Fragility functions for seismic risk in regions with moderate seismicity

Current seismic risk evaluations are mostly based on approximate vulnerability curves. Damage prediction is improved by considering structural characteristics of building. Such improved fragility functions do not exist for many structures, especially in countries with moderate seismicity. Typical structures of these regions, such as unreinforced masonry buildings URM, differ notably from structures usually considered in seismic risk. Due to low perceived seismic hazard, these types of structures have not been adequately investigated. Moreover, experimental investigations have shown that their deformation capacity is often underestimated. As a consequence, advanced methodologies have not yet been developed. This paper describes the results obtained with an analytical displacement-based methodology to assess the seismic risk in existing buildings through fragility functions. The study is based on URM existing buildings, common in North Western Europe. These low or mid-rise buildings with basements generally do not have significant structural plan irregularities. The impact and the accuracy of several parameters (such as the damage grade definition, the prediction of the ultimate drift, and the strength of the walls) are addressed. The fragility curves obtained are compared to those obtained using empirical methods in a typical existing URM building.

Fragility Functions for Seismic Risk in Regions with Moderate Seismicity

Marcelo Rafael Oropeza Ancieta

The seismic assessment of existing buildings has been underestimated in regions with moderate seismic hazard, such as the North-West Europe. Strong earthquakes, such as the 1356 Basel earthquake, already occurred in such regions and caused severe damage. The majority of the building stocks in those regions were built before the introduction of the modern building codes. Unreinforced masonry structures are widely used, and post-earthquake surveys show that such kind of structures are vulnerable under seismic actions. Moreover, existing reinforced concrete structures, which do not integrate current seismic features, are also highly vulnerable. Fragility analysis, which permits to estimate the probability (of an individual building or a building class) of reaching or exceeding a damage grade under a given earthquake load level, arises as a useful tool to evaluate seismic risk in existing buildings. Although several methodologies to derive fragility curves already exist, they consider building typologies which are not necessarily representative of regions with a moderate seismic hazard. The methodology presented in this dissertation allows engineers to derive fragility curves for unreinforced masonry and reinforced concrete structures, using a simplified and analytical methodology. Therefore, analyses for building classes or preliminary results for individual buildings can be obtained. The procedure is based on an improved static non-linear approach, and on analytical definitions of damage grades. In general, existing buildings can develop a moderate torsional behaviour; hence, the methodology integrates that phenomenon in terms of damage grades. With those damage grades, and considering the epistemic and aleatory uncertainties inherent in the analysis, fragility curves can be derived. Given that spectral parameters are better descriptors of earthquakes than other ground motion parameters, fragility curves are expressed in terms of spectral displacement. The methodology is capable to derive damage estimations in a reasonable amount of time, compared to more detailed analytical approaches. The results obtained in terms of capacity curves, modal parameters and probability of damage (an application of fragility curves) show a satisfactory correlation with experimental data. Finally, several real buildings are studied and the corresponding fragility curves and probability of damage under a seismic scenario are presented and compared to those of existing methodologies.

EPFL2011

Observations on out-of-plane behaviour of URM walls in buildings with RC slabs

Katrin Beyer, Marco Tondelli

In Switzerland many new residential buildings are constructed as unreinforced masonry (URM) structures or as mixed structures where URM walls are coupled with reinforced concrete (RC) walls by RC slabs. At present the boundary conditions of URM walls subjected to out-of-plane accelerations are still not well quantified. In the framework of a large research activity on RC-URM wall structures a shake-table test on a four-storey mixed structure was performed. The test specimen, which was built at half-scale, was designed in such a way to allow the collection of data on the effect of boundary conditions provided by the RC slabs and adjacent RC walls on the out-of-plane seismic behaviour of URM piers. This paper presents observations on the out-of-plane behaviour of the URM walls from the shake-table test and draws conclusions for the boundary conditions to be applied in numerical studies on URM piers in mixed RC-URM wall structures.

2014

Performance-based approach for the retrofit of URM wall structures by RC walls

Katrin Beyer, Alessandro Paparo

In several countries of moderated seismicity, with the re-evaluation of the seismic hazard most unreinforced masonry (URM) buildings with reinforced concrete slabs failed to satisfy the seismic design check. A possible seismically retrofit solution consists of adding RC walls to the existing structure or replacing selected critical URM walls with RC ones. Experimental and numerical studies have shown that this retrofit technique can be very effective since it modifies the global deformed shape of the structure, leading to an increase in the system’s displacement capacity. The paper proposes a Displacement-Based Design methodology for the retrofit of URM structures by replacing selected URM walls by RC walls. The methodology follows the Direct Displacement-Based Design (DBD) approach by Priestley et al. (2007) and is based in particular on the DBD procedure for frame-wall buildings (Sullivan et al., 2005 and 2006). The design procedure consists of three main phases: (i) A preliminary design check of the URM building by means of the DBD approach. (ii) If the structure does not fulfil the design check and exhibits at the same time a dominant shear behaviour, replacing the critical URM wall or walls with RC ones leads to an improved system’s behaviour. (iii) In the final phase, the DBD design of the mixed RC-URM wall structure, in which both the URM and the RC walls are taken into account, is carried out. The design methodology is then investigated through non-linear dynamic analyses of one case study.

2014

Fragility Functions for Seismic Risk in Regions with Moderate Seismicity

Marcelo Rafael Oropeza Ancieta

EPFL2011