Strengthening of 100 year old concrete arch bridge 'Kuhbrücke/Hildesheim'
Hermann Weiher1, Andreas Praus2 and Katrin Runtemund1
1matrics engineering GmbH, Nymphenburger Str. 20a, 80335 München, Germany;
2City of Hildesheim, Am Markt 5, Hildesheim, Germany; email:
Kuhbrücke/Hildesheim is an unreinforced concrete arch bridge built in 1910. The
bridge has been strengthened in 2016 to upgrade the capacity carrying vehicles with
maximum weight from 3ton to 40ton. The historic arch and the foundations are
further used. New webs had been added by horizontal prestressing to the arch. A bar
post-tensioning system (50 mm) has been used with innovative and very durable
Ultra High Performance Concrete (UHPC) anchor plates (Hybridanker). Finally a
reinforced concrete deck slab has been added to create a kind of box section. To
reduce thermal stresses in the integral bridge a bridge deck cooling systems has been
installed. To verify the efficiency of the cooling many temperature sensors had been
placed. The measure has been finished in June 2016 and bridge can be used now by
40 ton trucks. The article presents the strengthening concept, the construction and
finally the innovative aspects (UHPC anchor plates, bridge deck cooling).
Keynotes: Unreinforced concrete, arch, strengthening, post-tensioning
SITUATION AND TASK
Kuhbrücke had been built in 1910 as unreinforced single span arch to bridge the river
Innerste nearby the city of Hildesheim. The bridge has an important function because
it is the single access to an agricultural area owned by the City of Hildesheim.
Figure 1. Kuhbrücke (1910) before strengthening
Recalculation has shown that the bearing capacity is not sufficient to carry ordinary
agricultural machines and furthermore traffic loads had been limited to 3tons
maximum for vehicles. Furthermore the bridge and its equipment showed many
damages related to ageing.
City of Hildesheim investigated several alternatives, e.g. building a new bridge at the
same place, building a new bridge at an alternative place and strengthening of the
existing bridge. Because of limited financial capacities strengthening was the
preferred solution. Structural engineering firm matrics engineering was chosen to
search for a technical solution that
- Upgrades the bridge for load model “Brückenklasse 30” according DIN1072
- Minimize effort and costs for the construction measures
- Allows use of old bridge during harvest before strengthening is done
KUHBRÜCKE – 100 YEAR OLD STRUCTURE
The bridge has been built in 1910 as unreinforced concrete arch and is more than 100
years old. The arch is continuous and supported by two massive abutments with a
span of nearly 25 m. With a thickness of just 50 cm it is very slender by 1:50. The
bridge is very slender (1:40 ratio of midspan height to span length) and the arch very
flat (1:10 ratio of rise of the arch to span). Although more than 100 years in service
the bridge showed only minor deficiencies, e.g. a transversal crack of about 5 cm
depth at the bottom side of the arch in its centre along total width. This might have
come from overloading by traffic, temperature, shrinkage and horizontal movement
of abutments and is not limited by any reinforcement. Concrete testing has been done
to determine the concrete strength. Class C16/20 according to Eurocode 2 (2011) has
been finally found. Based on that strength calculations had been done and traffic
loading finally had been limited to vehicle loads of maximum 3 tons. For the
upcoming harvest in autumn 2015, when thousands of tons of sugar beet root were
expected, an urgent solution was needed. If using this bridge with its limited capacity
only a solution with conveyor belt and small equally distributed loads was allowed.
Finally the City of Hildesheim created a temporary access by concrete cylinders
thrown into the river and filled up with earth.
Figure 2. Sections of original structure
The main structural deficiency of the arch bridge is the limited bending resistance of
the arch both in longitudinal and transversal direction. It must be assumed that the
sandy filler above the concrete arch does not act as resistance although it seems that it
has some strength. The strengthening approach foresees to overcome this.
New cast in situ webs with height from lower bound of the arch to the traffic lane
level are added on both sides of the arch. For monolithically connection to the
existing arch the webs had been cast against roughened surface and stressed by 50
mm prestressing bars together (some 1.5 MN for each bar stressing force). Time
depending losses had been very small because of the age of the existing arch. This
prestressing force also solved the deficiencies in transversal direction. Finally a
reinforced concrete deck slab is added to further help distributing the loads and also
for durability reasons. The strengthened bridge still needs the high compression
resistance of the arch and actively uses it by transferring the forces through the webs
into the arch which acts like a bottom slab. Of course dead weight of the bridge is
fully transferred by the old arch. Old arch and new members webs and slab act fully
together similar to a box girder/arch.
