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Giada Gasparini

Associate Professor

Department of Civil, Chemical, Environmental, and Materials Engineering

Academic discipline: ICAR/09 Structural Engineering

Research

Keywords: Performance Based Seismic Design torsional effects in asymmetric structures wall structures structural functioning identification of the seismic input sistemi di dissipazione e di isolamento alla base

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. Analysis of the torsional effects induced by the seismic action in structures characterised by plan eccentricity between the centre of mass and the centre of stiffness.
2. Structural reliability analyses with specific reference to the Probabilistic Seismic Hazard Analysis and to the search for efficient Intensity Measures for the creation of a group of seismic records for a given site to be used, as earthquake input, for non-linear time-history dynamic analyses.
3. Study and application of the innovative methodology of the Performance Based Seismic Design (performance framework of seismic design).
4. Optimal damper insertion of viscous dampers into structures for the mitigation of the seismic effects.
5. Analytical developments to assess the action induced by grain on flat-bottom silos due to seismic input
6. Experimental research upon large lightly-reinforced concrete walls.



1. Analysis of the torsional effects induced by the seismic action in structures characterised by plan eccentricity 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: Starting from the governing equations of motion of linear elastic eccentric systems, a key system parameter which controls the maximum rotational response of such systems under free and forced vibration, is identified. This parameter, called “alpha”, is defined as the mass radius of gyration of the structure multiplied by the ratio of the maximum rotational to the maximum longitudinal displacement response developed by a one-story eccentric system in free vibration. A number of numerical, experimental (through shaking table tests of linear elastic and inelastic systems) and field data (from historically recorded structural responses) analyses have shown that the parameter alpha is capable of providing a tight upper bound for the maximum rotational response developed by the eccentric systems starting from the knowledge of the maximum longitudinal response of the “equivalent” non eccentric system.

2. Structural reliability analyses with specific reference to the Probabilistic Seismic Hazard Analysis and to the search for efficient Intensity Measures for the creation of a group of seismic records for a given site to be used, as earthquake input, for non-linear time-history dynamic analyses.
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. 

3. Study and application of the innovative methodology of the Performance Based Seismic Design (performance framework of seismic design).
TOPIC: The design of building structures capable of providing given seismic performances represents a difficult task due to the complex characterization of the seismic action (not a single action but a set of possible actions of different strength and probability of occurrence) and of the structural response. A viable solution to this task may be found in the recently proposed methodologies that fall under the name of Performance Based Seismic Design (PBSD). The core idea of the PBSD resides in the capacity of defining and satisfying given performance objectives, i.e. in the capacity of guaranteeing (within the limits of engineering precision) that a given structural system will perform in a selected manner (performance level) with a given probability.
CONTRIBUTION: Using the statistical characterization of the seismic inputs developed at the University of Bologna, a comprehensive and complete study of the seismic performances offered by an existing masonry structure (Teatro Galli in Rimini) has been performed both considering the building in its actual present state and under several retrofitting configurations. The work involved the detailed determination of the statistical characteristics of the input, the identification of the most suitable response parameters, the framing of the structural demand (and its dependence upon the Intensity Measure used), the search for suitable limits of the response parameters, the comparison with the performance expectations. This work (currently in further development) has so far led to the preparation of a final report for the research project founded by the Italian Ministry of Research titled “Adeguamento sismico di edifici monumentali tramite isolamento sismico e materiali innovative”.

4. Optimal damper insertion of viscous dampers into structures for the mitigation of the seismic effects
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: The researches we carried out at the University of Bologna in the last few years have focused upon the search for the system of added viscous dampers capable of maximising its dissipative effectiveness taking into consideration at once all possible dampers sizing and placement. These researches were performed using both physically-based and numerically-based approaches and have indicated that the mass proportional damping (MPD) component of a Rayleigh damping systems (which is actually physically implementable through a damper arrangement that sees dampers (a) placed so that they connect each storey to a fixed point and (b) sized proportionally to each storey mass) is capable of providing the best overall dissipative properties. This suggests a new and efficient way of inserting viscous dampers in structures to be built in seismic areas, which is alternative to the common (and less efficient) interstorey damper placement.

5. Analytical developments to assess the action induced by grain on flat-bottom silos due to seismic input
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: The research group has carried out analytical developments devoted to the evaluation of the effective behaviour of flat-bottom silos containing grain, as subjected to constant horizontal acceleration and constant vertical acceleration. Keeping the validity of the hypothesis (i), these developments aim to assess the effective horizontal actions that rise on the silo walls due to the accelerations, by means of analytical studies and on the basis of dynamic equilibrium and friction considerations. The results obtained show how these horizontal actions are far lower with respect to those that can be obtained using the hypothesis (ii). To better understand the physical meaning of the results obtained, a physical representation of the results in terms of portions of grain mass which actually weigh (in terms of horizontal actions) upon the silo walls is also provided, in addition to the analytical expression of the horizontal actions.

6. Experimental research upon large 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. 

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