Small and supposedly simple footbridges affect their particular adjacencies architecturally and structurally just as larger, more prominent ones. Especially at inner-city and urbanistic crossings a bridge can appear unobtrusive and integrated, but also importunate, clumsy or misplaced. In order to give a bridge the right and appropriate appearance in its particular context, the local situation should be studied thoroughly.
The three following bridges designed by schlaich bergermann partner show the result of a sophisticated consideration of each location. And they show that sometimes a simple approach, carefully executed, can bring up interesting and innovative solutions.
The new Fehrlesteg in Schwäbisch Gmünd is a key connection for pedestrians, as it links the inner city to the main station and the urban district Taubental. A two-span stress ribbon bridge was designed to cross the river Rems featuring a total length of 58 m with spans of 27 m and 19 m respectively. With its minimalist design the new 2.50 m wide bridge follows the directions of the approaching walkways with a sharp bend at the central support.
The shape of the central support, that is located on an island between the two arms of the river, clearly shows the horizontal forces deriving from the redirection of the stress ribbons. Thus, the abutment “leans” against the deviation, creating a balcony-shaped cantilever on the central island.
At both ends the stress ribbons rest on slender concrete saddles which protrude from the abutments that are concealed by the river banks.
The stress ribbons consist of two parallel 400 x 40 mm steel plates in grade S355, that were pre-curved in the workshop and anchored at the abutments and the central support. With a sag of 40 cm or 19 cm respectively, the maximum inclination is at around 6%.
The abutments serve for anchoring, adjusting and hence for pre-stressing of the steel plates. Following a specific curvature the stress ribbons are pulled over the concrete saddles and back-anchored in cast-in steel parts. The hammerhead-shaped anchorage allows for adjusting the final length to the last millimeter.
At the central support, the stress ribbons are connected as fixed anchors to a cast-in steel part with shear connectors on the bottom to transfer the high horizontal loads into the concrete structure.
The solid abutment blocks rest on rectangular pile caps that transfer the loads via inclined micro piles into deeper and sound soil layers.
The slab elements, comprising 12 cm thick cut and grit-blasted Alpendorada granite (Portugal) with dimensions of 290 cm x 80 cm, are placed on elastomeric strips onto the two steel stress ribbons. The slabs are laid with gaps, invisibly fastened to the stress ribbons by grouted stud shear connectors. Adjacent slabs are separated by a neoprene insert within the joints to improve the damping behavior of the bridge.
The guardrail on both sides of the walkway is a 1,20 m high modular railing made of vertical stainless-steel rods. The posts are bolted to the granite slabs with every individual handrail segments having sliding joints towards the adjacent one. The friction in these sliding joints has a positive impact on the damping behavior of the lightweight stress ribbon structure.
The handrail itself allowed for the integration of LED strip, illuminating the walkway, emphasizing the form and structure of the stress ribbon and reflecting the layout of the bridge in the nighttime.
The construction of the Fehrlesteg stands for a sophisticated and innovative structure. With its slenderness, transparency and shape being defined by the flow of forces, this exemplary structure meets high aesthetic and economic standards. The bridge is intrinsically connected with the urban setting. Despite its modest appearance, the outstanding structure makes it a unique and distinctive bridge. Its grace is underlined by the granite walkway and stainless steel railing.
In 2009 a bridge design competition was tendered in Esslingen, close to Stuttgart. The objective included a better connection between the inner city and the Maille-Park as well as the re-design of the public place in front of the listed Eichamt.
For crossing the Rossneckarkanal, an equally listed canal, the objective called for a slender, filigree form that smoothly integrates into the historic ensemble of the old municipal park, without infringing the river banks walls, or the adjacent historic buildings. However, it was just as important that the design kept to the given budget.
Design and Concept
The realized structure meets these requirements in a satisfactory manner. With an overall length of 29 m and a fixed end on one side only the new the bridge focuses structurally and architecturally on the eastern bank of the canal. Facing to the west at the connection to the Maille-Park the bridge only lightly rests on the bank like a cantilevering leaf spring.
The fixed support has been conclusively realized by splitting the fixed-end moment into a pair of compression and tension forces: the compression force is supported on the frontside of the abutment block, while the tension force is anchored on the back side of the abutment with a pendulum.
The 3 m wide steel superstructure, fabricated as a steel hollow section in monocoque-style, has a variable height following the size of the bending moment. Hence the height reduces from 65 cm at the fixed-end to merely 23 cm on the park-side.
