Proceedings of Bridge Engineering 2 Conference 2009
April 2009, University of Bath, Bath, UK
A CRITICAL OVERVIEW OF THE
VERRAZANO-NARROWS BRIDGE
W. T. Arnold1
1 rd
3 Year MEng Undergraduate Student, University of Bath
Abstract: This conference paper provides a critical analysis of the Verrazano-Narrows Bridge in
New York City. It analyses the design aspects of the bridge, along with the finished structure and
how it performs today. It looks at the aesthetics, structure, construction and serviceability of the
bridge, along with analysing the loading on the bridge in terms of the current British Standards.
Keywords: Verrazano-Narrows, New York, suspension, steel, Othmar Ammann
Figure 1: The Verrazano-Narrows Bridge
1 Introduction1
The Verrazano-Narrows Bridge is a double-decker
suspension bridge that connects the New York City
Boroughs of Staten Island and Brooklyn, and is named
after Giovanni di Verrazano, the first known explorer to
enter New York Harbour. It was the final bridge to be
designed by Othmar Hermann Ammann, a Swiss-born
structural engineer who was responsible for designing
many of the other large-spanning steel bridges in New
York and New Jersey, such as the George Washington
Bridge and the Bayonne Bridge.
1
Mr. William Thomas Arnold - [email protected]
Construction started on the bridge in August 1959,
with the upper deck being opened to the public on
November 21st 1964 at a cost of $320 million - $100
million over budget [1]. The lower deck was kept closed
to the public until June 28th 1969, nearly four years after
Ammann’s death, as there was no need for its extra six
lanes of capacity until then.
The design and construction of this bridge was
commissioned for several reasons. The first was to
provide a connection between Staten Island and the rest
of New York City that would not be hampered by
adverse weather conditions in the same way that the
current ferry system was. It was also deemed to be
desirable as it would form part of an expressway that
would link New England, New York and New Jersey
together; and form part of a route for express buses to
travel along when going between Manhattan, Brooklyn
and Staten Island.
Finally, the Triborough Bridge and Transport
Authority (TBTA) chose to commission a bridge to
cross the Narrows rather than a tunnel as it would be
quicker and cheaper to build, along with providing a
greater traffic capacity.
bridge, then the facade would far more flowing and
uncluttered, enhancing the appearance of the bridge
deck. The fact that the suspender ropes have been
designed in groups of four also detracts from the order
of this bridge.
2 Aesthetics
Whilst every bridge is individual and as such,
every bridge has its own unique reasons for why it looks
beautiful or ugly, it is widely accepted that in order for a
bridge to be as aesthetically pleasing as possible, Fritz
Leonhardt’s ten areas of aesthetics should be adhered to.
This section analyses the areas relevant to this bridge.
Figure 3: Close-up of the deck from a distance
2.1 Fulfilment of Function
When viewing the elevation of the bridge from a
distance, it is obvious straight away how the bridge
works. The strong looking towers at either end of the
deck carry all of the compression in the bridge, and the
purpose of the suspension cable is very clear to see.
However, the deck itself does appear to be
superfluously deep, but this is an issue that is
unavoidable in a bridge that carries two decks of traffic.
Figure 2: Profile of the bridge
2.2 Proportions of the Bridge
With a height to length ratio of nearly 1:10, the
Verrazano-Narrows Bridge looks slender and efficient.
There is a good balance between mass and void under
the bridge and it doesn’t look over-engineered. The
curve at which the bridge deck rises towards the centre
of the span helps to add extra space underneath the
bridge that helps add a sense of lightness and efficiency.
2.3 Order within the Structure
From a distance, this bridge appears to have very
good order about it, with the number of different
structural elements being kept to a minimum. However,
at closer inspection, the bridge loses this simplicity in its
order. The main problem isn’t the truss element of the
deck which is a necessity in order for the bridge to carry
two levels of traffic, but amount of structure elements
emerging from the deck, such as the vertical ties that the
hanger cables are attached to in Fig. 3, and the
horizontal struts that hanger cables are attached to in
Fig. 4. If these were removed from the design of the
Figure 4: Close-up of the deck from below
2.4 Integration into the Environment
As this bridge crosses the main access into Upper
New York Bay and the Hudson River, it was always
important that it had a high enough clearance
underneath it to allow as much sea-traffic through as
possible. For this reason, a suspension bridge was an
obvious choice as a simply-supported concrete beam
bridge wouldn’t give enough clearance underneath it for
huge oil-tankers to pass through.
