Engineering analysis and design software
Civil and Structural engineering

Case Study

Design and analysis of Gota Tunnel (Götatunneln)

  • Detailed 3D modelling of large concrete box tunnel structure
  • Creation of customised scripts for results processing
  • Smart load combinations and slice sectioning of model

Skanska Teknik AB, of Sweden, used LUSAS Civil & Structural to help analyse and design a 128m long, reinforced concrete section of tunnel which carries road 45 beneath the city of Gothenburg. Customised scripts were developed by LUSAS Consultancy Services for specific use on the project to automate the results processing operations, produce graphs and screen plots, and output results data for spreadsheet calculations of reinforcement quantities using Skanska Teknik AB’s own in-house software.

Overview

The main tunnel design contract was split into three with the Western cut-and-cover section at Järntorget being designed by Skanska Teknik AB. This contract comprised a 96m open cross-section, a 128m long twin reinforced concrete box tunnel with an integral access ramp structure, ventilation shaft and 30m high chimney, and a 276m long twin-box tunnel. The 128m long access ramp/tunnel structure carries two carriageways of traffic in both directions and is also subject to vehicle loading on its roof due to a car park and access roads above. The tunnel walls and roof slab are 1m thick. The floor slab varies in thickness from 0.8 to 1.5m. In all, nearly 1000 piles support the structure.

Modelling in LUSAS

Almost 22,000 elements were used to model the complex tunnel structure to the required level of detail. Thick shell elements - used so that out-of-plane shears could be extracted - modelled the concrete structure. Joint elements modelled the end bearing and frictional piled supports.

Load cases applied to the model included:

  • Differential settlement, modelled as a sine curve along and across the tunnel centre line.
  • Temperature gradient and shrinkage
  • Soil and water pressure
  • Varying water level
  • Vehicle loads - manually evaluated

ULS and SLS Load Combinations

In LUSAS, the Smart Combinations facility can automatically generate maximum and minimum load combinations for any selected design code from applied loadings and allows for adverse or relieving effects. From these load combinations, envelope, contour and deflected shape plots and results graphs can be readily obtained for any loadcase under consideration. However, on this particular project the Smart Combinations facility was combined with customised scripts written by LUSAS Consultancy Services to automate the output of the results processing and produce results for additional spreadsheet calculations.

Bending moments from axle loading on roof

Scripting in LUSAS

Of many custom scripts written, a model sectioning tool was developed to allow a cut section to be taken between selected points, nodes or at specified coordinates on the model. The location of the section could then be stored and named in a database for future use. Chosen results of interest for the selected loadcase were then automatically plotted on a corresponding graph. In this way plots of either stress, shear or displacement could be readily obtained for each active loadcase allowing rapid assessment of the structure along its length. By changing the active loadcase and re-selecting the same cut name from the database a whole series of results for the many load cases and load combinations could be obtained.

To calculate reinforcement quantities Skanska Teknik AB used stress resultants calculated by LUSAS in its own in-house Microsoft Excel spreadsheets. A load combination listing script in LUSAS exported basic and smart load combination definitions and envelopes (including load cases used and load factors applied) to a spreadsheet for quality assurance, and cross-checking purposes.

Benefits of using LUSAS

Mathias Uhlán, Structural Engineer at Skanska Teknik AB said: ‘Since the geometry was so complicated it would have been difficult to model the tunnel in 2D, and in order to do so a far more conservative approach would have had to be taken. But by using a 3D LUSAS model we could slice sections through the model and easily plot the results we wanted at our chosen locations.’ He continues: "Ultimately, the main benefit in using LUSAS for this project was to be able to take into account the complex geometry of the structure and to get all of the results we required from the one model."

"...the main benefit in using LUSAS for this project was to be able to take into account the complex geometry of the structure and to get all of the results we required from the one model."

Mathias Uhlán, Skanska Teknik AB


 

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