Integral Railway Bridges

Integral Railway Bridges - New Bridges in Germany
Mike Schlaich, schlaich bergermann und partner, Germany
1.
Introduction
The image of the bridges of German Railways the "Deutsche Bahn" is shaped on one hand by the
standardised bridges which result from the German Design Framework "Rahmenplanung" and, on the
other hand, by the classic attractive bridges of the early 20th century; but only the latter make it into
annual bridge calendars. However, at a closer look one quickly notices that especially recently a
multitude of interesting railroad bridges has been developed and built, which prove that design of
railroad bridges in Germany is again on a high level of quality.
This paper will give a short description of the trends in the design of railroad bridges in Germany in
the recent past as well as show some prominent examples. The Railroad Bridge Council,
"Brückenbeirat" created by the Deutsche Bahn and its "Leitfaden – Gestalten von Eisenbahnbrücken"
(guideline – design of railroad bridges) were an important impetus greatly influencing the bridges of
German Railways described in the following, especially the integral and semi-integral bridges of the
new railway lines. After a quick overview on international high-speed train bridges, five recently built
integral or semi-integral bridges in Germany are presented. The difficulties which occurred during the
approval procedure and the execution of these innovative bridges as well as generate ideas for future
research will also be mentioned.
2.
Some Recent Innovations
In addition to the traditional steel arch with its superstructure above the deck and thus mainly used for
crossing roads and channels, recent years saw mostly railroad bridges being subjected to the dictate of
replaceability. In order to allow the easy replacement of worn out superstructures, the bridges had to
consist of relatively small components if only for the sake of facilitating transportation. This required
numerous joints and bearings which led to the Design Framework of standardised single spans of
reinforced concrete box girders with 42m length and a construction height of 4m. However, the
number of joints and bearings reduces the life span of the structure making the demand for
replaceability a self-fulfilling prophecy. This requirement of replaceable superstructures is rendered
completely questionable considering that in 80% of the cases not only the superstructure had to be
exchanged but the entire structure [1].
But aside from these bridges constructed according to the Design Framework a number of noteworthy
bridges have been built within the past two decades. As an alternative to the steel arch bridges trough
structures, such as the Havel-bridge at the Spandau train station [2], were designed following the flow
of forces. This led to the design of composite bridges such as the bridge across the Danube in
Ingolstadt [3] or the railroad crossing Stephanitor in Bremen [4].
Fig. 01
6-track Havel-bridge at the Spandau train station, steel trough, 1998 (photo: sbp)
Fig. 02
Fig. 03
Bridge across the Danube in Ingolstadt with a composite cross-section, 2001 (photo: Roland Halbe)
Railroad crossing Stephanitor in Bremen with a composite cross-section, 2006 (photo: Michael Zimmermann)
The bridge across the river Main in Nantenbach is also worth mentioning. "The bridge across the river
is a three-span haunched composite truss with one of the lower concrete slabs in the range of the
negative moments (double-composite). With a central opening of 208m it is the bridge with the widest
span on the new railroad from Hannover to Würzburg and Mannheim to Stuttgart and the girder bridge
with the widest span in the entire railroad system of the Deutsche Bahn" [5].
Fig. 04
Composite truss bridge across the river Main in Nantenbach, 1993 (photo: lap)
For shorter spans composite bridges with precast girders (VFT) are highly successful. Lightweight
precast composite elements consisting of steel girders and parts of the deck are accurately
prefabricated at low cost and rapidly assembled on site. Up to now a surprising 600 bridges have been
built following this procedure [6]!
Fig. 05
Bridge across the river Lech in Schongau, VFT-method, 2001 (photo: ssf)
In another innovative step the German Railways permitted the use of cast steel in the construction of
their bridges, a method previously tested in high-rise and footbridge construction. The use of cast steel
nodes in railroad bridges allows loadbearing and robust steel structures while reducing the expenditure
for manufacturing and maintenance. They permit a design suitable for the structure and a multitude of
forms giving the structural engineer more freedom in the design and structural detailing. The
Humboldt harbor bridge in Berlin with its slender steel arches and cast steel nodes was built after
extensive testing [7]. In 2008 it received the German Bridge Award.
