SSUD13-311

Structural

Engineering Assignment 2

Samuel Beckett Bridge

STUDENT (SID):

Hongsheng YE (13262123)

2014/11/11 Tuesday

2014

Nick YEhs

1 / 9

Samuel Beckett Bridge

Executive Summary

Samuel Beckett Bridge (Irish: Droichead Samuel Beckett) is a cable-stayed bridge in Dublin that joins Sir

John Rogerson's Quay on the south side of the River Liffey to Guild Street and North Wall Quay in the

Docklands area.

It is the Dublin City’s newest bridge, and is now established as a landmark structure spanning the

maritime gateway to the City. The bridge is located East of the City’s centre and within the ‘heart’ of

the newly developed docklands’ area, facilitating an important urban transport link for private car use,

public transport, cyclists and pedestrians; and contributing towards the improved environmental,

commercial and social development of the urban area in which it is located.

Geometry

Describe the principal structural system (load-bearing form) adopted for the

structure.

The Samuel Beckett Bridge is a cable-stayed, steel box girder structure, with a span, across the River

Liffey, of 123 m. The bridge, which rotates through 90° to maintain shipping movements upstream, has

an asymmetric shape, with the base to the cable-stayed steel pylon set outside of the river’s central

navigation channel. The pylon curves northwards to a point 46 m above the water level with 25

forestay cables set in a ‘harp’ formation. An elevation of the bridge is shown in Figure 1.

Figure 1

The bridge deck (bridge beam) has a box-girder structure which provides rigidity. The load is

transferred and dissipated (spread) throughout the structure of the bridge deck and into the

cable-stays. The cable-stays are in tension and the load is transferred to the pylon which in turn

2 / 9

transfers the load to the bridge’s foundation, and in turn to the ground. The pylon is in compression.

When a load is applied to a member of the bridge structure, that member will deform to some extent

and when this deformation takes place, internal forces in the material of the structural member will

resist it. These internal forces are called stresses. The force transmitted across a section of the

structural member divided by the area of that section gives the intensity of the stress; it therefore has

the same units as pressure – pascals (Pa) or N/m2.

Outline the architectural inspiration/rationale that influenced the shape of the structure.

It was required to maintain a navigable river for shipping and to provide an iconic landmark structure

of a modern and unique design for the City. Having determined the requirements, Dublin City Council

sought a Designer of distinction capable of providing such a landmark structure, fulfilling the

requirements at this key strategic location. The concept design of the bridge examined a number of

ideas; one of which fully met the requirement set by the Client in achieving a landmark structure which

would act as a symbol for the City, that being of a bridge concept representing the shape of a Celtic

Harp; the harp being of great significance in that it is a symbol of Ireland. One can recognize the

resemblances between elements of the bridge with those of the harp, as seen in Figure 2.

Figure 2: the Bridge’s side elevation and Irish coin with harp symbol

Identify any geometrical constraints that influenced the shape during concept design or

3 / 9

detailed design.

It was a requirement to design the bridge in such a way that the navigable channel in the centre of the

river was maintained. This meant that an opening bridge was required. The option of rotating the

bridge in the horizontal plane was chosen by the Designer, which led to an architectural and structural

engineering challenge. Due to the navigable channel in the centre of the river, the axis of rotation was

positioned closer to the South river bank. The challenge for the structural engineers was to provide an

elegant solution in terms of balance and strength. In order to produce an architecturally ‘balanced’

impression the tip of the pylon was placed at the centre of the river, and using the lines of the front

and back cable stays, a triangle was created, which architecturally ‘balances’ the bridge.

Load

Identify all of the design live loads imparted on the structure.

The assessment of the requirements of the bridge dictated that the bridge was to provide for two lanes

of traffic in both directions, one of which would provide for public transport – in the initial years

facilitating a dedicated bus lane, and for possible future years a provision for a light rail system, and

was also required to provide for ample space for pedestrians and cyclists. Therefore, its biggest live

loads must be from people and public transport. And wind force should be another outstanding load.

Also, the river should be one of outstanding load.

Describe any structural issues that arose once all of the loads had been identified.

It was a requirement to design the bridge in such a way that the navigable channel in the centre of the

river was maintained. This meant that an opening bridge was required. The option of rotating the

bridge in the horizontal plane was chosen by the Designer, which led to an architectural and structural

engineering challenge. Due to the navigable channel in the centre of the river, the axis of rotation was

positioned closer to the South river bank. The challenge for the structural engineers was to provide an

elegant solution in terms of balance and strength. In order to produce an architecturally ‘balanced’

impression the tip of the pylon was placed at the centre of the river, and using the lines of the front

and back cable stays, a triangle was created, which architecturally ‘balances’ the bridge. The bridge

span between quay walls is 123m and the height of the pylon from the mean tide level is 46m.These

were principally to protect the environmental impact on the river and its quay wall infrastructure, and

on the urban area in which the bridge was to be constructed and operated.

