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The Emilia earthquake: Seismic performance of precast reinforced concrete buildings

On 20 and 29 May 2012, two earthquakes of MW5.9 and MW5.8 occurred in the Emilia region of northern Italy, one of the most developed industrial centers in the country. A complete photographic report collected in the epicentral zone shows the seismic vulnerability of precast structures, the damage to which is mainly caused by connection systems. Indeed, the main recorded damage is either the loss of support of structural horizontal elements, due to the failure of friction beam-to-column and roof-to-beam connections, or the collapse of the cladding panels, due to the failure of the panel-to-structure connections. The damage can be explained by the intensity of the recorded seismic event and by the exclusion of the epicentral region from the seismic areas recognized by the Italian building code up to 2003. Simple considerations related to the recorded acceleration spectra allow motivating the extensive damage due to the loss of support.

Shake table tests for seismic assessment of suspended continuous ceilings

After an earthquake, the failure of suspended ceiling systems is one of the most widely reported types of nonstructural damage in building structures. Since suspended ceiling systems are not amenable to traditional structural analysis, full-scale experimental testing is planned and executed. In particular, shaking table tests are performed in order to investigate the seismic behaviour of plasterboard continuous suspended ceilings under strong earthquakes. Two kinds of ceiling systems, named single frame ceiling and double frame ceiling, are tested. A steel test frame is properly designed in order to simulate the seismic effects at a generic building storey. A set of five accelerograms, used as input for the shakings, are selected matching the target response spectrum provided by the U.S. code for nonstructural components. Three limit states (occupancy, damage and life safety limit state) are considered in this study in order to characterize the seismic response of suspended ceiling systems. The tested ceilings show no damage at all intensity levels, evidencing a low fragility. Three main aspects may be the cause of this low vulnerability: (a) the continuous nature of the tested ceilings; (b) the dense steel channel grid that supports the plasterboard panels; (c) the large number of hangers that connects the ceiling system to the roof, avoiding any vertical movement of the ceilings. Finally, an interesting comparison is made with a previous vulnerability study on a different typical U.S. ceiling system.

Shake Table Tests on Infill Plasterboard Partitions

Shaking table tests are performed to investigate the seismic behaviour of plasterboard partitions. A steel test frame is properly designed in order to simulate the seismic effects at a generic building storey. The tests are performed shaking the table simultaneously in both horizontal directions. At this aim the accelerograms are selected matching the target response spectrum provided by the U.S. code for nonstructural components. To investigate a wide range of intersto- rey drift demand and seismic damage, the shakes are performed scaling the accelerograms at eight different intensity lev- els. The tested plasterboard partitions exhibit a good seismic behaviour, both in their own plane and out-of-plane, showing light damage up to 0.8% interstorey drift ratio and 2g top frame acceleration. Finally, an interesting comparison of the dynamic characteristics, i.e. fundamental period and damping ratio, between the bare frame and the infilled structure is also performed using different methods.

Influence of earthquake direction on the seismic response of irregular plan RC frame buildings

The nonlinear response of structures is usually evaluated by considering two accelerograms acting simultaneously along the orthogonal directions. In this study, the influence of the earthquake direction on the seismic response of building structures is examined. Three multi-story RC buildings, representing a very common structural typology in Italy, are used as case studies for the evaluation. They are, respectively, a rectangular plan shape, an L plan shape and a rectangular plan shape with courtyard buildings. Nonlinear static and dynamic analyses are performed by considering different seismic levels, characterized by peak ground acceleration on stiff soil equal to 0.35 g, 0.25 g and 0.15 g. Nonlinear dynamic analyses are carried out by considering twelve different earthquake directions, and rotating the direction of both the orthogonal components by 30° for each analysis (from 0° to 330°). The survey is carried out on the L plan shape structure. The results show that the angle of the seismic input motion significantly influences the response of RC structures; the critical seismic angle, i.e., the incidence angle that produces the maximum demand, provides an increase of up to 37% in terms of both roof displacements and plastic hinge rotations.

Floor response spectra in RC frame structures designed according to Eurocode 8

Nonstructural components (NSCs) should be subjected to a careful and rational seismic design, in order to reduce the economic loss and to avoid threats to the life safety, as well as what concerns structural elements. The design of NSCs is based on the evaluation of the maximum inertia force, which is related to the floor spectral accelerations. The question arises as to whether Eurocode 8 is able to predict actual floor response spectral accelerations occurring in structures designed according to Eurocode 8. A parametric study is conducted on five RC frame structures in order to evaluate the floor response spectra. The structures, designed according to Eurocode 8, are subjected to a set of earthquakes, compatible with the design response spectrum. Time-history analyses are performed both on elastic and inelastic models of the considered structures. Eurocode formulation for the evaluation of the seismic demand on NSCs does not well fit the numerical results. Some comments on the target spectrum provided by AC 156 for the seismic qualification of NSC are also included.

