Francesco Graziotti

Shaking table test on a full scale URM cavity wall building

A shaking table test on a two-storey full scale unreinforced masonry (URM) building was performed at the EUCENTRE laboratory within a comprehensive research programme on the seismic vulnerability of the existing Dutch URM structures. The building specimen was meant to represent the end-unit of a terraced house, built with cavity walls and without any particular seismic design or detailing. Cavity walls are usually composed of an inner loadbearing leaf and an outer leaf having aesthetic and weather-protection functions. In the tested specimen, the loadbearing masonry was composed of calcium silicate bricks, sustaining two reinforced concrete floors. A pitched timber roof was supported by two gable walls. The veneer was made of clay bricks connected to the inner masonry by means of metallic ties, as seen in common construction practice. An incremental dynamic test was carried out up to the near-collapse limit state of the specimen. The input motions were selected to be consistent with the characteristics of induced seismicity ground motions. The article describes the characteristics of the building and presents the results obtained during the material characterization and the shaking table tests, illustrating the response of the structure, the damage mechanism and its evolution during the experimental phases. All the processed data are freely available upon request (see http://www.eucentre.it/nam-project).

A nonlinear SDOF model for the simplified evaluation of the displacement demand of low-rise URM buildings

The determination of displacement demands for masonry buildings subjected to seismic action is a key issue in the performance-based assessment and design of such structures. A technique for the definition of single-degree-of-freedom (SDOF) nonlinear systems that approximates the global behaviour of multi-degree-of-freedom (MDOF) 3D structural models has been developed in order to provide useful information on the dependency of displacement demand on different seismic intensity measures. The definition of SDOF system properties is based on the dynamic equivalence of the elastic properties (vibration period and viscous damping) and on the comparability with nonlinear hysteretic behaviour obtained by cyclic pushover analysis on MDOF models. The MDOF systems are based on a nonlinear macroelement model that is able to reproduce the in-plane shear and flexural cyclic behaviour of pier and spandrel elements. For the complete MDOF models an equivalent frame modelling technique was used. The equivalent SDOF system was modelled using a suitable nonlinear spring comprised of two macroelements in parallel. This allows for a simple calibration of the hysteretic response of the SDOF by suitably proportioning the contributions of flexure-dominated and shear-dominated responses. The comparison of results in terms of maximum displacements obtained for the SDOF and MDOF systems demonstrates the feasibility and reliability of the proposed approach. The comparisons between MDOF and equivalent SDOF systems, carried out for several building prototypes, were based on the results of time-history analyses performed with a large database of natural records covering a wide range of magnitude, distance and local soil conditions. The use of unscaled natural accelerograms allowed the displacement demand to be expressed as a function of different ground motion parameters allowing for the study of their relative influence on the displacement demand for masonry structures.

Remarks on damage and response of school buildings after the Central Italy earthquake sequence

The seismic assessment of the vulnerability of existing public structures, especially school buildings, is a crucial issue in seismic prone countries. Recently, several national and regional programs and activities have focussed on the mitigation of Italian public buildings. They promote the scheduling of public buildings’ structural safety assessment and, when needed, the design and execution of strengthening interventions. Nevertheless, the three strong earthquakes that occurred in the last decade in Italy, Abruzzo (2009), Emilia (2012), and Central Italy (2016), confirmed the vulnerability of school buildings and the social importance of their quick re-opening after a damaging earthquake. In the present paper, the activities carried out on 1514 school building structures in the aftermath of the 2016 Central Italy earthquake sequence are reported and analysed. According to survey data collected by post-earthquake usability inspections, the paper analyses the school buildings characteristics, damage level and extent to structural and non-structural components as well as the correlation between seismic intensity and observed damage.

Dataset from the dynamic shake-table test of a full-scale unreinforced clay-masonry building with flexible timber diaphragms

This paper provides information related to the sensor measurements obtained from an unreinforced masonry building subjected to incremental dynamic shake-table tests at the EUCENTRE facilities in Pavia, Italy. These tests provide a unique data set that captures at full scale the in-plane and out-of-plane behavior of unreinforced masonry walls, and the influence of flexible diaphragms on the dynamic global response of a complete building. The authors made this information available to assist in the development of analytical and numerical models, necessary to estimate the dynamic response and the engineering parameters for the performance-based seismic assessment of unreinforced masonry buildings. All recorded data (acceleration and displacement time histories) and the videos of the tests can be requested online on the EUCENTRE repository at the URL www.eucentre.it/nam-project referring to EUC-BUILD-2. For further interpretation of the sensor recordings, and for a detailed discussion on the seismic performance of the building specimen, the reader is referred to the article entitled “Experimental seismic performance of a full-scale unreinforced clay-masonry building with flexible timber diaphragms” (Kallioras et al., 2018) [1].

