1 Robert M. Foster – rf227@bath.ac.uk

A CRITICAL ANALSIS OF THE NESCIO BRIDGE, AMSTERDAM

R M Foster1

1Department of Architecture and Civil Engineering, University of Bath

Abstract: The Nescio Bridge (Nesciobrug) is a self-anchored mono cable pedestrian suspension bridge

located near Amsterdam. This paper will seek to provide a comprehensive critical analysis of the Nescio.

The analysis will include a critique of the aesthetic, structural and construction solutions employed in the

design of this unusual bridge. A number of simple hand calculations are presented in order to evaluate the

performance of the bridge under various Ultimate Limit State and Serviceability Limit State conditions.

Although the concrete approach ramps are discussed where relevant, this paper is principally an analysis

of the steel main-span and foundations.

Keywords: Nescio, suspension bridge, mono-cable, self-anchored.

1 General Introduction

The Nescio Bridge (Nesciobrug) is a self-

anchored mono cable pedestrian suspension bridge

with a main span of 171m. Completed in 2006 on a

budget of only !12.2million, Nescio is the first

suspension bridge to have been built in the Netherlands

and has received several national and international

awards. The bridge connects existing pedestrian and

cycle routes across the Amsterdam-Rhine canal

(Rijnkanaal) and eliminates a 40-minute detour

between Oost-watergraafsmeer in the city of

Amsterdam and its Ijburg suburb.

Figure 1: Location plan of the Nescio Bridge.

Close collaboration was required between

Wilkinson Eyre, Arup and Grontmij to develop a

design that fulfilled the City of Amsterdam’s brief for a

bridge that would ‘demonstrate its commitment to

design quality, and sensitivity to the environment’, Ref.

[1]. Additional constraints on the design included

limited flexibility regarding the positioning of the

approach-ramps, stringent requirements for non-

interruption of the canal and the requirement to clear

the canal in a single span.

The main span is fabricated entirely from steel

while the long approach ramps are concrete cast in-

situ. The unusual bifurcated end design allows separate

pedestrian and cycle access. A winding steel stair

provides pedestrian access to the bridge, while cyclists

ascend a 275m concrete approach ramp to gain the

9.3m of elevation required to clear river traffic.

2 Aesthetics

2.1 Critical Method

The aesthetic critique conducted here is

undertaken principally with reference to Fritz

Leonhardt’s ten principles of bridge aesthetics. It is not

clear whether Leonhardt’s principles were deliberately

employed in the design process, but it is certain that

aesthetic considerations were fundamental to the

design.

2.2 Fulfillment of Function

The manner in which each component of the Nescio

fulfills its function is expressed with great clarity. The

composition of the bridge structure reflects Arup

Proceedings of Bridge Engineering 2 Conference 2009 April 2009, University of Bath, Bath, UK

Director Angus Low’s response to the challenge of

bridge design, expressed in his 2008 Milne Lecture,

Ref. [2], that ‘…despite their size they have relatively

few components to design. There are usually more

design criteria to satisfy than there are components to

shape. It follows that each component is serving

several functions and so there is a complex interaction

between all the design requirements, and the different

ways they can be satisfied.’ An illustration of this

challenge is the bridge’s deck that is required to act as

a long span beam during erection and an

aerodynamically stable, low radar profile, torsion

resisting, compression deck in the final condition.

Despite this, the arrangement of components that

comprise the Nescio is sufficiently simple that it is

clear, even to the lay bridge user, how the bridge

achieves its stability and span. Confidence in the

bridge’s stability is enhanced by the clearly expressed

role of the bifurcated ends of the bridge in providing

lateral stability. Similarly, the twin backstay cables at

each end of the span reassure the bridge user that the

slender pylons are securely restrained. The positioning

of the pylons between the legs of the bridge allows a

separation of elements that leaves no doubt that the

deck is suspended from the cable arrangement rather

than gaining direct support from the pylon, Fig. [2].

Figure 2: Clear separation of pylon and deck.

The deck section is principally governed by

structural requirements, with smooth curves

interpolated by the architect between critical sections

determined by the engineer, Ref. [3]. However, the

critical section at mid-span reflects the construction

requirement that the deck be erected as a single

component, simply supported at two points. In this

sense, it can be argued that the form does not represent

the honest fulfillment of the bridges functional

requirements in the final condition. The author would

not regard this as a fair criticism for the simple reason

that a bridge is not a solution to a single well-defined

problem or final load case. A bridge is a response to the

complete set of issues encountered in spanning a void,

from beginning to end, not just the final state

requirements. Therefore, a form that reflects an elegant

solution to the manifold of functional considerations,

including those of constructability, can be said to

express the fulfillment of its function rather more

honestly than a structure that purports to have been

designed solely for its final condition.