In order to reduce the weight of the structure, the complete superstructure has been manufactured using 10 - 12 mm thick metal plates. However, areas prone to buckling or wheel loads were strengthened with transversal stiffeners.
The loads are transferred via micro piles into deeper and sound soil layers. The compression bearing at the frontside of the eastern abutment is the longitudinally fixed point of the bridge. It consists of a robust stainless steel contact plate, welded to the superstructure, which is inserted into a steel crown that has been cast-in into the abutment.
The pendulums on the back side of the abutment connect the superstructure and the abutment with stainless-steel pins. Facing the park, at the western end of the bridge, stainless steel shear dowels ø 70 mm allow for the superstructure to slide in horizontal direction. These are inserted into a prefabricated steel abutment block that had previously been aligned and cast in its exact position.
The deliberate use of stainless steel in critical areas reduce maintenance requirements to a minimum and realize a durable and sustainable structure.
Finishing and Installation
The unobtrusive filled-rod railing was already welded to the superstructure in the workshop. Through numerous small sliding connection details a constraint-free installation and later use of the bridge is guaranteed.
Due to its relatively small length, the installation of the bridge was remarkably simple: the completely prefabricated superstructure, including railing, corrosion protection and walking surface, was lifted from the flat-bed truck and set onto the prepared substructure in just 3 hours.
The slender steel bridge across the Rossneckar provides a solution that naturally blends into the surrounding of both new and historic buildings. Conceptional design, material and choice of colour contribute to an adequate structure of high workmanship.
The new Bleichwiesensteg in Backnang connects the redesigned Bleichwiese with the arboreous backside of the Stiftshof. As an attractive link it thus establishes a direct connection between the historic centre of the city and the just recently finished Schweizerbau, a modern shopping facility.
The new bridge crosses the river Murr with a span of approx. 27 m, fulfilling an important function in traffic. As it connects two urban districts it is also of significance in terms of urban development and landscape design.
The brief was complex. The main requirements were to reuse the existing abutments and foundations (the bridge replaces a wooden truss-girder bridge), as well as the demand on a high level of prefabrication and an easy and efficient way to erect the bridge.
Hence, a simple and yet outstanding single-span girder with an effective width of 2.50 m was designed. Two lateral box sections which rise in height from 30 cm at the abutments to 1.30 m towards the middle of the bridge, serving both as primary structural element and as railing. The two box sections are interconnected and stabilized through an orthotropic deck.
The bridge’s structural and creative characteristics become apparent in the division of the superstructure into two almost identical parts. These are completely prefabricated, individually delivered, lifted and then assembled to a whole system on site.
The distinctive central opening is therefore not only eye-catching design but particular intelligent referring to efficient fabrication and installation procedures.
Using a compression strut as top chord and visible pinned connections the maximum bending moment is visually divided into a pair of forces, thus making the structure comprehensible. And besides: the emerging triangular recess makes the bridge appear light and graceful at its highest point.
Via elastomeric bearings it is supported on the in-situ enlarged abutments and the existing pile foundation. As the new bridge is altogether lighter than the previous wooden one, the existing piles could be used without any additional measures.
Seemingly detached from the superstructure the delicate railing floats above the lateral box sections. It comprises ø4 mm horizontally pre-stressed stainless steel cables that accentuate the dynamic linearity of the bridge.
In the dark, the walkway with its light toned surfacing is illuminated by LED lighting strips integrated into the lower edges of the box sections. At daytime too, the light hue of the walkway creates a contrast to the significantly darker box sections.
The new Bleichwiesensteg is a striking demonstration of how an innovative structure can be realised in such a short construction time by pursuing a straightforward approach to structural design. Moreover, the visualization of the flow of forces within the structure, generated by the opening of the superstructure at the bridge centre, makes it tangible for the user and thus contributes to its acceptance in the public.
The innovative steel structure across the river Murr comprises a graceful bridging between a modern residential area and the historic Stiftshof. With their filigree railing contrasting the sculptural shape the structure gains an air of elegance and distinction.
This article shows, that the architectural motto “less is more” especially counts with the design of small footbridges. Striving for simplicity and reducing bridge-design to the very necessary is always a desirable goal. At best, every single element is structurally essential, reasonably shaped and well-proportioned.
The three bridges introduced in this paper show, that following these principles will lead to quite satisfying and yet individual designs. Consequently all of these bridges show the desire for a simple appearance, an efficient structural behaviour, careful detailing and a sustainable construction. The combination of which leads to the superior goal, bridge design should focus on: finding an appropriate and self-evident solution for each particular situation.
This article is a short version of "Simple Bridges", published in Steel Construction 9 (2016)