comparison report - gladesville vs millau

Parth Gulati + Alex Dimakopoulos The key differences between the Gladesville Bridge and the Millau Viaduct are evaluated in this report. The Gladesville Bridge is an arch bridge that was built in 1964 (as a replacement for the old trussed structure) to reduce traffic congestion, and provide a fortified pathway between Sydney suburbs. The bridge was to allow for a clear navigation path that ships could take under the deck, given the functioning of the oil refinery at the time of construction. The Millau Viaduct on the other hand is the world’s tallest cable stayed bridge. The environmental deterioration, caused by the extensive automotive bottleneck in the iconic town of Millau during the summer months led to the construction of this state of the art structure. It connects Paris and Barcelona and has provided much relief for the citizens of the town of Millau, also cutting travel time between the two major cities by around 4 hours. The main features of both bridges are assessed as follows.

Civil Structures – Design and Construction

Parth Gulati Alex Dimakopoulos

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INTRODUCTION The essence of an arch is that ideally there should be no tendency for it to bend, except under live loads. It should be purely in compression, and for that reason it can be made of materials such as, masonry, cast iron and concrete, those that perform poorly in tension.

The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the center. A big difference between cantilever bridges and cable-stayed bridges is that the former usually have a suspended span, and the latter do not.

A cable stayed-bridge lacks the great rigidity of a trussed cantilever, and the continuous beam compensates for this to some extent.

This report discusses the contrasting features of two bridges: The Gladesville Bridge, which is an arch bridge, and the Millau Viaduct, which is a cable-stayed bridge. This report outlines the features of each of the two bridges and it is evident in the outlined characteristics and procedures of construction for both the components that distinguish one from the other, evaluations and impacts.

THE GLADESVILLE

BRIDGE

PURPOSE

The first Gladesville Bridge was built to accommodate for the increasing amount of people commuting goods across the Parramatta River by Ferry services or on punts. The new concrete bridge was designed to be part of North Western Express way that would have connected Sydney to Newcastle. There was a series of other bridges that were included in this expressway but although the bridges were built the actual expressway was never completed. The bridge that stands today is a replacement of the old iron truss bridge that could not handle the traffic demands of the time with traffic often coming to a stand still for hours. The old bridge carried cars, trams and also included a swing section to allow large ships and steamers passage along the river. The new bridge however only accommodates cars although its arch structure makes it easy for boats to pass underneath. The Bridge opened on the 2nd of October 1964.1

1 http://www.ozroads.com.au/NSW/Freeways/GladesvilleBr/gladesvillebr.htm


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In modern day context, Victoria Road (the road which the bridge carries) is one of Sydney’s busiest road transport corridors. Used by up to 200,000 bus passengers each week, it also accommodates over 75,000 vehicles per day and is heavily congested, particularly in Drummoyne and Rozelle in the morning peak. During a typical weekday morning peak, up to 170 buses with more than 8000 passengers travel between Gladesville Bridge, Drummoyne, and The Crescent at Rozelle. Therefore it was an important upgrade from the old bridge, and plays an important part in carrying several commuters to and from Sydney and the Inner West.

DISTINCT CHARACTERISTICS, AND DESIGN FEATURES

The Gladesville Bridge was, at the time of completion, the longest concrete arch bridge in the world. Some physical characteristics of the Bridge include:

- A 305m span

- The clearance is 40.7m above the water

- Total length of 488m

- Segmented concrete blocks make up the four arches

- Special inflatable rubber gaskets were placed in-between the concrete blocks, which aided construction when they were inflated and then filled with concrete to stabilize and strengthen the bridge (Abdunur 2011)

- 13 pairs of columns create a platform, along with the four arches, on which the roads rest.

- Rise to span ratio of 1:7.52

The design philosophy of the new Gladesville Bridge Project was to be a six-lane structure with ample footways on both sides of the roadway and it was to meet the following objectives:

• Navigation clearance above the rover of 36.5 m over a width of 61m to allow the passage of major pleasure craft and moderately sized shipping which then serviced an oil refinery upstream

• Traffic capacity and load carrying capability to ensure a service life of at least 100 years.

