Structural Design Generalities and critical elements of the design The structural design of the construction was made entirely by Besix Design Department. The high degree of complexity of this design is due to: • The project has numerous and very critical structural irregularities in the foundations and the superstructure. • The shape, the height as well as the presence of surrounding high rise buildings make it especially sensitive to wind effects. For these reasons, a wind tunnel test was made on a physical model. These tests have been used to define the wind load on the structure and to confirm that local turbulence does not affect the safety of pedestrians using the adjacent public spaces. Moreover the wind action induces dynamic effects increasing the forces in the structure and reduces the comfort of the occupants. On this last aspect, the literature specifies that the level of discomfort is reached when the horizontal acceleration that users are subjected to exceeds 0.02g or 0.2m/sec². Consequently, the design includes a detailed analysis of the wind effects, both on the induced forces and the horizontal acceleration. • The dynamic behaviour of a building as slender as the CBX tower depends also significantly on the soil - structure interaction that has therefore been thoroughly examined. • The asymmetrical and hybrid foundation system requires a precise calculation of the differential settlements. • The average vertical compression stress in the columns is much higher than in the core walls. This means, for high rise buildings, very significant differential displacements between the central core and the façade columns due to the elasticity, shrinkage and creep. The design includes a detailed examination of these displacements and required compensations at each level. Modelling and structural calculation A complete 3D model of the tower including all slabs, beams, walls, columns, piles and the raft was made with ESA-Prima Win. The details of the model incorporate even the main openings (up to 2 m²) in the walls and slabs, the different type of concrete grades, the cracked inertia of the elements subject to bending and the springs simulating the soil - structure interaction. The model includes all loads on the slabs as well as the wind forces from 36 different angles. Considering the exceptional size of the model, three different users worked, in a first stage, on three separate parts. These were subsequently assembled to constitute one complete model of the building. For the typical floor, the possibility to copy and paste groups of elements was invaluable for the efficiency of the model construction. These programming advantages enabled the engineers to complete a model running on PC within three weeks. The size of the mesh used for the general down load path calculation and the dynamic behaviour of the tower was of about 2m. This size was locally refined to detail the calculation of the forces in parts of the structure such as on one floor level or walls on three levels. The results obtained from the complete 3D model were compared with partial model and hand calculations. This has specifically been the case for the down load path, the transfer elements and the slabs. These comparisons showed that the different approaches were either corresponding or complementary. It is also to note that, for such large model, the various possibilities of graphical viewing are essential for the efficiency or the result interpretation and model control. The calculation of the horizontal acceleration resulting from the dynamic behaviour of the tower under the wind effects was deducted from the methods given in the Eurocode 1 EN 1991-14.6 and the CTICM n°3, wherein the parameters of frequency and static horizontal displacement under peak wind loads, were calculated with the complete 3D model The analysis of the settlements and soil-structure interaction, necessary to define the stiffness of the springs to locate at the foot of the model, was made with the geotechnical computer program PLAXIS and a hand calculation approach. 99
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