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