Saturday, June 29, 2013

158> 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!