2.3 Proportion

Figure 3: Elevation of the Nescio Bridge.

The height of the masts and the parabolic curve

described by the main suspension cable are elegantly

proportioned relative to the length of the main span and

the height of the deck above the water. However, it

could be argued that the 2m depth of the deck at mid-

span is greater than might be considered ideal for a

suspension bridge.

The abruptness of the end of the pedestrian deck

at the stairs is disconcerting from some angles. The

composition would appear more balanced if a long

ramp where employed here, as at the end of the cyclist

deck. The designers were no doubt concerned that a

long ramp would be an inefficient method of ascent

and might frustrate pedestrians.

Figure 4: Plan of the Nescio Bridge.

Despite the unusual curvature of the bridge on

plan, it is symmetrical about the mid-span axis and

conforms to a constant radius of curvature of 300m,

Fig. [3]. This lends a sense of deliberateness to the

proportions of what might have otherwise seemed an

arbitrary freeform design. The curvature on plan also

echoes the vertical curve of the main cable engendering

a sense of converging trajectories, in different planes,

to the path of the bridge. In conjunction with the

bifurcated ends that come together mid-span, this

reinforces the image of the bridge as a unifying and

connecting structure. Bringing the main cable down to

connect with the deck might have made this imagery

more striking and would have had the added effect of

stiffening the deck.

2.4 Order

The principal reason for not attaching the main

cable to the bridge deck was to avoid disturbing the

order of the bridge. The designers felt it important to

ensure ‘visual continuity of the curtain of hangers…

For the same reason the hangers were spaced at the

relatively close spacing of 5m’ Ref. [3]. Connecting the

two components would have also undermined Low’s

desire for ‘a clear separation of parts’ Ref. [2].

A number of elements disturb the otherwise

disciplined order of the bridge. The steel end columns

are positioned too close to the adjacent concrete

column, disrupting the rhythm established by the ramp

supports. This breaking-of-step is no doubt required to

achieve additional stiffness at the end of steel structure

by tying any laterally induced differential forces

between the bifurcated deck-ends directly into the

torsion box of the foundations. However, aesthetically

this is a disappointing solution.

The steel connectors, shown in Fig. [5], that are

used to tie the otherwise un-hung pedestrian deck to

the suspended cycle deck after the point of bifurcation

undermine the order of the soffit in this area. Although

relatively unobtrusive, these connectors give the

impression of ad hoc rather than integral design

elements. Some thought has gone into the aesthetic

design of these components to alleviate this impression

but this author feels that further stiffening the

pedestrian deck to negate the need to reduce the

effective span would have been a better solution

aesthetically. Presumably, additional stiffening was

found to be a less materially efficient solution and so

the connector was chosen.

Figure 5: Hangar connection plate at bifurcation.

2.5 Refinement

A number of refinements contribute significantly

to the aesthetic appeal of the bridge. The fluent

changes in section of the bridge deck and the clean

lines of the soffit emphasise the sinuous form of the

bridge and alleviate the sense of heaviness that might

have arisen from the substantial depth of the deck at

mid-span.

The end taper of the pylons lends a sense of

elegance and lightness to these strong vertical

components of the composition. The length of the

taper, combined with the inclination of the pylons at

2.8° off vertical, helps convey the initial architectural

conception of the vertical elements as ‘backward

raking cocktail sticks’, Ref. [3].

2.6 Integration into the Environment

In elevation the bridge is sensitively proportioned

with the pylons high enough to allow the main cable to

achieve a graceful and efficient parabolic curve without

being so tall as to overbear the surrounding tree line.

The scale and slenderness of the pylons is also

reminiscent of a ship’s mast while the soffit of the deck

conjures images of a ship’s hull – indeed, the deck

steelwork was constructed in local shipyards using a

number of boat building techniques. Therefore, the

form of the bridge can be said to have been very much

a product of its environment, both in terms of

fabrication and of the sweeping plan geometry that was

required by the site constraints.

2.7 Texture

The glossy texture of the painted steel deck edges

and soffit is unusual but conveys a feeling of sleek

newness, rather like the brilliant-white hulled yachts

that leave the nearby shipyards.

2.8 Colour

Figure 6: Deck edge as illuminated ribbon.

The choice of an all-white soffit and pylons

reinforces the sense that this bridge is a showpiece and

harbinger of modernity and regeneration for the area.

