ANALYSIS OF THE SUNNIBERG BRIDGE
Luke J Drinkwater1
1The University of Bath
Abstract: The Sunniberg Bridge in Switzerland, designed by Christian Menn, is a tall cable-stayed bridge with
low pylons. It is a an excellent example of the way that structural members, shaped in response to engineering
considerations can be both functional and have high aesthetically qualities. This paper examines the close link
between the aesthetics and the form of the structural elements; compares the loading used for the design with
loading from the British Standards; uses simplified structural elements to analyse the stresses in the bridge; and
examines the construction process.
Keywords: bridges, cable-stayed; concrete structures; aesthetics; Switzerland
1 Introduction
The Sunniberg Bridge is situated in the Lanquart
valley below the international Swiss ski resort of Klosters.
It is on of the largest bridges in the Alps, and the most
prominent part of the 6547m Klosters Bypass.
The Sunniberg Bridge is a harp arrangement cable-
stayed bridge with 3 main spans (the longest measures
140m) and 2 side spans. The reinforced concrete deck is
526m long and follows a tight of curve of radius 503m at
an inclination of 3.2%. The deck is 12.37m wide in total,
9m wide curb to curb, and it carries 2 lanes. The
piers/pylons are also constructed from reinforced
concrete, the tallest of which rises a total of 75m above
the valley floor, 62m up to the roadway and 15m above it
(figure 2).
Initial proposals for a highway by-passing the town
of Klosters, and hence a bridge at Sunniberg, were made
in the mid-1970s. However, the canton of Graubűnden
felt that environmental concerns were not satisfied [1]. In
1993, approval was given to a new by-pass scheme and
the canton invited 3 firms to compete for the design of the
Sunniberg Bridge. However, when the designs (figures
1a-d) were submitted, the eminent Swiss engineer
Christian Menn presented an alternative to the Highway
Department Architectural Consultant. The Highway
Department chose Menn’s design, but appointed one of
the three original firms (Bänziger Bacchetta Partner) to
complete the final calculations and drawings. 1
At 20million Swiss francs, the total construction cost
of the Sunniberg Bridge is approximately 14% more than
the construction cost of the most economical solution [1],
a traditional cantilever constructed girder. However, this
only added about 0.5% to the total cost of the Klosters
1 Undergraduate Student, University of Bath, Dept.
Architecture and Civil Engineering, Bath. E-mail:
Bypass project. Moreover, the Highway Department
clearly deemed that the exciting and innovative design,
which provided a bridge of elegance and grace that fitted
effortlessly into the sensitive landscape, justified the
increased cost. However, had the cost of the bridge been
more than 20% of the cost of the cheapest alternative, the
design would not have been considered viable and the
concept would have been scrapped.
Figure 1a: 6 span composite truss bridge
(Bänziger, Koeppel & Braendli, Chur)
Figure 1b: 6 span concrete cantilever bridge
(H.Rigendinger & W. Maag, Chur)
Figure 1c: 7 span triangular composite box bridge
(Branger & Conzett, Chur.; Grignoli & Muttoni, Lugano)
Figure 1d: 9 span continuous concrete beam bridge
(H.Rigendinger & W. Maag, Chur)
Proceedings of Bridge Engineering 2 Conference 2007
27 April 2007, University of Bath, Bath, UK
Figure 2: Elevation, plan, cross-sections
2 Aesthetically Driven Concept
The Lanquart Valley contains only one engineering
structure, the Sunniberg Bridge. As a result of its
prominent location, the citizens of Klosters requested that
the bridge be as thin and transparent as possible in order
to have the least visual impact on the idyllic alpine view.
Menn has stated that the design of the bridge and the
selection of its basic elements (the low pylons, the slender
piers, and the thin curved deck) came directly from this
consideration [2], and this has resulted in a number of
important technical and aesthetic consequences.
2.1 Conceptual Design
The curved plan of the bridge allowed the concrete
deck to be cast as a monolithic slab without expansion
joints at the abutments or bearings at the piers. As a result,
the piers are restrained both laterally and longitudinally by
the deck, rather than being cantilevered from their
foundations, thus enabling them to be slender and visually
unobtrusive.
