What is the upper arch of the Sydney Harbour Bridge for?

[andrewkirk]

I was looking at the Sydney Harbour Bridge the other day, trying to work out how the weight of the roadway was transmitted to the ground. http://www.exploreaustralia.net.au/New-South-Wales/Sydney/The-Rocks/Dawes-Point/Sydney-Harbour-Bridge.

You can see that there is an inner (lower) and outer (upper) arch. From underneath the bridge I saw that the lower arch abuts against the big granite pylons. So I concluded that the lower arch supports the weight of the road, via the vertical girders from the lower arch down to the roadway.

But I couldn't work out what the upper arch is for. It just ends without abutting against anything massive. What does it do? Could the bridge have been built just as effectively without the upper arch?

 

[russ_watters]

The upper arch and lower arch work together to form one big arch, that supports the bridge. Consider a rod or beam: a narrow rod/beam is easy to break, but a thicker one is stronger, even if it is hollow and contains the same amount of material. That's why buildings are held-up with I-beams instead of just smaller solid, square beams. The height of the beam makes it more rigid. This concept is described by moment of inertia: https://en.wikipedia.org/wiki/Moment_of_inertia

 

[Baluncore]

From underneath the bridge I saw that the lower arch abuts against the big granite pylons.

No. The tower structures at the ends of the arch are decorative, not structural. The mass and compressive forces of the bridge are carried by the lower chords on sandstone well below ground level. During construction, a loop of many steel cables passed from one side of the bridge, down a tunnel, around the rock pillar and back up another tunnel to the other side of the bridge. Tension in those cables supported both half-arches until they were finished. The cables were then adjusted to lower the two bridge halves a few inches into position for final joining. The cables then went off to build a couple of more bridges, in USA? then Africa.
http://sydney-harbour-bridge.bos.nsw.edu.au/engineering-studies/steel-structure.php

 

[Baluncore]

But I couldn't work out what the upper arch is for. It just ends without abutting against anything massive. What does it do? Could the bridge have been built just as effectively without the upper arch?

The upper arch was attached during construction to the support cables that passed down into the rock. They were essential tension members.
The upper arch was also used as the rails for the creeper crane that assembled the arches ahead of it's position.
The bridge deck was then hung from the arch, beginning at the centre, by the creeper cranes as they came back down.
The upper arch is now the upper chord of the arched truss that supports the bridge deck.

 

[andrewkirk]

No. The tower structures at the ends of the arch are decorative, not structural. The mass and compressive forces of the bridge are carried by the lower chords on sandstone well below ground level

I understand the towers that project above the bridge are decorative, but when I went on the weekend to look at the pins where the Southern arch end abutted a big granite structure I'm pretty sure that was against the lower part of the towers. The level of the pins is a few metres above head height as one stands on Hickson Road.

Perhaps the towers were built on top of the abutments, and finished so that there is not a clear dividing line. I'll have another close look next time I'm there

 

[Baluncore]

Perhaps the towers were built on top of the abutments, and finished so that there is not a clear dividing line.

The entire bridge was present before the towers were built. The tension cables and creeper cranes ran through where the towers are now placed. The abutments are made from concrete that is seated on sandstone. The towers stand above and behind the abutments.

The psychology of bridge users is important. Notice how most bridges were built with a very slight arch of the roadway. Although the arch does give a little more clearance below the bridge, it is not present for structural reasons. It is there to reassure users that the bridge is safe. Likewise, the towers make the bridge seem more solid and reliable. The towers are made from concrete but are clad in granite. Your initial focus on the towers demonstrates that they are still performing their psychological function well.

Bridge assembly and bridge operation are separate phases in the life of a bridge. Some artefacts of construction will remain during operation, others will be removed, such as the tension cables and creeper cranes. The aesthetic "granite" towers were never part of the original engineering design and calculations. They were added to the plans later because it made the bridge look better.