Issue 3

Due to expected capacity requirements for public transport during the 2006 Football World Cup, it was decided to enlarge the metro station Marienplatz, situated right beneath Munich city hall. The project concept finally approved provides for two extra tubes running in parallel to the existing ones. By connecting the new and old tubes with 11 cross cuts each approximately 3.10m wide, an intended doubling of the platform passenger capacity is achieved. The close vicinity of the new tunnel tubes to existing buildings – in particular the historic city hall ‘Neues Rathaus’ that was erected from 1867 to 1909 – puts a high demand on control and limitation of the soil movements to be expected during the construction process. While the original bid invitation proposed a lowering of the groundwater level, the solution finally chosen uses an innovative brine freezing technique, a specific proposal made by the executing consortium. As the analysis covers numerous non-linear construction stages involving parametric studies, it was essential to have a fast solution technique. On the other hand, simulation of the cross cutting process is a complex non-linear three-dimensional problem. Aiming at the best compromise between efficiency and required accuracy, a novel concept was adopted. It started with a classical two-dimensional finite element analysis of representative cross-sections using standard and approved techniques but was extended to account for the three-dimensional stress redistribution effects occurring during tunnel excavation. A special combined mesh generation and mapping technique is applied to create a three-dimensional model that inherits the load history induced by the tunnelling process and thus allows for simulation of the cross cut installation process. This combined approach enables the complex analysis task to be performed within a reasonable timeframe. Holger Heidkamp Casimir Katz SOFiSTiK AG, Oberschleissheim, Germany Christian Hofstetter Schmitt Stumpf Frühauf & Partner Ingenieurgesellschaft mbH, Munich, Germany

• Arches with low dead load, and therefore low prestress may be susceptible to traffic damage under loads currently applied. • Using current assessment rules these bridges appear to be safe. • Using a distribution model which is more in line with anticipated behaviour is capable of yielding results which allow an engineer to differentiate effectively between bridges which are safe and those which may be in danger. • A true representation of behaviour is not yet possible and may never be. • It is a matter of some urgency that the potential for over estimation of capacity is acknowledged and research put in hand to investi gate the causes thoroughly and test the models proposed here. • In the meantime, it is unsafe, and therefore inadmissible to suggest that the current model should be used until a better one is proved. It would be better to give no guidance at all, and allow individual engineers to make their own judgments than to continue to offer guidance which is known to be, at best, flawed. Bill Harvey, BSc PhD, CEng, FICE, FIStructE Director, Bill Harvey Associates Ltd, Exeter