Figure 3. Sections of strengthened structure
The bridge is far away from public road network and subsequently de-icing salts are
not used. To keep costs small no sealing has been applied. To improve durability,
calculated crack width in the slab was limited to 0.2 mm instead of 0.3 mm.
of old bridge
(concrete arch and
A - A
B - B
The arch bridge has no hinges and hence very high stresses can occur due to
temperature loading. To avoid massive reinforcing because of the high stiffness of the
new box arch girder the concept was to allow cracking and limit the crack width to
0.2 mm in the webs. Further limitation of stresses due to restraint deformation was
achieved by actively cooling and heating the bridge deck, see chapter “Tempering of
Construction works had begun in February 2016 with installation of scaffolding (see
Figure 4). The old bridge deck had been completely rebuilt. Only the arch and the
filling material remained and could be used as formwork for the new members. The
arch had been bored horizontally in transversal direction at the length of 4m to house
the prestressing steel bars. Vertical and horizontal deflection of borings had been very
small (see Figure 5). After reinforcing the webs and closing the formwork webs had
been poured with C30/37.
Deck slab had been poured in second stage and monolithically connected to the new
webs. Reinforcement was designed for a minimum calculated crack width of 0.2 mm
because no sealing was foreseen. For transversal prestressing of the arch a bar system
of BBV Systems GmbH has been applied according to ETA-16/0286 (2016) using
Macalloy prestressing bars and ‘Hybridanker’-anchorages.
It was first time that Hybridanker-plates have been applied for these prestressing bars.
Hybridanker are anchorages made of ultra-high performance concrete (UHPC). Using
these anchorages had been beneficial from durability point of view (no steel parts
exposed outside stainless steel cap, see Figure 7) as well as because of very small
edge distances. This is mainly because of special testing and its stiffness (large
thickness) when applied on concrete without extra confinement (e.g. spiral).
Figure 4. Erection of scaffolding
Figure 5. Drilling of tendon holes and detailing of webs (from left)
The Hybridanker-plates for this project consisted of a force transfer unit made of
ductile cast iron, confinement with rebar spiral and precast with Ultra High Strength
Concrete with a compressive strength around 200 MPa. Further features were:
grouting inlet, threads to connect the cap, trumpet made of polyethylene. The
technology is still new; its first application was in 2011. See Weiher et al. (2012) for
more details about general principles.
Construction had been finished in June 2016 with a fully strengthened arch bridge
(see Figure 8). The position of the arch still can be seen by following the anchorages.
Figure 6. Reinforcement of deck slab and installation of prestressing bars
Figure 7. Hybridanker-plate for anchoring 50 mm prestressing bar
Figure 8. Strengthened bridge finalized in June 2016
TEMPERING OF BRIDGE DECK
City of Hildesheim was very open-minded for a research project. The restraint
stresses due to temperature should be limited by tempering the bridge deck. For that
purpose plastic hoses had been installed (see Figure 9). By sending tempered liquid it
is possible to cool down in hot periods (e.g. summer) or heat up in cold periods (e.g.
winter) the concrete deck. By doing so, one may decrease stresses due to temperature
significantly. The project is used as trial project and permanent use is not foreseen.
Therefore, the design had been done in a way not considering the benefits of such a
tempering. Even higher effects of tempering can be achieved by this method for large
continuous girder bridges and integral bridges.
Figure 9. Plastic hoses and temperature sensors
A very old concrete bridge built in 1910 could be strengthened with little costs (<
30% of building a new bridge) to meet modern goals. For this purpose it was
beneficial that the bridge was unreinforced and furthermore had a static system (arch)
that offered hidden resistance.
The strengthening concept had been chosen in a way not to load the bridge during
construction very much, no heavy vehicle was allowed to load the arch. Innovative
aspects had been applied, such as the high strength concrete anchorages of
prestressing bars and bridge tempering trial to limit stresses from temperature
Client: City of Hildesheim
Contractor: Hoch- und Industriebau Celle GmbH, Hambühren
PT-system: BBV Systems GmbH, Bobenheim-Roxheim
Design: matrics engineering GmbH, München
Tempering: matrics engineering GmbH, München
tripleS GmbH, Mülheim an der Ruhr
DIN 1072. (1985) Straßen- und Wegbrücken. Lastannahmen. Beuth Verlag GmbH,
DIN EN 1992-1-1 Eurocode 2. (2011) Bemessung und Konstruktion von Stahlbeton-
und Spannbetontragwerken – Teil 1-1: Allgemeine Bemessungsregeln und
Regeln für den Hochbau; Ausgabe 2011-01.
ETA-16/0286. (2016) BBV 1030 post-tensioning bar tendon system, nominal
diameter 32 to 50 mm. BBV Systems GmbH, Bobenheim-Roxheim.
Weiher, H.; Tritschler, C.; Glassl, M.; Hock, S. (2012): Hybridanker aus UHPC -
Erstanwendung bei der Verstärkung der Rheinschleuse Iffezheim mit
Dauerlitzenankern. Beton- und Stahlbetonbau, Vol. 107, Nr. 4, Ernst & Sohn.