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:

ljd23@bath.ac.uk

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].