Fig. 06
Humboldt harbor bridge at the Berlin main train station, 1999 (photo: sbp)
The German Railways is not only open-minded about the choice of materials but also about innovative
structural solutions. As an alternative to the traditional steel arch, network arch bridges – originally
designed by Per Tveit from Norway – with spans up to 140m are also being built in Germany [8]. The
slightly more expensive assembly due to the optimal placement of the hangers is compensated by the
resulting favourable structural behavior and the efficiency of this method.
Fig. 07
Network arch bridge across the river Oder in Frankfurt, 2008 (photo: gmg)
After the referendum in November 2011, the dice have been cast concerning the controversial largescale project Stuttgart 21, and now two interesting new bridges will be built on the site. The bridge
across the river Neckar in Stuttgart has four tracks, two major spans of approximately 75m and three
shorter spans on each side. The bridge's distinctly innovative and individual character is due to the
necessity that the bridge needs to assert itself in the extremely obstructed knee of the river between the
Rosensteinpark and Bad Cannstatt. The thin bridge deck rests on slender supports on the banks and is
suspended across the Neckar on steel sails with concrete masts, thus typologically belonging in a
category between the cable- and extradosed suspension or a trough bridge with webs formed following
the moment diagram [9].
Fig. 08
Bridge across the river Neckar in Stuttgart-Bad Cannstatt, completion 2018 (animation: sbp)
From Wendlingen across the Swabian Alps to Ulm the railroad will cross the steep, deep valley of the
river Fils with two parallel 500m long bridges [10]. The Fils valley bridge is designed as a slender
concrete structure with V-shaped main pylons, one of the new (semi-)integral bridges of the German
Railways described in chapter 4.
Fig. 09
Bridge across the Fils valley, animation, completion scheduled for 2018 (animation: lap)
3. Bridge Council and Guideline
"Railroad bridges are truly high performance structures with respect to requirements for load bearing
capacity, durability and serviceability. During the past years these requirements together with
additional features regarding construction technology and renewability without interrupting traffic
have overshadowed the call for an appropriate design of the bridges. But these bridges in particular
shape the building culture in our country because so many people use them on a daily basis and also
take them for granted. It is therefore high time that the focus is placed on the aesthetical design of
railroad bridges and to substantially improve their integration into their surroundings but not at the
expense of functionality, durability or even expenditure. This is a major goal of the Bridge Council
founded two years ago (2007)" [1].
This quote describes very well the task of the Bridge Council, founded in March 2007 by Hartmut
Mehdorn, then CEO of the German Railways, and by Jörg Schlaich. Only one year later the Council
issued a design guideline the "Leitfaden – Gestalten von Eisenbahnbrücken" (guideline – design of
railroad bridges) [11], which might be called a small revolution. Written by renowned experts in the
field and legitimized by the prefaces of Wolfgang Tiefensee, then Minister of Transport Construction
and Urban Development, and Hartmut Mehdorn, this guideline bid farewell to the dictate of
exchangeability i. e. the Design Framework. This guideline includes the German Railways's selfcritical assessment of existing designs naming some of the bridges described in chapter 2 as examples,
encourages sophisticated designs for bridges developed for a specific location. The guideline is a
definite invitation to utilize the diversity in bridge design and thus contribute to the building culture
without losing sight of durability and cost-efficiency, i. e. the "life cycle costs". To the author's
knowledge there is no other country where the design of sophisticated railroad bridges is similarly
supported by a comparable document. The bridges on the German Railways' current new projects,
described in the following chapter, definitely profited from this helpful and inspiring guideline as well
as from the ensuing publications [12 et al].
4. Bridges for High-Speed Traffic
Since the 1960s the high-speed trains Shinkansen are operated in Japan. In Europe it is the French
TGV (Train à Grande Vitesse) since the 1980s. High-speed tracks also exist in Asia, for example in
Korea and China [13]. In Europe, after France mainly Germany and Spain built high-speed tracks for
trains with a design speed of 300 km/h. The dynamic stresses of the bridges resp. the stiffness
requirements for reasons of the trains' safety and comfort required new approaches in the design,
detailing and standardisation of the bridges. This resulted in numerous interesting new structures, such
as in Spain where several bridges were built for the AVE (Alta Velocidad España) [14, 15].
