The Frankfurt Bridges are architecturally diverse in design and execution

The Frankfurt will be designed differently to match the streets they cross and according to the width of the road: ultra-modern or with traditional craftsmanship. Some sections are made of steel trusses, others of red Main River sandstone or even of light-colored exposed concrete combined with modern art - every conceivable variation is planned. Accordingly, the large Main river bridges on the route are also designed differently: Both Main bridges will be artisan-crafted and will cite old German or European bridge structures. The bridge that spans the tracks behind the main train station in Frankfurt, on the other hand, will be ultra-modern.

The Frankfurt Bridges are constructed differently depending on the local conditions

The route for the Frankfurt Bridges is very heterogeneous: In some places, the underlying roads are not quite so wide, so that the Frankfurt Bridges have to be kept relatively slim or - when running through residential areas - even glassy; in other places, however, the roads are comparatively wide but are already crossed by railroad bridges, so that the Frankfurt Bridges have to pass over these crossbridges at a greater height.

For each of these local conditions, a correspondingly suitable bridge design must be conceived: In narrow places, a glass or filigree bridge structure made of truss steel is chosen instead of a wide bridge corpus made of concrete; where the bridges have to go up high, a viaduct-like design - for example, made of natural stone masonry - makes the most sense ; or else they float as high as gondolas above old trees. A wide variety of shapes and materials are used.

Frankfurt's bridges run in a wide variety of environments: sometimes over the river, sometimes over tracks of the main railway station or suburban train bridges in the city; and in a few cases they also run through narrow streets.

Each of these challenges requires its own solution.

Not all bridges are the same: In the course of Frankfurt Bridges, a wide variety of constructions come into play - depending on the environment

Unlike river bridges, the Frankfurt Bridges do not have large spans, but must primarily bear the load of the quarters, plants and traffic routes. Accordingly, they are built of concrete with steel reinforcement for most of their course. If the streets are not four or six lanes wide, but narrower in some places, or if the bridge piers are on narrow sidewalks and thus quite close to the wall of houses, then Frankfurt Bridges are not built of concrete, but of steel with piers that face away from the walls of the houses.

These different construction methods for a second level in the city were already being considered in New York in 1870, when the city was looking for new transportation routes that could provide another level of traffic through the street canyons of Manhattan.

Rufus Henry Gilbert - Never-built-New-York

The Frankfurt Bridges over the Main River will be built with steel - and for bottleneck sections they will be also constructed with steel

Most bridges in Germany were built of concrete after the Second World War. Today, after only 50 or 60 years, more than half of them are in need of renovation, while steel bridges from the 19th century are still doing their job. These are also easier to repair because you can get right up to the structure, whereas with concrete bridges, where the steel reinforcement is hidden under the concrete, it is often not apparent how dilapidated the structure may already be.

The Frankfurt Bridges cross the Main twice. In both cases, construction in steel is planned.

Steel trussing is also ideal for the two narrow pass sections through which the Frankfurt Bridges run, because steel trussing is filigree, lets in a lot of light and - when artistically designed - is aesthetically very pleasing.

Steel has the advantage of allowing filigree structures and its durability (when properly processed and maintained) makes it suitable for a highly sustainable bridge construction system

A renaissance of the steel truss would be the most sustainable solution - the only question is whether there are still companies existing today that can build it...

43 Alte Hamburger Elbbrücke - HHLA Hamburger Fotoarchiv

Robert Doisneau - Paris-les Halles Baltard - GAMMA RAPHO

Cervin Robinson - Library of Congress, Prints & Photographs Division - Penn Station NY 1962

Steel trusses are the method of choice not only for bridge construction, but also for hall construction

Buildings that can last for centuries and are beautiful: the old market halls in Paris (Les Halles), the Pennsylvania Station in New York (unfortunately demolished in 1969) or even the Eiffel Tower - they are all examples of an engineering art that has almost been forgotten.

Frankfurt has some beautiful steel truss constructions: the Honsellbrücke and the Eisernen Steg as well as the hall of Frankfurt's main train station

Arches and rosettes also have a function

One often finds strikingly beautiful worked arches or rosettes in steel trusses. Admittedly, these do not have a load-bearing function; but they are not simply for the sake of beauty either, but have a stiffening function: This not only makes the overall structure more stable, but also reduces the vibrations of the steel body due to an evenly distributed "extra" mass.

Especially with regard to steel construction and steel trusses, which flourished during the period of industrialization, there are fortunately still numerous records and documentation of calculations that can be evaluated and processed in the Master Academy on the Offenbach bridges.

An example: the Ardant steel truss

In 1839, engineer Camille Polonceau invented the structurally effective Polonceau truss. However, architects long refused to expose iron structures in public buildings for aesthetic reasons.

