MECHANICS OF A RAIL JOINT
In theory, the forces acting on a bolted rail joint depend on
several factors such as: joint bar type (e.g. long-toe angle bar or
short-toe joint bar), length of the joint bar, rail size, tie-ballast
support conditions, bolt tension, number of bolts, and joint
anomalies (such as rail end gap, rail height mismatch, tread
mismatch, and end batter). The mechanics of a bolted joint
must also account for dynamic impact loads.
In terms of structural performance, rail joints are
considered as a weak link because the section properties (i.e.
cross-sectional area and area moments of inertia) of the bars are
typically less than those of the rail itself. For example, Table 1
lists section properties for 90 ARA-A rail and its corresponding
long-toe angle bars. Table 2 lists section properties for 136 RE
rail and for short-toe joint bars associated with 132 RE rail.
These joint bars were originally dedicated to join 132 RE but
are often used to join 136 RE and 140 RE rail sections as well.
A consequence of reduced section properties is relatively
larger deflection at the joint as wheels pass over it. Further,
these large deflections can lead to and accelerate track
degradation, which in turn can affect the structural performance
of the joint assembly. When a single wheel passes over the
joint, the rail ends deflect downward one at a time, creating a
small step and mismatch in rail heights. The size of the step
varies depending on the original gap distance between rail ends.