This light grey suspension bridge fits seamlessly
into the environment, as it is a continuation of the built
up city surrounding it. A long masonry arch bridge,
concrete box-section bridge or timber truss bridge
would look very out of place as a span of this size
demands a bridge that is equally imposing.
2.5 Texture and Colour
Rather than paint the Verrazano-Narrows Bridge in
striking colours to help it stand out on the horizon, the
whole bridge is painted a light grey-blue colour. Part of
the reason for this monotony of textures and colours is
that the bridge is only used by road traffic. As there is
no pedestrian or bicycle traffic allowed over the bridge,
there is no need for details which would only be
experienced by a slow moving pedestrian or cyclist.
Instead, it makes more sense to save time and money by
painting the entire bridge in one colour.
Ammann has carefully used shadows on the bridge
towers to make these look more slender than they
actually are. This effect is apparent in Fig. 3, where it
can be seen how the shape of the towers gives makes
them look thinner on the outside of the bridge than they
look on the side nearest the deck.
harsh urban city in which the it sits. And whilst there is
no way to blend a bridge of this size into its
surroundings, the grey painted steel finish does fit in
with the majority of ships that pass under it every day.
2.6 Character
Whether a bridge has character or not is not
necessarily a matter of fact, and is often more one of
opinion. Calatrava’s bridges are often referenced when
talking about character, because of their individualism
and uniqueness when compared to other bridges of their
time. However, a logic-defying form of a bridge is not
the only thing that gives it character.
2.9 Aesthetics Summary
This is a bridge that is both beautiful and majestic,
because of its size and position. However, Mies van der
Rohe famously said that “God is in the details” [2] and
sadly, in the Verrazano-Narrows Bridge these beautiful
details are lacking, resulting in a bridge that is less well
ordered than it could have been.
3 Loading
When this bridge was first designed, it will have
been designed to cope with loading cases outlined in the
standards set by AASHTO – the American equivalent of
the British Standards. However, for the purposes of this
paper, I am going to analyse the loading on the bridge to
BS5400-2:2006 [3].
As the bridge has no facilities for pedestrians, no
pedestrian loading has been taken into account in these
calculations. There is a possibility that this will become
an issue in the future, but compared to the traffic loads,
it would still only make up a very small percentage of
the overall loading.
Figure 5: Satellite image of New York Bay
This bridge has great character because of its size
and position. Anyone who travels into New York
Harbour by ship will have to pass under this colossal
man-made structure that acts as a gateway to the
world’s financial capital. It’s the longest suspension
bridge in the USA, and can be seen from various points
all over New York City. There is no doubting the
impact that this bridge has made on the skyline of New
York Bay.
Figure 6: Load path diagram
3.1 Dead Loading
2.7 Complexity in Variety
The Verrazano-Narrows Bridge has very little
complexity to it as it just a large-span suspension
bridge. However, this should not be seen as a bad thing,
as its simplicity is what makes it such a graceful bridge
when viewed from afar.
Without this simplicity, the order of the bridge (see
section 2.3) would not be as easy to see, and as such the
bridge would not look as graceful as it currently does.
2.8 Incorporation of Nature
Whilst nature doesn’t influence this bridge in as
obvious a sense as many other bridges the world over, it
could be argued that for a bridge as un-natural and
prominent as this, the nature which has been
incorporated into the design of the bridge is that of the
Figure 7: Section through bridge deck
Reference [4] states that the total dead weight of
the suspended structure is 51,000tons, or 454MN. With
a total suspended bridge length of 2039m, this equates
to an un-factored dead load of 223kN/m.
