The stars finally aligned and I had the opportunity to watch the skyrail machine to see how it actually erects a span. It's b*y impressive.
Imagine you have four piers A, B, C, and D. The viaduct is completed beyond D back to Murrumbeena where the beams are assembled. The task is to place the next beam between Piers C and D. There are two machines: a straddle crane which carts the assembled beams from Murrumbeena to the piers, and a carrying beam which temporarily spans the piers.
The carrying beam is massive; it's about 2 metres wide, by about 3 metres deep, and about 110 metres long. At the start of our cycle, the beam spans piers B, C, and D, with about an 11 metre extension past Pier D. Supporting the carrying beam are three adjustable supports, two on Pier B and one on Pier C, and a support on the permanent viaduct at Pier D. Each adjustable support occupies half of the space on the top of a pier. The carrying beam itself rests on rollers on the top of each support. Under that are two hand operated vertical jacks. Under that are two hydraulic horizontal pistons. The jacks and the pistons allow the vertical and horizontal position of the carrying beam to be precisely adjusted. On top of the carrying beam is a cart. It seems small, but is actually about 10 metres long. It can run along the top of the beam on *very* heavy duty rollers. At the start of the cycle it is situated on the extension of the beam past Pier D - over the completed part of the viaduct.
The straddle crane is also enormous - it can carry a complete permanent beam which is about 45 metres long. I was told the permanent beam and straddle crane together weigh 400 - 500 tonnes, but that's unconfirmed. The straddle crane has two bogies - one at each end of the beam. Each bogie rides on eight massive pneumatic tyres, arranged in two rows of four.
When the straddle crane arrives at Pier D it moves forward so that the front bogie is over the cart on top of the beam (which, remember stuck out about 11 metres beyond Pier D over the completed viaduct). The front bogie is then attached to the cart (I know not know, but disconnecting it involved pneumatic tools).
The front bogie of the straddle crane, on the cart, is then moved along the carrying beam until it is beyond Pier C. The wheels of the front bogie at this point are dangling in mid air. A most impressive sight. At the end of this, the rear bogie is at the end of the completed section of the viaduct and the beam the straddle crane is carrying is above its final position. But the carrying beam is in the way.
The next step is to slide the carrying beam forward so that it spans Piers A, B, and C, leaving a gap between Piers C and D for the permanent beam. Remember that while it is being moved, it is supporting the front bogie of the straddle crane and half the weight of the permanent beam. The creaking of the beam as it is rolled forward is impressive. As the carrying beam is being moved forward, it carries with it one of the adjustable supports on Pier B. When the front of the beam gets to Pier A, this support is located over the front half of the pier. The carrying beam is then carefully aligned.
With the carrying beam out of the way, the permanent beam can be lowered onto Piers C and D and fixed in place.
The carrying beam is then moved back about 11 metres, so that the front bogie of the straddle crane is on newly added permanent beam. The front bogie is then disconnected from the cart, and the straddle crane returns to Murrumbeena to pick up the next beam (which will be used to extend the other viaduct).
The final step is to move the supports up. At the end of placing the beam, the supports are at Piers A, B, and D. The supports have upper rollers that roll along the top of lower flange of the carrying beam. The jacks on the support at D are lowered; this actually raises the feet of the support off the pier. It then rolls along the lower flange of the carrying beam to Pier C where it is located. The existing support at Pier C is similarly relocated to Pier A.
And the cycle starts again.