The unusual section of the bridge has the effect of

shading the soffit while the shallow side plates are

illuminated by sunlight, appearing as a thin white

ribbon snaking across the canal, Fig. [6]. This reduces

the apparent depth of the bridge, lightening the deck

and restoring the hierarchical supremacy of the main

cable over the bridge deck. The choice of a steely

coloured and permeable parapet reinforces this effect.

Figure 7: Use of colour and spacing in column

design.

The steel columns at the ends of the bifurcated

deck are coloured dark grey in a rather unsuccessful

attempt to diminish their presence. Figure [7] illustrates

both the colouring, and the cramped spatial relationship

between the steel column and its concrete neighbour as

highlighted in Section [2.4].

2.9 Character

The distinctive character of the Nescio Bridge

derives principally from its combination of a bifurcated

deck, pronounced curvature and skew on plan, and its

single main suspension cable. This combination has led

to a number of unusual features, such as the ‘parting

curtain’ that occurs when the hangers switch sides to

accommodate the curvature of the deck, Ref. [3].

2.10 Complexity

Although complex in its actions, and constantly

varying in its geometry, the Nescio manages to achieve

an admirable degree of visual simplicity. While some

may consider visual complexity to be an indicator of

intelligent design, this author regards it, generally, as

indicative of a designer’s failure to find a simple

formal solution to the set of problems encountered.

Mies van der Rohe’s famous assertion that ‘less is

more’ seems particularly apt for bridge design and it is

clear that this is the approach taken, with some success,

by the Nescio’s designers.

2.11 Incorporation of Nature

The creation of the wetland habitat in the area of

the winding approach ramp has led to the use of the

ramp by ornithologists and as a nature walk by local

people. The success of this aspect of the design raises

the question as to whether a long approach would

really have been inappropriate for pedestrians or

whether the stairs may have been unnecessary.

Trees break up the continuity of the concrete

approaches in a pleasing manner. This reduces the

apparent mass of the approach structure and increases

the sense of drama as the main span sweeps out from

the treeline and across the water.

3 Loading

3.1 Analytical Approach

The following analysis of loading conditions is

informed by the Limit State approach that will be taken

in Section [4] to evaluate the design strength of the

bridge. The load factors !fl applied in this section will

be those for Ultimate Limit State design unless

otherwise specified. Serviceability conditions are

discussed in Section [5]. Temperature, creep and wind

loading effects are discussed in Sections [8, 9&10].

3.2 Dead Load

The dead load (DL) of the bridge is taken as the

self-weight of the super structure, principally the steel

box section that comprises the deck. Due to the varying

section geometry of the deck over the length of the

span and the bifurcation of the deck towards the ends, a

value for DL per metre can been obtained by first

averaging the estimated cross sectional steel areas as

shown in Table [1]:

Table 1: Steel deck cross-sectional areas.

Midspan 0.201m2

At bifurcation 0.224m2

Footpath end 0.097m2

Cycle-path end 0.129m2

Footpath/Cyclepath ends combined 0.226m2

Average value 0.217m2

Taking the average cross sectional area obtained

in Table [1] and multiplying by the weight of steel, an

average deck weight can be estimated as 17kN/m.

Factoring by !f3 of 1.1 for steel, and !fl of 1.05

(appropriate for all adverse combinations), a uniform

dead load (UDL) due to the deck is obtained as

19.6kN/m.

The main cable is a 140mm steel cable weighing

1.13kN/m. Hangers are spaced every 5.5m with an

average length of 14.2m and a typical diameter of

36mm. This gives a suspension steelwork weight of

2.0kN/m on plan. Factored by !f3 and !fl as before, this

gives a DL of 2.3kN/m.

Summing these loads gives the total DL of the

structure as 21.9kN/m.

3.3 Superimposed Dead Load

There appears to be little topping material to the

bridge deck, simply an unidentified high friction finish

to avoid slips. Consequently the superimposed dead

load (SiDL) due to this topping is considered to be

negligible.

The lightweight, predominantly wire mesh,

parapet, provides little additional load. The SiDL due

to lighting and electrical cabling is uncertain, although

it is not thought that any other services are carried.

Consequently, the total weight of these components is

conservatively estimated at 1kN/m.

Additionally, the 130kN of tuned mass dampers

are to be considered as SiDL. It is possible that these

dampers may be altered, or most critically, added to, at

a future time in the bridge’s design life. The large !fl of

1.75 for SiDLs allows this possibility to be

accommodated more conservatively. Although the

dampers act as point loads at the mid and quarter span

positions, they will be taken here as a uniformly

distributed 0.8kN/m load across the main-span of

171m. This simplification is made because the actual

distribution of the total load of 130kN is unknown.

Thus the total factored SiDL is found to be

1.8kN/m.

