditions. The mechanical equipment will be located at the first set of anchor piers, mostly underground.

The clear structural hierarchy of the cable stayed bridge, with the tension elements being cables and the compression and bending elements being steel box sections matched in size and width to their structural requirements, results in the lowest weight for the bridge. The deck of orthotropic steel plate and steel cross beams spans between the primary edge girders and stiffens the structure horizontally, utilizing the full width of the bridge, so that no unsightly underfloor cross bracing is required. This reduction of weight makes for a slender and efficient looking bridge, and minimizes the requirements for lifting.

The bridge towers and cables will be illuminated at night creating a strong presence on the waterfront. The deck walking surface will also be illuminated with LED light strips located on the railings. The railings will be fabricated of stainless steel for durability and visual appeal. A transparent stainless steel mesh will be used to provide safety for users and visually enhance the bridge creating a very light appearance. This mesh will be hung from the railing and will follow the rectilinear shape of the bridge.

This bridge will use a minimum amount of modern, high strength materials to create a structure that is efficient, proven economical to build and maintain, and elegant. Its prominent location and unique shape will allow it to become a significant landmark in Chicago.