Saturday, 6 September 2014

Sydney Harbour Bridge




What is your structure? – describe it
The structure I have chosen to study for my case study is the Sydney Harbour Bridge. This is the bridge in Sydney that connects the two sides of the harbour.

Does it have a name?
Sydney Harbour Bridge

Where is your structure located?
Sydney

How old is your structure?
The Sydney Harbour Bridge construction started in 1924 and took 1,400 men eight years to build making it 82 years old.

What is your structure used for? What is its purpose? Why was it built?
This structure is used to carry passengers from one side of the Harbour to the other; it carries eight vehicle lanes, two train lines, a footway and a cycleway. In 1989 this bridge supported 182,024 vehicles / day.

Why have you chosen this structure – what interests you about it?
Sydney Harbour Bridge is the world's largest (but not longest) steel arch bridge. And it’s here in Australia.

Challenges faced when building this structure?
This project took approximately 6 million hand driven rivets this alone is an extraordinary feat in its self. Another major challenge was the lack of safety involved when building the bridge as 16 men lost their lives whilst constructing the bridge mostly due to falls.

Civil Engineer involved in the project


DR Bradfield (Australian Dictionary of Biography, 1979)
One of the civil engineers that worked on the Sydney Harbour bridge was Dr Bradfield; He was the chief engineer of the whole project. Dr Bradfield had a very impressive academic record ‘Dux of his school, he won a Queensland government university exhibition and in 1886 matriculated at the University of Sydney. From St Andrew's College, he continued his brilliant academic career, graduating B.E. with the University Gold Medal in 1889’ (Australian Dictionary of Biography, 1979).
After these great achievements Dr Bradfield didn’t slow down, he then went on to work as a draftsman in Brisbane and then after short amount of time he moved to the New South Wales Department of Public Works as a temporary draftsman ‘From May, Bradfield worked as a draftsman under the chief engineer, railways, in Brisbane. On 28 May 1891 at St John's Pro-Cathedral he married Edith Jenkins. That year he was retrenched and joined the New South Wales Department of Public Works as a temporary draftsman, becoming permanent in 1895. An associate from 1893 of the Institution of Civil Engineers, London, he graduated M.E. with first-class honours and the University Medal in 1896. He had been a founder of the Sydney University Engineering Society in 1895 and was president in 1902-03 and 1919-20. In his 1903 presidential address he drew attention to the competition, initiated in 1900, for the design of a bridge across Sydney Harbour; there had been agitation for a bridge or tunnel since the 1880s’ (Australian Dictionary of Biography, 1979).
Having shown a great deal of interest in the Design of the Sydney Harbour Bridge Dr Bradfield went on to become the chief engineer in the designing and constructing of the Sydney harbour bridge ‘Bradfield was [...] for over 30 years was the most active and influential person in promoting and overseeing construction of the Harbour Bridge. The Bridge was part of his grand vision for the electrification of the suburban railway network with a new electric train terminal at Sydney Central station and the city underground railway’(Board of Studies NSW, 2009).