3.4 Live Load

Live load for pedestrian and cycle bridges

spanning in excess of 36m can be taken as 5kN/m2

multiplied by a factor k, Ref. [5]. The factor k is

defined in Eq. (1), where HA is the nominal HA

uniformly distributed load for the loaded length in

kN/m, and L is the loaded length in metres.

!

k =HA "10

L + 270 (1)

Considering L as the main span and obtaining HA

from Eq. (2):

!

HA = 36" 1/L( )0.1

kN /m. (2)

A value for k is obtained in Eq. (3):

!

k =21.53"10

171+ 270= 0.5 (3)

This gives a value of 2.5kN/m2, indicating an

average unfactored live load of 13.8kN/m. Factoring by

!f3 and !fl for load combinations 1, 2 and 3 gives values

for Ultimate live load (ULL) shown in Table [2].

Table 2: Live loads for Loading Combinations 1, 2&3.

Combination 1 2 3

!f3 1.1 1.1 1.1

!fl 1.5 1.25 1.25

ULL [kN/m] 20.7 17.3 17.3

Although reductions for pedestrian bridges greater

than 2m wide are permissible, these will not be applied

as it is foreseeable that the bridge might be fully loaded

with crowds watching events on the river.

Bollards preventing motor vehicle access to the

approach ramps mean that accidental vehicular loading

does not require consideration.

3.5 Parapet Load

Parapet loads are required to resist a notional

design load of 1.4kN/m acting at the top of the parapet.

Scaling off drawings indicates the parapet height is

1.120m and the vertical supports are spaced at 1.940m.

Taking !f3 as 1.1 and !fl as 1.5, a factored design

moment, requiring resistance by each vertical support

is obtained as 5kNm.

4 Strength

4.1 Arrangement of Forces

Combination 1 loading is carried by the structure

as indicated in Fig. [8].

Figure 8: Simplified schematic indicating arrangement

of forces under Combination 1 loading.

The relatively uniform plan loading of the bridge

deck carried by the hangers causes the main cable to

form a parabolic curve, rather than the catenary that

would form if the cable were to hang under its own

self-weight. Unlike most suspension bridges, Nescio is

self-anchored. This means that the deck acts in

compression to carry the large horizontal forces

exerted by the backstay cables, rather than transferring

these forces into the foundations.

4.2 Hangers

The 36 hangers are 36-42mm Macalloy 460 type

bars spaced at 5m centres. This indicates that, from the

loadings calculated in Section [3], each hanger carries

an ultimate load of:

!

5" 21.9+ 3.5+ 20.7( ) = 230.5 kN .

The ultimate capacity of the smaller 36mm bars

can be estimated as:

!

460000" 0.0182" #

1.05= 445.9 kN .

Since 445.9>230.5, the hangers are safe and

perhaps a little conservatively sized.

4.3 Main cable

The maximum tension in the main cable will

occur at the pylons and can be determined by resolving

forces at the pinned connection to the pylon top. The

vertical force component V can be taken as the load on

half the span, amounting to 4.1MN. The horizontal

force component H can be found using Eq. (4), where

w is the [Combination 3] load per unit length on plan, l

is the span and s is the depth of the parabola.

!

H =wl

2

8s=46.1"171

2

8" 20.8= 8.1 MN . (4)

Having found H and V, the maximum cable

tension Tmax can be obtained:

!

Tmax

= H2

+V2

= 4.12

+ 8.12

= 9.1 MN .

The main cable is specified as a 140mm fully

locked vvs-3 Pfeiffer cable. The manufacturer’s tables,

Ref. [6], indicate a limit tension for this cable of

12.1MN. Since 12.1>9.1MN, this cable is safe.

4.4 Deck

4.4.1 Compression

Assuming that the foundations take no significant

horizontal forces, the force component H obtained in

Eq. (4) must be tied back into the superstructure by the

backstay cables in the form of a longitudinal

compression of the bridge deck. Taking the cross

sectional area A of the deck at mid-span an axial

compressive stress "c, ignoring bending effects, is

obtained in Eq. (5):

!

"c

=H

A=8100000

201000= 40.3 N /mm

2. (5)

Clearly "c is acceptable for a steel deck.

4.4.2 Buckling

In its action as a relatively slender compression

member the deck must be checked for susceptibility to

buckling. Lateral buckling is unlikely as stiffness in

this plane is enhanced by the end bifurcations. Lateral

torsional buckling is unlikely in a torsionally stiff

closed section. Therefore, it is buckling in the vertical

plane, about the x-x axis that must be checked. The

critical force for buckling, Pcrit, is found in Eq. (6),

where E is the Young’s Modulus of steel and Ixx is the

second moment of area about the x-x axis at mid-span.