The piers/pylons are constructed from two legs
connected at several points to form a vertical Vierendeel
truss. In the transverse direction the piers vary in width
with height, from 8.8m at the base to 13.3m at the
roadway, above which the pylons flare to 17.25m. This
creates a cupped form which cradles the road.
The continuity of the road deck gives stability to the
piers, while the narrow base of the tapering piers enables
them to tilt. Consequently, the roadway is allowed to
expand and contract due to temperature variations without
producing large moments at the base.
Figure 3: Lateral Stability
3 Aesthetics
This section examines the aesthetic qualities of the
Sunniberg Bridge according to Fritz Leonhardt’s ten rules
for bridge aesthetics.
3.1 Functionality
The simple structural form of the Sunniberg Bridge
bestows a delicacy and elegance that is rarely given to
20,000 tonnes of concrete. It has obvious load paths
which enable bridge users and onlookers to understand
how the bridge works.
3.2 Proportions
The pylon height for a typical cable-stayed bridge is
approximately ¼ the length of the main span, which gives
efficiency in terms of cable forces. For the Sunniberg
Bridge, this ratio would have required pylons which
projected 35m above the deck. Coupled with the tall piers
and relatively short spans, this would have produced a
design which was awkward and visually overpowering.
By reducing the height of the pylons to 9-10% of the main
span this aesthetic issue is resolved, however it generates
structural issues that require consideration. The low cable
angle considerably increases the cable forces. Under
unbalanced live loading this would produce large
deflections in typically flexible pylons, and hence large
deck deflections. As a result, the pylon has been stiffened
against longitudinal bending, thereby creating the eye-
catching distinctive flared pylons Fig…
3.4 Order
For the most part the Sunniberg Bridge has clean and
elegant lines that allow the eye to move easily along its
length. However, the stay cable connections at the edge of
the deck protrude below the edge beam creating a broken
soffit line. Nevertheless, their close, regular spacing
offsets any mental unrest and the protruding cable
connections enable the viewer to read the structure, allow
cables to be replaced easily, and also disguise the
thickening of the edge beam near to the piers. , Fig 4.
Fig 4: Cable-deck connection.
Due to the curvature of the bridge, the cables appear
to cross each other when driving over the bridge or when
the bridge is viewed from oblique angles. However, the
harp arrangement of cable provides a regular and clear
pattern through which to observe the continually changing
view when driving over the bridge (Fig 5).
Figure: 5 Crossing of Cables
3.5 Refinement
The perceived thickness of the deck is reduced by
setting the edge beams back in the shadows and by the
feathered edge created by the stay cable connections. The
edge beam also serves to hide the drainage pipes that are
slung beneath the roadway so as to remain accessible. The
simple handrail sitting atop the pre-cast concrete crash
barriers, which form a low parapet, serves to further still
reduce the perceived deck thickness, Fig. 6.
As the land rises toward the deck, the bridge spans
reduce near the abutments. Thus the aspect ratio of the
voids between the structural elements is maintained,
which is appealing to the eye.
Fig 6: Edge beam
3.6 Integration into the environment
The slender piers, low pylons and transparently thin
deck blend effortlessly into the magnificent alpine
landscape. When viewed from the valley floor, the narrow
pier legs blend into the wooded environment, giving the
impression that bridge has been grown rather than
constructed. Additionally, because of the low pylons the
bridge is below eye level. This allows the bridge to be
obscured by vegetation and to appear unobtrusive when
viewed from most locations in Klosters Fig 7.
Fig 7: Sunniberg Bridge viewed from Klosters
3.7 Surface texture, colour and shadow
The white concrete structure is clean and eye-
catching in its simplicity and creates a pleasing interplay
between sunlit and shaded areas. The white concrete has
also been used as a ‘canvas’ by a contemporary artist, Fig.
art.
Figure 8: Contemporary art
The reflective stay cables are highlighted or lost to
the background depending on the angle of the sun creating
interest and intrigue.
The pier/pylon has a T-shaped cross section (to give
transverse stiffness, discussed later) creating vertical
shadows which accentuate the curved shape and enhance
the slenderness of the piers, Fig.9 .
