Case Study 

Rail track / structure interaction analysis for the Dubai Metro light rail project

  • 2D modelling of rail / structure interaction modelling to UIC 773-4

  • Verification of spreadsheet calculations

  • Potentially problematic results proved by LUSAS to be within project design criteria

Aerial view of Dubai Metro Financial Centre station and footbridge

The Red and Green Lines of the Dubai Metro light rail scheme are being constructed as a Design-Build contract by a consortium of international contractors. Atkins, one of the world’s leading engineering and design consultancies, is the lead designer to the major civil contractor of the Dubai Rapid Link (DURL) Consortium and is carrying out the full multi-disciplinary design and project management of the civil works for the project. Atkins used LUSAS Bridge analysis software to verify its rail track interaction spreadsheet calculations to the International Union of Railways Code UIC 774-3, and to obtain more accurate and ultimately design-complying results for situations that the spreadsheet calculations had identified as exceeding project design values.

Overview

Deck segment at support showing deck cross-section, pierhead and pierThe 52km long Red and 24km long Green Lines of the Dubai Metro run mostly on elevated single span pre-cast post-tensioned concrete deck units of U-shaped cross-section. Deck crossbeams sit on elastomeric bearings, with fixed and free guided sliding pot bearings being used for two and three-span continuous decks. Standard spans are typically 28m, 32m or 36m in length, with each end supported on a flared pierhead sitting on a single, circular, reinforced concrete column. Rails are continually welded across all the decks and deck joints, and connected to concrete plinths running the length of each deck using regular track fixings.

A detailed assessment of the interaction of the track with respect to the deck structures was needed in order to derive stresses in the rails and forces and longitudinal displacements induced in the deck caused by both thermal and train loading.

Spreadsheet calculations

Based upon the formulae and design methods specified in the UIC 774-3 code Atkins set-up spreadsheets to calculate rail track/structure interaction effects for around 1200 spans on both the Red and Green Lines. Whilst based on the simplified techniques of the UIC code and producing generally conservative results this spreadsheet approach had the benefit of allowing a large number of span arrangements to be assessed relatively quickly.

Modelling with LUSAS

2D modelling conceptTo verify the results obtained from the spreadsheets Atkins used LUSAS to model and analyse a number of representative span arrangements. Models were built using thick beam elements to represent the piles, piers, crossbeams and concrete decks. Pile modelling was done using the equivalent cantilever method (as opposed to modelling the soil/structure interaction as a beam with elastic springs). Joint elements of an appropriate stiffness and at the required locations modelled the elastomeric and other bearing types. A single thick beam element offset by an appropriate eccentricity from the deck represented both sets of rails. Nonlinear joint elements modelled the track fixings. Thermal loadcases were applied to the elements representing the deck according to the UIC 774-3 code. Train loading of both vertical live loading and horizontal braking loads were applied to the elements representing the rails at locations of interest in the model.

LUSAS modelling of a multi-span arrangement including a 2-span and 3-span continuous deck

Rachel Jones, engineer on the project said: "For the LUSAS verification modelling we chose a selection of span arrangements that would give us a variety of effects for a combination of single, 2-span and 3-span continuous decks. The stresses in the rails obtained from the LUSAS analyses proved our spreadsheet calculations were being carried our correctly". 

However, for a small number of span arrangements the rail structure interaction forces obtained from the spreadsheet calculations were shown to exceed project design values. Each of these situations required additional LUSAS models to be created for a detailed analysis to be carried out. The primary difference between analysis to the UIC-773 code and using LUSAS is that modelling in LUSAS is more realistic (using nonlinear joints to model the track fixings for example) and makes no assumptions on the behaviour of the structure. Results obtained from the LUSAS analyses showed that the rail/structure interaction values of initial concern did not actually exceed the project design values. 

Loading configuration: temperature loading to deck and vertical live train loading with braking forces (not visible) in first two spans of station

Stress in rails from applied thermal and train braking loading

"By using LUSAS to investigate these span arrangements we got a more accurate assessment of the rail track interaction effects and obtained values that did not exceed design criteria"

Rachel Jones, Atkins

Automated rail track / structure analysis with LUSAS

If it had been available at the time Atkins carried out its own manual 2D modelling and analysis of rail track / structure interaction effects as described in this case study, the LUSAS Rail Track Analysis software option for LUSAS Bridge would have let Atkins carry out automated 2D modelling of track/structure interaction analysis to the International Union of Railways Code UIC 774-3.

With this option, track and bridge interaction models are built automatically in LUSAS from geometric, material property, and loading data defined in a MS Excel spreadsheet. Both thermal loading to the track and train loading due to acceleration and braking forces can defined. 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.

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 track fixing clips is nonlinear, a nonlinear analysis always needs to be carried out. After running an analysis, results can be quickly obtained in both spreadsheet or LUSAS results file formats.

A key benefit of using the LUSAS Rail Track Analysis software option is that it automatically updates the material properties associated with the track/structure interface based upon the position of the train or trains crossing the bridge - something not easily done by manual or other software methods.

 


Other analyses undertaken by Atkins on the Dubai Metro light rail project using LUSAS include:

 

Software Information


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