L’ is the effective length of the deck, taken as 0.5L for

the fixed ended condition. L is the total unrestrained

length of the steel deck, 205m.

!

Pcrit

=" 2EI

xx

( # L )2

!

=3.14

2" 200"10

9" 0.032

(0.5" 205)2

= 6.0 MN . (6)

Since 6.0<8.1MN from Eqs. (4, 6) indicates

Pcrit<H, the deck does not appear to be safe for

buckling. However, the value of Ixx used in Eq. (6)

assumes the section as a simple triangular hollow

section and neglects the longitudinal stiffeners that can

be seen in Fig. [9].

These longitudinal stiffeners appear to be 25mm

steel plate and provide the additional stiffness to

prevent the global buckling effect identified above.

Transverse stiffeners, also identifiable in Fig. [9],

contribute to the section’s resistance to local plate

buckling effects.

4.4.3 Torsion

Figure 9: Mid-span deck section.

The main cable passes over the mid-line of the

overall bridge structure, switching sides as the deck

curves on plan, preventing any net overturning effect.

However, at various points, notably mid-span, there are

significant torsional moments generated by the

asymmetric support provided by the hangers, as shown

in Fig. [10].

Figure 10: Deck supported on one side at mid-span,

inducing torsion in the deck.

The distributed Combination 1 load on the bridge

acts as a 46.1kN/m load at a lever arm of 2.5m. This

induces a moment of 115.3kNm/m in the deck that

must be resisted as a torsionally induced shear stress in

the deck section.

If the length of deck that is asymmetrically

supported as in Fig. [10] is taken as the 71m portion

between the bifurcations, then the total moment M to

be resisted is 3.3MNm. The trapezoidal deck section at

each end of the span considered can be assumed to

carry half of this moment. The maximum torsional

stress # can be approximated using Bredt’s formula,

Ref. [7], shown in Eq. (7), where A0 is the area

enclosed by the section and t is the plate thickness:

!

" =M

2A0t=

(3.3/2) #106

2# [(5+ 2.5) /2]#1.7# 0.012

!

= 10.8"106

N /m2

= 10.8 N /mm2. (7)

Despite the asymmetric support arrangement the

torsional shear stress in the section is acceptable.

5 Serviceability

Consideration of the issues affecting the

serviceability of a bridge is a crucial aspect of bridge

design. This is particularly true of pedestrian bridges

where users travel slowly and expect to be provided

with a positive experience rather than simply a viaduct.

Detailed modeling that is beyond the scope of this

paper is required to conduct Serviceability Limit State

checks to any meaningful degree of accuracy.

Deflection in the span should be limited to 1:240, but

checking this requires an analysis of the stiffness of the

structural as a whole, which is not feasible by hand.

Issues of vibration, durability and intentional damage

are considered in Sections [11, 12&13].

A governing serviceability consideration for the

structure was the slope of the bicycle approach ramp to

achieve the necessary 9.3m clearance over the

waterway. This rise is accommodated by the 275m

concrete ramps, giving a slope of 3.4% and eliminating

the need for landings. Allowing space for these ramps

was an important consideration during the initial

design process.

The estimated 1.120m high parapets are below the

minimum height of 1.400m suitable for a cycle bridge.

If this estimated height is accurate then these parapets

are not serviceable and should be replaced.

6 Construction

Construction method was determined by two main

considerations: a requirement to close the canal for no

more than 12 hours, and the need to self anchor the

main cable. Both of these problems were solved by the

decision to design the deck as a single component,

sufficiently stiff to span with only two temporary

intermediate supports.

Figure 11: Deck lifted into place by crane barges.

This allowed the deck to be lifted into position by

two crane barges, Fig. [11], and the ends secured to the

main cable in a single 10-hour shift. The deck then

spanned, without impeding river traffic, in its

temporarily supported state while the hangers were

connected over a much longer period, Fig. [12].

Figure 12: Deck supported at ends and two

intermediate positions during construction.

The deck is fabricated by a ‘coffin construction’

welding technique used for box girders that are too

small to weld from the inside, Ref. [1]. Basically, a box

is constructed and then the lid is welded on from the

outside. Careful welding is necessary to avoid locking

thermal stresses into the welded plates.

Deck sections were fabricated in local shipyards

before being assembled and floated down the canal in a

single piece.