Figure 9: Shaping of pier
3.8 Character
The Sunniberg Bridge can deffinately be said to have
character. The tapuring and flaring piers, combined with
the regimented lines of the harp paterned stay cables are
widely regarded as a piece of structural art [2]; a structure
which is is based on engineering criteria, hence being
efficient and economic, but has a higher than average
quality of aesthetics [2].
3.9 Complexity
For the most part the Sunniberg bridge is as simple as
possible. Where complexity has been included it has been
for a combination of structural, aesthetic and
construction/maintenance reasons, for example the cable-
deck connection as mentioned above.
4 Bridge Form and Design Calculations
Menn’s concept design, Fig.2 shows a road deck
0.40m thick with edge girders 0.80m deep based on an
approximately 10m transverse span between cables. Due
to considerations of cable size, deck stability (regarding
buckling from axial forces), and the pylons [3], cable
anchorages are spaced 6m apart.
4.1.1 Bridge Loading
Menn’s concept design calculations for the Sunniberg
Bridge were based on the following loading [3]:
� Dead load, g = 190 kN/m (including a 0.17m wearing
surface)
� Constant load, ∆g = 20kN/m
� Live UDL, q = 36 kN/m (4kN/m2 over 9m wide
roadway)
� Live concentrated, Qc = 300 kN
4.1.2 Loading According to British Standards
Had the bridge been constructed to British Standards
[4] it would have been subject to considerably higher
loading (as shown by the following calculation for live
loading). Type HA loading consists of a combination of a
uniformly distributed load (UDL) and a knife-edge load
(KEL), both uniformly distributed over the full width of
the lane.
For bridges between 50m and 1600m the nominal
UDL, expressed in kN per metre length of notional lane,
is given by: 1.0
136
=L
W (1),
which for the Sunniberg Bridge, at 526m in length, gives 1.0
526
136
=W
2.19= kN/m for each notional lane.
Multiplying by the number of notional lanes, applying
partial factors γfl and γf3 of 1.50 and 1.15 respectively, and dividing by the width of the roadway gives a UDL live
loading, q, of:
×××=
Roadwayof Width
f3 γ
fl γW LanesNo.
q (2)
2mkN11
9m
1.15 1.50 mkN19.2 3 q =
×××=
A KEL of 120kN is also applied to each notional
carriageway, and is positioned onerously.
Designed to BS5400 the bridge would be considered
to have 3 notional lanes because the distance between the
raised kerbs is 9m. However, it is worth noting that the
width of the carriageway in the tunnel immediately
adjacent to the bridge is only 7m and would therefore only
be able to accommodate 2 lanes of traffic.
To assess the structural dimensions, the following
section examines the effect of the critical loads (as
specified by Menn [3]) on pier P2 with
cantilevered/suspended deck spans on each side.
4.1.3 Temperature Effects
The location of the Sunniberg Bridge, high in the
Swiss Alps, means that it is subject to large temperature
variations throughout the year. The plan curvature of the
deck means that the bridge can respond to temperature
changes by expanding and contracting radially. The
combination of horizontal arch action and the flexibility
of the piers allowed the use of a monolithic deck slab,
without the need for expansion joints at the abutments or
along the span. This allows temperature induced
deflections without large internal forces being generated
[1].
4.1.4 Wind
The altitude and topography of the bridge location
could result in high wind speeds and funnelling effects.
However, the monolithic deck, which is restrained at the
abutments, acts to restrain the piers laterally, Fig. 3.
Additionally, the slim deck, the circular section stay-
cables, and the parapet rail present a small surface area
and hence minimise wind loads on the structure.
