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.
