ارزیابی مقاوم سازی لرزه ای ساختمان های غیرمسلح با استفاده از مدل های عددی و اندازه گیری های لرزه ای محیطی Evaluation of the seismic retrofitting of an unreinforced masonry building using numerical modeling and ambient vibration measurements

1. Introduction Since a large part of buildings around the World and especially in Northern Europe, were built without or with insufficient seismic provisions, seismic assessment of existing buildings is a critical and endless issue to be solved by earthquake engineering [1]. Cost-benefit approaches for the assessment and retrofit of existing buildings in Switzerland started to be used on a day-to-day basis in 2004 with the Prestandard SIA2018 [2]. This Pre-standard provides guidelines on the measures to undertake to improve the seismic safety of a building taking into account the seismic risk, and in case, if retrofitting measures are commensurate. The results provided by the prescribed method are, however, quite rough, and the effects of an eventual retrofit needs to be more finely quantified. Different seismic retrofitting measures have been proposed for unreinforced masonry (URM) buildings. Examples are: adding sandwich columns to partition brick walls [3], jacketing of columns, adding structural walls, and construction of a mat foundation [4], reinforced cement jacketing of the main load carrying walls [5] and using FiberReinforced Polymers [6,7]. The effect of those retrofitting measures has been assessed using a variety of methods including Finite Element numerical modeling [1,8] and vibration measurements [9]. One of the common measures to retrofit existing URM buildings in Europe is to create composite slabs by adding a reinforced-concrete (RC) layer over the existing wooden floor or even to replace it by a RC floor. This measure is generally proposed to improve the comfort and the soundproofing of the building, and is thought to be beneficial for the seismic behavior, as well. As a result, the masonry walls are linked together creating a diaphragm effect, which also prevents out-of-plane collapse of the walls [10,11]. This paper presents the evaluation of the retrofitting measures on a typical residential building in Switzerland [12]. The studied building is a 6-story simple stone unreinforced masonry building located in Lausanne Switzerland (Fig. 1) built in the end of the 19th century. The quality of masonry is poor since different bloc sizes and materials were used. The width of the walls varies from 25 to 70 cm, and its average story height is 3 m (Fig. 2). The mortar quality was also observed to be poor as it would crumble in hand under fingers’ pressure. The structure is regular and not surrounded by any other building. The structural plan is rectangular (14 m by 12 m), with wooden attics at the top (6th floor) and thinner walls at the 5th floor. Originally, no rigid diaphragm existed in the building, as shown in Fig. 3a. Retrofitting measures were taken primarily in order to improve the soundproofing of the building.