Construction materials

The materials used when constructing the Sydney Harbour bridge were concrete and steel. As far as C02 release goes these materials are up there with the worst. Steel requires a lot of energy to get the iron out of the ground, crushed then refined to get rid of impurities and then these even more C02 IS released when the iron is heated and shaped to suit the project it is intended to be used for. Then there’s concrete, concrete releases a lot of C02 this is due to the fact that it requires energy to create the cement and the cement the releases C02 when it’s being formed, furthermore this concrete then needs to be mixed which also requires more energy.
The reason why these materials were chosen over more economically sustainable ones was for multiple reasons. Firstly the steel was chosen over materials such as wood because woods strength is nowhere near as reliable as steel, they would have had to use much larger quantities of wood to get a similar sort of strength, they would have had to use steel brackets at the joints regardless so steel would have been required anyway and lastly steel will last a lot longer than wood and when the project was decided to be this big they didn’t want to use a material that would have to be replaced in the near future.
The reason why concrete was chosen was there isn’t really any other viable option as no other material can withstand such huge amounts of compression, can be shaped as easily and it was a material that was easy to obtain large amounts of.
There were several ideas implemented to the Sydney Harbour bridge that were reasonably innovative, for example the cement that was used for the crossing is not just normal cement as they thought this would be to heavy ‘In the case of the deck structure of the Sydney Harbour Bridge, weight saving was obtained by using a special form of concrete which had a lower, though acceptable, strength than traditional concrete’ (Board of Studies NSW, 2012). This ‘coke cement’ was expected to lower the weight of the bridges crossing surface by up to 44% if chosen over normal concrete.
Another innovative material used was silicone steel. This material was used for the main structure of the bridge to increase strength ‘The steel used by Dorman, Long & Co. to construct the bridge was therefore higher in carbon content than mild steel making it stronger and tougher by having a greater proportion of Pearlite present. Added to that, their steel had relatively high quantities of Silicon and Manganese that increased the strength of the Ferrite in the microstructure. These facts account for the fact that their silicon steel has an average yield strength 1.3 times that of the average yield strength of mild steel’(Board of Studies NSW, 2009).


Green Building Design
The Green star organisation was obviously not around when this structure was being built as the structure is about 82 years old and the green star organisation was only launched in 2002. The green star organisation is an organisation dedicated to increasing sustainable building in Australia. Green building are extremely beneficial to make, green building usually cost more than a normal building to make but over a long period of time the green building takes less energy to maintain, for example green building use a lot of natural light meaning you save on electricity, they also use recycled water where they can, to lower water costs. Because of these reasons tenants are more attracted to green building as they are cheaper to live in. The green star system does not really apply to the Sydney Harbour Bridge as a lot of the criteria for obtaining green starts have to do with its ability to recycle water and how much natural light it uses where these thing are not applicable to a bridge. There are some aspects of the green star scheme that do however apply to the Bridge for example use of recycled material. The Sydney Harbour Bridge was made from predominantly steel imported from England. One way to increase its green star rating would have been to source recycled steel from Australia. Another place where the Bridge could have obtained a higher green star rating is use of innovative material. If they had the research we have today they could have used a different type of cement for the crossing, something that was still strong but also used recyclable material like rubber cement. Also another place there could have been improvements is the rivets, there was 6,000,000 rivets used in the creation on the bridge where I think with today’s technology they would use more welding and less rivets.


Structure loads and load paths
The dead loads of the bridge would simply be self-weight of the structure this is made up of the bridges arc, cables, steel frame, cement crossing and its rivets. These are regarded as the bridges dead weight as these weights are constantly applying a force to the bridges supports.
The live loads that act on the bridge are predominantly cars. The bridge has eight vehicle lanes, two train lines, a footway and a cycleway this means there can be a number of live loads acting on this structure at the same time. This bridge has been made to distribute all of the weight acting on the crossing threw all the steel columns to its huge steel frame and then down through its support columns.
The main environmental load this bridge had to withstand was wind load. Due to the Sydney Harbour Bridge’s large span and huge steel arch this bridge dose catch a bit of wind. When constructing the supports where all the bridges loads where going to be dispersed they had to make sure the footings could support the live, dead and environmental loads of this bridge, this is why 95,000 cubic meters of concrete was used for the supports of this bridge. We can see from the photo to the right that the steel frame arch doesn’t have anything between the frame, this would mean the wing affecting the bridge would be lowered. If the arch were to be covered over by a solid cladding more precautionary steps would have had to be taken to make sure this bridge was going to be safe.





The way this bridge transfers dead/live and environmental loads to the ground is the loads act upon the crossing (shown above in blue), this crossing begins to bend under the weight of the loads. When the crossing begins to bend this causes the columns (green) to be in tension and take the weight of the crossing. As the columns get pulled this in turn pulls down the steel frame structure (black). From here the steel frame structure distributes the weight along its internal structure until it gets to the cement columns (red). From here the cement columns which are buried deep into the ground take this weight and transfer it to the ground.



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