Spectre de réponse

Résumé

Les spectres de réponse tirent leur origine et leur intérêt de l'assimilation, en première approximation, du comportement sismique d'un bâtiment à la réponse d'un oscillateur simple à un degré de liberté. La représentation en spectre de réponse vise à donner accès directement aux mouvements subis par le centre de gravité de la structure. La représentation graphique d’un spectre de réponse est construite point par point en faisant varier la fréquence d’un oscillateur à un degré de liberté et en reportant cette fréquence en abscisse et le maximum temporel de la réponse à une sollicitation sismique en ordonnée. On obtient ainsi un spectre de réponse pour un amortissement fixé. Les spectres de réponse élastiques sont très utilisés en ingénierie parasismique car, en première approximation, les structures simples peuvent être assimilées à un oscillateur à un degré de liberté dont la fréquence propre et l'amortissement sont à peu près connus. Les mouvements au centre de gravité peuvent al

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Seismic risk analysis, at large scale and local scale, is fundamental support for mitigating the investment costs and recovery works, impacting society's economy. The presented article's main purpose is to present a mechanicalbased model to evaluate the fragility curves through nonlinear static analysis. The results achieved from statistical analysis allows defining a representative building stock for two height classes. Material properties for typical masonry typologies that characterise the unreinforced masonry building heritage for the specific area of investigation are assigned to the prototype buildings. The paper proposes a procedure for fragility function evaluation to reduce computation time and limit the results' loss of reliability. A technique based on macro-element formulation and threedimensional equivalent frame model is proposed to evaluate the assessed buildings' seismic response. The proposed model includes implementing the macro-element formulation in open-source software, Opensees, that allows higher versatility and the possibility to include in the analysis sources for better reliability and representation regarding the real building. Equivalent frame modelling allows an efficient way to represent the seismic response and a compromise between computation costs and high accuracy. Modelling technique validation is proposed by comparing results derived on specified studies concerning, ambient vibration and seismic response. A limited set of capacity curves is generated from the aleatory variation on the material properties. Then, the number of capacity curves is expanded by a correlation between the seismic response parameters. The increase in capacity curves aims to cover the variability on the structural capacity. Fragility curves are then derived from a stochastic selection of response spectra, by using GMPE allowing to estimate the probability distribution of each damage state, as a function of the reached displacement. Reliability of the mechanical fragility curves is proved by comparing recent studies, involving both empirical and mechanical based fragility functions. The assessment's feasibility is demonstrated by applying the procedure over most representative prototype building configurations for the Switzerland city of Basel.

2021

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Steel concentrically braced frames (CBFs) as lateral load resisting system are common in steel construction, for their high strength and architectural versatility. However, the brace connection details have to be well designed to ensure a ductility of the frame under seismic excitation. This Master Thesis studies the behavior of a steel CBF, located in Sendai (Japan). It experienced significant damage at brace connections after the 2011 Tohoku-Oki Earthquake in Japan. They are mainly due to the eccentricity created by the single-lap gusset plates along the long duration of the imposed ground motion hazard. The seismic response of the CBF is assessed by means of nonlinear analysis with the performance-based approach according to ASCE/SEI 41-13 (2014). As the braces do not meet any performance criteria, seismic retrofit is required. Three retrofit solutions are investigated and compared. Two of them are considered as conventional solutions. They consist to replace the braces and gusset plates, once with new Chevron braces and once with new X braces. The new braces are designed with the Capacity Design rules. The third solution modifies the CBF into a Rocking Braced Frame (RBF). This innovative high-performance system allows the column bases to uplift in order to dissipate the seismic energy and localize the damages in easily replaceable fuses to protect the primary members. The retrofit works are mainly located at the column bases. Dynamic analyses of the initial frame and its retrofits are computed for a seismic input recorded during the 2011 Tohoku-Oki Earthquake. The peak storey drift ratio (SDR), residual SDR and story shear forces are compared. The new Chevron CBF has the worst performance of the three retrofits. New X-bracing CBF and the RBF show a similar performance, the RBF being slightly better.

2020

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Uplift mechanics of unanchored liquid storage tanks subjected to lateral earthquake loading

Konstantinos Bakalis

Motivated by the seismic response of unanchored liquid storage tanks, their uplift mechanism under strong lateral loading is examined. Using three-dimensional finite element models, nonlinear static analysis is conducted to define the moment-rotation relationship of uplifting tanks resting on rigid foundation and describe the evolution of critical response parameters with increasing level of lateral loading. Meridional and hoop stress, as well as their distribution and evolution with increased uplift are computed. Comparing the numerical results with the corresponding results from anchored tanks, striking differences are observed on the values of compressive meridional stresses and their distribution around the tank circumference. Cyclic analysis, associated with repeated uplift at both sides of the tank, is also performed, to obtain the corresponding hysteretic response and verify the assumption of nonlinear-elastic tank behaviour, used in several previous works. Finally, an analytical solution is developed, capable of describing tank uplift in an efficient manner. The analytical solution accounts for the special features of uplift, obtained from the finite element solution, and can be used for simple and reliable assessment of seismic performance in unanchored liquid storage tanks.

ELSEVIER SCI LTD2021

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2021