3.2 Superimposed Dead Loading
3.4 Wind Loading
On top of the dead load that is provided by the
structural elements of the bridge, a superimposed dead
load needs to be assumed. As this is so difficult to
accurately predict, BS5400-2 gives very high safety
factors. These factors can be found in Table 1 of Ref.
[3].
The superimposed dead load on the VerrazanoNarrows Bridge is assumed to be made up from the
following: 50mm of bituminous asphalt (23kN/m3),
200mm of saturated sand fill (21kN/m3), and an added
1kN/m2 for services. The road surface and fill are
assumed to be covering a carriageway width of 28m in
total (see section 3.3), and the services cover a 31m
width.
This gives a total un-factored superimposed dead
load of 181kN/m.
3.3 Vehicular Live Loading
The total width of the bridge deck is 31m [1]. From
this and satellite images, it is fair to assume that the total
carriageway width is about 28m.
The total length of the Verrazano-Narrows Bridge
is 2040m. According to clause 6.2.1 of Ref. [3], for a
loaded bridge length over 1600m, the HA UDL needs to
be agreed with the relevant authority. However, as this
is not possible, for this analysis the HA UDL will be
calculated from the following equation:
The maximum wind gust is calculated as being:
(2)
Due to a lack of sufficient data, the mean hourly
wind speed, v is assumed to be 30m/s. The bridge’s
height above ground level is 70m [4], and the total
wind-loaded length of the bridge is 2040m. This gives a
wind coefficient, K1 of 1.53 and a gust factor, S2 of 1.39.
The funnelling factor, S1 is taken as 1.00 as funnelling is
highly unlikely to occur for this bridge.
As Fig. 4b) in Ref. [3] explains, due to the cross
section of this bridge, wind tunnel tests would be
required in order to calculate the forces acting upon it.
However for the purposes of this report, all forces shall
be calculated using the equations that are applicable to
the bridge sections shown in Fig. 4a) of Ref. [3].
3.4.1 Transverse Wind Load
The horizontal transverse wind load is given by:
(3)
where
(1)
HA loading = 16.8kN/m of notional lane
KEL = 120 kN (placed for most adverse effect)
(4)
This means that the dynamic pressure head, q is
equal to 2495N/m2. Figure. 10 shows the profile of the
deck when viewed from the side.
Due to this bridge’s size, a full HB loading of 45
units will be used in the analysis. This is a loading of
112.5kN per wheel.
Figure 10: Sketches of deck elevation [5]
Figure 8: HA and HB loading on upper deck
Figure. 8 shows the loading layout on one of the
bridge decks only. It is highly unlikely that the bridge
would be designed to cope with HB loadings acting on
both decks at once, so the loading layout on the other
deck is shown in Fig. 9 below.
Figure 9: HA and HB loading on lower deck
Both A1 and CD depend on the depth of the section,
and this value depends on whether or not live load is
included in the calculations. The left half of Fig. 6
shows the elevation of the deck with no live load, and
the right half shows it with an added live load.
With no live load, the depth of the section equals
9m, and the projected surface appears to be
approximately 50% solid.
When the live load is considered to be acting on
both decks of the bridge, the depth of the section can be
increased by adding a 2.5m high vehicle to both upper
and lower decks. This would increase the section depth
to 10.5m, with the projected surface increasing in
solidity to about 75%.
The largest transverse wind force acting on the
bridge is obviously going to occur when live load is
present, due to the much higher depth and solidity of the
section.
load (where the depth of live load is 2.5m) and CD
changes to 1.45, as outline in clause 5.3.4.3 of BS54002 [3].
m2
3.4.4 Wind Load Combinations
There are four combinations of wind load, all that
must be considered to find the worst case scenario.
These are outlined in the table below.
The transverse load acting on the piers should also
be taken into account, also using Eq. (3). The piers t/b
ratio has been assumed as 1. The breadth of the piers
has been taken conservatively as 14m as there is
insufficient data available. Data available from Refs. [1,
5] give the maximum clearance at the centre of the
bridge, the maximum longitudinal grade of the bridge,
and the length of the main span. From these, the pier
height can be assumed to be about 55m.