Nevertheless, engineers recognized the great structural possibilities of iron roofs, which led them to experiment with curved trusses to avoid the stick roof trusses reminiscent of industrial buildings.

Today, though, everyone knows the Polonceau truss, while its aesthetic counterpart, the Ardant truss, is known to few. This curved truss uses an inscribed arch tangential to the main rafters of a gable roof to increase the rigidity of the roof structure and avoid distracting tie straps.

In keeping with the spirit of the times, this solution, with its high decorative potential, seemed to offer a satisfactory answer to both the aesthetic and structural problems of iron roof trusses in public buildings.

As a result, it was widely used in swimming pools, stock exchange buildings, schools, platform roofing, palaces, shopping malls, etc. (https://onlinelibrary.wiley.com)

The Ardant truss is a roof truss structure consisting of a "spandrel arch" attached tangentially to the main rafters and gable frame.

The design was intended to be an aesthetically pleasing alternative to the poloceau truss, a conventional truss popular for its optimal truss use of elements and minimization of material. The poloceau truss relies on a horizontal tie rod spanning the roof to balance the horizontal thrust caused by the roof loads.

Figure 14 shows the principle used by Vierendeel (Belgian engineer 1852 to 1940): some pseudo-stretched truss joints (in blue) and the actual curved truss elements (in red).

Steel trusses for bottlenecks: Most of the time, the Frankfurt Bridges run over very wide four- or six-lane roads - but there are also bottlenecks along the entire route

One of the two most critical bottlenecks is a section of Gartenstrasse: Although it, too, has four lanes, it has comparatively narrow sidewalks in some areas, which means that a particularly filigree structure has to be chosen for the Frankfurt Bridges. A glass structure is a possibility here, or steel trusses with a high quality craftsmanship finish.

A glass structure in narrower sections of the street looks comparatively plain, but has the great advantage for residents of creating a minimum of shading

A comparatively filigree steel truss bridge leads through Gartenstraße

The design is structurally flexible: Different arch sizes ensure that driveways are left free and -where possible- parking spaces remain unaffected.

Arches can be steel-framed with bracing rings and arch elements. Wrought iron lanterns can enhance the bridge section in the evening with insect-friendly amber light.

This could also be equipped with a plantable arch - top view of a section through a steel truss bridge in Gartenstrasse

The section in Gartenstrasse is "Frankfurt Bridges light": There are no buildings or other structures on the bridge section, but it only serves autonomous traffic on the sides and strollers in the middle, with a total corpus width of only 9.5 meters. Overgrown with climbing plants, the bridge offers residents a green space in front of their windows.

Here is a variant without an arch: In bottlenecks, bridge elements such as the arch can also be omitted. In any case, it is important that as many light windows as possible are let into the bridge floor

The lighter and more filigree, the better. This also applies to the body of the bridge: Wherever possible, it is equipped with a walkable glass floor, which is either interspersed in the form of round light holes or - as in the Gartenstrasse example - extends as a strip under both the roadways and the walkways, allowing plenty of light under the bridge.

With the help of a shadow analysis, it was possible to determine that the daylight quotient is reduced by 4% to 11% for a bridge without an arch, and by 18% to 20% for a bridge with an arch.

It can be seen from the figures that it is not so much the incidence of light at the front of the window that is affected, but rather the incidence of light into the depth of the room (turquoise areas). The values below (Dmin and Dmax) denote the "daylight quotient": The daylight quotient (D) describes the ratio of indoor illuminance (Ei) to outdoor illuminance (Ea) under overcast skies, and is a measure of the supply of natural light to a space. While at present daylight penetration in ground-floor apartments on Gartenstrasse reaches the back of the rooms in some places, this is reduced by 4 to 11 percent even in the case of a filigree bridge without an arch.

In order to reduce the impact on the daylight quotient to a minimum, a "glass bridge structure" was designed (cross-section shown here)

However, the decision as to which form is chosen is to be left to the residents in all sections: whether a glass unadorned bridge or a filigree steel truss, whether with or without a plantable arch - all this can only be assessed by the residents once comprehensive shadow analyses are available for all variants. The only important thing for the bridge route is that even through the bottlenecks, the traffic route for the bridge transport system remains secure.

Another bottleneck on the entire route is located on the Höhenstraße

Similar to Gartenstraße, the distance between opposing houses is only 19 to 20 metres.

For Hoehenstrasse, therefore, an even slimmer construction is proposed than for Gartenstrasse: the lanes of the autonomous traffic run one above one another here, so that the lower bridge corpus is only 8 metres wide, and the upper bridge corpus is even only 5 metres wide.

