Foto del docente

Stefano Silvestri

Associate Professor

Department of Civil, Chemical, Environmental, and Materials Engineering

Academic discipline: ICAR/09 Structural Engineering

Research

Keywords: cultural heritage probabilistic seismic hazard analysis for the identification of the design seismic input seismic vulnerability assessment torsional effects of plan asymmetric structures structural reliability Performance Based Seismic Design viscous dampers seismic behaviour of flat-bottom silos containing grain-like material seismic protection systems shaking-table tests structural functioning hysteretic dissipative devices wall structures hysteretic steel bracing devices semi-engineered earthquake-resistant structures for developing countries seismic response of masonry vaults hysteretic dissipation devices

The research group works, since many years, in the field of the earthqauke engineering, with specific reference to new methodologies for the seismic design of civil structures. In detail, the research topics are the followings ones: 

1. Use of viscous dampers for the mitigation of the seismic action upon structures.
2. Torsional phenomena in building structures characterized by eccentricity in plan between the centre of mass and the centre of stiffness.
3. Development of “Crescent-Shaped Braces” capable of satisfying multiple seismic design objectives in the Performance-Based Seismic Design framework (analytical and numerical studies, quasi-static experimental tests).
4. Seismic response of silos: analytical developments and design, carrying out and interpretation of experimental shaking table tests ("SEismic Response of Actual steel SILOS ‐ SERA‐SILOS" European Project, SERA call, 2018-2019).
5. Experimental research, analytical developments and analytical/experimental correlation study on the cyclic behaviour of structures composed of lightly reinforced concrete walls.
6. Probabilistic and deterministic seismic hazard analyses (analytical and numerical studies).
7. Methodologies for the probabilistic identification of reference design earthquake inputs for non-linear dynamic analyses in the Performance Based Seismic Design framework (analytical and numerical studies).
8. Design, carrying out and interpretation of experimental shaking table tests on a 3-storey full-scale building, realized with a structural system composed of squat r.c. sandwich panels at the EUCENTRE lab of Pavia (SERIES project, European FP7 program).
9. Assessment of the structural safety, seismic analyses and interpretation of the monitoring data of historical monumental buildings, with specific reference to the Cathedral of Modena and the 2 Towers of Bologna.
10. Pseudo-static and shaking-table experimental tests on scaled cross-vault models as subjected to in-plane shear deformation ("SEeismic BEhavior of Scaled MOdels of groin VAults made by 3D printers - SEBESMOVA3D" European Project, SERA call, 2018-2019).
11. Earthquake-resistant constructions in developing countries: study of the seismic behavior of gabion-box wall buildings, "bhatar" buildings and rubble-stone masonry buildings.


DETAILED DESCRIPTION 

1. Use of viscous dampers for the mitigation of the seismic action upon structures.
TOPIC: Dissipative systems have widely proven to be able to effectively mitigate seismic effects on buildings. However, still the issue is open of how to insert viscous dampers into shear-type structure in order to reach the best dissipative performances of the dynamic system (structure + dampers). In fact, most of the research works available in literature regarding the problem of damper system optimisation deals with the search for optimal damper sizing for given traditional inter-storey damper placement.
CONTRIBUTION: Analytical identification of the superior dissipative properties of the so-called MPD (Mass Proportional Damping) systems as applied to shear type frame structures. Development of strategies for their implementation in actual building structures. Development of a simple original procedure for the identification of the mechanical characteristics of the manufactured viscous dampers which allow to achieve target levels of performances. Identification of the force reduction factors to be used for highly damped structures, for the intermingling of the inexpensiveness due to the ductility capacities of the structural elements and the efficiency due to the added viscous dampers.

2. Torsional phenomena in building structures characterized by eccentricity in plan between the centre of mass and the centre of stiffness.
TOPIC: Structures characterized by non coincident centre of mass and centre of stiffness (eccentric structures) when subjected to dynamic excitation, develop a coupled lateral-torsional response that may increase the local peak dynamic response. This behaviour has been investigated by many researchers since the late 1970s. Nevertheless a number of issues still remain unresolved in the areas of inelastic response and development of simplified, yet physically-based design procedures. In particular, in order to effectively apply the performance-based design approach to seismic design, there is a growing need for code oriented methodologies aimed at predicting deformation parameter.
CONTRIBUTION: Development of predictive formulas for the estimation of the increment of the displacement demand in such eccentric structures (with respect to the corresponding non-eccentric structures). These formulas make use of simple synthetic parameters based upon the static characteristics of the systems. The effectiveness of the proposed formulation has been verified both numerically and experimentally, these last through shaking table tests.

3. Development of “Crescent-Shaped Braces” capable of satisfying multiple seismic design objectives in the Performance-Based Seismic Design framework (analytical and numerical studies, quasi-static experimental tests).

4. Seismic response of silos.
TOPIC: In the general issues concerning the actions provoked by earthquake ground motion on the walls of flat-bottom grain silos, the assessment of the horizontal actions seems to be of particular interest. These actions are usually evaluated under the following hypotheses: (i) stiff behaviour of the silo and its contents (which means considering the silo and its contents to be subjected to ground accelerations); and (ii) the grain mass corresponding to the whole content of the silo except the base cone with an inclination equal to the internal friction angle of the grain is balanced by the horizontal actions provided by the walls (supposing that the seismic force coming from the base cone is balanced by friction and therefore does not push against the walls). This design approach is not supported by specific scientific studies; as a matter of fact, even though there are many papers on the behaviour of liquid silos under earthquake ground motion, there are no examples of scientific investigation into the dynamic behaviour, let alone under earthquake ground motion, of flat-bottom grain silos.
CONTRIBUTION: Development of predictive formulas for the estimation of the horizontal actions exerted by grain-like materials upon the walls of flat-bottom squat silos during earthquake. These analytical formulas (based on the assumptions made by Janssen and Koenen for their “static” analyses) are currently the object of specific shaking table tests at the Earthquake Centre of Bristol (European SERIES project). The experimental results obtained so far confirm the effectiveness of the proposed formulation in better capturing the actual seismic actions on the silo walls with respect to the Eurocode 8 prescriptions.

