Today tourists are enjoying a beautiful climb on Sydney Harbor Arch Bridge. However around 90 years back there was a big challenging day for the workers of Sydney Harbor Arch Bridge. The joining of two half arches. They braved the elements and riveted The Arches together on the same day; otherwise, they might not have had another chance.
They built a sturdy arch bridge that can withstand the weight of trains. Have you seen the lovely curve in this bridge? The intriguing thing is that if they had planned the bridge in a circular shape, it would have collapsed catastrophically.
Obviously, a long road deck cannot be supported in this manner. This is why Arch Bridges utilise such massive support structures. The road deck is supported by hangers on the support structure. But what ensures that the support structure will stand strong without collapsing?
We need a strong self-supporting structure. Remember that the support structure is only supported at the ends, not along its entire length. This would impede the movement of the ships below. Here’s the thought-provoking question out of these shapes which one is the best self-supporting structure?
Very soon we will find out the answer for this logically with the help of an interesting King Kong example. Now let’s find out the answer using a simple experiment. When we apply a weight to the circular wood block Arrangement it just collapses. However if you place weight on a parabolic shape it stands strong.
In short, the best self-supporting structure is the parabolic arch. Now it’s the engineer’s duty to convert this Parabola to a practical structure.
Welcome to the Sydney Harbour Arch Bridge Construction Innovations.
The Beautiful Engineering behind the Arch Bridges
The construction developed as half arches from both ends until they were linked in the middle. Let’s observe the construction phase more closely. Can you find something wrong here in below image? If the bridge is built in this manner, the arch will collapse due to gravity pull.
Please note the arch is supported only at the bottom points. To address this issue, several support cables were dug into the rock near the half Arch’s end. A fleet of creep and floating cranes were used to execute this construction feat. Individual Arch members were transported on a pontoon boat from a workshop. They were later hoisted and erected with the help of creep cranes.
The arch members were first joined using bolts. After adjusting the final angle in position rivets were used to secure the entire structure. The bridge is held together by around 6 million rivets. The weight of this Arch is around 38,000 tons. Even after all the structures were erected The Arches had a gap of three feet between them. Now comes the trick just release the support cables of the Arches one by one, their own weight causes the Arches to slowly move down and eventually touch each other.
This impressive arch spans 1654 feet and stands 440 feet from water level. Now it’s time to prove logically why the parabolic shape is the best self-supporting structure.
Why is the parabola the best self-supporting structure?
Let’s string a massive chain across the New River Gorge’s mountains. A parabola is formed by this chain. In fact the true shape of a freely hanging chain is a cadenary. Even if you try to hold it in a different shape, it will not stay there.
A chain can support only a normal tension force. Now the tricky part. Let’s solidify this parabolic chain. Now let’s flip the direction of the gravitational force. Because the external force has flipped its direction, the internal stress will also revert to a purely compressive state. Now just flip this entire arrangement.
Wow it becomes an exact Arch Bridge. The compressive Force developed by this parabolic structure is exactly at the middle of the section. Such structures are always stable. You can see why a structure may not be stable when the forces do not have this quality. This is why the parabolic arch is the best self-supporting structure.
Now let’s construct a road deck below this Arch. However there is one more way to achieve the same objective. Just shift the supporting Arch down. Here again the force induced in the arch is the same. If you were the chief engineer in charge of this project which design would you choose?
Obviously the above deck design right. The reason is simple you will save a ton on construction materials. Does this mean that the engineers must always choose the above deck design?
Let’s try this design in the Sydney Harbor Bridge location.
The Sydney Harbor Arch Bridge
As you can see we quickly run into an issue. There’s an insufficient amount of space between the road deck and the river for the arch. Shall we try an underwater Arc design? No such a design would block the ship movement of a huge area. That’s why in this location the only design possible was the road deck below design.
Here’s an interesting question for you do you think it’s possible to construct a road deck that is completely below the arch?
There is an issue with that the arch must be connected with the ground. How can we modify this design to achieve that? Only by extending a part of the arch below the road deck can it be connected to the ground. This is precisely what Engineers did with the Sydney Harbor Bridge. Such bridges are called through Arch Bridges.
The most suitable type of Arch Bridge to be constructed depends entirely on the geography of the location. Did you notice these enormous bearing hinges at the bottom of the bridge?
Interestingly during temperature change these hinges allow the arch to change angle as shown. If there’s a temperature increase the length of the arch must also increase.Because of the bearings, the bridge can simply modify its angle to meet the length adjustment. The engineers cleverly avoided the issue of thermal stress in the bridge in this manner.
You’ll be amazed to know that the Sydney Bridge Rises by almost 7 inches during summer time. Along with this Arch expansion the hangers also expand due to the temperature changes. This ensures that the road deck’s position is not compromised.
In this illustration below you can clearly see how this enormous hinge is fitted with the ground as well as the details of the foundation. We already know how they built the arch. One clever construction trick they played here is that they constructed one side of the arch faster than the other.
The concept was that if a mistake was made in the design or construction of the arch, it could be fixed on the other side. It’s now time to connect the hangers. The v-shaped end of these hangers was the first to be affixed to the arch construction.
Let’s have a look at how the steel workers join the deck’s cross girders to the other end of the hangers. They began assembling the road deck from the centre and worked their way towards the ends to achieve balance. The road deck structure does not actually contact the main Arch structure, as can be seen.
This clearance ensures that the road deck is unaffected when the arch changes shape with temperature fluctuations. The total width of this 12-lane deck is a whopping 160 feet. The road deck has four railway tracks six lanes for road traffic and two lanes for pedestrians.
The cool thing is that The Pedestrian lands are positioned outside the arch. Upon completion of the bridge 96 locomotives were positioned in different ways on four lines to test it. In 1958 two of the railway tracks were converted into roadways.
Now a question arises. Why didn’t they go with the suspension bridge in Sydney Harbour, which is less expensive and easier to build? Suspension bridges waver all the time. When the bridge must support a big load, such as a goods train, an arch bridge design is required. The arch bridges are more rugged. The under construction Chennai bridge in India is one of the best examples of it. It’s also the world’s tallest rail Bridge.
The load acting on the bridge structure in this case the weight of the train has a big impact on the internal stress developed and stability of the bridge. The previous research simply examined the bridge’s self-weight.
we already saw how the internal stresses flow perfectly along the Body Center when the shape is parabolic. However when there is an external load on the arch the internal forces deviate from the center line even though the arch shape is a parabola.
We can create a line of thrust by connecting the sites of these internal forces. For good stability of the arch bridges the engineers make sure that even unde extreme load conditions the line of thrust remains in the middle third of the arch section.
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