VALIDATION OF DISPLACEMENT-BASED ASSESSMENT PROCEDURES FOR ROCKING ELEMENTS IN MASONRY STRUCTURES

Abstract eng:
The observation of damage caused by recent and past earthquakes has demonstrated the high seismic vulnerability of historic masonry structures, especially referring to the activation of out-of-plane mechanisms in masonry walls and rocking response of stone or masonry elements. These latter include both free-standing elements in ordinary and monumental buildings (such as parapets, battlements of fortresses, soaring portion of church façades), and artistic artefacts (as statues, pinnacles or balustrades), all characterized by an intrinsic seismic vulnerability, due to the high slenderness and the small size (thickness). The complexity of nonlinear dynamics of rocking systems promoted in the past research aimed to understand the peculiarities of the rocking dynamic response of rigid bodies. However, since the use on nonlinear dynamic analyses is rather demanding and not practice-oriented, more recently different seismic assessment procedures have been proposed within the ambit of the Displacement-Based Approach (DBA). DBA methods have been sometimes already adopted in Standards, but a systematic validation and comparison of their results have not been provided yet. Furthermore, many Authors raised doubts on the actual reliability of DBA procedures due the inherent dynamic instabilities and chaotic behaviour characterizing the rocking response. In this framework, the paper aims to assess the reliability of four DBA methods currently available in literature and Standards and based on the equivalent static approach. The considered DBA methods are: New Zealand Guidelines for Assessment and Improvement of Existing Buildings, 2006; its updated version as issued in April 2015; Italian Technical Building Code, 2008; a recent paper by Lagomarsino, 2015. With the aim of validation, Incremental Dynamic Analyses (IDA) have been performed by considering several real accelerograms and three different blocks. The blocks were characterized by various size and slenderness, in order to be representative of typical masonry elements prone to rocking like pinnacles, parapets or planebelfries. Furthermore, they have been alternatively assumed resting on the ground floor or at the top of a masonry main building. The results of the IDA (in terms of Peak Ground Acceleration leading to the overturning of the block) have been considered as reference solution and then compared with the ones obtained by applying the considered DBA methods. These latter differ in the assumed ultimate displacement capacity and the equivalent period used for the evaluation of the displacement demand from the accelerationdisplacement response spectrum. The comparison in terms of fragility curves and failure probability assessment allowed to evaluate safety margins, robustness and reliability guaranteed by the static previsions. The results of the validation showed that not all the proposals present in literature are equivalent and, in particular for two of them, highlighted that the equivalent static approach provides a coherent and reliable estimate of overturning of rocking masonry element, with the aim of checking the Near Collapse Limit State. In fact, under proper definitions of the ultimate displacement capacity and of the procedure to evaluate the seismic displacement demand, the DBA approach turned out reliable for both ground and atop rocking elements. Moreover, the performed nonlinear dynamic analyses clearly highlighted that the displacement demand is very sensitive to the intrinsic features of each record and the assessment cannot be made by assuming only a mean spectrum, as usually adopted in Standards. To this aim, a new proposal is currently underway to correct the mean spectrum and improve the reliability of the selected DBA procedure.

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