Table 9 of BS5400-2 [3] therefore gives the
coefficient of drag for the piers as 1.4. So, ignoring
shielding effects of the upwind piers on the downwind
piers, the transverse wind load on each pier is
3.4.2 Nominal Vertical Wind Load
The vertical downward force caused by the wind is
calculated as
(5)
Where the plan area, A3 is 57120m2 and the
coefficient of lift, CL is ±0.64, found from Fig. 6 of Ref.
[3].
3.4.3 Longitudinal Wind Load
Due to the solidity of the bridge deck’s elevation
being so high (75% when live load is included), the
longitudinal wind load, PL is calculated using equations
that refer to bridges with solid elevations, rather than
bridges with truss girder superstructures.
(6)
(7)
(8)
In Eq. (7), q, A1 and CD are all the same as in Eq.
(3). However, in Eq. (8), A1 is purely the area of live
1
2
3
4
Table 1: Wind loading combinations
Pt alone
68.9MN
Pt ± Pv
160.1MN or -22.3MN
PL alone
32.9MN
0.5Pt ± 0.5PL
50.9MN or 18.0MN
Taking the worst case combination of wind loads,
the un-factored wind loading for this bridge is 160.1MN
total.
3.5 Temperature Loads
Because of the bridge’s size and orientation, during
the day one side of the bridge often warms up more than
the other, causing unequal expansion throughout the
bridge. To ensure that this didn’t affect the surveying of
the construction, all precision measuring of the bridge
took place during the night, when the bridge was at a
constant temperature throughout [4].
3.5.1 Effective Temperature Loading
This loading is due to an overall increase or
decrease in temperature between night and day. The
maximum and minimum temperatures for this location
have been taken as 40o and -20o Celsius respectively, for
a 120-year return period. However, these maximum and
minimum temperatures take place over a whole year,
whereas we are primarily interested in the diurnal
temperature difference of the bridge. This is taken as
25oC.
The coefficient of thermal expansion for both steel
and concrete is 12 x 10-6 /oC. Assuming that thermal
expansion joints are in place where the deck crosses
each tower, the effective length of deck is 1300m. From
these, the change in length can be found from Eq. (9)
provided that the expansion joints are fully working.
The apparent compressive stress that the deck will
experience can be calculated from Eq. (10), and will
occur if the expansion joints are prevented from moving
at all (and δ = 0).
(9)
(10)
An overall temperature change will also affect the
length of the main cables and suspender ropes
throughout the year. Using Eq. (9) again, but with ∆T
set to 60oC (the difference between the maximum and
minimum temperatures over a 120-year return period),
we can see that the length of the main cable can change
by up to 1.6m.
3.5.2 Temperature Difference Loading
Temperature difference loading is the loading
caused by a variation in temperature between the top
and bottom surfaces of the bridge deck. Figure. 11
below shows the difference in temperature throughout
the deck section. The two parts of Fig. 9 show how the
top of the deck will be approx. 21oC hotter in the
morning and 5oC cooler in the evening than the bottom
of the deck is. It also shows how the temperature
gradient varies throughout the steel deck and girders.
These variations in temperature will also cause apparent
loads and moments to be felt by the deck. Whilst only
the top deck is shown in the figure, the bottom deck will
follow a similar patter, though with less of a difference
in temperatures due to the shading provided by the
upper deck.
3.6.4 Parapet Collisions
This is based on 25 units of HB loading, i.e.
1000kN colliding with the parapet. Analysis of this
loading based on momentum and impulse forces is
required in order to calculate the actual force “felt” by
the parapet during a collision.
3.6.5 Substructure Impacts
Due to the size of the ships passing under the
bridge every day, it is vital that the tower piers are well
protected, as one impact from a passing ship could
render the entire bridge unsafe. For that reason, the base
of the piers are very well protected by artificial islands
of rocks designed to absorb the impact of a ship to a
level where the bridge will not be affected by a
collision.
Figure 12: Impact protection around the substructure
3.7 Load Combinations
Figure 11: Temperature difference diagram
3.6 Secondary Live Loads
The secondary live loads all occur due to the
acceleration and collision of traffic in different
directions. The effects of these secondary live loads are
used in load combination 4 (see section 3.7) and include
the following.