7 Foundations & Geotechnics

Very soft ground to a depth of around 20m meant

that horizontal forces from the backstay cables had to

be tied into the superstructure, discussed in Section

[4.4.1], rather than into the ground. Similarly, force

differentials, due to lateral wind loading, between the

deck ends acting at the top of the steel end columns can

cause moments to be induced in the foundations. The

netting out of these moments is achieved by the use of

reinforced concrete torsion box that effectively

prevents moments being transferred into the piles. The

box acts as a torsionally stiff ground beam and pile cap

that ties the legs of the superstructure together. Figure

[13] clearly shows this push-pull effect on the pairs of

end columns.

Figure 13: Arup model of first deformation mode

under lateral wind loading.

The steel end columns have fixed moment bearing

base connections, necessitating the use of the torsion

box. The pylon bases are nominally pinned, using a

50mm thick, 420mm diameter steel disk as a simple

rocker bearing. This connection prevents significant

moments developing, allowing a slender mast-like

pylon to be designed and meaning that moments in the

underlying raked pile groups are minimal.

8 Temperature

Making the reasonable assumption that thermal

conditions near coastal Amsterdam are similar to

conditions on the English coast at the same latitude:

Table 3: Values for calculating thermal stresses.

Maximum Minimum

120-year 33°C -14°C

50-year 31°C -12°C

50-year[group1] 31°C -10°C

Since footbridges may be designed to 50-year

return temperatures, a design range $T, adjusted for

group 1 construction type, can be taken as 41°C. For a

205m continuous steel deck this entails a maximum

thermal expansion/contraction e of:

!

e ="L#T

= 12$10%6$ 205$ 41= 0.101 m.

If the deck were initially restrained at the median

of the range, this expansion would induce a maximum

stress in the deck "T of:

!

"T

=±eE

L=

±0.051# 200#109

205

!

= ±50"106

N /m2

= ±50 N /mm2.

This stress entails an additional [Combination 3]

compressive force of 14.2MN in the deck. In addition

to the [Combination 3] 7.5MN compressive force

obtained similarly to Section [4.4.3], this gives a total

of 21.7MN and a clear risk of failure through buckling.

It is likely that it is for this reason that the steel deck

end is not connected to the concrete approaches,

allowing for independent movement. The steel end

columns, being less stiff than the deck, will elastically

deform to allow end movement and avoid the buildup

of these stresses.

Differential heating across the section is unlikely

to be significant as the section is relatively small and

the inclined soffit likely to receive some sun along with

the top of the deck.

9 Creep

Creep effects in steel are negligible in the range of

temperatures experienced by bridge components.

10 Wind

Wind conditions are assumed to be similar to

conditions on the English coast at the same latitude.

Thus the maximum wind gust vc, reduced for

footbridges at Nescio’s height of approximately 10m

above ground, can be found in Eq. (8). The value v

denotes the mean hourly wind speed, K1 is a wind

coefficient and S2 is the gust factor. Funneling effects

are unlikely at this site so a funneling factor S1 of 1 is

used.

!

vc

= 0.8vK1S1S2

!

= 0.8" 28"1.37"1"1= 30.7 m / s. (8)

The horizontal wind load Pt and the vertical wind

load Pv are calculated in Eqs. (9, 10). The value q is the

dynamic pressure head, A1 A3 are the characteristic

areas, and CD CL are coefficients of drag and lift. CD is

taken here as the minimum value of 2 for a footbridge.

However, Nescio’s relatively aerodynamic deck profile

is likely to exhibit a lower CD value even when the

additional 1.25m of live load height is considered.

!

q = 0.613Vc2

= 0.613" 30.72

= 577 N /m2

!

Pt = qA1CD

!

= 577" 3.15" 2 = 3.6 kN /m. (9)

!

Pv = qA3CL

!

= 577" 5" 0.4 = 1.2" kN /m. (10)

The longitudinal wind loading Pl is insignificant.

Therefore, the most onerous design combination of

wind loads is Pt±Pv. Factored for Combination 2

loading, Pv contributes only 1.5kN/m while other live

load is reduced by 3.4kN/m from Combination 1 to

Combination 2. Consequently, Combination 1 can be

seen as the governing vertical load case.

Figure 14: Plan. Arrangement of forces in legs due to

wind load.

The bifurcated deck causes Pt to induce a tension

in the upwind leg and compression in the down wind

leg, Fig. [14]. The factored Pt load of 5.5kN/m for half

the structure can be simplified as acting as force F1.

This allows forces T1, C1 to be estimated by statics as

±641kN. Due to the heavy compression exerted on the

deck [4.4.1] by the self-anchored arrangement of the

backstay cables, T1 will not lead to a net tension in the

deck. However, the ±641kN reactions in the line of

each leg of the deck acting at over 10m above the top

of the foundations will generate a pair of opposing

moments in the order of 7MNm each. This necessitates

the use of the torsion boxes discussed in Section [7].

