What makes suspension bridges unsuitable for railways?

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I recall reading in an old issue of Model Railroader an article about railroad bridges. In it, the author mentioned that you shouldn't have a model suspension bridge for a railroad track on your layout because no such arrangement would be prototypical.

In his own words, "suspension bridges and trains don't mix."

I realize that the author probably wasn't an engineer (at least in the sense of this site), but his remark did get me thinking. I am somewhat of a railroad buff, and I can't think of a suspension bridge for trains - the closest thing that comes to mind are the trolley tracks that were on the Brooklyn Bridge*.

I understand that there wouldn't be many suspension bridges for trains, since other bridge types would be better suited (cheaper, easier to build, etc.) but why are they so rare?

*Bear in mind, though, I live in the US, and am pretty unfamiliar with foreign railroad infrastructure.

Dave Coffman

Posted 2015-02-03T23:43:30.253

Reputation: 193

PATCO - Ben franklin bridge in Philadelphia. – None – 2015-03-13T11:46:25.880

Are you asking about bridges that are purely for railroads, or that happen to have them as a secondary feature?HDE 226868 2015-02-03T23:45:32.477

@HDE226868 Both, I suppose. I can't think of many examples of either.Dave Coffman 2015-02-03T23:51:43.650

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There is a rather long discussion on the topic at cs.trains.com. No conclusive proof, but the consensus is that the advantages of a suspension bridge don't work well for large, moving, concentrated loads.

Chris Mueller 2015-02-04T01:07:23.303

Answers

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There are a few main reasons why suspension bridges aren't used for railroads.

The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers.

The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%.

Third is that trains don't really have suspensions, especially freight trains. This means that any movement in the bridge itself has little opportunity to be dampened before it reaches the train. Suspension bridges are relatively flexible by design which makes transferred motion even more of a problem. You do not want the bridge to be rolling under a train!

None of these are impossible engineering issues to overcome. But by the time that you have accommodated all of them, you might as well find a different location or build a truss bridge instead.

hazzey

Posted 2015-02-03T23:43:30.253

Reputation: 7 817

trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Can you explain what this means? If a train car weighs 100 tons, its weight is 981 kN. How can the load on the bridge under that car somehow increase to 1275 kN?DrZ214 2016-02-23T01:18:56.183

@DrZ214 As the train bounces, the effective force on the bridge gets increased. Instead of doing complicated momentum and energy calculations, the at-rest load is increased by a percentage to account for the added force.hazzey 2016-02-23T01:51:23.300

1Not only are trains heavy, but freight locomotives are especially heavy. In a typical suspension bridge, portions of the bridge deck balance other portions. If there is a large point load in the middle of a bridge, the towers will be stressed more if there's nothing elsewhere on the bridge than if the remainder is also loaded. A cluster of freight locomotives pulling a bunch of empty rail cars will for loading purposes be very close to that worst-case scenario.supercat 2015-04-02T20:18:57.830

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Possibly the reputation of railway suspension bridges was fatally damaged by the poor performance of the very first one?

In 1830, the Stockton and Darlington Railway built a suspension bridge over the River Tees, designed by Samuel Brown.

enter image description here

(Illustration by William Miller, from The North Eastern Railway by William Tomlinson, 1915.)

Intended to carry trains of 150 tons, testing showed that the bridge towers sustained damage when the load exceeded 66 tons. Chris Lloyd describes the occasion:

As the first engine, pulling 16 coal trucks, edged onto the bridge, the deck wobbled and shook, and the pillar on the Yorkshire side swayed and cracked. As the train neared the centre, the deck rose up in the middle, creating a mini-mountain, with eight trucks going up the Durham slope while, simultaneously, the other eight rolled down the Yorkshire side. The coupling in the middle snapped, and while the front eight wagons and the engine proceeded across to Yorkshire, the eight rear wagons ran away, speeding down the hump into Durham.

Trains were thereafter limited to four wagons at a time connected "by means of chains with couplings which kept them 9 yards apart." In 1844 traffic was transferred to a cast iron girder bridge designed by Robert Stephenson, and the suspension bridge was demolished in 1880.

By contrast, two modern suspension bridges carry railways over very long spans:

Tsing Ma Bridge

(Public domain image of Tsing Ma Bridge by Snaveed, 2006.)

Gareth Rees

Posted 2015-02-03T23:43:30.253

Reputation: 376

2Note that these bridges are designed for passenger (intercity or heavy transit) rail, which imposes much less load than freight rail can.ThreePhaseEel 2015-08-31T23:33:31.050

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Light rail, and a floating rather than a suspension bridge, however I suspect many of the issues are the same: http://bulletin.pbworld.com/volumes/2014_04/taking_light_rail_over_floating_bridge.aspx

"The bridge destined to host light rail is the Homer M. Hadley Memorial Bridge, one of the longest floating bridges in the world, which currently carries the Interstate 90 highway over Lake Washington. Two of the bridge’s existing lanes will be replaced with dual-track light rail in order to take the region’s light rail system over Lake Washington as part of Sound Transit’s East Link extension from Seattle to Redmond. While passenger rail systems have been developed that cross cable support structures and encounter multi-dimensional movements, there is no other bridge carrying passenger rail that accommodates the wide range and magnitude of movements that will be required of the Hadley Bridge."

So it seems that development is ongoing in this area.

(Sorry, not enough reputation to add as a comment yet, but I thought it might be of interest)

welf

Posted 2015-02-03T23:43:30.253

Reputation: 400

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If you look at the Øresund Bridge they state that the bridge was designed with heavy rail traffic in mind:

A girder and cable-stayed design was chosen to provide the specific rigidity necessary to carry heavy rail traffic, and also to resist large accumulations of ice.

The trains run in a deck below the road trafic.

As the article in wikipedia lacks a reference for this statement, I can't give the actual reason. But it seems that a suspension bridge is too wobbly. Personally I have heard that trains on a suspension bridge would push a wave of bridge in front of them, tearing the tracks and the bridge apart.

Secondly suspension bridges are often rather steep out to the middle, something trains do not like, trains need quite a bit less incline than what a car or a lorry can handle.

Bent

Posted 2015-02-03T23:43:30.253

Reputation: 141

It's worth pointing out that the Oresund Bridge is not a Suspension bridge. It is a cable-stayed bridge. They look similar but in principal they function differently, particularly under dynamic loads.Smeato 2016-06-29T09:05:38.823