4.2 Cable Stress Cross-section
The critical loading for the determination of the cable
cross-section is given by combining full dead loading,
UDL live loading, and 60% of concentrated live loading
(assuming the two neighbouring cables take 40% of the
load). The concentrated live loading also has +80%
impact factor and +80% eccentricity factor (representing a
concentrated (lorry) load, Q, in the lane closest to the
cable being designed). Given that the cable anchorage
spacing, ya, equals 6m, the vertical load applied to a cable,
Qv, is given by:
( )( )( )0.61.81.82
Q
ay
2
q∆ggQ c
v
+
++= (3),
( )( )( )0.61.81.82
300kN/m6m
2
36)kN/m20(190
+
++=
kN 030,1=
Figure 10: Component forces of cable tension
Hence from the cable geometry shown in Figure 10 the
resultant tensile force in the cable T is:
kN 257,53.11sin
970
θsin
QT v =
°== (4),
Assuming an allowable cable stress of σσσσc = 0.5 fsy, where
the maximum allowable stress fsy=1.6kN/mm, gives: σσσσc
=0.8kN/mm.The minimum cable area, Amin is hence given
by:
2mm6571
.8kN/mm0
kN4950 TA
2c
min σ=== (5)
In the final design the cable area used was between
4810mm and 6157mm [3]. This suggest that the loads and
load factors used for the initial design were slightly
conservative, especially since final member sizing
complies with Swiss maintenance provisions which
permit any single cable to be removed while the bridge
remains open [5].
4.3 Deck Girder Stress
Each cable contributes a component of horizontal
force, hence the stress in the girder can be ascertained
from the cable forces calculated above. The critical
section of the road deck, where the axial force is largest,
occurs between the first cable and the pier connection.
The compressive force in the girder, N, at this point is
given by:
( )( )Cables ofNumber QNH
= (6),
Where the horizontal component of the cable force, QH, is
given by:
kN155,5tan11.3
1030kN
θtanQH
=°
==
v
Q (7),
Hence:
kN ,10003120kN155,5N =×=
Note that the axial force carried by the girder at this
point is due to the cumulative axial forces from 20 cables,
10 along each side of the roadway.
For the majority of the bridge, the girder consists of a
0.4m thick slab spanning two 0.8m deep edge beams.
However, the area of the section between the first cable
and the pier connection has been increased to
approximately 9m2 [3]. The axial stress in this section of
the girder, σσσσG,N, is given by:
2
26
3
mmN5.11
mm109
N10100,103NG,
σ =×
×=
(8)
Figure 11: Moments
Furthermore, it is assumed [3] that the concentrated load
on the 6m span closest to the pier connection is taken by
bending of the girder. Assuming a typical beam, fixed at
one end and simply supported at the other (as shown in
Fig. 11), the hogging moment at the fixed end and the
sagging moment under the concentrated load are given by:
××=
16
yc
Q3M a
pier (9)
675kNm16
6m600kN3M
pier=
××=
××=
32
yc
Q5M a
mid (10)
kNm56332
6mkN0065M
mid=
××=
The maximum moment occurs at the pier, hence the
maximum stress induced by the point load, σσσσG,M, is given
by:
=
Z
Mpier
MG,σ (11)
where the section modulus, Z, of the girder at the pier, is
approximated to:
=
6
bdZ
2
(12)
Assuming an average girder thickness of 0.73m (from Fig.
2), for a metre width of deck girder the section modulus
is:
( ) 32
m0888.06
0.73m1mZ =
×=
hence:
2
39
6
mmN6.7
mm100.0888
Nmm10675MG,
σ =×
×=
The maximum compressive stress in the girder, σσσσG,
MAX, is found in the bottom fibres at the pier/pylon
connection and is calculated by combining σσσσG,N and σσσσG,M.
Hence:
222 mmN1.19mmN6.7mmN5.11MAXG,
σ =+=
The maximum compressive force in the girder is
lower than the average concrete strength of 64N/mm2
given by the contractor [6].
Note that the critical axial girder force for buckling would
need to be checked to ascertain if the proposed section
was suitable.
4.4 Pylon Form
The shape of the pylons is a direct consequence of
their need to be able to resist axial stress as well as
longitudinal bending stresses (due to unbalanced live
loading) and transverse bending stresses (due to the
curvature of the bridge). This has resulted in Menn’s T-
shaped arrangement (Fig. 12), where the cross-section
dimension were estimated from assumptions that the
longitudinal bending was taken by the flanges, the
transverse bending is taken by the web, and the axial
forces are taken by both the web and flanges [3].
