Engineering analysis and design software
Bridge design and engineering

Case Study

DART Blue Line Extension

  • Design of bridges on a 4.75 mile light rail extension

  • Rail track-structure interaction analysis of two multi-span precast concrete beam structures

  • Detailed modelling and analysis of a steel through girder span

KCS/N. First Street Bridge

As part of a design-build team, Gannett Fleming Engineers and Architects P.C. provided design services for Dallas Area Rapid Transit's (DART) Blue Line Light Rail Transit Expansion which runs from the downtown Garland Station to the city of Rowlett, Texas. In addition to the systems design, including track work, and overhead catenary systems, Gannett Fleming was also responsible for the design of a number of new bridge structures along the route and used LUSAS Bridge analysis and Rail track analysis software to assist with its design of the two longest structures.

Overview

The Dallas Area Rapid Transit (DART) light rail system is the largest electric light rail system in the United States. It currently totals 85 miles (137 km) of track and 61 stations between its Red, Blue, Green and Orange Lines, with plans in place to further increase its size. The 4.75 mile (8km), $360 million, twin track extension to the Blue Line opened to the public in December 2012. Its construction required six new bridge structures to be designed, the longest of which is a 28 span, pre-cast concrete beam structure over Rowlett Creek. This and an 11 span bridge carrying the extension over the Kansas City Southern (KCS) Railway were both analysed using customized LUSAS Rail Track Analysis software to assess track-structure interaction effects and verify that key values were within acceptable limits.

Rowlett Creek Bridge

Rowlett Creek bridge is a 2,565 foot (782m) long structure that takes the DART Blue Line extension over Rowlett Creek. Its deck spans are formed from AASHTO Type IV prestressed beams that vary in number according to span length with six beams used on the two 105'-0" (32m) spans and four beams used throughout the remaining 91'-0" or 92'-0" (28m) spans. The beams are supported on a flared pierhead sitting on a single, 6'-0" (2m) diameter reinforced concrete column with 2" (50mm) expansion joints between each span. Direct-fixation fasteners restrain the track to the concrete plinths that run the length of each 33'-2" (10m) wide reinforced concrete deck.

Track-structure interaction analysis

By using LUSAS Rail Track Analysis software the track-structure interaction model was built automatically from geometric, material property, and loading data defined on separate worksheets in a MS Excel spreadsheet. Thermal loading to the track and train loading due to acceleration and braking forces could be defined, and rail clips, bearings and pier stiffnesses could all be included in the analysis model. Model building dialogs allowed for modelling either one train crossing, or for multiple trains crossing the structure. Deck temperature loading could be considered in isolation for subsequent analysis of multiple rail configurations, or a full analysis could be carried out considering the combined temperature in the deck and rail loading. Because the response of the track fixing clips is always nonlinear, a nonlinear analysis was required. 

Rowlett Creek Bridge during construction

Rowlett Creek Bridge during construction

After running a track-structure interaction analysis, results can be produced in either Excel spreadsheet along with automatically created graphs, or in standard LUSAS results file format. Enveloping of results is carried out automatically inside Excel, or by specifying user-defined load combinations inside LUSAS. With Excel, separate worksheets within the results spreadsheet contain results for specific areas of interest. These worksheets include:

  • Raw results data in summary, graph and tabular form for each track and deck component

  • Envelopes of raw track and deck data in summary, graph and tabular form for combinations of temperature and trainset rail loading

  • Tables of railbed displacements, longitudinal reactions, and rail stress values - providing key results in summary form and allowing the quick determination of which analysis is causing the worst effects for each of the checks that need to be carried out. 

Structural elements such as the track, decks, bearings, piers and foundations are stored as individual groups within LUSAS and this allow for easy selection by structural type when viewing displacements, stresses or reactions etc. If additional spot checks need to be performed at specific locations on the tracks, the areas of interest can be selected and analysed automatically. 

The use of track-structure interaction software has many benefits over manual methods: Automated model building guarantees correctly-built models compared to manual model creation; the material properties associated with the track/structure interface are automatically updated according to the position of the passing train or trains; and overall it provides a much faster assessment of thermal and / or train loading track interaction effects on multi-span structures. Eric Dues, Principal Engineer at Gannett Fleming said: "An independent verification of the thermal interaction results obtained using LUSAS for the Rowlett Creek bridge was carried out by engineers in another Gannett Fleming office using different software. The results were strikingly similar, verified the LUSAS modelling method, and illustrated just how efficient LUSAS is at modelling track-structure interaction effects for these types of bridges."

 

owlett Creek Bridge completed

Rowlett Creek Bridge

KCS/N. First Street Bridge

KCS/N. First Street Bridge is a 1,054 foot (321m) long structure that carries the two DART light rail tracks over the KCS Railroad and North First Street in Garland. Its deck spans are formed from AASHTO Type IV prestressed beams that typically span 85-90 feet (26-27.5m) between crossbeams supported by a pair of rectangular reinforced concrete columns.

Its 155 foot (47m) long second span is a welded through girder structure comprising two welded plate exterior girders with 3" x 30" (762mm) top and bottom flanges and 1"x105" (2.67m) webs and a single interior welded plate girder with 3"x30" (762mm) top and bottom flanges and 1"x126" (3.2m) web. The girders are braced with W14x14 floor beams that also support the 8" (203mm) thick reinforced concrete deck slab. As for the Rowlett Creek Bridge, direct-fixation fasteners are used to restrain the track to the concrete plinths running the length of the deck. An aesthetic non-structural arch is affixed to each outer girder.

As well as carrying out a track-structure interaction analysis for the whole structure, various detailed beam and shell element models of the second span were built in LUSAS to assess different span solutions. Particular attention was made to the modelling of the crossbeam/girder connections to determine dead and live load stresses for use in the design which was to the AASHTO standard specification, as modified by the DART Design Criteria Manual. Moving train load cases were enveloped and combined with dead loads to determine maximum in-service effects. In addition to the determination of the stresses, eigenvalue analysis with LUSAS helped determine the natural frequencies of the structure.

KCS/N. First Street Bridge during construction

KCS/N. First Street Bridge during construction

Illustrative model and results plots

LUSAS rail track-structure interaction model for KCS/N First Street Bridge

 

LUSAS modelling of steel through girder span

LUSAS modelling of steel through girder span

 

Stresses in girders, deck and floor beams for a particular train loading position

Stresses in girders, deck and floor beams for a particular train loading position

 

Through girder span during construction

KCS/N. First Street Bridge during construction

Through girder span during construction KCS/N. First Street Bridge during construction
 
KCS/N First Street Bridge on completion
KCS/N First Street Bridge on completion

"An independent verification of the thermal interaction results obtained using LUSAS for the Rowlett Creek bridge was carried out by engineers in another Gannett Fleming office using different software. The results were strikingly similar, verified the LUSAS modelling method, and illustrated just how efficient LUSAS is at modelling track-structure interaction effects for these types of bridges."

Eric Dues, Principal Engineer, Gannett Fleming Engineers and Architects, P.C.


 

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