Shake-table test on a four-storey structure with reinforced concrete and unreinforced masonry walls

With the introduction of higher seismic design forces in the Swiss loading standard of 2003 most unreinforced masonry (URM) buildings failed to satisfy the seismic design check. For this reason, in new construction projects, a number of URM walls are nowadays replaced by reinforced concrete (RC) walls. The lateral bracing system of the resulting structure consists therefore of URM walls and some RC walls which are coupled by RC slabs and masonry spandrels. Within the scope of a FP7-Series project a four-storey RC-URM wall structure is tested on the shake-table at the TREES laboratory of the EUCENTRE in Pavia (Italy). The test is conducted at half-scale and is part of a larger research initiative on mixed RC-URM wall systems initiated at EPFL. The key objective of the test is to gain insights into the dynamic behaviour of RC-URM wall structures and to provide input for the definition of a performance-based design approach of such mixed structural system. This paper presents details on the structural system, the instrumentation and the selected ground motion and discusses preliminary results of the shake-table test.

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

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

Evaluation of the seismic retrofitting of an unreinforced masonry building using numerical modeling and ambient vibration measurements

Amin Karbassi, Pierino Lestuzzi, Clotaire Michel

Ambient vibration measurements and 3-D nonlinear time-history numerical modeling are used to assess the retrofitting measures conducted in a 6-story unreinforced masonry building (URM) built in the end of the 19th century in Switzerland. Retrofitting measures were taken in order to improve the soundproofing and possibly the seismic performance of the building. Reinforced concrete (RC) footings were added under the walls and horizontal steel beams were added to link the walls together with a RC slab at each floor, though the wooden beams were left in place. Several ambient vibration recordings were performed before, during and after the retrofitting work in order to monitor the evolution of the dynamic behavior of the structure. Moreover, numerical models representing the state of the building before and after the retrofit work have been developed to perform nonlinear dynamic analyses using various ground motion records. The change in the modal vibration frequencies, mode shapes, and failure mechanism are presented and discussed in further details. According to ambient vibration measurements, the performed retrofitting resulted in an increase of about 25% of the fundamental frequency. From the results of both the numerical modeling and the ambient vibration measurements, it is confirmed that the in-plane behavior of the slabs evolved from non-rigid floors with in-plane deformation to rigid floors with diaphragm effects. The ambient vibration measurements show that the new stiff slabs could lead to torsion behavior in the building as the result of the diaphragm effect and to higher seismic demand. However, the numerical models show that the displacement capacity of the building increases as a result of those new stiff slabs. Consequently, higher deformation capacity, indicated by the inter-story drift values, on average, are observed for all the damage grades in the post-retrofit state of the building. Finally, the overall seismic safety was only slightly improved.