In Hoehenstraße the vehicle lanes are one above the other, whereas in Gartenstrasse they are side by side

If one wants to create a closed ring traffic around Frankfurt's city center, there is hardly any alternative to these two road sections. An interruption of the ring route connection would significantly impair the effectiveness of the autonomously driving traffic system or possibly even make it unattractive. The slim bridge variants are a possible solution here to make the bridges' travel and pedestrian paths continuous while sacrificing as little space and light as possible – and can be still replaced by glass bridges, if the filigrane construction turn out create too much shadow for the people living along the brigdes.

Alternativ können fast komplett gläserne schlichte Varianten für die Engpässe gewählt werden

Brightness under the bridges is maximized mainly by walk-on glass shelves in the bridge body

A wide variety of designs are possible for the bottlenecks

But no matter which truss construction you choose: It is important that it looks as filigree as possible If the construction still casts too much shadow, a glass bridge body can be chosen instead

The bridge sections with walkable glass can also feature beautifully crafted cast-iron structures

When positioning all bridge sections, a “field of vision”- check must be carried out in advance

The structural system of the bridge pillars is kept for all sections that were statically checked in the feasibility study in such a way, that the supporting pillars of the bridges can always be moved slightly variably up to one meter to the right or left.

If they are then still in the field of vision at intersections, then their design must be constucted in the form of steel trusses so that they are permeable to view.

Main bridge 1, the "Freiheitsbrücke“ (Bridge of Freedom) will run with parallel course to the existing "Friedensbrücke„ (Bridge of Peace)

The Frankfurt Bridges arrive at the Friedensbrücke with a clear height of 6.20m above street level on the banks of the Main River. In the further course, however, they do not pass over above the Friedensbrücke, but run as a second, newly built bridge alongside the Friedensbrücke. On the one hand, this simplifies construction, as no existing structure is affected: If the route were to be routed over the Friedensbrücke, the latter would have to statically support the new upper section, which it is not designed to do; moreover, the Frankfurt Bridges on the Main provides a visually beautiful backdrop behind the otherwise rather grey drab Friedensbrücke.

The traffic in Frankfurt is massively relieved by the approximately 40 million passenger trips that the autonomously driving system takes over on the Frankfurt Bridges - which affects the „Friedensbrücke“ in particular. Anyone who has already been stuck in traffic jams there with nice regularity will appreciate the relief provided by a second bridge next to it.

The Freiheitsbrücke (Freedom Bridge) next to the Friedensbrücke (Peace Bridge) is made of steel trusses, based on the example of Frankfurt‘s Eisernen Steg (Iron Bridge)

Corresponding to the Peace Bridge, the higher bridge next to it bears the name "Freedom Bridge".

Freedom as an important complement to peace is thus honoured: freedom of opinion or freedom of speech - a valuable basic element of our cultural area in Europe.

In terms of design and construction, the bridge is based on the most prominent bridge in Frankfurt: the Eiserner Steg (iron footbridge), built in 1868 and immortalized in one of his paintings by the famous expressionist painter Max Beckmann.

The Freedom Bridge is planned as a cantilever bridge made of steel trusses - just like the Eiserne Steg

Even though the Iron Bridge looks like a suspension bridge, since it was renovated and raised in 1912 it is no longer a suspension bridge but a so-called cantilever bridge.

The first cantilever bridge with articulated girders in the world already crossed the Main, some 150km from Frankfurt: Heinrich Gerber built the Hassfurth Main Bridge there in 1864, the design of which he patented in 1866.

Two other cantilever bridges were built in the late 19th century: the Quebeck Bridge and The Fourth Bridge in Scotland. Both bridges are still standing today.

The Freiheitsbrücke (Freedom Bridge) over the Main has two levels: The vehicles of the autonomous bridge traffic drive on the upper level, the lower level is reserved exclusively for cyclists and walkers.

Two "relatives" of the Frankfurt‘s planned Freedom Bridge: in New York and Budapest

The steel framework of the Freedom Bridge on the Main is shown off to its best advantage at night thanks to discreet lighting.

- similar to its namesake, the Freedom Bridge in Budapest, or to the Queensboro Bridge in New York, which also has two traffic levels on top of each other.

Main bridge 2 - the "Gerechtigkeitsbrücke„ (Bridge of Justice): It leads the Frankfurt Bridges from the Deutschherren-Ufer over to the Schoene Aussicht

The Frankfurt Bridges cross the Main a second time to close the ring: from Walter-Kolb-Straße / Deutsch-Herren-Ufer on one side to the Schoene Aussicht on the other bank.