3.6.1 Horizontal Centrifugal Loading
This can be ignored for the Verrazano-Narrows
Bridge as the bridge is straight in plan.
3.6.2 Longitudinal Braking Loading
This load consists of a horizontal force of 8kN/m
applied over one notional lane, plus a single 200kN
force. It also includes 25% of the total HB loading
applied over two axles.
So in this case, the total longitudinal breaking load
applied to the bridge is 16.5MN and the total vertical
breaking load is 450kN applied over two axles.
3.6.3 Accidental Skidding
This load is modelled as a point load of 250kN
acting horizontally in any direction in one notional lane,
plus the associated HA loading.
The five different combinations of load that need to
be checked are outlined in the table below. These each
need to be checked at serviceability limit state (SLS)
and ultimate limit state (ULS) to find the worst case
loading combination. To do this, the loads need to be
multiplied by two partial factors, γfl and γf3. These safety
factors can be found from Table 1 of BS5400-2 [3].
1
2
3
4
5
Table 2: Loading combinations
All dead loads plus primary live loads
Combination 1 plus wind loads
Combination 1 plus temperature loads
All dead loads plus secondary live loads and
associated primary live loads
All dead loads plus loads due to friction at the
supports
3.8 Loads in the Cables
The worst case loading scenario can then be used
to calculate the maximum tensile forces experienced by
the suspender and main cables.
Note that in Fig. 13, the sets of suspender cables
(of which there would be eight under each pair of main
cables) have been simplified to show one support at
either side of the deck.
Taking moments about V2 (Fig. 13):
4.1 Materials
(11)
The main structural elements of the bridge are
constructed out of steel due to its high strength and easy
workability. The foundations and anchorages are all
made out of concrete, a large proportion of which is
reinforced.
Table 3 shows a rough distribution of the amount
of each material used in construction. Most of the data
is taken from Ref. [1], but Ref. [4] provided a better
idea of the breakdown of the use of steel in the towers
and suspended structure.
Table 3: Approximate material quantities
Steel
Tonnes
Towers
92,000
Suspended structure
15,000
Reinforcing steel
31,000
Approaches
18,000
Concrete
Cubic meters
Anchorages
289,000
Tower piers
150,000
Approaches
99,000
Bridge deck roadway
15,000
Figure 13: Typical loading condition
These values are total UDLs for the pairs of cables
on either side of the bridge. Dividing the larger of the
values (V1) by two to share the load between the two
main cables on that side; the maximum tension in the
main cables can be found as so:
(12)
(13)
(14)
4.2 Bridge Design
4.2.1 Anchorages
The two anchorages are each 40m high, 49m wide
and 91m long. However, due to differences in the
ground conditions at each end of the bridge, the
Brooklyn anchorage contains 158,000m3 of concrete,
whilst the Staten Island one only contains 131,000m3.
The anchorages exist to house the ends of the four main
suspension cables that carry the enormous weight of the
bridge deck, as well as supporting the two approaching
decks of bridge on the landward ends.
Where the maximum UDL on the cable, w =
147.5kN/m; the plan length of the cable, L = 1300m;
and the midspan sag in the cable, f = 140m. This gives
values of Vmax = 191.8MN, Hmax = 222.6MN, and
therefore Tmax = 294MN.
Note that asymmetric loading does not give a
worse case of loading on one side of cables, this is
because the total load on the bridge has decreased
dramatically enough to counter-act the increase in
loading on the side in question.
4 Design and Construction
Whilst the Verrazano-Narrows Bridge was a
pioneering bridge in terms of its sheer size, the
construction of it did not stray too far from the tried and
tested methods that were at that time well known by the
thousands of construction workers who built the bridge.
This section discusses the design and construction
of each of the bridge’s components.
Figure 14: Inside one of the anchorages [7]
The anchorages resist the total lateral load from the
cables purely because their weight is high enough to
prevent them from slipping at all. As can be see in Fig.
