Rail buckling

[nw-mailing-list at nwhs.org]

Rail buckling is caused by the compressive forces generated by the heat
expanded rails in one section of rail.

 

The buckling is a stability issue in engineering, not one of strength or
internal temperature.  Simplified, as the rails heat up on a hot day, they
increase in length and push against their neighbors causing a compressive
load in the rail (two neighbors push inward on one chunk of rail causing an
internal compressive load).  Taking the 'chunk' as dead straight, the
compressive loads can be increased up to a critical load where the rail will
immediately bow sideways.  This critical load value changes as the two
compressive loads are applied through the centerline of the rail or just
slightly off centerline, eccentricity, in the rail.  More eccentricity gives
lower critical loads.  Also the slenderness ratio, very crudely, length over
width, matters, too.

 

As the problem is, more-or-less, load and geometry of the system, no
buckling will occur until the critical load is reached, then lots of deflect
can occur for a slight increase in load.  

 

Since rail and track is rarely straight and the rail will grow at different
rates depending on local conditions, sun angle and all sorts of other stuff,
the critical load will vary bunches over a long length of track.  In other
words, unpredictably.  Hence the track inspection trucks.  

 

Ribbon rail and jointed rail have mostly similar geometries over a long
length, so the critical buckling loads are not much different.  Never the
less, there are a large number of variables to consider in this problem.

 

The simple model is to stand a soda straw on end and carefully press
downward on the end with your hand.   Slowly increasing the force will cause
nothing much to happen until the cross section of the straw folds and the
straw will easily bend sideways with the slightest increase in load.

 

Gary Rolih

 

  _____  

From: nw-mailing-list-bounces at nwhs.org
[mailto:nw-mailing-list-bounces at nwhs.org] On Behalf Of
nw-mailing-list at nwhs.org
Sent: Wednesday, October 04, 2006 9:51 PM
To: NW Mailing List
Subject: Re: Trackwork

 

Asketh Ron:

 


>> 


Thanks Abe, but wouldn't that have happened with jointed rail also? 

<< 

 

I'm an operating guy, Ron, and not a track expert.  And I'm a philologist,
not an engineer. So I speak from very limited knowledge.

 

Did you ever walk along "jointed track" in the Summertime, and notice how
much the rail had "run" longitudinally in the tie plates, as indicated by
the shiney abrasions of the spike heads on the base of the rails?  Each rail
joint is in effect an "expansion joint" for taking up and relieving
longitudinal stresses.  

 

That doesn't happen on track with welded rail.  The rail is anchored tightly
to the ties with Woodring anchors or Pandrol clips, so that it will not
"run" longitudinally.  When the expansive forces and pressures build up
under these conditions, the rails move laterally and move the ties with
them, which amounts to a "kink" in the track.

 

I remember some years ago, not long after ribbon rail became popular, that
railroads were having numerous derailments on "buckled track," but most of
them were happening about 15 or 20 car lengths behind the engine.

 

The DOT Pueblo facility fitted some welded rail track with all kinds of
instrumentation and even took movies of track buckling in the hot sun.  What
they discovered is that the sun heated the rail to the high-130's, and then
the friction caused by a train moving over the track added a couple more
degrees to the rail temperature and pushed the track over the critical
temperature threshold, and the track buckled horizontally right under the
train.  I recall seeing an AAR movie (viedeo ???) of these experiments in
the late 1970s.

 

This showed the railroads why they were having "track buckled derailments,"
and the response was to "regulate" gobs of ballast against the ends of the
ties on continuously welded track, to keep the ties from moving laterally.

 

That's a brakeman's explanation for you.  I'm sure a civil or mechanical
engineer could put perfume and a ribbon my my pig of an explanation.

 

-- abram burnett

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