Engineering analysis and design software
Bridge design and engineering

Case Study

Automated track - structure interaction analysis

  • Automated model building and analysis from user-defined spreadsheet data
  • Fast assessment of track-structure interaction effects
  • Evaluation of forces in rails from thermal and in-service train loading

With a large percentage of track on modern high speed railways comprising continuously welded rails there is a requirement for accurate modelling of the interaction of the track with respect to any supporting bridge structures, and in particular, to calculate stresses and forces induced in the track by both thermal and train loading. A LUSAS product option, created initially for the Korea Railroad Technical Corporation (KRTC), permits track-structure interaction analysis to the International Union of Railways Code UIC 774-3. It allows KRTC to automatically build models from data defined in MS Excel spreadsheets, run an analysis, and quickly produce results in spreadsheet or LUSAS results file formats.

KRTC

KRTC is a subsidiary organization of Korea National Railroad. It is involved with performing planning, feasibility studies, design, and providing supervision for major rail and high speed rail construction projects throughout South Korea. Its stated aim is to enhance its competitiveness by developing advanced railroad technology. By using LUSAS Bridge to automate its track/bridge interaction analysis it is achieving this to good effect.

Model building

KRTC’s track and bridge interaction models are built automatically in LUSAS from geometric, material property, and loading data defined in a Microsoft Excel spreadsheet. Both thermal loading to the track and train loading due to acceleration and braking forces can defined. In accordance with the UIC774-3 code of practice, a user-specified element length is used to define the longitudinal embankment and bridge features. Rail clips, ballast movement, bearings and pier stiffnesses are all included in the analysis model. The model building dialogs allow for either one train crossing one or more structures, or for multiple trains crossing the same structure.

Viaduct data defined in spreadsheet

Spreadsheet input data
Complete embankment and viaduct model built in LUSAS

Complete embankment and viaduct model built by LUSAS

Enlarged view of first span

Exploded isometric view showing embankment support and first span

 

Enlarged view of first span

Exploded isometric view of part of the model showing embankment support and the beginning of the first span

Analysis and results

When running an analysis, deck temperature loading can be considered in isolation for subsequent analysis of multiple rail configurations, or a full analysis can be carried out considering the combined temperature in the deck and rail loading. Because the response of the ballast and/or clips is nonlinear a nonlinear analysis always needs to be carried out. Results can be produced in either Excel spreadsheet or standard LUSAS results file format. User-defined load combinations can be specified. Spreadsheet results include axial forces in the rails for deriving in-service stresses and forces and moments in the deck structure.


Typical axial force results plot from LUSAS showing force in continually welded rails from temperature loading to the deck

For multi-span and multi-deck structures such as that shown above the interaction between the embankments / abutments and other decks means that the behaviour can be complex. As an example, when considering only thermal loading on a structure having continually welded track it is common to see a reversal of the axial force / stress in the rails.

Whilst not apparent from the size of the model shown, the structure illustrated above is 325m long and consists of two decks, each with two 25 m spans followed by three decks, each with three 25 m spans. One pier / bearing support for each deck is represented by a restraining spring which takes account of the deflection characteristics of the pier / bearing system in accordance with UIC774-3. All of the remaining piers / bearing supports are roller supports allowing longitudinal movement of the decks. Under the effect of temperature on the structure the axial force / stress in either of the tracks / rails can be seen to vary, primarily due to the interaction between the movable and fixed structural components. 

 


 

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