Seismic Isolation of the Great Mosque of Algiers

Abstract eng:
The Great Mosque of Algiers (Djamaâ El Djazïr) will be the third largest in the world and shall be completed in 2016/2017. The mosque building has a plan size of 145 x 145 m and a height of 65m while hosting up to 32,000 people. This main building was decided to be placed on 246 nos. seismic curved surface sliding isolators to mitigate the specified seismic PGA of 0.65 g. The isolation period of the system was chosen to be 3,1 s, to achieve sufficient reduction of incoming ground accelerations. The isolators must carry more than 27,000 kN each. The necessary damping for displacement limitation is provided by approx. 30 % from 3 % sliding friction within the isolators and by approx. 70 % from additional hydraulic dampers. This system limits the horizontal seismic displacements within a specified value of +/-500 mm, which was even magnified by the g x reliability factor of 1.2 (EC 8) to +/-600 mm. The additional 80 nos. hydraulic dampers with 2,500 kN capacity not only provide additional damping to limit the max. seismic displacements to 500 mm, but they allow a more smooth damping and shear force depending on the seismic impact. Thus the damping performance can be assumed to be adaptive. The dampers display a soft force development for velocities up to 400mm/s with a damping exponent of 0.4, what creates less response forces for minor to medium earthquakes. This characteristic was also not possible to be created alone – without dampers - by the applied curved surface sliders with defined high friction levels of approx. 8%, as the high friction would be always active – also for minor earthquakes and results in bad re-centring combined with great shear. For the maximum credible earthquake the dampers have been equipped with an integrated force limiter function starting from 1,000mm/s and being active up to 1,200mm/s in order not to overload the damper, the brackets, the anchoring or the structure. The CE marking and testing according to EN15129 in University of California San Diego and EU Center Pavia was a must for this project. A theoretical service life span of 500 years was proven by wear and fatigue testing. The author will present in this paper the design considerations and testing of curved surface sliders and hydraulic dampers to be applied within the huge Djamaâ El Djazïr Mosque in Algiers.

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