Fig. 10
Bridge across the Ebro for the high-speed track Lerida-Saragossa, [14] (photo: cfcsl)
Fig. 11
Sant Boi Viaduct for the high-speed track near Barcelona [15] (photo: pdelta)
5. Integral High-Speed Train Bridges in Germany of the New Railroad Corridor EbensfeldLeipzig/Halle
In Germany integral bridges have been investigated for some time now [16, 17, 18] and possible new
types of bridges have been outlined. Integral bridges were recommended to the German Railways as
early as 1990 [19] such as the bridge across a "wide flat valley of average depth" later included in the
guideline. The superstructure is a robust two-webbed prestressed concrete beam with spans around
20m, a series of continuous beams with an l/h-ratio of approximately 12. Thin steel supports are
monolithically connected with the superstructure and the foundation. Structural joints are placed at
120m intervals for the tracks to continue also without joints and track expansions. The braking forces
are transferred by intersecting steel supports acting as brackets. A prominent example of this type of
bridge may be seen at the Berlin main train station.
Fig. 12
Initial ideas of integral railroad bridges (from [19])
The German reunification presented the unique opportunity to build an entire series of railroad bridges
under the heading of the project "Verkehrsprojekte Deutsche Einheit (VDE)" (The German Unity
Transport Projects) [20, 21, 22]. The new high-speed track of the German Railways with the projects
VDE 8.1 (from Ebensfeld to Erfurt) and VDE 8.2 (from Erfurt to Leipzig/Halle) includes a total of
five remarkable, large bridges: the Scherkonde valley bridge (2010), the Stöbnitz valley bridge (2011),
the Gänsebach valley bridge (2011), the Unstrut valley bridge (2012) and the Gruben valley bridge
(2012).
The common denominator of these bridges is that they are integral or semi-integral bridges, i.e. built
completely or partially without bearings and expansion joints and that their design was inspired by the
above-mentioned guideline resp. mentored by the Bridge Council. All of these five structures have
been successfully completed or are nearing their completion. This paper leaves only room for a short
description with photographs and sketches of these well-designed and revolutionary bridges. For
further, in-depth information about these bridges please refer to the references at the end of this paper.
The Scherkonde valley bridge [23], 576m long, was the first of these bridges to be completed. The
intervals of the supports had to follow the regular span of 44m given in the project approval. The solid
supports were connected mostly monolithically to the haunched superstructure, a prestressed concrete
slab. The result is a slender, transparent semi-integral structure, gently blending in with the natural
environment of the Scherkonde valley.
Fig. 13
Scherkonde valley bridge, cross-section and view (photo: DB AG)
The integral Stöbnitz valley bridge [24] is being perceived as a simple engineering structure befitting
its location. In the guideline the Stöbnitz valley bridge spanning 297m falls under the category
"Narrow flat valley of low depth". The horizontal forces are transferred by a monolithic Vierendeelgirder consisting of the superstructure, a concrete slab, concrete supports and pile caps.
Fig. 14
Stöbnitz valley bridge, cross-section and view (animation: lap)
The 1.001m long Gänsebach valley bridge [25] spans the shallow and wide valley of the same name.
With a maximum of transparency the bridge tries to blend in with the surroundings. The structure is
divided into 10 blocks, each a 112m long integral prestressed concrete bridge with a twin-webbed Tbeam. The axial forces due to braking and acceleration are transferred by a characteristic braking
trestle in the middle with V-shaped plates beneath the webs. The width of the individual braking
trestle varies depending on the height of the superstructure above ground and soil conditions. Vshaped plates are responsible for the cross-stiffening of the bridge between the supports and the joints.
It is quite obvious that this bridge might be seen as the interpretation of the bridge with steel supports
described in fig. 12 translated into a pure concrete bridge.
Fig. 15
Gänsebach valley bridge, cross-section and view of 2 blocks (photo: DB AG)
The integral design of the Unstrut valley bridge [26] calls for a series of four continuous 10-span
supports without joints and bearings, with a regular span of 58m and the overall length of 2.668m as
given in the project approval. The fix points are located at each mid-span where it merges with the
vertex of a truss-like reinforced concrete arch. Extensions are still required on both ends. The standard
supports are slender pier-plates, effectively stiffening the deck transversally and with sufficient
flexibility along the bridge to absorb the deformations due to temperature changes.
