UKC

Any Civil Engineers on here? Un-civil ones need not reply. I have exhausted the usual search engines so I have come to the authority on everything. Today the last section of the new Forth Road Bridge was lifted into place. From the footage it appears that there was still a gap of 10-20 cms when it was at the right height. So what happened next - how do they close that gap and fix the new section in place?

In reply to keith-ratcliffe:

Duct tape.

In reply to keith-ratcliffe:

Im not a civil engineer but i suspect that could be an expansion gap of some sort and may be filled with a flexible caulk

just a guess though

In reply to keith-ratcliffe:

When the second severn crossing was being built there was a massive gap both horizontal and vertical when the last section was offered up. Apparently it was an issue with having all the weight of the construction machinery lifting the section into place on one side so they had to do things with ballast to get things to come together.

In reply to keith-ratcliffe:
The whole deck of the bridge is hanging from the towers, and where it passes them it is pinned in such a way that it can't move sideways too much in the wind, but can still move along it's length. At either end are big expansion joints (I'm pretty sure there are only two on the new bridge), which allow the ends of the deck to move without making gaps. Before they lift the last deck section, they use some huge winches and cables to pull the gap open wider, shoving both halves of the deck back onto the expansion joints. Then lift the deck segment on a nice calm day - 20cm isn't much clearance! They'll join it onto one side, then on another nice calm day they'll pay out the cables until the gap closes, and join it up.

At least I think that's how it works... I'm neither civil nor an engineer :-P.

In reply to WildCamper:

Well it won't be caulk, that much I can guarantee. If it is a movement joint to go in the gap, I imaging it'll be a very heavy duty, stainless steel 'toothed' element able to cater for both expansion and contraction. They probably have several along the length of the entire bridge.

JLS should hopefully make an appearance on this thread and clarify the issue.

In reply to keith-ratcliffe:


Polyfilla shares ! BUY BUY BUY !

In reply to keith-ratcliffe:

semi-educated guess:
the gap that you see on the footage is closed by pulling the final section onto that end of the deck; you can see the bolt fixings. this'll leave a wider gap at the other side of the section that'll be closed by welded sections.

comparing one bolted steel arrangement to another:
in flange bolted steel pipe runs - a short section of cut-to-suit pipe ("pup") has to be fashioned into the final connection - tolerances for bolt connections are quite tight and things are never millimeter perfect.....
(it's construction: the solution is always pretty mundane and involves a man in a boiler-suit hitting things with a hammer)

I don't know where the expansion joint(s) are but I don't think they're at this section - that would leave a massive cantilever and/or a well complicated joint. (even more complicated that a 5 skin camberwell carrot).

In reply to daWalt:
Re: The CC - I thought carefully about the title of this thread and dismissed 'joints on the Forth Bridge' for such reasons

In reply to daWalt:

"even more complicated that a 5 skin camberwell carrot"

I never understood why stoned people try to construct increasingly complicated structures the more stoned they get.

In reply to gethin_allen:
It is because the chemically expanded brain has a greater capacity to deal with the complexities.

In reply to keith-ratcliffe:

??? http://www.transport.gov.scot/sites/default/files/documents/rrd_reports/upl... ???

In reply to keith-ratcliffe:


Also, it would have been inaccurate because it's not the Forth Bridge, it's the Queensferry Crossing. The Forth Bridge is the big red-painted cantilever bridge to the east. (The suspension bridge in between that and the new cable-stayed bridge is the Forth Road Bridge.)

The Forth Bridge was actually the fifth railway bridge built over the Forth. There's one near Aberfoyle, two in Stirling (side by side just to the north of the station) and there was one near Alloa, the Throsk Viaduct. That one was closed in the 1970s: you can still see the bridge piers on Google Maps' satellite view.

Of course the river above Stirling is non-tidal so in fact there have only been four railway bridges built over the Firth of Forth. (You can't count the first one that Bouch started to build, which would have been the fourth over the Firth, and which probably have meant that they wouldn't have needed the fifth.)

In reply to FesteringSore:
Nice photo of a sawtooth expansion joint - great for bridges with a large movement range, such as the second Thelwall Viaduct, which at 4500ft is the longest continuous steel girder bridge deck in the world, and can move up to 4" per day and 24" over an extreme annual thermal cycle.

I suspect, as others have indicated, that the gap has been created deliberately to ease the construction, and will be closed later. Unfortunately I don't know enough about the Queensferry Crossing project to be sure, but it would be normal to have the deck continuous at midspan. I do know that there are other cases where a bridge has been constructed piecemeal at height and the last section has proved problematic - notably the Lune West Bridge on the Heysham to M6 Link Road in Lancaster.

Bridges are wonderful things in that they really do move under load - one only has to see the photos of the old Forth Road Bridge being load tested early last year.

In reply to stewieatb:


See e.g. https://twitter.com/forthroadbridge/status/674543631772979200

There's also some video footage somewhere of the Forth Road Bridge visibly flexing as traffic moves along: that thing moves more than you think!

Back to the new bridge: I assume that somewhere there is a large 'Made in China' sticker on it?

In reply to keith-ratcliffe:
Thanks for your thoughts on this one. I did some more specific Googling and think I have a solution. I found an article that described the closure of a similar structure and it uses the expansion joints to create a gap. Essentially you compress the joints using clamps and that provides a gap into which you slot the final piece then you release the expansion joints to close the gap and weld up the steel girder sections.
On the Queensferry Crossing the expansion joint allows for 2.3 m expansion on the North side and 1.7 m on the South side - this easily covers the observed gap of about 20 cm.
I also did some calculations based on lifting the two sides up to expand the gap. My estimate is that 1m of lift gives you a 0.5 cm opening 2m gives 2.0 cm and 3m gives 4.5cm. To obtain a 10 cm increase in gap needs nearly 4.5m of lift. I don't think that this is the method but I was interested in the result.

In reply to Rob Parsons:


It's probably fallen off by now, what with the rust forming behind it.

In reply to Lusk:

The bridge will be of a modern design using the latest techniques and technology. Key features which enhance reliability include:
• A dehumidification system inside the box girder to reduce moisture, preventing corrosion

So it appears that engineers do learn from previous mistakes. Who knew?

And don't forget that, because it's a cable-stayed bridge:

The cables can be replaced with more ease than on the Forth Road Bridge – it can be done as part of normal maintenance works without closing the bridge

http://www.forth-bridges.co.uk/queensferry-crossing/facts-figures-queens.ht...

In reply to keith-ratcliffe:

For bridge fans there is a company that do nothing but physics based bridge building games:

http://www.chroniclogic.com/

I haven't played with the more recent ones, but carefully designing an elaborate structure only to watch it collapse and drop the first train to cross into the sea was quite good fun.

I never did manage to do a decent suspension bridge, although cable stayed designs are relatively straightforward.