• Construction using minimum maintenance material to maximize durability and minimize maintenance costs over the theoretical 100-year life of the bridge.

PRINCIPAL FEATURES OF THE BRIDGE: • The roadway width is 22m, which

permits dual 3-lane traffic with no emergency stopping lanes or dual 2-lane traffic with full emergency stopping lanes. Raised footpaths on both sides of the bridge have clear widths of 1.85m

2 Abdunur, C. Arch'01:troisième Conférence internationale sur les ponts en arc Paris, 19-21 sept. 2001. Presses des Point, 2011.


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• The deck consists f 16 spans of 30 m, using precast, post-tensioned girders made continuous at deck level. Total length of the bridge between abutments is 580 meters and the longitudinal profile is made up from 6% approach slopes with a central transition curve over 91.4m.

CONSTRUCTION AND MATERIALS

USED

The Gladesville Bridge was seen to be stretching the limits of bridge design when Maunsell and Partners of London proposed the original idea to the contractors. Eugene Freyssinet, a famous bridge architect, approved their idea, also giving some important suggestions for the bridge design and construction processes. Construction began in 1959 and it took 6 years to complete. The first step in the lengthy construction process was to erect the four arches that would support the main roadway. This was accomplished by first excavating on both river banks and using cofferdams where used on either side of the river in order to excavated under the water, to allow for massive thrust blocks which would carry the concrete arch.

The second step was to use temporary framework to support individual, hollow, precast, concrete blocks, which would make up the arches. Temporary steel framework was erected across the river, with deep trusses leaving a 61 m opening for navigation. Each day 5 or 6 voussoir (a wedge shaped element used in building arches) units, up to 6.1 by 6.9 m, were hoisted from barges into position and concreted together, and the complete arch was then jacked away from the formwork. This process was known as centering and was an important concept in ensuring the successful and economical construction of the bridge.

Between each block inflatable gaskets were placed, a recommendation of Freyssinet’s who had used this method on previous bridges he had designed. These special gaskets where inflated with synthetic hydraulic fluid once all blocks where in place.

This lifted the blocks off the framework and made a self-supporting structure. The gaskets were then filled with concrete, which further stabilized and strengthened the arch. At 15-meter intervals ‘diaphragms’ are place instead of block units, this is designed to tie the final arch ribs together and support the columns on which the road would rest. When one arch was completed the framework was simply moved across along the river and the next arch was constructed. The process was repeated four times to create the four supporting arches of the bridge. In order to let the arch carry its own weight the removal of the temporary framework (the centering) was necessary. This process is known as decentering. The decentering of arch ribs was the most critical operation in the construction of the bridge.

The deck is made of precast, prestressed concrete and designed to be as light as possible, given its diaphragm structure, as it has to bear a number of point loads on the arch. The concrete used in the construction of the Gladesville Concrete Arch Structure comprised of 1 part sand, 1 part cement and 2.75 parts coarse aggregate (10 to 40 mm).


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Refer to the table for the strength profile that was provided when a mixture of the above concrete was used.

Strength at 28 days 50MPa

Minimum acceptable strength

42MPa

Actual mean strength 52MPa

Standard deviation 3.5MPa

Coefficient of variation

6.8%

The total cost of the bridge and approaches was $9 Million in 1964. In the 47 years since being opened to traffic, the bridge has performed exceptionally well, with only minimal maintenance required. Sealing of shrinkage cracks in the diaphragms within the arch ribs was carried out in 1971. In addition, there has been some distressing the fixed connections of the deck girders to the abutments and arch crown. Cracked concrete has been repaired and is being monitored.

The bridge is continuously being inspected fully at 5-year intervals, including internal inspection of the arch ribs. The bridge is performing adequately under current legal loads and permit overloads with a satisfactory reserve of strength. The National Trust of Australia classified the bridge as a heritage item in 1990.

THE MILLAU VIADUCT

PURPOSE The Millau Bridge or Viaduct as it should technically be known as provides the final missing link in the A75 AutoRoute ultimately connecting Paris to Barcelona. Prior the viaducts construction traffic would have had to descend the Tarn Valley causing a bottleneck in the town of Millau especially during the summer months of July and August.

