Modal identification and dynamic analysis of a 1000 years old historic minaret of Kalaa Beni-Hammad

From structural and material point of view each historic monument is a textbook case, as it differs in shape, age, material characteristics and level of preservation. New means and approaches are used for geometric and shape survey, material and modal characterization, which helps restitute detailed information regarding the structural elements of the edifices. This paper investigates a 1000 years old historic minaret having a square base of 6.50 m of side and 24.70 m height. An ambient vibration survey has been carried out and the results were used to update a Finite Element (FE) model of the minaret in two stages. First, the elastic modulus of the rubble stone masonry has been determined by matching the experimental and numerical fundamental frequency. In a second phase, a frequency response function (FRF) correlation is carried out iteratively by adjusting the damping ratios to minimize the FRF error. The elastic modulus of the rubble stone masonry is found to be in the lowest range of values given in the literature, which confirms a deteriorated quality of the material. A seismic performance of the structure has been carried out using the updated FE model subjected to a set of acceleration time histories having the mean spectrum matching a compatible site spectrum. The stress concentration zones have been determined and a steel tie system to retrofit the structure is proposed. Beside the proposed investigation framework, the characterization results are valuable facts that can be added to the available database of historic monuments.