The name of this second Main bridge is "Gerechtigkeitsbrücke" (Justice Bridge or Bridge of Justice) - as a supplement to the first Main bridge, the "Freiheitsbrücke" (Freedom Bridge): Alongside Liberté, Fraternité and Egalité were central values in the French Revolution for Europe's departure towards democracy. The concept of justice is a reminder of these fundamental values: Striving for social justice in all dimensions of our society will be one of the great challenges of the future, alongside the aspects of freedom.

In the spirit of fraternité and egalité, the Frankfurt Justice Bridge for Social Justice is modeled on the Pont- de Bir Hakeim in Paris

The main similarity between Pont de Bir Hakeim and the Justice Bridge: Both have transport routes on two levels - below is pedestrian traffic and above is light rail traffic

Pont Birhakact - www.paris1900.lartnouveau.com

nikonaft - Pont de Bir Hakim - istockphoto.com

Traffic on two levels across the Main is thus provided by both bridges: the Freiheitsbrücke (Freedom Bridge) next to the Friedensbrücke (Peace Bridge) and the Gerechtigkeitsbrücke (Justice Bridge) between the Alte Brücke (Old Bridge) and the Ignatz-Bubis-Brücke (Ignatz Bubis Bridge).

The Justice Bridge also provides protective canopies for pedestrians and cyclists on the lower level, while autonomous traffic travels on the upper level.

The Bridge Triad: Freedom Bridge, Peace Bridge, Justice Bridge

Frankfurt has always been a free trading city and has always been distinctly international. As a major transport hub and financial centre in Europe, it performs important functions. The anchoring of the fundamental values of "freedom, peace and justice" in its bridges fits both the tradition and the future of the city.

Anna Opoleva - bildagentur.panthermedia.de

In terms of design, the Justice Bridge takes up the arches of its two neighbouring bridges: the Old Bridge on one side and the Ignatz Bubis Bridge on the other

The third major bridge crosses the tracks of Frankfurt Central Train Station: the "Deutsche Bahn Bridge of the Future"

In order to close the ring for the traffic circle route on the Frankfurt Bridges, they also have to cross over the tracks behind the main train station. This span of over 230 meters is spanned by a bridge that uses the most modern design of bridge construction to turn towards the city of the future, in which the space above the major traffic routes is used for living on a second level in the city.

Design objective for the bridge over the tracks: The arch shape of the main station barrel roofs is to be taken up

The beautiful arch shape of the main station roofs provides the natural reference point for an equally arch-shaped bridge over the tracks. The one span of 230 meters can be represented with an arched bridge. However, the possible locations for piers of the bridge are diagonally offset from each other, which should be compensated by the bridge design.

Accordingly, the JK Bridge in Brasilia was the model for the Deutsche Bahn Bridge of the Future

The first of the three arches alone spans a distance of more than 230 meters - the Deutsche Bahn Bridge of the Ruture, on the other hand, spans just a total of only 234 meters.

Unlike its model in Brasilia, Deutsche Bahn’s Bridge of the Future does not have a surface of exposed concrete, but a shiny metallic one - corresponding with the rails beneath it and providing a perfect backdrop for its illumination

Without the cooperation of Deutsche Bahn, the Frankfurt Bridges could not be built

The Frankfurt Bridges not only have to cross the tracks of Frankfurt's main train station, but also cross smaller Deutsche Bahn railway bridges at six points along their route - on a third level, so to speak, above the second level "railway bridge". This can only be done in cooperation with Deutsche Bahn and not simply over them, in the truest sense of the word.

In the case of particularly high sections that run over DB railway bridges, viaduct architecture is used

In Frankfurt there are numerous railway bridges in the urban area, which - similar to motorway bridges - cross wide roads. The Frankfurt Bridges must still pass over these railway bridges at an appropriate distance, as they are not allowed to cross them wiht an intersection. If the Frankfurt Bridges usually run at a clear height of 5.2 metres, here they must run up to a clear height of at least 11.2 metres.

In the case of such bridge sections, which run particularly high, the construction method of conventional viaducts, which were traditionally also often built of natural stone, is more suitable.

Accordingly, it should be examined whether the concrete structures can be supplemented or partially replaced by natural stone at these points.

Slim valley bridge constructions are also suitable for spanning high treetops.

Viable and aesthetically pleasing solutions can be found for all the challenges of the route

The Frankfurt Bridges do not use a uniform construction method, but a wide variety - adapted to the respective challenges of the route.

This allows bridge sections to be perfectly integrated into their surroundings.

For the bridges that cross the Main, bridge forms were specifically chosen that are aesthetically pleasing with two traffic levels: a lower one at street level and an upper one at the level of the Frankfurt Bridges.

The Bridge World

Affordable housing

Special quarters

Resident compensation

Architecture

Green on the bridges

The green metropolis of the future

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