DISTINCT CHARACTERISTICS, AND DESIGN FEATURES

The multi-span cable stayed bridge passes over the Tarn valley at its lowest point between two plateaus. In order to do this it had to become the tallest road bridge in the world creating the world’s tallest bridge piers standing at 242m, the structure rising to 343m at the top of the pylon. The bridge


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also holds the title of the world’s longest multi-span cable stayed bridge with a total length of 2460m. There is a slight gradient of 3% from North to South as well as a slight curve about a radius of 20,000m.

The bridge takes the form of a multi-span cable stayed bridge. Having multiple spans there are no backstays as with most cables stayed bridges to anchor the pylons to a rigid support. Instead adverse loads on one span directly interact with the next as the pylons bend to accommodate this

Aesthetics was also a major concern when construction and design were signed, and therefore many aspects of the bridges features have been fabricated and structured such that their aesthetic appeal remains peak, and blends in with the natural environment that the bridge has been built in. By choosing to have a cable-stayed bridge the serious consideration that had to be given to make the bridge look as natural as possible, was accomplished. The morning mist and low-lying cloud hide the concrete piers and gives the impression that the deck is delicately floating on them. This natural feature would have been noted and taken into consideration in the design to produce this effect. The apparent thickness added to the deck by the windshield helps to enhance this.

Usually when crossing a valley it is good to keep the aspect ratio of rectangles between ground, piers and deck constant. However this hasn’t been done on Millau and with good reason. With differing spans between the piers there would have to be different numbers of cables supporting the deck between each pier, which would not look good. Also the pylons may also have to differ in height if constant spacing of cable anchorage at the deck is to occur.

When looking longitudinally along the bridge in the presence of sunlight it appears that the piers are of equal thickness to the pylons. The piers have been deliberately made hexagonal in shape to produce this effect with the leading face of the hexagon reflecting the sunlight and the other sides in

shadow. All measures mentioned above help to reflect the idea that the surrounding landscape is very much untouched and itself delicate.

BASIC LOADS APPLIED TO BRIDGES

WHICH WERE CONSIDERED BEFORE

CONSTRUCTION INCLUDE: • Dead loads such as the deck and

the cables • Super imposed dead loads such as

crash barriers and concrete/black top surfacing

• Live traffic loads (The Bridge currently has two lanes of traffic and a narrow hard shoulder in each direction. The total width of carriageway is 23.3 meters including the steel crash barriers on the outside. Factor of safety requires 6 lanes of appropriate lengths to be considered.)

As well as these, the geometry and design of the bridge leads to other loads and effects that needed to be considered. The constant curvature introduces horizontal centrifugal loading and the single plane of cables requires consideration to be given to torsion effect.

KINETIC LOADS CONSIDERED FROM

TRAFFIC; FIGURES INCLUDED: • Crawlers which wheeled the pylons

onto location: 8MN(4MN + 4MN with a crawler located on each side of the pylon)

• According to standards used to build the bridge, total loading per heaviest vehicle on bridge was taken to be equivalent to 1.8 MN spread over 4 axles each consisting of 4 wheels, i.e. 112.5kN per wheel point loading.

• Secondary live traffic loading considered separately

• Centrifugal loading generated by the curvature of the bridge, which was calculated to be insignificant given the precise make and large


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radius (about 20000 km) of the arch, formed. (1.49kN)

• Braking from trucks can cause horizontal loading on the deck with a force of 8kN/m being assumed as acting along one notional lane.

• Accidental skidding from vehicles considered as causing a point load of 250kN.3

• Collision with the steel parapets on the outside and concrete ones on the inside4

The height of the bridge introduced another collision loading in the form of impact from aircraft but each pylon is placed far away from another to hold its own weight, and given the unlikelihood of such an event, the consideration was dismissed.

The deck was designed and fabricated such that it could resist the subjected wind loading after advanced computer model tests in conjunction with wind tunnel results.

Given the length of the deck at 2460 m, temperature loading was another big consideration while constructing and designing the Millau Viaduct.

