دانلود رایگان مقاله انگلیسی مقاوم سازی لرزه ای ساختمان های با مدل های عددی و اندازه گیری لرزه ای محیطی - نشریه الزویر

ABSTRACT

Ambient vibration measurements and 3-D nonlinear time-history numerical modeling are used to assess the retrofitting measures conducted in a 6-story unreinforced masonry building (URM) built in the end of the 19th century in Switzerland. Retrofitting measures were taken in order to improve the soundproofing and possibly the seismic performance of the building. Reinforced concrete (RC) footings were added under the walls and horizontal steel beams were added to link the walls together with a RC slab at each floor, though the wooden beams were left in place. Several ambient vibration recordings were performed before, during and after the retrofitting work in order to monitor the evolution of the dynamic behavior of the structure. Moreover, numerical models representing the state of the building before and after the retrofit work have been developed to perform nonlinear dynamic analyses using various ground motion records. The change in the modal vibration frequencies, mode shapes, and failure mechanism are presented and discussed in further details. According to ambient vibration measurements, the performed retrofitting resulted in an increase of about 25% of the fundamental frequency. From the results of both the numerical modeling and the ambient vibration measurements, it is confirmed that the in-plane behavior of the slabs evolved from non-rigid floors with in-plane deformation to rigid floors with diaphragm effects. The ambient vibration measurements show that the new stiff slabs could lead to torsion behavior in the building as the result of the diaphragm effect and to higher seismic demand. However, the numerical models show that the displacement capacity of the building increases as a result of those new stiff slabs. Consequently, higher deformation capacity, indicated by the inter-story drift values, on average, are observed for all the damage grades in the post-retrofit state of the building. Finally, the overall seismic safety was only slightly improved.

5. Conclusions

A numerical modeling and ambient vibration measurements were used to assess the retrofitting measures conducted in an URM building aiming at improving its soundproofing and also its seismic behavior. The major part of the retrofitting work concerned the addition of a stiff slab at each floor. From the results of both the numerical modeling and the ambient vibration measurements, it is confirmed that the in-plane behavior of the slabs evolved from non-rigid floors with in-plane deformation to rigid floors with diaphragm effects. This will be relevant under earthquake only if the connections work well, something that could not be assessed from either the numerical modeling or the ambient vibration measurements. The numerical modeling pointed out that the failure mechanism of the URM building in the pre-retrofit status transformed from a local (out-ofplane) to a global (in-plane failure of the walls) behavior when subjected to strong motions. The numerical models also showed that the displacement capacity of the building increased as a result of the stiff slabs, and on average, higher inter-story drift values were observed for all the damage grades in the post-retrofit model. The ambient vibration measurements showed an increase of about 25% in the modal frequencies of the URM building due to the retrofitting work. However, this stiffness increase (or loss of flexibility) could not be explain with the modeling which is the strongest limitation of our study. Such a change has an influence on the seismic demand that the structure could resist. According to the scenarios performed with the numerical model, the seismic safety was only slightly improved: 50% of the scenarios end up with the same level of damage, while about 20% led to higher damage and 30% to lower damage. It should be mentioned that this study did not account for the local hazard that might influence the resulting safety for a given location.

Even if all discrepancies between the model and the observation could not be resolved, the simultaneous use of these tools allowed a better understanding and quantifying of the seismic vulnerability of the structure. This combination is necessary to assess existing buildings since their seismic behavior remains poorly known.