11 Natural Frequency

The natural frequency, f0, of the bridge can be

estimated using Eqn. (11) based on the Rayleigh-Ritz

method for beams, where m is the mass per unit length

and l the length of the beam considered.

!

f0

= "n l( )2EIxx /ml

4Hz. (11)

In order to apply this method to the Nescio, a

number of simplifying assumptions are required. It is

assumed that the deck is prismatic in section; not

curved on plan or elevation and does not bifurcate. It is

also assumed that only the bridge deck contributes to

stiffness. The section geometry chosen as indicative for

the purposes of this analysis is that at mid-span. The

value of

!

"nl( )2 is taken as that of a beam rigidly fixed

at both ends. Taking the values: (!nl)2 = 22.37, Ixx =

0.032, m = 1570kg and l = 170.690m, the value of f0

can be estimated as:

!

fo = 22.37200"10

9" 0.032

1570"170.6904

= 1.54 Hz. (12)

The value of f0 obtained in Eqn. (12) is

significantly below the guideline acceptable value of

f0>5Hz. Therefore, it is necessary to conduct a check

for the most adverse vertical acceleration, a. To be

considered acceptable, a must satisfy the requirement

that:

!

a < 0.5 f 0 m / s2. (13)

Substituting the value of f0 from Eq. (12) into Eq.

(13) it is clear that, in order to be considered acceptable

for vibration:

!

a < 0.5 1.54 = 0.62 m / s2. (14)

The value of a at a given position can be

determined by:

!

a = 4" 2 f0

2ysk# m / s

2.

Where ys is the vertical deflection at mid-span due

to a notional 0.7kN load, k is the span configuration

factor and " is the dynamic response factor due to the

dampening characteristics of the bridge materials. The

value of ys is obtained conservatively from the standard

deflection formula for a simply supported beam:

!

ys =700"170.690

3

48" 200"109" 0.032

= 0.011 m.

The value of " is obtained by extrapolation from

Ref. [5]. Taking the values: f0 = 1.54Hz, ys = 0.011m, k

= 1 and " = 20 in Eq. (15), the value of a mid-span

can be estimated as:

!

a = 4 " # 2 "1.542 " 0.011"1" 20

!

= 20.60 m / s2. (15)

The value of a obtained in Eq. (15) is

unacceptable according to the limiting value obtained

from Eq. (14) as 20.60>>0.62m/s2 and would clearly be

extremely dangerous for bridge users. However, in

obtaining this value of a, it has been assumed that the

bridge acts solely as a beam. In reality, of course, the

parabolic cable arrangement will limit downward

deflections of the deck at the hangar attachment points.

This will interfere with the vibration modes that have

led to the very high value of a, obtained in Eq. (15). If

a is recalculated with the span reduced to that of the

5.048m between hangers, taking " as 5 while f0

remains that for the overall span:

!

ys =700" 5.048

3

48" 200"109" 0.032

!

= 2.931"10#7

m / s2,

!

a = 4 " # 2 "1.542 " 2.931"10$7 "1" 5

!

= 1.37"10#4

m / s2

(16)

Clearly the value of a obtained in Eq. (16)

satisfies Eq. (14) as 0.000137<<0.62m/s2. The actual

value of a will fall between the values obtained in Eqs.

(15, 16). Obtaining a more accurate value of a is

beyond the scope of this paper, but would be necessary

to demonstrate the serviceability of the bridge.

It is worth noting that the value of 1.54Hz for f0

obtained in Eq. (12) is outside of the typical range of

1.6-2.4Hz for pedestrian motion indicated in Ref. [4].

This indicates that normal pedestrian motion may not

excite the bridge deck. However, forced vibration at

1.54Hz would be easy to achieve, making the bridge

susceptible to vandalism. This frequency is also a

concern for wind excitation.

The possibility of Synchronous Lateral Excitation

was also considered by the designers, as indicated in

Refs. [2, 3], and tuned mass dampers are utilized at the

half and quarter span positions to mitigate this effect.

12 Durability

As a steel bridge with concrete approaches,

Nescio’s deck is unlikely to face any particular

durability issues provided that regular maintenance and

repainting is undertaken. Hatches in the top of the deck

provide maintenance access to the void and electrical

cabling.

The structural round steel wires in the main cable

are hot-dip galvanised and the bundle permeated with

zinc paint. A triple layer of profiled and interlocking

outer wires is GALFAN coated to provide additional

corrosion resistance, Ref. [6].

