Counterweight or not, structures deflect under load in the direction of the load. The two ends of a swing bridge counterbalance each other but each individually is a cantilever beam and both ends will drop down some when they leave their end supports. The form of the bridge truss has a pretty big moment of area but the cantilevered ends also weigh a lot.
Fair point. But a swing bridge has support when closed, and presumably has some kind of ‘home’ support when open? Surely they don’t leave them hanging in the middle of nowhere for any period of time?
You mistook my intended, and I guess poorly worded, description. I thought I was describing a typical swing bridge, that would have the pivot in the center of the waterway, and move the pivot onto land. So the waterway would be one-half of the area available if the pivot were over the water. I wasn’t intending to describe a bridge that pivots on the end.
But now I know that they exist! So that’s a plus for me.
I don’t mean to say that the elastic limit is exceed so as to cause a permanent deformation.
However since the ends will drop down on opening they have to be lifted back up on closing. This lifting is probably done on a track in the abutments and wheels on the end of the bridge. Such things require constant maintenance, as for example lubrication of the wheels, which is a nusance. But I gess “a man’s gotta do what a man’s gotta do.”
What distance would they be likely to drop, across say a 50 metre span? I’d have thought it was no more than the typical tolerances allowed in ordinary steel bridges to account for high winds etc.
And no, the ones I’ve seen, as far as I could tell, had no special mechanism for the ‘lifting’ on reaching the far end of the turn. Maybe a few wheels were involved - but that’s inconequential compared to the main mechanism of either a drawbridge or turntable.
Check out the links I posted. (And an even better picture here.) The swing arms meet in the middle of the span, with no support underneath them. They may have some way of locking together when the span is closed, but I don’t know what it is. One of the sites I came across while looking for pictures mentions that from the closed position, the swing arms raise before turning.
There’s definitley some confusion over the different layout of swing bridges. the ones I’m talking about, the ones I’m most familiar with (being common around here 
) are a turning platform built on a stage in the middle of the water, with a symmetrical bridge structure, often a basic cantilever, on both sides. This is the system that I meant early on when I said it was basically a loco turntable with a bridge on top.
While Googling for this thread, I’ve realised that many other swing bridges are built on one bank, with a huge counterweight. And I can’t for the life of me see any particular benefit to that layout - except possibly when the river formed a boundary line and they wanted to keep the construction to one side of it.
Here’s agood article about King brand swing bridges. It says briefly that swing bridges fell out of favor around the turn of the century but doesn’t say why.
As far as sag goes, I’ve seen a video of aging bridges that talked about the problem. On older swing bridges, the deflection can increase enough that the bridge can no longer close properly. I think the video was about railroad safety inspections but I’m not positive.
I can tell you that current day bridge builders always avoid movable bridges of any type if possible. Not only is there the worry about breakdowns and maintenance, the cost of having someone staff bridge operations adds up very rapidly. It winds up being cheaper to build a bridge a hundred feet higher.
I don’t know and since working out the deflection is a lot of work I don’t intend to do it. My only point was that if you replace a swing bridge (middle) with a single span of the same length that swings from one end the deflection on the free end might be troublesome.
The picture of the bridge with the pivot on shore seems to me to show an ordinary swing bridge with a long and relatively light end balanced by a shorter and heavy section that forms part of the approach. It looks like the channel is relatively narrow and a bridge in the middle might restrict the size vessel to be accomodated.
I know the kind of bridge you mean, GorillaMan, but even when it’s open there’s still an obstruction right in the middle of the river. It’s great if the river traffic is skinny sailboats. The one I posted links to (does anyone ever follow one of my links, ever?) swings completely clear of the river. And if you look at the high-level bridge next to it, you can see there must be some decent sized ships going through there.
Or maybe they were just showing off. One link described Seattle as “a city known for its innovative uses of concrete.”
OK, in all off both of youre examples they’re ‘one sided’ swing bridges (dammit, will nobody ever turn up and explain the terminology properly?!). But each of those could surely be replaced with a drawbridge which requires far less engineering? If so, I wonder why the swing bridges were ever used? (Allowing for my note about the history of drawbridges earlier…)
(And a final note - I realised that the droop question isn’t relevant to the bridges I know, because nowadays they spend most of their time ‘closed’. At least one has an ‘opening times’ display for masted boats.
I’m still coming to the conclusion that nineteenth-century technology prefered swing bridges. Twentieth century developments (including that realisation that swing bridges didn’t stay horizontal) meant that more drawbridges were built. I guess that’s also historical, because we now tend towards suspension bridges.
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(And a final note - I realised that the droop question isn’t relevant to the bridges I know, because nowadays they spend most of their time ‘closed’. At least one has an ‘opening times’ display for masted boats.
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It has occurred to me that one way to solve the sag problem is to have the ends unsupported whether the bridge is open or closed. That way the bridge can be built so that the bridge and roadway are level. However, this brings up another problem (that’s called the Law of The Conservation of DIfficulty). When a heavy vehicle like a fully loaded semi, or any vehicle for that matter, reaches the unsupported end that end will deflect causing a thump when the roadway is hit. I suppose such a problem is manageable. At any rate swing bridges are built and they work. But there seems to be more to the bridge-roadway transition than first meets the eye.
For example about that gap that was spoken of in connections with the on-shore pivot. There has to be a gap. Otherwise the ends would interfere as the sections turn. If the bridge is 30’ wide the swing arm is 15’. When the bridge swings left by 1[sup]o[/sup], for example, the right edge moves about 3-1/8" toward the opposite shore.
And if your speculation about the tie between turntable and bridges isn’t true it certainly is a reasonable guess.
Thunder Bay, Ontario, has a jacknife type drawbridge. Here is a pic: http://www.lakesuperior.com/online/185/185tbaybrdg.gif
That’s no problem - the end has a curved profile to match the circle of the swing!