5. Experimental research, analytical developments and analytical/experimental correlation study on the cyclic behaviour of structures composed of lightly reinforced concrete walls.
TOPIC: Buildings made up of reinforced-concrete walls represent a structural typology which has been widely used in economic public housing. In these structures, the walls are often characterised by small thickness (15 - 25 cm) and by small percentage values of steel reinforcement. These buildings have shown excellent strength resources even against strong earthquake ground motions: the structural overstrength allows to reduce the ductility demand. However, still few experimental and analytical studies have been performed up to now with the aim of evaluating the ultimate (near-collapse) seismic performances of buildings realised using large lightly-reinforced concrete walls.
CONTRIBUTION: The research group has recently organised, designed and interpreted (by means of appropriately-developed analytical models capable of capturing the experimental behaviour) a series of experimental tests with cyclic horizontal loading (conducted at the laboratory of the European Seismic Centre EUCENTRE in Pavia) upon a peculiar typology (with nonreturnable block-formwork) of lightly reinforced concrete walls. The construction process of such structures sees the realisation of bearing walls through the casting of ordinary concrete inside wood-cement block-formworks. Due to the peculiar conformation of the block-formwork, the structural wall so-obtained is characterised by the presence of lightening alveolar zones. Inside the blocks, before casting the concrete, appropriate horizontal and vertical reinforcement steel bars are placed, so that the structural walls is actually a reinforced-concrete wall. To obtain an adequate characterisation of the seismic behaviour (stiffness, strength, ductility) of such walls, experimental pseudo-static tests with constant vertical loading and increasing horizontal loading have been carried out both upon single walls and upon a H-shaped 2-storey structural system. The results obtained show a good ductile behaviour, yielding horizontal loads comparable with applied vertical loads, and the maintenance of strength to vertical loads after damaging. Degradation of material is substantially acceptable.

6. Probabilistic and deterministic seismic hazard analyses (analytical and numerical studies).
TOPIC: In a Performance Based Seismic Design framework, it is of fundamental importance the determination of the “demand” imposed upon the structure by the seismic action. Structural demand is generally evaluated through series of non linear dynamic analyses (Probabilistic Seismic Demand Analysis) obtained using as inputs selected (typically historically) seismic records which are correlated to specific hazard levels. A key step in the whole procedure is represented by the correct probabilistic evaluation (on the basis of Probabilistic Seismic Hazard Analysis) of the reference (design) acceleration time histories. For each design level (the typical PBSD procedure is articulated in 4 design levels corresponding to a probability of 50%, 30%, 10% and 2% of occurrence upon the life span of the structure), it is thus necessary to identify a group of reference inputs (often referred to as “bin”). Bins are typically identified on the basis of a set of ground motion parameters referred to as Intensity Measures (IM), both scalar or vectorial.
CONTRIBUTION: The research group has developed two procedures for the Probabilistic Seismic Hazard Analysis (PSHA). The probability functions (PDF and CDF) of a selected ground motion parameter (e.g., peak ground acceleration, peak ground velocity, peak ground displacement, spectral acceleration, …) at a specific site, over a given observation time, are analytically developed and elucidated. Both procedures are developed according to Cornell's widely upheld approach (1968). The first procedure is characterized by the treatment of the distance R from the epicentre to the site as a continuous random variable, while the other treats R as a discrete variable. Both procedures lead to closed-form analytical expressions for the PDF and CDF of the selected ground motion parameter. Grounded on the results of the developed PSHA, a vectorial Intensity Measure has been proposed as composed by the PGA and the PGV. Non-linear time-history dynamic analyses carried out using, as earthquake input, groups of seismic records obtained on the basis of couples PGA-PGV have shown to lead to results characterised by smaller dispersion with respect to the results obtained through analyses carried out using, as earthquake input, groups of seismic records obtained on the basis of other suggested Intensity Measures.

7. Methodologies for the probabilistic identification of reference design earthquake inputs for non-linear dynamic analyses in a Performance Based Seismic Design framework (analytical and numerical studies).

8. Design, carrying out and interpretation of experimental shaking table tests on a 3-storey full-scale building, realized with a structural system composed of squat r.c. sandwich panels at the EUCENTRE lab of Pavia (SERIES project, European FP7 program).

9. Assessment of the structural safety, seismic analyses and interpretation of the monitoring data of historical monumental buildings, with specific reference to the Cathedral of Modena and the 2 Towers of Bologna.am).

10. Pseudo-static and shaking-table experimental tests on scaled cross-vault models as subjected to in-plane shear deformation ("SEeismic BEhavior of Scaled MOdels of groin VAults made by 3D printers - SEBESMOVA3D" European Project, SERA call, 2018-2019).

11. Earthquake-resistant constructions in developing countries: study of the seismic behavior of gabion-box wall buildings, "bhatar" buildings and rubble-stone masonry buildings.