Gennaro Magliulo

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.

Effect of the Seismic Input on Non-Linear Response of R/C Building Structures

The effects of the variability of seismic input on system structural response are investigated. Non linear time history analyses are performed using as input sets of accelerograms satisfying the compatibility criteria with an elastic design spectrum according to the Eurocode 8. The response of elastic SDOF systems, in terms of spectral acceleration, is analyzed; the same parameter, for corresponding non linear SDOF systems as well as ductility demand and strength reduction factors, is also discussed. The analyses are extended to a r/c multi-storey space frame, which is designed according to Eurocode provisions. The main conclusion of the paper is that, when the input is selected according to the Eurocode 8 provisions as recorded accelerograms, a “natural” mean CoV of spectra ordinates is expected; it proportionally conditions the CoV of the parameters characterising the non linear response of structures.

Influence of cladding panels on the first period of one-story precast buildings

During the recent severe seismic events, as L’Aquila (2009) and Emilia earthquakes (2012), the collapse of cladding panels system in precast buildings has been frequently observed due to their connection system failure. Such a damage have demonstrated the deficiency of the actual design approach, that considers cladding panels as non-structural elements, neglecting any interaction with the structure under bi-directional and dynamic seismic excitations. This paper investigates the influence of vertical cladding panels on the first vibration period of one-story precast concrete buildings, with floor rigid in its own plane. At this purpose, a bare structural elastic model and an innovative elastic model of the building with cladding system are implemented and a parametric study is performed. The results of the parametric study show a high influence of the panels on the first period of the analyzed buildings, as well as the inadequacy for this typology of buildings of the simplified relationship for computing the fundamental period proposed by some codes. More suitable formulas are proposed to evaluate the first period in a linear static analysis of one-story precast buildings, both in the case of bare buildings and of buildings with cladding system.

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.

Seismic performance of R/C frames with overstrength discontinuities in elevation

The paper presents the results of a research study concerning the seismic response and design of r/c frames with overstrength discontinuities in elevation. The discontinuities are obtained assigning overstrengths either to the beams or to the columns of a “regular frame” (assumed as reference). Two “regular frames” are designed: one according to the Eurocode 8 (EC8) medium ductility class (DCM) rules and the other one according to the EC8 high ductility class (DCH) rules. For all frames the criteria of vertical strength irregularity of many international seismic codes are applied. Non linear static and dynamic analyses are performed; mechanical non linearity is concentrated at the element ends. These analyses are carried out according to EC8 provisions: for non linear static analysis the N2 method is applied; in the case of non linear time-history analyses, seven real earthquakes, selected in order to fit on average the elastic design spectrum, are used as input. The seismic response of frames characterised by the assigned overstrength is not very different with respect to the “regular frame” one; furthermore all the frames satisfy the Ultimate Limit State, verified by the application of non linear static and dynamic analyses. This demonstrates that the sensitivity of frames, designed according to EC8 medium and high ductility classes, to overstrength vertical variations is low. Consequently, international code provisions on vertical strength regularity should be reviewed.

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.

Cyclic shear tests on RC precast beam-to-column connections retrofitted with a three-hinged steel device

Recent European earthquakes demonstrated that the seismic response of RC precast structures can be significantly influenced by the connection systems. Moreover, during past seismic events, many failures of the beam-to-column connections occurred due to their inadequate strength under seismic loads. The seismic safety of these connections has a crucial role in the overall seismic capacity of existing precast structures. A new connection system is employed as a retrofitting solution for a damaged beam-to-column connection and its cyclic shear performance is investigated by means of two cyclic shear tests on two different configurations. In both the experimental tests, the results demonstrate an efficient behavior of the retrofitted connections under horizontal cyclic loads. The comparison between the performance of the investigated connection and the response of a typical beam-to-column dowel connection allows to discuss the main critical features of the dowel connection system.

Seismic performance of non-structural elements during the 2016 Central Italy earthquake

Non-structural elements represent most of the total construction cost of typical buildings. A significant portion of the total losses in recent earthquakes worldwide, has been attributed to damage to non-structural elements. Damage to non-structural elements occurs at low levels of ground shaking, and can significantly affect the post-earthquake functionality of buildings. However, in Europe, limited prescriptions are provided in the codes for seismic design of non-structural elements and this may partially explain why it is so common for these elements to perform poorly during earthquakes. This paper describes the observed damage to non-structural elements following the 2016 Central Italy earthquake. The most commonly damaged elements were partition walls, ceiling systems, non-structural vaults, chimneys, and storage racks. As a result, it was highlighted the need to introduce seismic regulations devoted to improving the seismic performance of non-structural elements and to reduce the associated economic losses, loss of functionality, and potential threats to life safety.