6, the main cables enter the anchorages and are then
split up into their 66 individual strands. These strands
are each individually attached to the anchorages by
inclined steel girders, buried deep within the concrete.
The strands are firmly attached to these girders by the
steel ‘shoes’ that are bolted on the ends of them. This is
shown in Fig. 13.
The anchorages also contain rollers for the main
cables to pass over on the side nearest the river. They
allow the cables to expand and contract freely when
there are changes in temperature or loading on the
bridge, without exerting any extra forces on the wrong
parts of the anchorages.
4.2.2 Piers
The piers and foundations were designed to support
the 2.5GN provided by the loads on the bridge, and to
complicate the design of the foundations further, no
bedrock could be found underneath the Narrows, even
as deep as 100m below the surface [6]
Instead of founding the bridge on rock, it is
founded on a deep compacted band of glacial clay.
Caissons were sunk into the ground whilst protected
from the water pressures by steel cofferdams. This is
shown in Fig. 15. These caissons are pressurised
chambers that allow workers access to the material
under the river, whilst keeping out river water by use of
pressurised air. Muck and sand could then be dug up
and removed from the caisson through a separate shaft
(known as muck tubes). Once a suitable depth of ground
had been removed, the caisson could slowly be lowered
down, with concrete being added on top in 3m depth
increments [6].
Once the predetermined depth had been reached for
the caisson (52m for the Brooklyn foundation and 32m
on the Staten Island side), the muck tubes (of which
there were 66 for each foundation) could each be filled
with water. The caissons were then sealed with concrete
[4]. It took nearly two years to fully construct the two
foundations for the Verrazano-Narrows Bridge, but by
1961, the construction of the bridge towers could begin.
Figure 15: Typical caisson schematic
4.3.3 Towers
The two 211m tall towers were each constructed by
a different steelworks company. The Brooklyn tower
was constructed by Harris Structural Steel Co. Inc. and
the Staten Island tower by Bethlehem Steel Co. The
towers were prefabricated in large sections and bought
to site by barges. The bottom three sections of tower
were lifted into place by floating derricks that were
anchored alongside the tower piers.
Once these three sections were in place, towering
about 37m above the piers, the floating derricks were
removed and replaced by “creepers”. These were
derricks that were attached to the towers by rails bolted
to the side of them. The creepers could then lift the next
tier of tower up the existing structure, allowing it to then
be bolted in place at the top. When finished in 1962, the
top of each towers stood 212m above the mean water
level.
4.3.4 Cables
Each of the four cables are 910mm in diameter and
are made up of 61 strands. These strands are each made
from 428 galvanised steel wires, each 5mm in diameter.
Figure 16: Suspender ropes connections [8]
The spinning of these wires was carried out in the
usual manner of passing the wires back and forth
between each end of the bridge on a 1.2m diameter
wheel. On a good day, the wheel would pass back and
forth 50 times. Workers would grab the wire as it passed
and clamp them into place, laying them side-by-side and
on top of each other. Once all 428 wires had been
carried across the bridge by the wheel, these would be
bound together into a strand. When all 61 strands had
been bound, they were squeezed into a cylindrical shape
by hydraulic jacks and permanently encased in steel
castings that provided the saddles for the suspender
ropes to sit in.
Once all four main cables had been constructed,
262 suspender ropes were attached on to the cable.
These steel ropes were prefabricated off-site, so that
each one had been correctly cut to length before it even
reached the site.
4.3.5 Bridge Deck
The final phase of the construction process was the
installation of the prefabricated bridge deck. The steel
structure of the deck had already been constructed at the
American Bridge Company’s steel yard that was four
miles upstream in New Jersey. Each of these 60 sections
was floated to site by large barges before being lifted
into position by hoisting machines on top of the towers.
The deck sections were lifted one at a time, starting
with the central piece and working outwards towards the
towers. This was done so that the cables would keep a
roughly symmetrical shape throughout the construction
process, and the deflections in the towers would be kept
to a minimum.
