Selection and ranking of the main beam geometry of a freight wagon for lightweighting

The traditional freight wagons employ I-beam sections as the main load-bearing structures. The primary loads they carry
are vertical (from loading units) and axial (from train traction and buffers). Ease of manufacturing has played an important
role in the selection of the I-beam for this role. However, with lightweighting increasingly becoming an important design
objective, an evaluation needs to be done to assess if there are other existing or new section profiles (geometry) that
would carry the same operational loads but are lighter. This paper presents an evaluation of 24 section profiles for their
ability to take the operational loads of freight wagons. The profiles are divided into two categories, namely ‘conventional
– made by wagon manufacturers (including the I-beam)’ and ‘pre-fabricated’ sections. For ranking purposes, the primary
design objectives or key performance indicators were bending stress, associated deflection and buckling load.
Subsequently, this was treated as a multi-criteria decision-making process. The loading conditions were applied as
prescribed by the EU standard EN 12663-2. To carry out structural analysis, finite element analysis was implemented
using ANSYS software. To determine the validity of the finite element analysis results, correlation analysis was done with
respect to beam theory. Parameters considered were: maximum stress, deflection, second moment of area, thickness,
bending stiffness and flexural rigidity. The paper discusses the impreciseness related to the use of beam theory since the
local stiffness of the beam is neglected leading to an inaccurate estimation of the buckling load and the vertical displacement.
Even more complicated can be the estimation of the maximum stress to be used for comparison when features
such as spot welds are present. The nominal stress values computed by means of Navier equation lead to an inaccurate
value of the stress since it neglects the variations in the local stiffness, which can lead to an increase in the bending stress
values. The main objective of the paper is the applicability of particular section profiles to the railway field with the aim of
lightweighting the main structure. Sections commonly adopted in civil applications have also been investigated to understand
the stiffness and strength under railway service loads. The common approach reported in literature so far
makes use either of the beam theory1 or topological finite element approach2 to determine the optimised shape under
the action of the simplified loading conditions. Although the previous approaches seem to be more general, the assumptions
made affect the optimisation process since the stress state differs from that attained under the actual service load in
the real structure. In this paper, the use of complex shape cross sections and detailed finite element models allows to
take account of the real behaviour in terms of stiffness distribution and local stress effects due to manufacturing features
like welds. The structural assessment carried out with the detailed models also allows for the proper comparison among
the considered sections. Analysis of the results showed that three out of the 24 section profiles have the highest
potential to be fitted as the main load-bearing beams for freight wagons, with the pre-fabricated Z-section being the
optimum of the three.