As far as instrumentation is concerned, the viaduct is state of the art. The pylons, deck, masts and stays are equipped with a multitude of sensors. These are designed to detect the slightest movement in the viaduct and measure its resistance to wear-and-tear over time. Anemometers, accelerometers, inclinometers, temperature sensors are all used for the instrumentation network.

Twelve fiber optic extensometers are installed in the base of the second pylon, which being the highest is also under the most intense stress. These sensors detect movements on the order of a micrometer.

3 http://en.structurae.de/structures/data/index.cfm?id=s0000351

4 Leonhardt, F, Cable Stayed Bridges with Precast Concrete, PCI journal, special report.

Displacements of the deck on the abutment level are measured to the nearest millimeter. The stays are also instrumented, and their ageing meticulously analyzed. Additionally, two piezoelectric sensors gather traffic data: weight of vehicles, average speed, density of the flow of traffic, etc. This system can distinguish between fourteen different types of vehicle.5

CONSTRUCTION AND MATERIALS

USED Constructing the worlds tallest road bridge was always going to be extremely difficult. There are traditionally two methods used for constructing cable stayed bridges, incremental launching and cantilever construction. Working at such height poses significant risks as well as the cost involved in lifting sections of the deck over 200m. The design of the bridge also deems this method inappropriate, as a single pier and pylon cantilevering deck from either side would be very unstable and susceptible to wind.

The decision was therefore made to launch the deck incrementally which itself posed many risks. A launch of this size had never been undertaken before and new technologies had to be developed to slide the deck out into position.6

Firstly the foundations for the piers had to be constructed. The concrete for these and the piers was produced in newly built plants close to site to minimize transportation costs.

5 http://en.wikipedia.org/wiki/Millau_Viaduct

6 Virlogeux, M, 1999. Recent Evolution of Cable Stayed Structures, Engineering Structures, Vol 21, pp. 737-755


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In order to reduce construction time emphasis was initially placed on the piers closest to the abutments so the launch of the deck could commence whilst the remaining piers were still being constructed. The formwork for the piers was a revolutionary self-climbing device using hydraulics.

The piers are of post-tensioned reinforced concrete and the deck and pylons are of steel. The piers have been given a rough finish and the deck and pylons a smooth one. The texture helps distinguish between materials used with the concrete left untouched and the steel given a glossy white finish. There is no structural confusion in how the bridge works; as it is clear what role each component has to play. Concrete piers support the deck at 342m intervals (204m at end spans). Cables attached provide additional support to steel pylons located above the concrete piers. The cables are of a semi fan arrangement whereby they are fixed at equal distance over a certain area of the pylon, neither all at the top nor equally spaced over the entire height of the pylon.

Each pylon of the viaduct is equipped with a monoaxial layer of eleven pairs of stays

laid face to face. The stays were made of high tensile steel cables, or strands, themselves formed of seven strands of steel (a central strand with six intertwined strands). Each strand has triple protection against corrosion (galvanisation, a coating of petroleum wax and an extruded polyethylene sheath). The exterior envelope of the stays is itself coated along its entire length with a double helical weather-strip. The idea is to avoid running water, which, in high winds, could cause vibration in the stays and compromise the stability of the viaduct.7

The steel deck was fabricated offsite. The deck was transported to site in sections by road. This resulted in high cost, as over 2000 police escorted convoys were required to transport them.

When considering that concrete plants where required onsite for the piers and foundations it seems that the sensible thing to do would have been to use a concrete deck. However various other factors are likely to have been taken into account when deciding this. Considering environmental impacts the emissions caused using either material are high. This is taking into account the general rule of thumb that for every tonne of concrete produced a tonne of carbon dioxide is released into the atmosphere. There are obvious effects associated with the long distance transportation of the steel deck. The high cost associated with the transportation of the steel deck would also somewhat correspond to the cost of pre-stressing the steel deck prior to launch. The overall time taken to construct a concrete deck is likely to be longer than for a steel deck. This is because the concrete will need to be at construction grade i.e. left to set for 28 days prior to launching the deck. With a steel deck the prefabricated deck sections can be welded together relatively quickly.8

