Why do train tracks always use a swing bridge rather that a draw bridge when going over water? I can’t see how it’s any stronger more stable. Actually, a swing bridge seems less stable. So, what’s the deal?
Always?
The Chicago River is crossed at two places with draw bridges that carry the “L”, the elevated light rail commuter trains.
I believe draw bridges are a bit more expensive to build than swing bridges.
A draw bridge must be built strong enough to go from a horizontal position to a near-vertical position, and back again. A swing bridge only has to rotate while staying in the horizontal position. Fasteners on the draw bridge have to deal with stresses in 2 directions: downward when in the down position, sideways when in the up position. In swing bridges, the stress is always in the downward direction.
There might be an operational cost difference, too. For a drawbridge, the whole bridge must be raised to a vertical position. That probably takes more power than just rotating a swing bridge in the horizontal position. (But maybe counterweights on a draw bridge can negate this.)
A real engineer will probably be along soon, with an authorative answer. (IANAE).
Here is a list ofrailroad bridges in New Jersey.
There is quite a mix of types, swing, vertical lift and bascule (counterweighted draw).
At the time railways were being developed, swing bridges had an added advantage, in that they used expertise they railways already had. The turning mechanisms were in regular production, to build turntables for repositioning locomotives. Just put one of them on a stand, build a structure on top, and there’s your bridge.
One of several railroad drawbridges in Cleveland. Cleveland has something like a dozen different types of movable bridge spanning the Cuyahoga River, though I can’t remember any more than the three basic categories. There’s another image about halfway down here, with a swivel bridge in the background (the swivel bridge is for cars, though, not trains).
Oh, another explanation for choosing swing bridges - they were generally left in the ‘open’ position, except when needed by trains. I’d guessing this might be less easy with drawbridges. (This is UK-oriented, YMMV)
Takes less energy to raise and lower a drawbridge.
At least in Chicago, the drawbridges (and Chicago has many of them) use massive counterweights. The motors to raise and lower the bridges are surprisingly small.
And in addition to the two drawbridges that accomodate both cars and trains (on two separate levels, yet!), there are several more in the area. In fact, I can’t recall seeing a swing bridge around here, but drawbridges are everywhere.
It’s probably just a regional preference thing.
It could take more total energy to operate a counterweighted drawbridge vs. a swing bridge – but not necessarily MUCH more. In either system, the energy primarily overcomes friction losses in the bearings. The design of those bearings would be the major determinant of the losses.
The biggest secondary factor would be the mass of the bridge. Obviously a counter-weighted bridge will have significantly more dead mass than a similar bridge with no counterweights. It may be unexpectedly easy to “lift” a counterweighted bridge, but you still have to “budge” the inertia of all that dead weight. On a swing bridge, you only need to budge the dead weight of the loadbearing structure, which is arranged to counterbalance (bout not counterweight) itself.
Of course, the two types of bridge are structurally very different. The swing bridge has twice the effective span, All things considered, both can use surprisingly small motors for such large structures, if you are willing to accept a slow opening speed, The difference in structure probably determines the final answer – after all, cable operated swing bridges can use counterweights, too.
You can think of the counterweight as a mechanism for storing energy. When the drawbridge comes down, the counterweight comes up, storing that energy for use when the drawbridge needs to go up again. However, as in most energy storage systems, the storage comes at an energy cost (inefficiency) so a system that doesn’t need that system (and its losses) can be more efficient. Often the energy cost per opening isn’t large enough to be a primary consideration: e.g. the size, operating life, and maintanence on the motor probably outweighs energy costs.
One of the RR bridges in LaCrosse was recently repalced (was a aswing, now a bascule
http://www.edkraemer.com/Uploads/Bridges/500217%20CPRAIL%20LACROSSE.pdf
http://www.edkraemer.com/news/news_detail.asp?id=70
Brian
It does? Unless you’re not comparing it to a symmetrical drawbridge arrangement?
I was thinking about a swing bridge with a single central pivoting support and two counter balancing spans, one on each side, vs. the two halves of a symmetrical “two lift” drawbridge. (“Twice as big” is often more than twice as “hard” to operate) You’re right, there are many other very different configurations.
Some wags:
Draw bridges cause a automatic barrior to cars running into the water, it ain’t going to stop a trail though. THis also serves as a visual warning that the bridge is open and you must stop.
RR Bridges are normally narrower then car bridges, remember that a swing bridge is a circular section, the wider the bridge (multi lane) the larger the circle must be to allow enough room for ships to pass w/ enought clearence on the outside of the roadway. RR bridges over water are normally 1 or 2 tracks even if the tracks to and from the bridge are 2-4 tracks. While car bridges usually are the same # of lanes of the road.
In most cases, by the time you saw it from the train, you’d have no chance of stopping in time.
I’m confident in suggesting that the scenarios where a couple of lanes’ width makes all the different are few and far between. But in any case, I don’t see how comparing railway bridges to road ones is of much use: neither type of bridge is common for roads, for a different reason - they can climb a steeper gradient, so that in most situations it’s possible to leave enough clearance for the largest ships.
Our own baby blue bascule Johnson Street Bridge
And a slightly wistful history of same.
A couple more random thoughts on the different uses of the two bridges…
Clearly, there’s plenty of railway drawbridges. But I’m struggling to find any that are pre-20th century, and Tower Bridge in London was certainly pushing the limits of bridge engineering only a few years before then. So any railway built much before 1900 would presumably not have used them, and such railways will have only acquired them where there was a particular reason to change systems.
PLus…swing bridges require no extra land other than that for the railway, whereas drawbridges do. A consideration, where land is expensive.
Another possible consideration is that a swing bridge, at least all I’ve seen, have their support in the middle. And this is usually on piers or a man made island in the middle of the waterway. Drawbridges are supported at the ends, leaving more room for ships.
I guess there’s no reason the swingbridge couldn’t swing from the shore. Any pictures of such?
I doubt such a thing exists. Such a structure would be a cantilever beam and the abutment to support it would have to extend far from shore and deep into the ground. In addition, as soon as the free end cleared its support the beam would sag considerably what with Hooke’s Law about stress and strain. And this gets me to thinking. How do swing bridges handle this sag when returning to the closed position? Any bridge designers around?
Not so fast, David Simmons.
The West Seattle Low-Level Bridge (don’t know if that’s an official name or anything) is a “hydraulically-operated, double-leaf concrete swing bridge”. There are two pivoting sections, one on each pier on either side of the river. The movable elements are asymetric (240 ft. on the ends that meet to form the main span, 173 ft. on the other side) but constructed so that it still balances about the pivot point. It’s hard to describe.
Google ‘swing bridge’, and you’ll find many. It’s the same situation as with any other overhanding structure - you need a counterweight. Once you’ve got a big lump of concrete balancing the long steel structure that’s forming the bridge, you’re free to put it on a rotating platform and swivel to your heart’s content.