| dc.description.abstract | Before seismic events of Northridge (Los Angeles, 17 January 1994) and
Hyogoken-Nanbu (Kobe, 17 January 1995) MRFs were supposed to be the
most reliable seismic resistant systems due to the high number of dissipative
zones that are able to develop Before these earthquakes, especially in United
States, MRFs were realized, generally, by adopting fully welded connections,
which, at the time, were retained to perform better compared to other joint
typologies. In addition, the economic advantages deriving from the adoption of
field fully welded connections, strongly influenced choices of building owners’
and, as a result, led to the adoption of this joint typology in almost all pre-
Northridge steel MRFs. After Northridge earthquake, even though the loss of
life was limited, the unexpected amount of damages occurred in structures
adopting as seismic resistant system welded Moment Resisting Frames put
into question the role played by welded connections on the whole structural
behavior.
Therefore, after the seismic events, two strategies were identified to improve
behavior of fully welded connections. The first one is related to the
improvement of the welding technique, usually strengthening the critical area
subjected to fracture. The second one is based on the possibility of
concentrating the energy dissipation in the beam, reducing the bending
resistant area of beams by properly cutting the flanges in a zone close to
beam-to-column connection. This weakening approach is commonly called
RBS. A new design approach, which has been the subject of many studies in
last decades, has gained growing interest in last years. In fact, Eurocode 8
has opened the door to the idea of dissipating the seismic input energy in the
connecting elements of beam-to-column joints. It has been recognized that
semi-rigid partial strength connections can lead to dissipation a and ductility
capacity compatible with the seismic demand, provided that they are properly
designed by means of an appropriate choice of the joint component where the
dissipation has to occur.
In this work, the attention is focused on this last approach. The first part of the
work is descriptive and deals with the historical development and, in general,
with the seismic behavior of Moment Resisting Frames. In the same chapter
general concepts concerning the component method, as introduced by last
version of Eurocode 3, are given. Finally, the influence of the joint behaviour on
main characteristics of partial strength and/or semi-rigid MRFs is evaluated by
properly accounting for existing literature. Third chapter deals with an
experimental analysis on the cyclic behaviour of classical partial strength
beam-to-column joints. The main scope of the experimental campaign is to
show how to control the dissipative behaviour of joints by properly designing
the weakest joint component and by over-strengthening the other connecting
elements. Therefore, a design procedure is pointed out and the comparison
among the results obtained by cyclic tests is presented in terms of energy
dissipation capacity. In addition, by monitoring during the experimental tests
both the whole joint and the single joint components it is shown that the energy
dissipated by the joint is equal to the sum of the energy dissipated by the joint
components. This result assures that the first phase of the component
approach, i.e. the component identification, has been properly carried out and
that interaction between components under cyclic loads is negligible. Chapter
4 represents the continuation of the work carried out in previous chapter. In
fact, on the base of the obtained results, the goal is to provide a mechanical
cyclic model for the prediction of the overall joint behaviour, starting from
existing literature models. Hence, a state-of-the-art review is first presented
and then, model employed to set up a computer program devoted to the
prediction of the cyclic behaviour of steel beam-to-column joints is shown. In
particular, cyclic model adopts Kim & Engelhardt model for shear panel, Cofie
& Krawinkler model for Panels in Tension and Compression and Piluso et al.
model for the prediction of the T-stub behavior. Finally, in chapter 5, an
innovative approach to improve the seismic behavior of bolted beam-tocolumn
joints, which are affected by strength and stiffness degradation, is
presented. The development of a dissipative device representing the
application of ADAS concept to T-stubs, is detailed. First, a mechanical and
finite element model able to predict the whole force displacement curve of the
so-called hourglass T-stubs are set up. Next, an experimental analysis aiming
to compare the hysteretic behaviour and the dissipative capacities of
rectangular and dissipative T-stubs is carried out. Finally, as a consequence of
the study of the joint component, a further experimental analysis concerning
the application of such devices to partial strength beam-to-column joints is
presented and the results, in terms of moment-rotation curve and energy
dissipation capacity, are discussed and compared to those obtained by the
cyclic testing of classical joints.[edited by author] | en_US |