The prefabricated deck sections followed a
conventional double-decker design, with vertical
stiffening trusses separating the two decks, and lateral
braces giving lateral stability. The decks themselves are
supported by longitudinally spanning steel I-beams. The
depth provided by the second deck means that the
rigidity of the bridge is greatly increased over that of a
single deck – an issue which had been at the front of
ever bridge designer’s mind since the 1940 collapse of
the Tacoma Narrows Bridge.
Once construction of the steel deck had been
completed, the final part of the construction process was
to install the concrete roadway and add the finishing
touches, such as the road surface, services and painting
the steelwork.
5 Strength
Othmar Ammann designed all of his bridges in
steel because of its low weight when compared to other
materials such as concrete. The bridges he created were
all very light and inexpensive compared to other bridges
of the time and this was due to his use of deflection
theory in his designs.
From this theory, he believed that the dead load
provided by the deck and cables would provide enough
structural strength against the effects of other loads
applied to the bridge such as wind loading and live
loading. This meant that he was able to design a bridge
with relatively small stiffening trusses compared to
other suspension bridges on the time.
The main areas of strength that the bridge needs to
be checked for are strength in bending and shear in the
deck, and tensile forces in the main and suspender
cables.
5.1.1 Bending moments and shear forces
To analyse the moments in the deck of this bridge,
it is easiest to consider the bridge as a continuous beam,
with fixed supports at the anchorages and piers, and
flexible supports positioned where the suspender cables
would be supporting the deck.
It’s fair to say that once the steel structural deck
elements were bolted in position, there were very little
moments within the deck structure caused by its self
weight, due to the fact that the deck was raised into
position in sections with the suspender cables elastically
deforming under its weight and thus eliminating any
moments in the deck. For that reason, the only loads that
need to be considered when analysing the bending
moments in the deck are those provided by the live load,
the superimposed dead load, and also the concrete that
was poured onto the deck after the prefabricated steel
had been lifted into place.
Figure. 17 shows how a bending moment diagram
for the bridge might look. As can be seen, the maximum
moments in each span follow the same pattern that
would be expected from a beam if the intermediate
supports were all removed, but of a far smaller
magnitude.
A good way to get an approximation of the bending
moments in the deck is using Table 3.5 of BS8110-1
[9]. This table gives the ultimate bending moments and
shear forces in a multi-span beam.
The table states that the maximum bending
moment in a multi-span continuous beam will occur
above the first interior support, and will be equal to
0.11Fl, where F is the total ULS load and l is the
effective span between hanger cables. The maximum
shear force also occurs at the first interior span, and is
equal to 0.6F.
When F = 51MN and l = 35m, Mmax = 196MNm
and Smax = 30.6MN.
5.1.2 Tensile strength of the cables
The maximum stress induced in each of the cables
due to the tension applied in them is given in Eq. (15)
below.
(15)
For the main cables, Tmax is given by Eq. (12) and
for the suspender ropes, it is the value of w in Eqs.
(13,14), multiplied by the distance between two
suspender ropes, 34m. In both cases, A is the cross
sectional area of the cables in question, 0.66m2 for the
main cables, and 0.018m2 for the suspender ropes (with
an assumed diameter of 150mm).
6 Foundations and Geotechnics
As discussed in section 4 of this report, the ground
conditions at the bridge’s location are very poor, even at
great depths under the ground. For that reason it is quite
a feat of engineering that this bridge stands up at all.
Whilst the tower foundations had to be built to
depths of over 30m in order to create massive amounts
of friction to help support the loads, they also rely on
the strength of the glacial clay on which they stand to
hold them up. The anchorages however, work entirely
on friction being generated by the huge mass of
concrete involved at both ends to keep them stable.
Because the primary ground condition for these
foundations was clay, which has a very low settlement
rate, it is important that differential settlement was also
considered during the design of the bridge.