Fig. 16
Unstrut valley bridge, cross-section and partial view (photo: DB AG)
The semi-integral Gruben valley bridge [27] spans 215m across the Gruben valley with its dense
woods; its only access is a country lane and a trail. This valley is situated in the Thuringian forest
north of the Goldberg tunnel. The design, an arch bridge – in unavoidable adherence to the tender
design in compliance with the project approval – features the characteristic connection between super
and substructure without any joints and bearings as well as joints and longitudinally displaceable
bearings at the abutments. The truss-like twin-hinged arch with a distinctive crown allows almost
identical continuous spans of the superstructure, a twin-webbed T-beam prestressed longitudinally and
horizontally. The tapered slender pier-plates are designed to absorb the deformations due to
temperature changes and shrinkage without any bearings.
Fig. 17
Gruben valley bridge, cross-section and view (animation: sbp)
The new (semi-) integral bridges of German Railways
Concept Design
Scherkondetal Bridge
DB ProjektBau
Stöbnitztal Bridge
Leonhardt, Andrä und Partner
Detailed Design
Büchting + Streit
Leonhardt, Andrä und Partner
EBA-Checking Engineer
Contractor
Structural Type
Deck Type
Deck Height [m]
Max. Span
Spans [m]
Total Length [m]
Max. Length between joints [m]
Cost [Mio €]
Literatur
M. Curbach
Adam Hörnig
Semi-integral
prestressed conc. slab, haunched
2,00 resp. 3,50
44
27 + 2 x 36,5 + 10 x 44 + 36,5
576,5
576,5
20
[23]
Gänsebachtal Bridge
schlaich bergermann und partner
schlaich bergermann und partner/
SSF Ingenieure
H. Hartmann
K. Geißler
Alpine Bau
Adam Hörnig
Integral
Integral
prestressed concrete girder
prestressed concrete girder
1,95
2,08
24
25
22 + 3 x 24 + 6,5 + 4 x 24 + 6,5 + 3 x 24 + 22 52,5 + 8 x 112 + 52,5
297
1001
102,5
112
8
25
[24]
[25]
Unstruttal Bridge
DB ProjektBau /
schlaich bergermann und partner
Nord-West Planungsgesellschaft /
SMP Ingenieure im Bauwesen
H.-P. Andrä
Alpine Bau / Berger Bau
Integral with 4 arches
prestressed concrete box girder
4,75
108 (arch)
3 x 58 + 4 x (4 x 58 + 116 + 4 x 58) + 3 x 58
2668
580
60
[26]
Grubental
schlaich bergermann und partner
schlaich bergermann und partner
V. Angelmaier
Züblin
Semi-integral with arch
prestressed concrete girder
2,40
90 (arch)
25 + 25 + 90 + 25 + 25 + 25
215
120
[27]
The example of these five bridges clearly shows that integral and semi-integral designs offer a range
of advantages:
− The slender structural elements increase the elegance and transparency of the structure.
− Bearings and joints may be omitted, at least partially, resulting in a reliable, robust and
maintenance-friendly bridge.
− Superstructure: Shorter spans allow solid cross-sections such as T-beams. They favorably reduce
the longitudinal stiffness of the bridge, are easily accessible and, contrary to the box girder, may be
concreted as a whole without construction joints. The continuous superstructure increases the
vertical stiffness and the dampening of the bridge.
− Piers: Designed slender and solid, thus rendering internal formwork unnecessary (as with solid
superstructures)
− Foundation: The longitudinal stiffness of the entire structural system may be reduced using single
files of piles.
The experience gained up to now with thes five bridges described here raises the expectations that the
integral design will indeed keep its promise of economic efficiency. However, German Railsways
cautiously still considers integral bridges a non-proven technology and, therefore, a UiG
(Unternehmensinterne Genehmigung – in-house approval) and an extensive ZiE (Zustimmung im
Einzelfall – approval for each individual case) are required [28]. It also cannot be denied that in the
case of these first of their kind bridges all parties involved were confronted with difficulties and had to
learn to cooperate in the interest of the matter at hand.
