Page 135 - Handbook of Materials Failure Analysis

4 Current Analysis and Design Methods 131

capping layer, and formation conditions within the area were considered reasonable.
The ballast layer depth at this location is about 285 mm on top of the dense capping
layer. Certain level of fine contamination in ballast and its densification could be
observed and the subsoil drainage was deemed not to be very good. However, there
was no sign of distress and failure that could be observed from the substrata.

4 CURRENT ANALYSIS AND DESIGN METHODS

Turnout bearers undergo a lot more stress than conventional sleepers in railway
tracks, as the train is moving from one track to another. Standards have been set
in Australia regarding the design load on concrete sleepers, which can be found
in AS 1085 Part 14, and for RailCorp in SPC 233 [16–19]. The Australian Standards
1085, part 14 (AS 1085.14) sets out the requirements for design, manufacture, test-
ing, and installation of prestressed concrete sleepers and fastenings for railways
across Australia [16]. Under its Appendix A, the standards outline the details of turn-
out bearers, the manufacturing details, loadings and design, fastenings, and testings.
Turnouts consist of bearers of various lengths, with rails attached at predetermined
locations to allow tracks to be connected to adjacent tracks. Crossovers may have
discontinuous bearers along the connection, depending on how far apart the adjacent
tracks are to each other. In addition to the standards set out in AS 1085.14 for stan-
dard sleepers, further considerations must be made for the extra dynamics of turn-
outs, including:

(i) Same method for standard bearers shall be used for distribution of axle load;
(ii) The dynamic loading on the bearer shall be designed for at least 2.5 times the
static load for the impact factor;
(iii) Effects of centrifugal force should be allowed for curved rails;
(iv) Forces and moments shall be considered from point motors as well as other
equipments required;
(v) Special load distributions specified by the designers;
(vi) Generally, bearers extend further than standard sleepers, and this must be
accounted for in the ballast layout. In cases where they extend beyond the
ballast shoulders, the moments and shears should be calculated assuming the
ballast and subgrade behave as elastic foundation;
(vii) In the case of negative bending due to the extended bearer lengths, there must
be adequate bending capacity built into the bearers to deal with the dynamic
shear forces and bending moments. The bearer should be designed to deal with
negative bending that is at least two-thirds of the maximum positive bending
moments;
(viii) Transverse reinforcement should be provided at the bearer ends should there
be substantial bending moments at the ends.

Based upon the AS 1085.14, the standard specifies the exact conditions and dimen-
sions for concrete bearers. It has specified shear force and moment envelopes set out
for the design of the bearers, as outlined in Figures 6.5–6.8 [17].