Analysis

Annotate diagrams of the structural system showing load paths for permanent

(dead) and imposed (live) loads. Note: choose only one live load for discussion (e.g.

wind load, earthquake load, occupancy floor load).

4 / 9

Figure 3

Figure 4

Live loads from people and public transport

The biggest live loads must be from people and public transport, because the bridge is located east of

the city’s centre and within the ‘heart’ of the newly developed docklands area, facilitating an

important urban transport link for private car use, public transport, cyclists and pedestrians; and

contributing towards the improved environmental, commercial and social development of the urban

area in which it is located. The gravity of all private cars, public transport, cyclists and pedestrians is a

kind of live loads.

Horizontal Transfer

Gravity

Water Load (Flow)

Water Load (Flow)

Gravity

Gravity (live) Gravity

5 / 9

Identify and discuss the choice of the stability system

Beams& Cantilevers& Deck Design

A beam bridge is essentially a rigid structure supported at both ends. Various techniques are used to

maximize strength while minimizing weight. A girder bridge can be thought of as a variation of the

beam bridge. I-beam and box steel girders are used in this bridge; these shapes are much less prone to

bending.

The main fore deck structure, the ‘front span’, is a multi-cell box girder, made up from relatively thin

steel plates stiffened internally using a combination of longitudinal bulb flats, angle sections and

trapezoidal stiffeners. Cantilevered from this main box section is the ribs and steel decking, which form

the pedestrian and cycle tracks (Figure 5 shows that). The top of the box consists of a 14 mm thick

plate with 12 mm trapezoidal stiffeners. The 36 mm mastic asphalt layer was taken account of in the

fatigue check for this orthotropic deck. The back span, which houses the counterbalance, is also a

multi-cell box girder but made up from unstiffened steel plates. The cells in the back span were

generally to be filled with a combination of lead shot and concrete, which also supports the top and

bottom plates, preventing them from buckling locally. The ballast material was subsequently changed

by the contractor to a combination of steel blocks and heavyweight concrete. In order to achieve the

final bridge balance the amount of steel ballast placed on site during construction in these cells was

adjustable. This allows for the addition or removal of mass in order to balance any future changes

made to the superimposed dead loads on the bridge. An important structural and aesthetic feature of

the bridge is the single, central, line of forestays supporting the main span from a curved pylon. Such

an arrangement tends to lead to large torsional forces in the deck due to unbalanced live loadings

either side of the line support. Therefore, an advantage of using a multi-cell box section is its inherent

torsional rigidity.

Figure 5: Section through front span

6 / 9

Design

Identify a critical connection detail and describe how design forces are transferred

through the connection.

Figure 6

Figure 7

Critical Connection

Critical Connection

7 / 9

Figure 8

Identify any ‘value-engineering’ opportunities that arose and the influence on the

structural detailed design process (e.g. cost saving opportunities, preferred contractor

construction methodology/fabrication/erection sequence).

Value engineering

After the contract had been awarded, but before work commenced on site, the contractor proposed

some alternative solutions that would provide savings to the client, improve the programme and

sequencing of the work, reduce risk during construction and offer maintenance benefits over the

lifetime of the bridge.

For example, value engineering for Ballast Materials:

At the tender stage, the materials specified to provide the necessary counterweight to balance the

bridge were lead shot and concrete. At that time the price of lead, quoted at the London Metals

Exchange, had been rising markedly and was continuing to increase. Significant increased costs could

be avoided if the balance of the bridge could be achieved using alternative materials.

The contractor developed a proposal to replace the lead and concrete ballast material with the

alternative combination of steel blocks and heavyweight concrete with a density of 3.9 t/m3 using

magnetite aggregate.

Support

Direction indicates the force

transfer through the Joint

Compression

Tension Tension

Gravity

8 / 9

References

Cutter, J. & Flanagan, J. W. & Brown, P. & Rando, M. & Mo, G. (2011), ‘Samuel Beckett Bridge, Dublin,

Ireland’, Ice Proceedings.

Cutter, J. & Flanagan, J. W & Mo, G. (2010), ‘The Realisation of the Samuel Beckett Bridge – Dublin, Ireland’,

IABSE.