The literature referred to in this paper shows that the designing engineers had to deal with a whole
range of new subjects which usually do not occur at all or are of minor importance in the case of
project approval. For example proof of the track-stresses which determines the jointless length of the
bridge (block length); taking into account the vertical deformations of the slender superstructures by
investigating the dynamic behavior and the resonance frequency; the fatigue checks for the
reinforcement and the concrete as well as the feasibility of the clamping areas between support and
superstructure. In case of the Gänsebach valley bridge full size model nodes were constructed.
However, the designing engineers were extremely challenged by the following issues, which proved to
be both strenuous and time-consuming:
−
−
−
−
−
The parties involved lacked the experience in jointly designing integral bridges: one statement
describes the cooperation between designing engineers and the Federal Railway Authority (EBA)
as haggling over every centimeter in the height of the superstructure [22]. There has to be more
mutual trust and understanding between the parties involved.
The investigation of the limit values and the sensitivity analyses for parameters such as the Emodulus of concrete and of the foundation resistance caused unusually extensive computing time,
not only in the case of the ultimate limit state but also in the case of the serviceability tests (e. g.
proof of the track anchorages).
The detailing and the analysis of integral bridges reached the limits of common knowledge and
led to discussions about standards not intended for this type of bridge. For example the
specifications for the verification of the lateral stability – limit value of 1.2 Hz for the relevant
horizontal frequencies according to Tröschel and Döring [29] – apply to single-track steel bridges
only.
An exact fatigue analysis is impossible since no data is available concerning the estimated
volume and mix of rail traffic.
According to the technical reports there even has to be an elastic analysis for bridges under
constraint, although an assessment of the state of strain involving the cracked areas would be far
more realistic.
When tenders for the "original design" of some bridges were issued, they fortunately and explicitly
allowed the entry of special proposals for integral designs, thus proving that the integral design is the
most efficient solution. However, the ambitious author of such a special design was facing the
challenge of having to cover the entire scope of the design under the extraordinary conditions
described above, for the fee of a standard detailed design. It is therefore safe to say that these
consultancies contributed heavily not only with technical know-how but also financially to the
introduction of the integral bridge design.
Now most of the issues mentioned above have been clarified and the results published, thus making it
easier in the future for engineers to design integral bridges. In the meantime there are also research
projects under way such as the investigation of rail tensions conducted at the University of Hannover.
Further research is definitely necessary and would be quite helpful for example in the area of
dynamics regarding the interaction between train and structure, and in the field of system response of
the entire system consisting of integral bridge and soil. Another important factor is the comparison of
measured and calculated values for the completed bridges. For this purpose the researchers at the
universities should be supplied with the complete data and analyses.
6. Summary
There seems to be a general agreement regarding the potential of these newly completed bridges:
"Therefore, starting in the early stages of the planning process due consideration is to be given in the
future to possible holistic approaches of alternative bridge designs, with substantial support from the
guideline on the design of engineering structures issued by the DB AG in 2008. […] The attainable
results are well worth it." [20]
The integral method will prevail in many areas of railroad bridge construction due to the favorable
behavior of the structures, their efficiency, their economy, and their sustainable, holistic and aesthetic
qualities. Fortunately, all parties involved agree on this. It was a worthwhile effort. The new railroad
bridges and the efforts of the Bridge Council are a major contribution to the building culture and also
provide ideas for the design of road bridges as well as setting new international standards.
This paper is an adjusted translation of "Die neuen Brücken der deutschen Bahn" presented at "22.
Dresdener Brückenbausymposium", 2012, Germany.
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This paper is to commemorate the late Dr. T.N. Subbarao, the great Indian engineer and bridge
designer. Already in the early seventies of the last century he was - together with us - strongly
involved in two major bridges in India: the Second Hooghly Bridge in Kolkata, now called
Vidyasagar Setu, the first cable-stayed bridge designed with a composite deck, built indigenously
with local labor and local material and the Akkar Bridge in Sikkim with an all-concrete deck
and with cables fabricated on site. Amongst many other honors he received a well-deserved
honorary doctor´s degree from the University of Stuttgart, Germany. We will not forget him.