Figure 12: pylon x-section[3]
The worst load case for transverse bending and for axial
force will occur when the roadway is loaded on both sides
of the pylon. The worst load case for longitudinal bending
is when only the main span is loaded.
The transverse bending moment, Mt, at the pylon
base can be calculated using the sum of the cable end
loads and their lateral distance from the pylon base. Using
average eccentricities ei= 0.21m and eo= 2.54m [3],
where i and o denote inside and outside cables
respectively, the transverse bending moment at the pylon
base is given by:
it,ot,t MMM += (13)
where:
viit, Q cables ofnumber eM ××= (14a)
voot, Q cables ofnumber eM ××= (14b)
hence:
970kN202.54m)(0.21mM t ××+=
kNm350,53=
Figure 13: Cable eccentricity [3]
4.5 Pier Form and Stresses
Longitudinal bending moments are induced in the
pier when the bridge is subject to unbalanced live loading
and they reach a maximum when the main span alone is
fully loaded. The ridged deck-pier connection causes
moments to decrease linearly to zero at approximately
one-third height, increasing towards to half the maximum
value at the ground. Assuming that the flanges take the
longitudinal moments, the spacing between the flanges
then follows the variation of bending moments down the
piers (Figure…pier). The dead load of the structure should
be sufficient to compensate for the stress in the tensile
flange, however, if it is not then pre-stressing of the piers
may be required.
Effect and Spacing of Pier Cross Beams
The cross beams in the piers act to stabilise the long
slender pier legs against buckling. Additionally the top
cross beam also transfers transverse bending moments at
the pylon base into axial forces in the piers through
bending, and must therefore be able to resist the full
transverse moment calculated above (Fig Bending to
Axial)
As before, pre-stressing of the pier web may be
required if the dead loads are not sufficient to overcome
tensile axial forces.
Figure 14: Pier/Pylon P2: (a) longitudinal cross-section;
(b) horizontal cross-section; (c) transverse cross-section
[3]
Figure 15: Conversion from bending moments to axial
forces
6 Serviceability
The allowable vertical deformation due to loading
was set at 1/400 of the span distance. Employing a
serviceability live loading, consisting of a 2kN/m UDL
and a concentrated load of 360kN in the most onerous
location, the main 140m span was calculated to deflect
downwards by 235mm (amounting to 1/600 of the span),
while the neighbouring spans experienced upward
deflections of 60mm. 40% of the deformation was
attributed to the distortion of the pylon, while the other
60% to cable deformation [6].
7 Construction
Starting in June 1996, the construction of the
Sunniberg Bridge took less than the scheduled two and a
half years. The piers/pylons were constructed sequentially
starting with pier P1, the pier closest to the existing
Landquart to Klosters road. Once the pylons were
completed and the initial section between the pier/pylon
had been cast, the edge beams and deck were erected
using suspended cantilever construction.
The contractor had their own on-site concrete
production plant which enabled flexibility and maximum
efficiency. Micro silicate was added to the structural
concrete (pier/pylon and deck girder) to enhance
workability and to accelerate the development of strength
(43 N/mm² after 3 days and 64 N/mm² after 28 days), thus
allowing rapid construction.
7.1 Foundations
The geological profile (Fig. 17) shows that the site is
characterised by alluvial deposits, landslide material, base
moraine, and the stable Casanna rock mass overlaid by
river sediments. The foundation solution consists of earth-
filled concrete structures for the abutments, two small
concrete shafts for pier P1, and 6 bored pile foundations
1.5m in diameter and between 16m and 14m deep for
piers P2, P3 and P4 respectively. Note that because the
inner leg of the pier carries much greater load than the
outer, the 3m thick concrete pile caps are located
eccentrically towards the inside of the curve (Fig. 2)
7.2 Pier/Pylon Construction
The casting of the piers/pylons, with their complex
cross-section (changing with height) and elegant curves,
which are so integral to the structural performance and
aesthetics of the bridge, needed a clever solution from the
contractor. The answer was to construct the piers/pylons
using rectangular frame elements with timber formwork
inserts to create the required cross-section. As a result, the
inserts could be manufactured off-site under factory
conditions, to ensure a high level of accuracy. The
rectangular frame system was jacked up or craned up the
pier/pylon in 4m intervals and attached to holding points
cast in the concrete.