The simplicity of the bearings at the base of the

pylons, discussed in Section [7], minimises the

maintenance required at this point. The complete

separation between the steel deck and the concrete

approaches eliminates many maintenance issues at this

movement joint. However, an issue could arise if the

8mm thick steel cover plate that prevents obstruction or

injury were to be removed or damaged by vandalism or

accident.

13 Intentional Damage

13.1 Graffiti

The concrete columns that support the approach

ramp are the components that would typically be most

vulnerable to defacement by graffiti. However, the

creation of the marshy wetland habitat in the area of

the approach ramp means that columns are typically

positioned in shallow water, discouraging vandals.

The pedestrian stairs at each end of the bridge do

appear to have been used as platforms from which to

graffiti the side of the bridge deck. However, the

painted steel surfaces are easily cleaned or repainted,

minimising the disruptive effect of such vandalism.

Figure 15: Vulnerability to graffiti at stairs.

13.2 Terrorist Action

In order to span efficiently Nescio has, like any

suspension bridge, very limited capacity for

redundancy. Therefore, serious damage to a principle

element such as one of its pylons or back-stay cables

would likely cause failure of the structure. However,

despite being something of a local showpiece, Nescio

is an unlikely target for terrorist action at present.

14 Future Changes

There are unlikely to be any changes to the design

in the foreseeable future. The bridge is relatively new,

having been completed in 2006, and was designed to

be compliant with Eurocodes. As a pedestrian and

cycle bridge, there is unlikely to be any need to

upgrade the capacity of the bridge to meet future

demand. If this were to be required, it is difficult to see

how this would be achieved using the existing

structure.

Tuned mass dampers have been utilised to

mitigate Simultaneous Lateral Excitation and could

presumably be retuned if such effects were found to

continue to occur. Consideration in [3.3] of the tuned

mass dampers as SiDL with !fl of 1.75 allows for

86.7kN of additional load if the dampers are

reclassified as DL with !fl of 1.05.

Addition of further masses would require detailed

analysis of the structure to check that there is sufficient

spare capacity.

15 Suggested Improvements

Overall, this design is a very successful response

to the challenges of the City of Amsterdam’s brief.

There are some, relatively minor, improvements that

this author can suggest could have made the design

even more successful.

Pulling the first concrete ramp-support column

back from the steel end column would relieve the

congestion of vertical components highlighted in Fig.

[7] and remove the need to forlornly ‘hide’ the column

by darkening its colour.

Removing the bifurcation support plate

highlighted in Fig. [5] would restore the order to this

area of the bridge. Compensatory stiffening to the

pedestrian deck would doubtless have been required, as

discussed in Section [2.4]. Nonetheless, the order of

the deck soffit, and lack of functional clarity in the

anomalous hanger detail, would be markedly

improved.

Acknowledgements

The author would like to express his gratitude to

Angus Low at Arup for providing a number of

technical drawings and journal articles without which

this paper would have been very much poorer. The

author would also like to thank Dr Mark Evernden for

his supervision in the preparation of this paper and

Professor Tim Ibell for his invaluable blue book. Any

errors of fact or analysis are doubtless attributable to

this author, while any insights should be credited to the

guidance of the gentlemen mentioned above.

References

[1] Low, A., and Ichimaru, Y., 2007. Nesciobrug,

Amsterdam, The Structural Engineer, IStructE, 6th

March 2007, p.22.

[2] Low, A. M., 2008. The Milne Lecture: Designing

Bridges, The Structural Engineer, 87 (5), 3rd

March 2009, pp.26-31.

[3] Low, A., and Ichimaru, Y., 2007. Nesciobrug,

Amsterdam: A Symbol For A New Suburb,

inspirational and functional, Improving

Infrastructure Worldwide: IABSE Symposium –

Weimar 2007, No. 93, pp.242-245.

[4] Melchor Blanco, C., et al., 2005. Structural

Dynamic Design Of A Footbridge Under

Pedestrian Loading, 9th SAMTEC Users

Conference 2005, p.2. Available at:

http://mecanique.in2p3.fr/JUsamtech/proceedings/

02_11_ULB_Bouillard/02_11_ULB_Bouillard.pd

f [Accessed 3rd April, 2009].

[5] BD37/01: Loads For Highway Bridges. Design

Manual For Roads And Bridges, Vol. 1, Section 3,

Part 14, Appendix A: Composite Version Of

BS5400.

[6] Pfeiffer, 2007. Fully Locked Cable – GALFAN,

Cable Structures: Data Sheets.

[7] Blake, A., 1990. Practical Stress Analysis In

Engineering Design [2nd Ed.], CRC: Basel, p.24.