1. THE INFLUENCE OF CONNECTIONS IN THE SEISMIC RESPONSE OF
STEEL STRUCTURES
Fernando Palladino1
1
University of Naples Federico II, Department of Structures for Engineering and Architecture E-
mail: fernando.palladino@alice.it
Keywords: Moment resisting steel frames, dual concentrically braced frame, semi-rigid
connections, joint modelling, steel structures, steel connections.
Abstract
The strength and stiffness of beam-to-column joints generally affect the overall seismic response of steel
multistorey frames. Therefore, it is necessary to account for the joint mechanical behaviour in numerical
analyses devoted to assess the seismic performance. What is more, beam-to-column joints have a fundamental
importance in case of seismic moment resisting frames, because dissipative zones must be located at the
beamends, so that their rotational ductility supply is strictly related to the detailing of connections. At the
present time, EN 1993:1-8 provides models to compute the strength and the stiffness of connections but no
reliable and effective analytical tools are available to predict the rotation capacity and the cyclic performance
in relation to the connection typology. On the other hand, in order to carry out the seismic assessment of frames
with either partial strength or dissipative bolted joints it is necessary to account for the joint behaviour by using
refined models.
This consideration motivated the present study, which is devoted to assess the seismic performance of two
frame typologies (dual concentrically braced frames and moment resisting frame) with four bolted beam-
tocolumn joints types in terms both of strength and stiffness: a) full strength full rigid joint (with haunch); b)
full strength partial rigid joint; c) equal strength partial rigid joint; d) partial strength partial rigid joint. The
assessment of these frames was carried out through non-linear static analysis (pushover) and through time
history analysis where all the components who has an influence in the seismic behaviour, such as the web
panel and the connection were taken into account.
On the basis of numerical results, the following concluding remarks can be drawn:
- The influence of joint behaviour on seismic performance of MRFs and D-CBFs was investigated by
means of static and dynamic nonlinear analyses.
- Results from Pushover show that, considering the presence of joints (both full strength and full rigid),
the evaluated behaviour factor is larger (Avg. values: 3 storey=9.26; 6 storey=6.76) than that
determined assuming zero-length joints in the models (Avg. values: 3 storey=8; 6 storey=5.95). Also
frames with equal strength and partial strength joint show satisfactory behaviour, with behaviour
factors larger than the EC8 recommended value. The only cases where the behaviour factor is smaller
than the value given from the code is for 6 storey 3 span frame with only one moment resisting frame
system.
- Non-linear dynamic analysis showed a global behaviour of MRFs and D-CBFs. The influence of joint
properties are less affecting the overall response, while some differences can be recognized at local
level. This outcome can be explained in different manners for each structural scheme. For MRFs the
need to satisfy DL requirements leaded to design stiff structure with deep beams, which behave almost
elastically. For CBFs, the presence of stiff bracing significantly reduces the lateral drift demand and
the role of MRF spans are less influential.
- Therefore, semi-rigid connections can be used without affecting the overall response (provided that
the deformability is correctly accounted for the design) also reducing the constructional costs. In
addition, enforcing plasticity into the connections allows using deep beams that are beneficial for the
lateral stiffness and stability, while avoiding the severe capacity design requirements for columns that
will be designed for the strength of connections only.