7 http://en.wikipedia.org/wiki/Millau_Viaduct

8 http://www.bath.ac.uk/ace/uploads/StudentProjects/B


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The launching of the deck was undertaken essentially using a lift and push system which occurred all in one motion. This system was especially developed for the launch, as due to the magnitude of the task, no such device existed capable of doing the job. 9

Also, with the extensive amounts of concrete used in the construction process, one of the most important loads to be considered was the creep of the concrete. Advanced GPS placement techniques used in the construction of the piers to ensure that accuracy was maintained and damage caused by the creep of concrete, which is inevitable but reducible, was minimized.

With the deck complete the remaining pylons were erected. These were each transported onto the deck in a horizontal position using crawlers. Once in position they were raised and connected to the deck. The final stage of construction was to dismantle the temporary supports. 10

The bridge's construction cost up to €394 million. The builders, Eiffage, financed the construction in return for a concession to collect the tolls for 75 years, until 2080. However, if the concession is very profitable, the French government can assume control of the bridge in 2044.

The project required about 127,000 m3 of concrete, 19,000 tonnes of steel for the reinforced concrete and 5,000 tonnes of pre-stressed steel for the cables and shrouds.

ridgeconference2007/conference/mainpage/Saxton_Millau.pdf

9 Enerpac Launching system http://www.enerpac.com/html/Projects/Millau/Millau_Launching_Systems.html.

10 http://www.bath.ac.uk/ace/uploads/StudentProjects/Bridgeconference2007/conference/mainpage/Saxton_Millau.pdf

CONCLUSION After several years of service in an aggressive near the coastal zone of the city of Sydney, the Gladesville Bridge is still in excellent structural condition and is likely to continue to serve its purpose well beyond the projected life span of 100 years. The lifetime of the Millau Viaduct on the other hand is projected to be around 120 years. In terms of its modernity, it certainly defeats the Gladesville Bridge, given the most advanced technological implementations employed to build the Viaduct. However, if put in context, and viewed from an objective perspective, the Gladesville Bridge was built 47 years ago, and taking into account the technology of the time, has been able to provide a projected life span of 100 years, portraying exceptional engineering properties and value. The Millau Viaduct is young in that comparison, and it is definite that it too, like Gladesville, will surpass its projected lifespan. Both bridges, therefore in their own right surpassed several touchstones of their time, and are wondrous civil structures.


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BIBLIOGRAPHY Unknown. Gladesville Bridge. http://users.tpg.com.au/dougfam/gladesville_1.html (accessed November 22, 2011).

Wikipedia. Gladesville Bridge, Sydney. http://en.wikipedia.org/wiki/Gladesville_Bridge (accessed November 22, 2011).

Various. Millau Viaduct. http://en.structurae.de/structures/data/index.cfm?id=s0000351 (accessed November 20, 2011).

—. Millau Viaduct. http://en.wikipedia.org/wiki/Millau_Viaduct (accessed November 22, 2011).

Australian Government. The New Gadesville Bridge. http://aso.gov.au/titles/sponsored-films/the-new-gladesville-bridge/clip1/ (accessed November 22, 2011).

Abdunur, C. Arch'01:troisième Conférence internationale sur les ponts en arc Paris, 19-21 sept. 2001. Presses des Point, 2011.

Brown, David J. Bridges: Three thousand years of defying nature. MBI Publishing Company, 2011.

—. David J. Brown. MBI Publishing Company, 2011.

N/a. http://en.structurae.de/structures/data/index.cfm?id=s0000488 . (accessed November 20, 2011).

Mahmoud, Khaled M. Recent Developments in bridge engineering. Taylor & Francis, 2003.

—. Recent Developmentsin bridge engineering. Taylor & Francis, 2003.

Sydney Architecture. Gladesville Bridge. http://www.sydneyarchitecture.com/NOR/NOR09.htm (accessed November 21, 2011).

Saxton, J L. Report on the Millau Viaduct. April 2007, 2007.

comparison report - gladesville vs millau

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