7 Durability
7.1 Serviceability
Whilst a bridge can be checked under its ULS and
proved to be safe enough for use, that doesn’t
necessarily mean that it will be perceived as safe by the
public. Extreme oscillations and vibrations caused by
wind loading or moving traffic can cause the bridge to
sway and move in ways that would make its users feel
unsafe. Most suspension bridges are designed to be far
stiffer than needed to avoid this, as the public perception
of what is safe is often very different from what is
actually structurally sound (a good example of this
being the Millenium Bridge in London when it was first
opened). Having said that, it is important to recognise
the potential issues caused by a flexible bridge, and it is
important to ensure that enough stiffness is built into a
bridge to prevent it from collapsing due to it oscillating
at its natural frequency, as occurred on the Tacoma
Narrows Bridge in 1940. In the case of the VerrazanoNarrows Bridge, the truss separating the decks and the
horizontal bracing is sufficient to keep its natural
frequency high enough to prevent these kinds of
oscillation from occurring.
that covers the whole of the other four boroughs. If this
link is made, it is very likely that the cheapest and
therefore most viable option will be to connect these
along the Verrazano-Narrows Bridge, as has already
been done with the Manhattan Bridge and Williamsburg
Bridge.
There is also currently a lot of discussion about the
possibility of adding a cycle path onto the bridge,
running along the gap between the two main cables and
their suspender cables.
9 Conclusion
The Verrazano-Narrows Bridge was, and still is a
magnificent feat of engineering. It was not only the
longest suspension bridge in the world when it was
constructed, but is still the longest one in the USA
today. Whilst it is neither the most refined nor elegant
of suspension bridges to have been built, it is definitely
one of the most prominent and impressive due to its size
and position. The fact that it is still standing nearly 50
years after its construction with very little modification
is a credit to the people that designed and built it.
10 References
7.2 Repainting
Because of its size and position, the bridge has to
be scraped of rust and dirt and repainted every ten years,
a process which usually takes five years to complete [3].
Various different sources quote this as costing different
amounts of money, but they all range around the $50
million mark, which is the equivalent of about $10 for
every square foot of bridge.
During repainting, only small parts of the bridge
are closed at once. As the bridge has 12 lanes in total,
this means that the repainting works cause very little
disturbance to the traffic flow.
7.3 Access
Access is provided to the inside of the anchorages
so that the condition of the strands that make up the
main cables can be checked on a regular basis. These
anchorages underwent a $31M rehabilitation in 2002
along with the bridge approaches. As part of this
rehabilitation, the anchorages were sealed to protect the
concrete
from
deteriorating,
and
complex
dehumidifying systems were installed to prolong the life
of the cables.
It can be assumed that access to the towers is also
available, as it is important to check the condition of the
steel in the towers regularly.
8 The Future of the Bridge
As Staten Island is currently the only borough in
New York not to be linked to the New York Subway, it
is quite likely that at some point in the future, the Staten
Island Railway will be linked to the existing subway
[1] ANDERSON, S. Verrazano Narrows Bridge
Historic Overview [online]. Eastern Roads.
Available from: http://tinyurl.com/dgfqah
[Accessed 30th March 2009].
[2] THE NEW YORK HERALD TRIBUNE. 1959.
Restraint in Design, the New York Herald Tribune,
28 June.
[3] BS 5400-2:2006. Steel, concrete and composite
bridges – Part 2: Specification for loads. BSI.
[4] TALESE, G. 1964. The Bridge. New York:
Walker Publishing Company, Inc.
[5] FAUST, S.F. 1976. A Bicycle / Pedestrian Path for
the Verrazano-Narrows Bridge: A Demand and
Feasibility Study [online] New York: Unknown
publisher. Available from
http://tinyurl.com/ccmwtm [Accessed 18th March
2009].
[6] SCOTT, R. 2001. In the Wake of Tacoma
(Illustrated Ed.). Reston: ASCE Publications.
[7] JANBERG, N. 2009. Structurae: Verrazano
Narrows Bridge [online]. Ratingen: Nicholas
Janberg ICS. Available from
http://tinyurl.com/crblpy [Accessed 2nd April
2009].
[8] O’DONNEL, P.S. 1994. Bridgemeister [online].
Unknown publisher. Available from
http://tinyurl.com/dmkrwf [Accessed 31st March
2009].
[9] BS 8110-1:1997. Structural use of concrete – Part
1: Code of practice for design and construction.
BSI.