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

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.

Seismic Behaviour and Design of Mixed RC-URM Wall Structures

Alessandro Paparo

In several countries of central Europe, many modern residential buildings are braced by reinforced concrete (RC) and unreinforced masonry (URM) walls coupled with RC slabs. Such mixed constructions, in fact, behave better under seismic loading than buildings with URM walls alone. Similarly, the insertion of RC walls is a technique used to retrofit existing modern URM constructions that feature RC slabs, since both strength and displacement capacities of the repaired structure increase with respect to the un-retrofitted configuration. Although modern mixed RC-URM wall buildings are rather common, very little is known about their seismic behaviour and codes do not provide adequate design and assessment guidelines. Hence, the thesis focuses on four objectives: (1) to provide experimental data on the seismic behaviour of mixed RC-URM wall structures; (2) to formulate recommendations for setting up numerical models for structures with both RC and URM walls; (3) to develop a mechanical model that represents the interaction between RC and URM walls; (4) to propose a displacement-based seismic approach for the design of new mixed RC-URM wall structures and the retrofitting of URM wall buildings by adding RC walls or replacing URM walls with RC ones. An experimental investigation of two mixed RC-URM wall substructures leads to new findings with respect to the actual distribution of the reaction forces between RC and URM walls and gives insight into the displacement profile of mixed RC-URM wall structures. The results are used to validate two numerical strategies and recommendations for setting up models for structures with both RC and URM walls are formulated. A simple mechanical model, which takes into account the most important parameters influencing the seismic behaviour of mixed RC-URM wall structures, is proposed and validated. The model is based on the shear-flexure interaction as URM and RC walls display dominant shear and flexural deformations, respectively. In the last part of the thesis a displacement-based design approach for mixed RC-URM wall structures is developed. The design method is verified through inelastic time-history analyses (ITHA) of three-to-five-storey case study buildings. Comparison between the design values and the results from ITHA suggests that the design methodology controls the horizontal deflection of the structures, being almost linear over their height, and avoids concentrations of deformations in the bottom storey, a typical feature of URM wall structures. On the other hand, for the three-storey configurations, it was observed that the approach overestimates the maximum horizontal displacement.

EPFL2015

Development of a displacement-based design approach for modern mixed RC-URM wall structures

Katrin Beyer, Alessandro Paparo

The recent re-assessment of the seismic hazard in Europe led for many regions of low to moderate seismicity to an increase in the seismic demand. As a consequence, several modern unreinforced masonry (URM) buildings, constructed with reinforced concrete (RC) slabs that provide an efficient rigid diaphragm action, no longer satisfy the seismic design check and have been retrofitted by adding or replacing URM walls with RC walls. Of late, also several new construction projects have been conceived directly as buildings with both RC and URM walls. Despite the widespread use of such construction technique, very little is known about the seismic behaviour of mixed RC-URM wall structures and codes do not provide adequate support to designers. The aim of the paper is therefore to propose a displacement-based design methodology for the design of mixed RC-URM edifices and the retrofit of URM buildings by replacing or adding selected URM walls with RC ones. The article describes also two tools developed for estimating important quantities relevant for the displacement-based design of structures with both RC and URM walls. The tools are (i) a mechanical model based on the shear-flexure interaction between URM and RC walls and (ii) an elastic model for estimating the contribution of the RC slabs to the overturning moment capacity of the system. In the last part of the article the proposed design method is verified through non-linear dynamic analyses of several case studies. These results show that the proposed design approach has the ability of controlling the displacement profile of the designed structures, avoiding concentration of deformations in one single storey, a typical feature of URM wall structures.

Techno-Press2015

Fragility functions for seismic risk in regions with moderate seismicity

Pierino Lestuzzi, Clotaire Michel, Marcelo Rafael Oropeza Ancieta

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.

2010

Seismic Behaviour and Design of Mixed RC-URM Wall Structures

Alessandro Paparo

EPFL2015