Dynamic Shake-Table Tests on Two Full-Scale, Unreinforced Masonry Buildings Subjected to Induced Seismicity

This paper presents the results of an experimental campaign which is part of a wider research project, aimed at assessing the seismic vulnerability of buildings in the Groningen region of the Netherlands. The area, historically not prone to tectonic ground motions, has been subjected to seismic events induced by gas extraction during the last two decades. As part of this campaign, unidirectional dynamic shake-table tests were performed at the EUCENTRE laboratory on two full-scale, unreinforced masonry buildings, designed without specific seismic considerations or detailing. The first specimen simulated the end-unit of a two-story terraced house, built with unreinforced masonry cavity walls. These walls were composed of an inner load-bearing leaf, made of calcium silicate bricks supporting the floors, and an outer veneer, made of clay bricks with aesthetic and weather-protection function. The floors consisted of reinforced concrete slabs, providing rigid diaphragms; the pitched roof was made of longitudinal timber beams, supported by end gables perpendicular to the shaking direction, and wood boards. The second specimen represented a one-story detached pre-1940’s house, built with double-wythe unreinforced clay masonry walls. The structure included large openings and a re-entrant corner, causing significant horizontal irregularities; the two facades perpendicular to the shaking direction simulated two common gable geometries. The first floor was made of timber beams and planks, resulting in flexible diaphragm; the steep-pitch roof consisted of timber trusses connected by wood purlins and boards. Two similar incremental dynamic tests were performed up to the near-collapse conditions of the two buildings; the selected input ground motions represented realistic scenarios of seismic events for the examined region. This paper summarizes the main characteristics of the two specimens and the shake-table experimental results, illustrating the evolution of the damage mechanisms and the hysteretic response of the structures.

Experimental Seismic Response of a Half-Scale Stone Masonry Building Aggregate: Effects of Retrofit Strategies

A unidirectional shaking-table test on the half-scale prototype of a natural stone masonry building aggregate was performed as part of an extensive experimental and numerical research project, investigating the seismic vulnerability of historical buildings in the city of Basel, Switzerland. The prototype structure was characterized by architectural and construction details typical of Basel’s heritage residential building stock. The specimen represented a building aggregate consisting of two adjacent structural units, poorly connected on one side as if they were built at different times. The building specimen was three-story high and was built using double-leaf random stone masonry walls with undressed blocks and river pebbles. Timber floors, simply supported by the transverse walls, constituted flexible diaphragms in their plane. The two units were covered by roofs with different truss configurations, pitches, and side-gable wall heights. Steel wall-to-diaphragm connections were pre-installed but not initially fastened to the prototype; similarly, longitudinal and transverse steel tie-rods were also pre-installed at each floor without anchor plates. Both retrofit strategies were activated after a significant damage level was reached during the dynamic tests. This paper describes the seismic behavior of the prototype, analyzing the response of the bare masonry structure at various stages of the incremental dynamic test and investigating the effects of the retrofit interventions. The analysis focuses in particular on the dynamic behavior evolution, on the hysteretic response, and on the lateral displacement demand, in relation to the damage limit states associated with the observed mechanisms.

Numerical assessment of the dynamic response of a URM terraced house exposed to induced seismicity

This paper presents the results of a numerical study aimed at extending the utility of a shake-table test on an unreinforced masonry building prototype for the seismic assessment of terraced houses in the Groningen region of the Netherlands. The area, with no recorded tectonic activity to date, has recently experienced ground shakings induced by natural-gas production. Local buildings are mostly made of unreinforced masonry, often built with cavity walls, which were not specifically conceived for earthquake resilience; hence, they do not exhibit any seismic-resistant detailing. Numerical models were first generated and fine-tuned based on data obtained from incremental unidirectional dynamic tests on a full-scale building specimen, performed up to near-collapse conditions. The structure represented the end-unit of a typical Dutch terraced building with cavity walls and a flexible timber roof. Several issues concerning the numerical simulation of the dynamic response of unreinforced cavity-wall systems were addressed in the context of employing an equivalent-frame modelling approach. Analyses were conducted also considering the effect of the nonlinear out-of-plane response of walls. The calibrated single-unit model was then enlarged to numerically assess the effects of human-induced earthquakes on an entire row of terraced houses. A cloud method was selected to establish the probabilistic relationship between ground-motion intensity and nonlinear structural response, using a large suite of records characteristic of induced-seismicity earthquakes. The question of selecting an appropriate and comprehensive measure of shaking intensity for correlation with structural performance is also discussed.

ENERGY DISSIPATION INVOLVED IN THE OUT-OF-PLANE RESPONSE OF UNREINFORCED MASONRY WALLS

The assessment of the out-of-plane response of masonry has been largely investigated in literature assuming walls responding as rigid blocks or assemblies of rigid bodies. Several studies developed numerical integration of single-degree-of-freedom and multi-degree of freedom systems for the simulation of the OOP dynamic rocking response of simple mechanisms. Modelling the energy dissipation involved in such mechanisms is extremely important to capture the dependence of the damping phenomenon with the system frequency. Some studies, recurring to the classical hypothesis of the impulsive dynamics, simulated the energy dissipation by means of the coefficient of restitution assuming as the overall reduction of energy were concentrated at the instant of the impact. In other works, the damping force has been modelled as a velocity dependent acting force through a constant, variable or stiffness proportional damping ratio. The two damping models are compared highlighting advantages and shortcomings of each system. This paper proposes a numerical formulation for the direct equivalence of the two damping approaches for simple OOP one-way bending mechanisms assuming as force-displacement relationship nonlinear elastic tri-linear curves. 2996 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 2996-3010