118 thema The European Central Bank 1 The new building for the European Central Bank (ECB) (fig. 1) consists of two peculiar towers connected by diagonals. It is located in the east of Frankfurt, between the historic wholesale market, the Großmarkthalle and the Main River. The Großmarkt- halle, an impressive brick building and monument from 1928 designed by Martin Elsaesser, was restored as part of its integra -tion within the construction of the new ECB towers. Due to the twist of the towers, also torsion effects had to be considered. The locations of the diagonal elements were generated parametrically in an evolutional process. Dynamic and exciting structure with twisted towers connected by diagonals thema The European Central Bank 3 2017 119 The architectural design process The design of the new building for the European Central Bank was developed during a competition in 2002. The geometry of the striking 180 m high double towers follow the 'deconstructi- vistic' idea of the architects Coop Himmelb(l)au to divide a block diagonally, then turn the inner faces outside and the bottom to the top (fig. 2). The two parts are combined into one unit by a full height glass atrium. The global structure design process The stability system is based on the core walls of each tower. In longitudinal direction the stiffness of these cores (fig. 3) was sufficient to resist lateral loads. In cross direction the core width is only 6 m. Thus the structural systems of each tower had to be extended from two single cores to a global system consisting of the two cores and a spatial truss system which connects the cores via the atrium (fig. 4). The cores act as Alexander Berger, Manfred Grohmann, Klaus Bollinger B+G Ingenieure, Bollinger und Grohmann GmbH 1 The Großmarkthalle and the ECB towers credits: Paul Raftery2 Design of the towers credits: Coop Himmelb(l)au 3 Element of the structural system: the cores credits fig. 3 t.m. 10: Bollinger + Grohmann4 Horizontal platforms and bridges and diagonal struts 1) Deconstructivism is a movement of postmodern architecture which appeared in the 1980s. It gives the impression of the frag- mentation of the constructed building and is characterized by an absence of harmony, continuity, or symmetry (Wikipedia). 2 3 4 The European Central Bank 3 2017 The European Central Bank 3 2017 120 thema upper and lower chord of a global truss system. While the loca- tion of the platforms, elevators and bridges in the atrium were geometrically fixed, all diagonal elements were generated in a parametric and evolutional process. The first models show certain design steps of the process, the last one is the final one (fig. 6). The governing criteria in this process were the eigen- frequency and the stiffness. Torsional effects Due to the inclined façade torsional effects have an impact on the towers. As both towers twist in the same direction, the global structural system, apart from wind loads, has also to resist these loads. Slabs and columns Due to the twist, all slabs have a different geometry and diffe- rent span (fig. 5). The columns near to the façade follow the incline, while inner columns are vertical and straight. 5 Slab geometry 6 Parametric design of the truss system 7 Construction stage: while the cores are nearly finished, the truss system follows up from approx. 60 m below 8 Installation of the trusses 9 Reinforcement of the foundation slab 10 Bracing, bridges and platform above 11 The finished structure inside the Atrium credits: Paul Raftery 6 5 7 8 121 Deformations and Pre-camber The vertical and horizontal deformations of each column and slab were calculated considering the construction process as well as time-dependent factors like creep and shrinkage. These deformations were considered during the construction process by pre-cambering of these elements. Construction process During construction the structural system changed from pure core systems to partially finished truss systems (fig. 7, 8 and 10). For each construction stage the resistance of the structure had to be proved. The foundation All the loads of the towers are transferred into the ground by a combined pile-raft foundation (fig. 9). Conclusion With the construction of the new European Central Bank in Frankfurt a highly dynamic and exciting structure was realized. The design of the architects Coop Himmelb(l)au has been accompanied by the B+G structural engineers from the very beginning, i.e. already during the competition. Thereby, an opti- mized structure for this building has been developed. Through its expressiveness, it emphasizes and makes the seat of ECB archi- tecturally appealing and standing out from afar (photo 1). ? ? REFERENCES 1 Brensing, C., Bollinger, K., Grohmann, M., & Stracke, M. (2015). Architektur und Tragwerk in symbiotischer Beziehung - Der Neubau der europäischen Zentral- bank in Frankfurt. Ingenieurbaukunst 2016, Berlin: Ernst & Sohn. 2 Santifaller, E. (2013). Symbolic metamorphoses ? The European Central Bank building. Detail Engineering 3, (1st edition), pp. 42-51. 3 König, G., Liphardt,S. (2003). Hochhäuser aus Stahlbe - ton. Betonkalender I, Berlin: Ernst & Sohn, pp. 1-66. 4 Ruscheweyh, H. (1982). Dynamische Windwirkung an Bauwerken. Teil 1: Grundlagen der Anwendung. Teil 2: Praktische Anwendung. Wiesbaden: Bauverlag. 5 Krebs, A., Constantinescu, D. (1999). Der Einfluss der Axialverformung der vertikalen Tragglieder von Hochhäusern. Tagungsband Darmstädter Hochbau- Seminar, Hochhäuser. 6 Eurocode 2, Design of concrete structures - Part 1-1: General rules and rules for buildings. German version EN 1992-1-1:2004 + AC:2010. 9 10 11 The European Central Bank 3 2017