Landslip Deposits Washed Moraine Moraine
Figure 17: Site Geology
Figure 16: Pylon and deck formwork
7.3 Girder/Deck Construction
The initial 13m section of bridge girder (1m between
the pier/pylon and 6m either side) was constructed by
supporting the formwork on the horizontal pier crossbeam
below the roadway. The construction of the girder from
this point onwards is shown below (Fig.18):
Alluvial Deposits River Sediments Casanna Rock Mass
Figure 18: Construction
1. The steel reinforcement is placed and the concrete is
poured for both edge beams and the central slab area of
the previous stage.
2. The longitudinal pre-stressing bars in the edge beams
are tightened.
3. The stay cables are attached and tightened to between
2100 to 4000 kN
4. The cantilever construction carriage is moved forward
by 6 m to the next construction stage
The rapid development of concrete strength enabled the
construction of the suspended cantilevers to progress at
regular one week intervals (6m per week) [6].
It should be noted that the weight of the construction
carriage was considerable (in the region of 35 tonnes),
which could have been a crucial factor when specifying
cable and concrete capacities.
7.4 Creep
Concrete creep, the plastic deformation of an element
subjected to long term loading, is an unavoidable factor in
concrete construction and could result in a rippled or
bowed road deck. However, because 95% of concrete
creep occurs during the first year after construction, the
effects of creep can be designed out by initially building
in a slight camber to compensate. This would have been
an especially viable option for the Sunniberg Bridge since
it was only used by site traffic for the construction of the
adjacent tunnel for approximately the first seven years
after opening.
8 Durability
As with any suspension structures, cable corrosion is
a major concern. The cables used on the Sunniberg Bridge
consist of parallel galvanised steel strands that are 7mm in
diameter and sheathed with robust polyethylene that
contains rust inhibiting material. The rust inhibiting
material can be flushed and replaced if necessary, and
cables can also be replaced if corrosion occurs (see
above).
10 Possible future changes which the bridge might
have to undergo
The design of the Sunniberg Bridge does not allow
for the addition of any lanes in the future. However, lanes
are extremely unlikely to be added as the bridge is located
on the Klosters bypass, adjoining the 4200m long
Gotschna tunnel, and amid a long section of single
carriageway mountain road.
The stay cable construction would allow for the
weight capacity of the bridge to be increased if necessary
by increasing the tensile capacity of the cables. However,
the capacity of the piers/pylons and deck would need to
be assessed and may need to be increased. On the other
hand, in doing so, there would be a serious risk of
spoiling the aesthetics of the bridge.
11 Conclusion
From the above discussion it is clear that from
careful consideration of structural form during the
concept design stage seriously aid the creation of a bridge
that is both aesthetically outstanding, structurally and
economically sound.
References
[1] Figi, H., Menn, C., Bänziger, D.J., and Bacchetta, A.,
1997. Sunniberg Bridge, Klosters, Switzerland,
Structural Engineering International, Vol. 7, No. 1,
pp. 6-8.
[2] Gottemoeller, F., 2005. The true goals of bridge
aesthetics [online]. American Institute of Steel
Construction, Inc. Available from:
http://www.steelbridges.org/pdfs/.%5CGottemoe.pdf
[Accessed 18 April 2007].
[3] Honingmann, C. and Billington, D.P., 2003.
Conceptual design for the Sunniberg Bridge, Journal
of Bridge Engineering, American Society of Civil
Engineers, Vol. 8, No. 3, pp. 122-130.
[4] BS 5400-2: 2006. Steel, concrete and composite
bridges. Specification for loads. BSI
[5] Wells, M., 2002. 30 Bridges, New York : Watson-
Guptill.
[6] Umfahrung Klosters [online], 2007. Tiefbauamt
Graubünden. Available from:
http://www.tiefbauamt.gr.ch/projekte/index.htm
[Accessed 18 April 2007].