Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
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• Joint panel zones often develop a maximum strength that is significantly
greater than the strength at first yield. This additional strength has been
attributed to strain hardening and to contributions of the column flanges in
resisting panel zone shear forces. Large inelastic panel zone deformations
are typically required in order to develop the maximum panel zone
strength.
• Panel zone deformations can add significantly to the overall deformation of
a moment resisting frame, for both elastic and inelastic ranges of
behaviour.
• Panel zone stiffness and strength can be increased by the attachment of
web doubler plates to the column within the joint region. The effectiveness
of doubler plates is affected by the method used to connect them to the
column.
• In the inelastic range, panel zones can exhibit very ductile behaviour, both
for monotonic and cyclic loading. Experimentally observed hysteresis loops
are typically very stable, even at large inelastic deformations.
Current US building code provisions (AISC, 1997; FEMA 350, 2000) permit the
formation of plastic hinges in the panel zones of steel moment frames under
earthquake loading. Thus, rather than forming plastic flexural hinges only in the
beams or columns, a primary source of energy dissipation in a steel moment frame
can be the formation of plastic shear hinges in the panel zones. Consequently, an
accurate analytical model is needed to predict the response of the panel zone in
order to predict accurately the response of a steel moment frame under earthquake
loading,. The traditional center-to-center line representation of the frame must be
modified to include panel zone deformation in frame analysis.
Several researchers, including Krawinkler et al. (1971) and Wang (1988) proposed
relationships between panel zone shear force V and panel zone deformation γ for
monotonic loading. These relationships have been used as the basis of
mathematical models for non-linear rotational springs representing the panel zone.
Krawinkler’s V–γ relations have been adopted in several building codes (ICBO,
1997; AISC, 1997) as a basis for computing the shear strength of panel zones.
However, it was pointed out by Krawinkler that a new model might be needed for
joints with thick column flanges since his V–γ relations were derived from
experimental and analytical results for panel zones with relatively thin column
flanges. Wang also showed that Krawinkler’s V–γ relations may overestimate panel
zone shear strength for panel zones with thick column flanges.
The mathematical model for strength and stiffness calculations is shown in Figure
4.12. It consists of an elastic-perfect plastic shear panel surrounded by rigid