Saturday, June 29, 2013

157> More on Bridges

In my last post I echoed one reader’s suggestion that bridges might be extremely beneficial in rolling out a PRT system.  After all, a bridge for PRT could carry the same number of passengers as much more expensive traditional bridges and could be routed more flexibly on either side.  By solving these particularly vexing transit problems, PRT could offer cities a much more compelling value proposition than would otherwise be the case with a limited starter system.  Like an aircraft that needs to surpass a certain speed to achieve lift-off, PRT systems need a certain number a stations and a certain amount of track to make enough economic sense to be worthwhile.  These numbers, unfortunately, are high enough that there are currently almost no interested parties.  A bridge or two might tip the balance.

This raises the obvious question of just how much cheaper a PRT bridge could be. One of the first tricks to getting the most span for the money is to use cable instead of trusses, as in suspension bridges.

Here is a transportation system where they have taken the concept a bit far, at least for my tastes.  (It’s called Aerobus, and this was a temporary installation, though it did carry quite a few passengers in its short life...)  Note that this is a two-way (double) track, so that the “M” supports are actually a pair of “A” supports.  Also note the arching of the track in the far section. Presumably weight of the vehicle is pulling on the main support cables, which are then lifting adjacent sections.  The bottom pic shows what is, apparently, a switched, off-line station.  The one thing I like about that top picture, though, is that it points out something about suspension bridges (not to be confused with cable-stayed ones) that is worth pondering.  I am referring to the thinness of the track itself between the vertical “hanger” cables.  Since these cables act as simple skyhooks and are closely spaced, the track needs very little structure to stiffen it.  In this way a suspension bridge can be extremely long, yet the “roadway” portions each only need sufficient girth to support themselves between hangers…  at least in theory!  (There are wind loads and natural resonance issues to factor in.)

Long bridges for lighter loads than for automotive use are actually quite common.  This is because pipelines (and the occasional ore conveyor) also must also cross natural obstacles.  Here is a pipeline bridge in China, for example. 

Notice the cabling on the sides to prevent movement from crosswinds.  This is actually a fairly standard feature in pipeline bridges.  Here is another example, although I have to say that I find the parallel main cable design puzzling for a single pipe.  I like the bottom one, which splays those support cables to handle cross-winds.

Since bridges for transit are almost certain to be bidirectional, and since I have never really explored double track for PRT, I built a couple of 3D models.  Note that there is no triangulated truss structure, since such short spans do not require it.  Such sections would simply bolt together end-to-end, with each spanning the vertical support cables. 

While working on the section above, I realized that a pedestrian walkway would be a simple addition, and so I stopped working on that model and started playing around with adding precast decking sections. 

While working on that, however, I got yet another thought, which is the matter of emergency evacuation.  I guess it doesn’t pay to start modeling without the full vision of what you really want.

Even with vehicles that are sufficiently autonomous to render system-wide failures impossible, if a bridge becomes impassible for any reason there is the prospect of backing up and turning away a lot of traffic.  Depending on the span, number close stations, time of day, etc. this all might take some time.  If the bridge is equipped with U-turns at each end, however, and traffic is instantly diverted away from even approaching those, it seems like a bridge could be emptied fairly quickly by backing up, taking the U, and going back toward the station of origin.  There is also the matter of evacuating the occupants of a hypothetical vehicle which is blocking the way and cannot, for some reason, even be pushed.  And let’s not forget about ordinary maintenance.  This brings up another thought that I had long ago, which is to have some way to roll a small hanging platform along the tracks without blocking them.  Another thought is simply adding an extra lane.  With crossovers, a middle lane could be used for diverting traffic for any reason.  Wider GRT vehicles would be able to fit on such a bridge as well.  Maybe I’d better ponder this all a bit more before starting yet another 3D model!


Juho Laatu said...

I think this part of PRT planning is quite straight forward since these bridge designs are quite independent of other PRT design decisions. You could use almost similar bridges even for supported and suspended PRT tracks.

The key point is of course the cheap prize tag. It would be useful to have some realistic estimates on how much a PRT bridge of certain length or certain capacity would cost, compared to traditional highway bridges. There are numerous places around the world where people and politicians would like to have bridges, and wonder if their bridge can ever be built or not. For them a cheap (and credible) price tag could be a very interesting proposal. Maybe someone could even build a single (elevator like) PRT bridge that simply takes people from one end of the bridge to the other. It would be easy to expand that proposal to a slightly wider PRT system.

Some more thoughts:
- the third lane is useful for many needs, e.g. to carry rush hour traffic in one direction
- one can have bridges with PRT + pipelines + electricity
- one could build PRT bridges over congested areas
- it may be possible to change the track structure of an early bridge (=> early technology decisions will not limit possibilities in the future)

Bruffie said...

Cycling is receiving a lot of ink and even more money these days, Dan, hence my suggestion: make that 'roof' potion BRT (Bike Rapid Transit). It may be just what the doctor ordered.

Dan said...

Juho, I wrote a lengthy reply and thought I posted it... I guess not. In summery, yes, 3 lanes seems compelling, and that's a good point about utilities. Whereas the idea is not new, it's got a new twist in this instance. I wonder how it would be worth to the electric company, for instance, to get such a crossing. I guess it would eliminate a bunch of transformers, if nothing else...

Bruffie, actually there are a couple of lines that were supposed to indicate a bike lane in the bottom picture. Pretty narrow for anything like serious bike traffic.

Sorry to be brief guys... Time is tight.

Andrew F said...

The problem with three lanes is that you will eventually run out of vehicles on one side of the bridge. Vehicles can't be returned/reused as quickly as they are being used to cross the bridge.

Dan said...

Thanks Andrew! You've given me a good idea for a post... This is a matter that goes way beyond bridges...

In this specific instance I would point out that the direction of the middle lane could switch back and forth pretty quickly without humans to get confused.
Also, the lane could be used for empties only, with the added benefit of a emergency or maintenance access. It does, though, raise the question of where "pods" go, at night, to roost!

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