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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
1.25 specified by the seismic provisions of AISC (1992) and therefore, hardening of
cyclic stress-strain curve is expected.
The performance of structural joints under recent strong earthquakes pointed out
the significant importance of weld design and execution. Therefore, beam stubs
were connected to end plates by means of fillet welds, executed with special care
by licensed welders. The welding technology was Flux Cored Arc Welding (FCAW)
with Metal Active Gas (MAG) shielding and no preheating. A filler metal was
selected, characterized by a nominal yield stress fy equal to 420 MPa and a
nominal ultimate strength fu of about 520 MPa, respectively. This deliberate
strength overmatch by the filler metal aimed at shifting the expected failure planes
to the base metal adjacent to the weld. The test values of the yield and ultimate
tensile strength for the weld metal samples extracted from virgin specimens comply
well with the nominal ones as reported in Table 3.4.
3.4.2 Fracture mechanics-based characterization
A series of laboratory tests was carried out to characterize the end plate material in
its different microstructural states. As a matter of fact owing to the filler metal and
the uneven temperature distribution, a welded joint is a compound of three different
metallurgical regions: the fusion zone, the heat affected zone (HAZ), and the
unaffected base metal. The HAZ is the area adjacent to the fusion zone, where the
material has undergone a thermal cycle that alters the microstructure of the base
material, though the temperature is too low to determine fusion.
The microstructural characterization was performed by means of the optical
microscope in order to determine the microstructural state in the different zones.
Then, Vickers hardness measurements were carried out on different regions of the
specimens highlighted by etching. Such results are reported in Table 3.5
A further important characteristic in seismic design is the material toughness, in
particular in the HAZ. In order to meet this requirement filler material was selected
with nominal toughness, as obtained through a Charpy V-Notch impact energy test
(ASTM, 1988) greater than 70 J at -20 °C and 50 J at -40 °C.
A series of Charpy impact tests was then carried out to characterize the notch
toughness values both of base metal and of weld metal in two different directions.
Columns 3 and 4 of Table 5 collect sample values extracted along (L) and
orthogonally (T) the rolling-mill direction as shown schematically in Figure 3.5.
More specifically, one may observe that large differences exist between the
relevant impact energy values. This trend may be explained recalling that along the
rolling-mill direction, the material microstructure is an aligned multi-layered
structure with ductile behaviour made up of ferrite and perlite.
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