Mechanical Properties of Plasterboards: Experimental Tests and Statistical Analysis

Plasterboard components are widely used in current buildings worldwide. Despite their extensive use, the lack of a comprehensive test campaign on plasterboards in current literature is noted. An extensive test campaign consisting of 302 tests on plasterboards is performed both in tension and compression. A set of five plasterboard typologies is selected. The tests are performed in two different load directions: parallel or transversal to the direction of production. Tensile strength of the boards was systematically smaller than compressive strength, whereas elastic modulus values in compression and in tension are similar. Two different regression laws are defined for compression and tension behavior. An orthotropic behavior was exhibited in cases where the boards are loaded in tension. The significant influence of board thickness on their mechanical properties was also highlighted. Finally, the most appropriate probability distribution function was estimated for several mechanical parameters, and the corresponding data dispersion is evaluated. The performed activities can be used as reference for future numerical studies involving plasterboards.

Seismic Design of Single-Story Precast Structures for P-Δ Effects

A systematic parametric study is performed in order to both (a) investigate the influence of P-Δ effects on the seismic response of RC precast one-story structures and (b) assess the efficiency of the corresponding code provisions. At this aim, different design approaches are considered in order to critically review current design provisions included in current building codes with particular focus on Eurocode 8. Numerical analyses demonstrate the significance of the P-Δ effects on the seismic demand in precast structures in terms of displacement ductility. A modification of the approach of current building codes is proposed, which is demonstrated to ensure both a safer behavior and more economic structures.

Seismic Demand on Acceleration-Sensitive Nonstructural Components

Nonstructural components should be subjected to a careful and rational seismic design, in order to reduce economic loss and to avoid threats to the life safety, as well as what concerns structural elements. The design of nonstructural components is based on the evaluation of the maximum inertial force, which is related to the floor spectral accelerations. The question arises as to whether the European Building Code, i.e. Eurocode 8, is able to predict actual floor response spectral accelerations occurring in structures designed according to its provisions. A parametric study is therefore conducted on five RC frame structures designed according to Eurocode 8. It shows that Eurocode formulation for the evaluation of the seismic demand on nonstructural components does not well fit the analytical results for a wide range of periods, particularly in the vicinity of the higher mode periods of vibration of the reference structures. The inconsistent approach of current European building codes to the design of nonstructural components is also highlighted. For this reason a parametric study is conducted in order to evaluate the seismic demand on light acceleration-sensitive nonstructural components caused by frequent earthquakes. The above mentioned RC frame structures are therefore subjected to a set of frequent earthquakes, i.e. 63 % probability of exceedance in 50 years. A novel formulation is proposed for an easy implementation in future building codes based on the actual Eurocode provisions.

SEISMIC FRAGILITY OF FREESTANDING BUILDINGS CONTENTS MODELLED AS RIGID BLOCKS

The loss of functionality of health care facilities, which should be guaranteed particularly in the aftermath of moderate-to-severe earthquake ground motions, is typically caused by damage to nonstructural elements, such as freestanding cabinets. The assessment of the seismic fragility of such components assumes a key role in the evaluation of the performance of a healthcare facility. The present work is aimed to assess the adequacy of the rigid block modeling approach in predicting the seismic response of freestanding nonstructural components with rockingdominated response. The outcomes of the numerical analyses show that the considered modeling technique can provide a reliable prediction of the occurrence of rocking mechanism and predict the occurrence of the overturning. In particular, the overturning PFA is slightly underestimated in case a 1.0 coefficient of restitution is considered. But the question then arises as to which intensity measure (IM) is well correlated to the seismic performance of rigid blocks. A fragility study on a number of rigid blocks is therefore conducted in the present paper. Comprehensive incremental dynamic analyses on different rigid blocks highlight that the dimensionless intensity measure PGA/(g tanα) is an efficient intensity measures to predict rocking occurrence in a generic rigid block. The intensity measure pPGV/ (g tanα) is the most efficient one only for large, say R larger than 2.0 m, rigid blocks. Very small, say R<1.0 m, rigid blocks tend to overturn as soon as they start rocking and are therefore ‘‘PGAdominated’’. PGA/(g tanα) is therefore more efficient for such blocks. The use of these intensity measures allows assessing a unique fragility curve for rigid blocks characterized by different geometries, which may serve as a simple tool for the estimation of the damage occurred in rigid blocks after earthquakes. 2926 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 2926-2936

Evaluation of the seismic capacity of nonstructural components

Nonstructural components (NSC) economic impact and the extensive damages due to NSC after an earthquake motivate the research studies conducted in the past few years at the Department of Structures for Engineering and Architecture, University of Naples Federico II on this topic. The seismic qualification of continuous ceiling systems, plasterboard and brick internal partitions via shake table tests is described in the paper. The test campaign on continuous ceiling systems highlights the low fragility of the tested specimen, primarily caused by: (a) the continuous nature of the ceiling, (b) the dense suspen-sion grid, and (c) the large number of hangers being used. In order to test the internal partitions, which are mainly displacement-sensitive components, an appropriate steel test structure is designed. This structure simulates the behavior of a generic floor in a structure that exhibits an interstorey drift equal to 0.5 % for a frequent earthquake, according to Eurocode 8 prescriptions. Three possible damage states are considered in the study and correlated to an engineering demand parameter, i.e. the interstorey drift ratio, through the use of a damage scheme. Extensive tests show an excellent seismic performance of the plasterboard partition walls, which are characterized by innovative anti-seismic details. In fact, they show minor damage when subjected to interstorey drifts even larger than 1 %. The shake table tests performed at different intensity levels on hollow brick partitions, widespread in the European zone, denote significant damage in the tested specimen for 0.3 % interstorey drift and extensive damage for drift close to 1 %.