The Sydney Harbour Bridge is one of Sydney's best-loved icons. But we can take for granted the enormous engineering feat that created the bridge. I was reminded of this yesterday when our colleagues at State Records NSW tweeted an 'on this day' fact:  On this day - 19 August - in 1930 the two halves of the Sydney Harbour Bridge were joined at 10pm. Lawrence Ennis, the director of construction for Dorman Long, the fabricators, recalled there was a gale where there was just 1m between the two arches. It looked like the two arches were swinging wildly, but there only moved about 3 inches (7.6 cm). He went on to describe how the arches came together:

The first closure was effected at 4.15 pm in the afternoon of the 19th August 1930, but there was a subsequently slight opening with the contraction in the cool of the evening. Slacking of the cables was continued without intermission, and the final closure was made at 10pm the same day. Next morning the Union Jack was flown from the jib of one creeper crane, and the Australian Ensign from the other, to signify to the City that the arch had successfully closed. We felt that the arch had become not only a link between the two shores of a beautiful Harbour, but a further bond of Empire. Quoted in Peter Spearritt, The Sydney Harbour Bridge: A Life, UNSW Press, 2007, p 65

The two half arches were gradually fabricated from steel in workshops before being loaded onto barges and towed into position. The bits of the arches were then lifted up by two 580 tonne electrically operated creeper cranes. As the part-arches reached over the harbour, cables were continually re-tensioned to allow for the increasing weight of the structure they were holding, until the arches met. Steel decking was then hung from the arches over the next nine months. State Records have two terrific photos of the joining of the bridge here and here. The progress of building the harbour bridge was something few Sydneysiders could ignore. The technical details of how it was being built was explained and illustrated in all the newspapers and magazines of the day.  In fact, the Sydney Harbour Bridge is probably one of the most documented pieces of public infrastructure built in Sydney in the twentieth century.  State Records has about 2,000 photographs related to the building of the Sydney Harbour Bridge digitised and catalogued. They have created a photo montage of the building of the bridge and they also have a stunning selection of photographs on flickr. The Reverend Frank Cash, rector of Christ Church Lavender Bay, North Sydney, was a keen photographer and in the perfect position to document the bridge. He took hundreds of photographs and self-published Parables of the Sydney Harbour Bridge in 1930. And I'm sure you've got a photograph of the bridge somewhere in your family albums too. Bridge facts:

• The arch spans 503m
• The top of the arch is 134m above sea level
• Clearance for shipping 49 metres
• Height of the pylons 89 metres about mean sea level
• number of rivets  approx 6,000,000
• weight of the arch 39,000 tonnes

Lisa and Mitch talk all things Sydney each Wednesday morning  from the Dictionary of Sydney on 2SER Breakfast, 107.3 at 8:20am. You can catch up on this morning's segment here.