Seismic Assessment of Single-Block Rocking Elements in Masonry Structures

Abstract eng:
The paper focuses on the seismic assessment of single-block rocking elements in masonry structures, prone to rocking and overturning failure. They include both free-standing elements in ordinary or monumental buildings (such as parapets, battlements of fortresses, soaring portion of church façades), and single artistic artefacts (as statues, pinnacles or balustrades). As demonstrated by recent and past earthquakes, they are all affected by a significant seismic vulnerability, due to their high slenderness and small size (thickness). Since the use on nonlinear dynamic analyses is rather demanding and not practice-oriented, despite the complexity of nonlinear dynamics of rocking systems, 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 of their results have not been provided yet. The latter motivates the focus of this paper, that is the reliability assessment of four DBA methods available in literature and standards, that 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. To pursue the validation aim, three different blocks were analysed: by exploring various size and slenderness, they have been conceived to be representative of typical masonry elements prone to rocking like pinnacles, parapets or plane-belfries. Incremental Dynamic Analyses (IDA) have been performed by using several real accelerograms. Their results, in terms of Peak Ground Acceleration leading to overturning of the block, were considered as reference solution, to be compared with the ones obtained by applying the considered DBA procedures. These latter differ in the assumed ultimate displacement capacity and the equivalent period to be used for the evaluation of the displacement demand from the acceleration-displacement response spectrum. The three blocks were assumed alternately resting on the ground floor or at the top of a masonry main building. The comparison in terms of fragility curves allowed to evaluate safety margins, robustness and reliability guaranteed by the static previsions. The results of the performed analyses (which could be expanded in the future by considering further case studies and ground motions) showed that not all the proposals present in literature are equivalent and, in particular for two of them, confirmed the potential reliability of the equivalent static approach instead of a more demanding dynamic one for the seismic assessment of rocking elements. 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.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket